ist it

SYNTHESIS OF la,Scx-C')':CLOSTEROIDS

by

YI REN

Athesissubmit t ed toth e Sc hool of Gr aduate Studies in parti al fulfilment of th e

requirement s for thedegree of Master ofSci e n c e

DepartmentofChemist ry Memori a l Univers ity of Newf ound land

St. Jo h n' s. Newfoundland August 1992

1+1

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i i

Abstract

Epoxidationof 4,4-dimethylcholesta-l,s-dten- j -c n e with m- ch loro pe rben zoicacid gavea mixture of theep i me ri c 50.,6 a- epoxy - and 5~,6~ -epoxy-4,4-dimethylcholest -I -en-3-one.

unexp ec t ed ly, the major product wa s found to be the 5p,6p·

epoxide,in contradiction towha t hadbe e nreport e d by others. The unambiguo usassignment of the struc t uresoftheseepox:ides was based upon 'H NMR expe riments and X-raycrystallographic analysisof the5a, 6a-epox i d e . In thisthesis, the chemistry of the Sp,6 p-epox ideis describedinthe contextof at tempts ae the synthes isof the correspond ingi«,Sa -cyclosteroids.

The synthes isof anewla, 5a - cyclo s t e r o i d , 4,4-dlmethyl- icc,Sa -cyclocho l esta-3,7-dione. by lithium or ytt e r b i um in liq uid ammonia reduc t i on of the bis-a., p- unsa t u r a t ed ketone 4,4-dimethylcho l e s t a- l,5-die ne-3, 7-dione, is described. The ch emist ryof the reductivecyclizationis discussed. The 'H- NMR spect r a of th e mono- and di hydroxy derivatives of the la,Sa-cyclost e roidrevealan unus uallyhigh-fieldsig na l due toH-9.X-r aydif f ractionanalysisof7~ -hYdroKY -4,4-dimethy l- 1a,5a- cyclocholestan-3-one indicates tha t ring B of the st e r o i d nuc l eus pos s e s ses a boa t conforma tion, and that H-9 part ially eclipses the cyclop ropyl ring.Anani s o tropi c ring

i i i

current effect is postulated to account for the chemical shifts of H-9 inthese cyclosteroids. The chemistryof these compounds are described.

various other attempts at the synthesis of la,5a- cyclost eroidsare also described.

iv

Acknowledgements

I would like to extend my sincerest appreciation to my supervisor. Dr. P. E. Georghiou, for countl e s s and ve ry help ful instructionand discussions during the courseof my research project. I also am greatly appreciative of the relationshipthat we have had.

I wish toexpress my sincerestapp r e cia t i o n to Dr.D.J.

Burnell formany suqgestions, for his superb graduatecourses from which I benfitted so much, and for his ongoing encouragement. Or . C. R. Jablonski and Ms. N. Brunet are gr e a t l y thankedfor th e i r generousco n t ri b u t i on s in the form of numerous high-qualityhigh resolutionNMR spectra.Dr. B.

Grego ry and Ms.M. Ba 9g s arethankedfor theirmass spectral contribu tions.Dr.C.E.Loader is sincerelythanked for his helpin the use of th e pes during the processof my thesi s writing.

Specia l thanksare givento Dr.'long-JinWU andDr. set- 'ling taufor their support. encouragemen t and friendshi p.

I am de ep l ygrat e ful to my fianceeMiss SylviaZ. Zhao for her he lp in typing and for her lo v e, support and

encouragement. These are always warm in my heart.

F'inancial support from Department of Ch emi s t ry an d Memorial Un i v e r s i t y , as well as from Dr. P. E. Georghiouis gratefully acknowledged.

vi

Contents

Title .

Abst r a ct". . . .. ... .. ...•...••.... .. Acknowledgements.

Contents .

i i Lv

List of Figures vii

Listot Tabl es ii t

INTRODUCTION•••. .•• •..•.•• •.••••.• •••• ••• •.•••.•. ...••• •• 1 CHAPT ERONE: S\"NTH ESI SOFSP,6 !l -EPOX'l- 4, 4-DI METHYLCHOLEST-l-

EN-3-0NEAND UNEQUIVOCAL ASSIGNMENTOFITS STRUCTURE•.•••.• ••• ••• • ••••••• • • ••• ••.• ..• . . 13 CHAPTER '!WO: APPROACHES TO lao 5a-CYCLOSTEROIDSFROM

Sp.613-EPOXY-4,4-DIMETHYLCHOLEST-I -EN-J-ONE•••18 CHAPTER THREE:OTHERSYNTHETICAPPROACHESTOWARDS

zu, Sa-CYCLOS TEROI DS...••.•• •. .•• ..•• ••..•. 34 CHAPTERFOUR:SYNTHESISAND CHEMISTRY OF4, 4-DIMETHYL -

r«,5a.-CYCLOCHOLE$TA-3,7-DIONE.. .. . .•.•••• 39 CHAPTER FIVE :OTHER SYNTHETIC INVESTIGATIONS.. ...... 57 EXPERIMENTAL.•.••••••••.••• •.•••• ••• •.. .•. . ..• ••.••. 66 Referen ces.•...•... .. . ...• . .•..•• •.•.. ....•••.•.• • •.• 97 App en dix.. .... .. ... .•••... •.•••. .• •... . ..•..•• •.••. .• 102

vii

Figure1. X-Raycrystal st ru c t u r e of (321 16 Figure 2. 300MHz tH_NMRspectrumof (62)•••. .••.•.•• • •• 44 Figure3. )00 MHzlH-NMR epeccrumof (65a) 46 Figure 4. SelectedNOEcorrelationsfor(65a) 47 Figure5. X-Ray crystalstructureof(65a) 48 Figure 6. SelectedNOEcorrelationsfor(66a)•.• •...• •••49 Figure7. Plut odrawi ngsshowingthetorsion angles

H~a-C&-C~-Cl (l e f t) andH,-C,-C10-C 1(ri ght)

in(65a) 51

viii

TableI. NOED datafo r (62), (6 5 a)and(66a) .... . 102 Tabl e II. Calculated torsionangles.. ... ... . 10 3 TableI I I . Crystal data fo r (32)... .. .... .... .... ..... . ¹⁰⁴ Tab leIV . Crystaldata for(1;5 &)...... 105

INTRODUCTION

There ar e eig ht poss i bl e bicyclo(3.1.0Jring-Aste ro ids. Of theseonl ythe 10..So.-. 1P.S P-. 2P. 4P-. 30..So.- and 3P,SP- cyclost eroids hav e been re po r ted. The following- is a brie f review of somere presentati ve examples.

(i l 2j.\,4 j.\-Cyclo steroids.

Templeton andwie^h reportedthe syn thesesof17j.\-acetoxy- 2~,411-cyclo-Sa-androstan-J j.\-ol (la) and 17~-ace toxy-2p.411- cyclo-Sa-androstan-Jo.-ol (l bl. They empl oy e d a zinc-copp er coup le to effec t a 1.3-elimi n",t i o n of br omine from the cor r e s pondi ng 2a.4a-dibromi d e s(2a)and (2b) respecti vely.Or r et al^1bcould effe ct thesame tra nsf o rmati onbyusing lithium in liquid arm'lOnia (Scheme 1).

(i ii30.,50.- and3P,SP-Cyclos t e roids .

The 30..Sa-cyclosteroidsand3J5,Sll- cy c los t e roidsar ethe bes t known of the bicy clo [3.1.01 ring-Asteroids. They are pr e pa r edbyeit hersolvol y t ic²•1or pho t ochemical·routes,The sol volysis of cholesteryl tosyla te (3) gives 30., 50.- cyclocholesterol 14. ReB).whe r eas, irradiationofcboreae e- 3.S-di en e (5 )yieldS 3P.SP- cy c locho l este rol (6, R_B) (Scheme 2).

~

^{R ~}

^;;,

(2a):R-OH,R.H (2b):R.H, RoOH

Scheme 1

(18):R:.OH,A'.H lIb):R=H,RoOH

(3)

(5)

d:l

OR (6 )

(iii) lp, sj3- and lo..5a -Cyclosteroids .

1j3.Sj3-Cycloste roids can be prepared by photochemical methods via a [a'.xiJcyc Lcaddition reaction. Scheme 3 shows two examples.

r.t.

30days

(8)

niOJlaIle. r.t •.55min 250Wquartz1&11)

(9)'

(10 )

Scheme 3

By comparisonwith the 1j3,Sp -cyclosteroids, the la.5o.- cyclocholesteroids are relatively rare. La ing and Syk e sl.9

reported the synthes is of la.5o.- cy cloc hol e s t-2-ene (11) directlyfrom the in- s ieu formedp-e ctueneeurpncna eeof 50.- chol es t-l-en - 3 ~-ol (1.2) {Sche me 4J.

(12)

pTBCl

Sch eme .

(11)

To account for the fo rmation of (11) from (l:l.a). Laing- and Sykes proposed the mechanism shown in Scheme S. The ini t ial step inv o l ves a C- Sa to C-3 a tr a ns annular hyd r i de displacement of the C-3 tosylate in (12b) resulting- in the formation of a bridg-ed non-classic alion. Protonelimina t ion follo wedby e- bcn d rear rangement resultsin the forma tion of thela,Sa -bridqe (Scheme SI .

(128) : R=H

(11)

Scheme 5

(12b ): R=Ts

H~

The same authors efeo re p o r ted^lOthe synth e sis of 3~­

aceta xy -!a,Sa·cyclocholes tan-6-one (131 by tr eatment of3P- acetQxy-5 ~-hydroxycho les tan-6-one (14.)withthionyl chloride inpyridi ne (Sche me6).

(14)

Scheme6 (13 )

Since compound (131is functionalized at C-6 it could be used to study the potential for a cyclopropylcarbinol- homoallylic rearrangement with concomitant introduction of a functional groupat c-fvia the reactions in Scheme 7,

tUI

Scheme7

(LSI

The resulting 1-substituted cholesterol (15) could be envisioned as a synthetic precursorfo r l-substituted -(e.g .1- hydroxy-) procholecalciferol.s (-provitaminsD)-l ,nou Scheme 8 shows the biosynthesis of l(S) •25-dihydroxycholecalciferol

(1,2 5 - DHCCor "ceLcit.rLo L'"] (16..) from cho lest erol.

procllolecdc:ituol

(p~ltuLlnD3 )

l ^.lm

^{OD.l10btll:l l1.}

c:bolecalc:ituoi(V! t a&1l!.D3)

scbeee 8

(lh .),R.oJr,l, 25-IB:.'C (1 6b) . R••,r-ace

ner.uca" and Barton ec aj.I: re po r t e d tha t a synthet ic analog of 1.25-DHCC (16a). the mono- hydroxyl ated 1(S)- hydroxycho lecalciferol (1-HCC) (1 6b) had biologicalactivi ty comparablewith thatof (16a) itself. Hence, (Ubi as wellas its potential precursor (15) are thems e l ve s impo r t a nt synth e t i c targets .

Georghiouand Just^Hreportedobtaining different results when they used the procedure of Laing and Sy ke s⁹to synthesize (1 3 ). Instead of ob t a in i ng (l3) they obtaine d an isome ric compound(17) (s e e Scheme 10 ) .Compound(17) had spectraland analytical data that were similartothose reportedfor(13). exce p t for ado ub l e t at 15 4.98 ppm dueto H-4 whichwas not no t e dbyLaingand Sykes.

In the i r peper," Laing and Sykes did not pr e s e nt a mechanism to accoun t for the putative formation of (13). Georghiou1Stherefore proposedthe followingmechanism(Scheme 9).Treatment of hydroxy-ketone(14 )with th iony l chloride in pyridi neat low temperature sho u l d produce thechlo r o s ul ph it e (18). In order to form the cycLost.ezo Id (1 3) it woul d be ne c c e s s a ry to havepr o t on abstract ionof the a-hydrogen atC-1 occur to form a carbanion at C-1. An intramolecular nucl e o p hi li c displacement of th e chlorosulphitegroup (19 )by the carba n i o n wou l d result in the formation of the 1a..Sa-

br-idge .

~4!

^{pyridineSOC12}

^{• ~ ¢} _s ^I

⁰

or" ~

(U) (l B)

Pyl~

~-V ^I

⁰

^~~

⁰

Q\,

(19) (13)

Sche me 9

10

Howev e r , i f inst ead, a prot on is removed from the much more acid ic C-7 pos i t ion , an intra moleculardisplacementof chlorid e (40) from the cholrosul phit e grou p co u l d occur resulting intheforma tionof the cyclicenol-sulphite (21) (Scheme 101.In fact, GeorghiouandJust^Uwer e ableto isolate (21) as the exclusive product fr om the low temp era ture re ac t i on of (4) wi t h thionyl chl o ride in pyridine. This compoun dwas very labile and decomposedon neut ra l alumina to afford amon got he r products. compound (17).

Sc h eme 10

Theon l y ot her synthes isof a la,Sct-cyclosteroidis that of Ch ri ste ns e nand aeusch"wh o synthe s i zedIp-hydroxy- l a.5 u- cycloc ho l e s t a n-7 -o ne (22) by lithium in liquid ammonia redu cti on of chol e st -S-en e-l,7-di o n e 1231 (Sc h e me 111.

(23)

11

Sch eme11

(22)

The slL6p-e poxide(24.)can he envisionedesapotential prec ur sor for the synthesis of the cyclosteroi d (25). an anal o g of (13) . As depicted in Scheme 12. (2Ss) could in principlebeformedbyanint ramolecular ring openingof th e epoxygroupby the in t e rmedia t e dianio n (2' ). whi ch in turn mi g h t be generatedby metal-liquidamnon ill.reduction.

Acyclopropy lcarbinol -homoallylic re a rra ngement of the cycl o s t eroid(25b) couldthentra nsf o rmit into.for example . IIl- hydr o xy -4,4-dimethylcho l es t - S-en-3-one (2.7). Compound(27 ) ofcour s ecouldbe a precur sor of a vitamin oJ an alog.

12

o W ^~

^·n

_~3~ 0 41

~

^{(2 5b), R-T t}

^o~

^{Vl t}

^-,-

^{..u. DJ}

acneae 12

The result s of ouref forts toformic.ec -eyereeeeeetus using5/3,6j3- epox ide (2~) andot her potential precursors are the sub jec t of this thesis.

13

CHAPTBll 0llB

srN'l'lUSIS

0'

Sp.~-.PO.IY-".4-DIJIrtHYLCHOUS'1'-1·l!N - J- OJmAND OMfQUIVOCALASSI GNJaN'l'orIT S STRtrC'2'OR4'

Choles t.ero l (2 8) was chosen 45 the star ting compo un d for the synt hesi s of sp.6~ -epoxide (24). Oppenauer ox i dat i o nllof (2 8 ) (Sche me13) yiel de dcholese- 4-en-3-one(29) in7S\yield.

Treatment of (291 withpotassiume- bu e c xtde"in e-bucencj, and in aj ru trappin; of the anion formedat C-4 withiodome thane ga v e 4.4-dimet hyl c h ol e st- 5- e n -) -on e (3 0) in 80\ yield.

Benze ne sel e nenic anhyd r i deUoxi da t ionof4,4- di me thyl c holest- s-e n -a-cne(30)affordedth e 4,4-d i me e h y lcholes ea-l.5-d i e n·)- one e3l) in good yield. Reac tion of (31 ) wit h m- chloro per o xy b enz oi c acid (rrCPBA) in re f luxingdichloromethane solution for four ho ur s ga ve a mix t u r e of the

Sp .6P-

and Sa,6a-e poxides. (2f,. And (32) res pect.ively. A small amount.

112\) of the epoxy -lactone(33) was also formed.

Surpri singly. themajor produc t. wa s t.h e Sp.6p-epo xi de (24.), whichcomp ri sed71% of the mixt.u r e.The Sa.6a -epo xi de (32) compris edonly 9%. Theassignment of st.ructu res to the

epimeric epoxides initially based Cro s s'

cbe erve eIc ne-"-" of the H- 6chemi c al shift value s of othe r

511.61'-

and Sa,6a- ep o xi des. The maj or product (2f,) had the

14

lowe r fiel d che mica l sh ift for theH-6 sign al 183.32 ppm) . which was a br o a d singl e t. The minor product (32) hadthe hi 9 he r fieldchemical shift.for theH-6 signal (Ii 3.10 ppml which was a sharp doubl e t (J=3.6 Hz ).

o 0

I I

:-.::.-:.

o W . o~ . .~

Scheme 13

Nucl earOverhauser ef f ectdifference (NOED) experiment s bo t.h epoxides suggested that the se assignments were

15

cor r e ct.Separate.selectivesaturat.ionof the signalsdue to thea- andP- C-4 methyl groupsof theSa.6a -epoxide (3 :11atIS 0.9 3and 1.36respectively, ea chenha nc e d the H-6signalatIi 3.10 (16' and )\ respectively). By ccneraee, only when th e signalforth ea-C -4methylgroup of the Sp,6J}-epoxid e (24.) at Ii0.94, wa s saturated was th er eany enhanc eme ntof the H-6 signa l at Ii 3.3 2 (20tl. No corre s po ndi n g enhan cement was observe dwhe n the signal for thell- c-4me thyl groupatS1.33

(24 )was ir ra dia t ed.

Brynj o lf f s sen et 41.::reported obtaining a60\yield of 132 ) when they tre ated (3 1) wit h rnCPBA in reflwci n go di chl o r omet hane. The melting poin t and IH_NMR data of what theypre s umedtobe thesa.6a-epoxide(3 2)were identicalwith our data for (2 4). Since our NeED experiments were not nec essa rily unequivocal, direct proof by crystallogra phic analysis wasobtained.Althoughboth(2")and

(32) were crystalline. only crystalsof the lat ter compound were sui ta ble fo r x-ray crys t a llogrllophy. The structure obta i ne d(seeFigure1) confi rmed our origi nllolassignmentthllot (32)was inde e d thesa, 6a-epox ideandth a t theassignme nt of Bryn j olf fss en ec e.l,was inc or re c t.Using their condit i o ns, we ob ta i ned(32 1on l yas aminer produc t.The mlI.jorproductwa s diffe r entfrom theexpect ed.(2 "1.Elementalanalysis,spectral and mass spec t ro me tr i c data ofthi s compou ndwes co n s i s te nt

16

withit being the epoxy-lact one (J31. Thisproductwas most lik e ly formedby a Baeyer-Villageroxidati on of (24)since th e rnCPBA was us e din excess.

Pigure 1. X-Ray crysta l structu reof(32).

17

FromtheX-raystructureof Sa,6a -epoxide(32 ) depicted in Figu re 1, H-6 is situatedeq u i d i s t a n tly from boththen- and the ~-C-4methylgroup_As a re s ul t eachJr.ethyl showed d positiveNOEwhen the signaldue forH-6 was saturated . The torsionangle ofH6 -C6 -C7-H7ais-3 5°,whilethat of H6-C6-C7- H7j3 is 82°. These re su l t s are in acc o r danc e with th e pr e di ction s fromthe Karpluscur v e for the observedcoupling const an t s and spli tt ing pattern forH-6.

In th ecas e of the s/3,6 11-epoxide(24 ). since only thea- C-4methyl hada positiveNOE with H-6,i timp l i e d that theA- ring of 5/3.6p-e p o xi d e (24) couldadopt a chair conformation wh i l e the B-ri ng was still a boat. The absenceof an NQE be t we e n the P-C- 4 methyl and H-6 does not necessarily mean tha t they are not in close proximity. Molecular models indi c a t e tha t both the H6-C6-C7- H7a and the H6-C6-C7-H7~

torsionangle are approximatelySO" .The signal for H-6 isa broa dsing l e t as would be expected from the Karpluscurve, indicating a smaller coupling constant for H-6 thanfor the co r r e s p ond i ng protonin the Sa,6a- e pox i de (32 ).

18

CHAP'l'BR TWO

APPROACHJ!S2'0rc ,Sn-CTCLOSTBROIDS'ROll

513,6!5-BPOxr·4,4- DINBTHYLCHOLBST- l - BN- 3-Ola

Epoxide(2~)wee treated with lithi umin liqui d ammonia unde r several diff e r e nt conditions. For exampl e, higher tempe ratures (- 3 5 °C) . and the use of hexamethylphos phoro us triamid e (HMPA) weretr i e d.The productswhi chwere obtained were mixtureswhich weresepa r a tedby fla s h chro matographyto afford Sp.6p-epoxy -4,4-dime thylch olestan-)-on e (3'). and a mixtu r e of the co rre s pon d ing epi meric )[1- (35a) and 313- alco h o ls (35b) (Scheme 14).

Scheme14

The mi xture of (35a) and (35b)wa s ox i d i ze d dir e c t l y to (34) wit h pyridini wnchlor och ro mate (pe e).Thest ructureof epoxy-ketone (34 1wa s con fi rme dbycompa r i s on wit hthe prod uct obt a inedbycatalyt i chyd rogena t i onof (24).Reactionof (24)

19

withytt e r b i um in liqui dammaniaHgave thesame res ul taswi t h lithium in liqu i dammonia.Thus,althoughthe a,~-unsat ura t ed cerbonyI sys t em could be reduced , presumably via the dica r ba n i on (26 ) (se e Scheme 12), cyclization to (2 5) by in tramo l e c ul ar epoxi de op e n ing did not occur.

predhen" has reviewed the ste r eo c h emis t ry and mechanism of reduc t ion of cyc l ic satu r a t e d and a,~-unsaturatedketones wit h alkali metals in proti c sol v e n ts inc lud i ng liqui d ammo nia. Among the exa mples reviewedby Pradhan is the study sho wninSc h eme 15.

f""h-""",-f""h

oA.;J-.I o~

"

Sc heme 15

The results were interpreted by consideration of the in t e r a c t i ons involving the singlyoccupiedmolecularorbit a l (SOMO) ini tiallyformed whenthe electron was added to the n"

20

orbi tal of the a.~-unsaturated ket.one. The direction of pyramidali za t ionof the radical (and the ensuing carbanionl orbitalwas influenced by interactionswith thea- f r a mewo r k of the molecule.When the angular substituent is hydrogen, the carbanion which is formedwill be mainly pyramidalized in the a-face resulting in the formation of trans product. With a methyl group at the angular position, the interaction of the radical orbital with the It"-orbitalpxedomi net e....since it is almost parallel to it. The resulting carbanion becomes pyramidalized in the~-faceresulting in the formation of the cis product.

IlS. lo.od( ] Sbl 115'"

Scheme 16

21

I tis pos sibletha t the desired transannularcyclization (Sc h e me12) from the51},6t!-epoxide(24 ' di dno t occurbecause the intermediat e car ba nion {261 wa s not pyramidalized favo urably. That is, i fitwere preferen t iallypyramidalized at c-t inthe13-face (26s) as oppo sed to in the a-face(:aSb) it would not possess th e co r r e c t stereochemistry for antiperiplana rattack on the epox.ide, This wouldbe thecase as a re s ult of the interactions with the a- f r a mewo r k, es p eci a lly withtheC-19methyl group.Protonabst r a c t i o n from ammonia mustthereforehave occured faster thanint r amo l e c u l a r cyclization.

It is known that reductions employi ng so d i um or lith ium inethy!aminecan yield dif f e r e nt re sul t s than when they ar e employedin liquid ammonia .15.26 Hallsworthand Henbest;" found that the course of reductive ring ope n i n g of 513,613 - epoxycholestane could be alteredwhen lithium was used with ethylamine. They proposed that a C-5 carbanion was formed direct lyby the reductive ring-opening of the epoxideunder these conditions. Epoxide (24) was therefore treated with lithium in ethyl amineat O°C withthe hope that a carban ion formed at c-s in int ermediate (36 ) would undergo an intramolecularMichael addition tofo rm a 1a.5a -cyclosteroid (37) (Sc h e me 17).

22

(31) C31J

(30)(11'11) USl 150lirol

Scheme17

(UII1:l'1

Amore complexmixturewas obtained than those from the corresponding lithium-liqu idammonia reductions .The mixtures we r e simpli fied considerablybyoxidation withPCC to afford only thr e e compounds , none however being the desired cyclosteroid. The compounds were the C-5 epime r i c 4, 4-

2J

dimet hyl choles t a - ] , 6 - di on e s(38, 3 9)and4,4 -dimethyl-cholest- s-en-a-on e (3 0) . The Sa-diane (38) was the major product (50%) .The SJ}-dione(39), whichwas obt a inedin 32%yield. was epim e ri z e dto (38) by treatmentwi thsodiumme t h ox i de . Thu s, boththe u.~-unsatura t edcarb onyl system andth e epoxidewere reducedunde r these conditions.

The ini tia l complex mixture which was formed re s ul t e d from the fact that three asynmet ric cen tres were produced during the reduction. sec oxidationremovedthe twoasymme tr ic cent res atC- 3andC-6 by conve r ti ng theepimericdiels into thecorres pondingket ones .

Tha t compo u nd (30) obtained ind i ca t e d that elimi nationof the epoxy oxygen had also occurr ed duringthe re act ion withlithi uminethylamine . Hallswo r t h andaenbeet."

re port e d an analogousfindingwhentheyobta inedapp rox imately 11% of chole s t erol when 5~,6~ -epoxychol e s t erol wa s trea ted with lit h ium-et hylami n e under similar cond it ions. The remai nder of the ir reaction product unrea c t ed epoxychole st e r ol. They explained their re su l t by suggest ing tha t car ba nionfo rma tionat c-s was inh i bit edby theproxi mity toC-S of the oxya nio nat C-3 of th e intermediate ((0).Thu s, the on l y rea c t i on they obs erved wasthe formationof ""small amount of de o xygena tion prod uct (c holes t e r ol ).

gDl eLi

(40)

24

2~ ^eLi

(41)

In our case, reductionof the epoxide ring was found.

Carbanioo formation at C-S would not be inhibited as was presumed to have been the case with 51l.6~-epoxycholesterol becausethe enolate oxyanionat C-3 of thein t e rme d i a t e (41) is sufficientlydistantfromC-S due toth e presenceof the rigid double bond and the hindranceof the two methyls at C-4 .

The absenceof any cyclized pr od uctcan be rationalized by th e following ar g ume n t. Form at ion of both (38 1and 1391 suggests that the carbanion at c-s was pyramidalized in approximately equal amounts in the a- and ll-faces. The ca r bani oo thatis pyramidalizedin the ll-fa c e is not suitably oriented for transannular Michael additionandit undergoes

25

pre f erent.i al pr o t onabstraction from theet hy lamine.When the angular substituent is hydrogen. the carbani on formed in the intermed iatewill mainlypyramidalizeinthea- fa ce,resultin~

inthe format i on oftrans produc ts.Withamethyl groupat. the angular po s i t ion the situation cha nges. The C-C a-or bital dominates because the methyl is almost pa r a lle l to the conce r ned orbital so that the carbanionisnow pyramidalized in the ~-d i rec t i on resulting in the production of the cis pro d uc t s.

Onthe other hand. if theC-S carban i onwas fonne dand pyramidalizedin thea-fa c ewh il e the a,p,-unsat ura ted ketone wa s still present. the transannula r C-S to C- l cycliza t i o n wouldha v e been aS- en do- t r i gtyp e cyclizat ion.Accordingto Baldwin's rules. it is a dis favoured process. In our case, there for e, pro t.on abstraction fromethyl aminewouldbeamuc h more favourableprocess than the cyc li za tio n.

Triphe nyit i n hydride ITPTHI and tributyl tin hydride (TBTH) reduc e u,j3,· uns at urate d ketones by ra d i c a l me c ha n i s m.^ll.:lt It was the r efor e of int e rest to determine wh e t h er a radical~induced intramol ecular cyclization¹⁰could be effectedwith oursys t em with atin enola t e radical such as (&2) (Sc h emeIS ).

26

Whe n a dilute solutionof (2:4) wastreatedwithTPTH and azobis-isobutyronitrile {Al BN! , a mixtureof four compounds was obtained. The major product wa s the enol-epoxi de (431 (76%1. the keto - e pox i de (34' (1 4 %)and small amounts ()%and 4%respectively) of the correspond ing epoxide ring-opened products, diol (UI and hydroxy-ketone(.51.

. W

Scheme 18

The fa c t that(441 and(015) were obt a inedalbeitin small amoun ts. indica t e d thatthe epoxi de ringco uld be opene dby

27

TPTH/AIBN . Previous examples of epoxide ringop e ni ng whic h have been re po rted include a,!3-epoxy ket one^{l 1} and thi onodmidezo l Ldes" prep a r e d from ex,!3-epoxyalco hol s . The st ruc t u re of th e hydroxy- k e ton e (4 5 ) was con f i rm e d by its conve r s ion to ^{(3 8 )} by Pee oxida t i o n (Sc he me 17). The enct- epoxide (4 3) was ide n t if i e dby compar i sonwit h t.he pro du c t ob ta ine dbyNaBHj-reduction of (24.), and by pee ox i da t i on of (43)back to (2").

These re s u l ts can again be rationalizedby consi d e r i ng tha t theradical formedat c-f in intermediate (42) was mainly pyr amidalized in the j}-face. This of course, would not be su ita b l e forth e des iredtransannular cycli zat ion(Sch eme IS). A.lternatively ,whateve r it sdire ct i o nof pyramidalization , the C-l radical could, onc e fanned, abstrac t hy d roge n from TPTH fa s t e r than it couldundergo cyclization.

sincewe were unable toef f e c t transannula rcyclization using reductivemethods on the c,p-unsaturatedketon e-epoxide (2 "), we explored thepot e nt i a l for a more typical radical cyc liza t i on route. Thus , as depicted in Sch eme 19, if a bromidewere present atC- S(U), TPTH co u l d remove the halide and the resultingc-srad i ca l in the intermediate (0)might unde r go transannularcycli"zation¹²to the double bond atCl -C 2 toyie ldthe corresp onding la., Sa - cy cl os t eroid (5 0 ).

28

React-i o n of (~ f,1 with HBr in aceti c add gave the bromohy d ri n (U). its correspondingacetate ( 7) and the60- bromo - S!i-hydroxybrornohydrin(48) .The bromohydrinscouldnot bepurif iedbychrotl\altog raphy.and during attempte dseparation bothreverte d back to startingmaterial (:aU. OnlyIn )cou ld be is o l ated and characte rized . Theref ore.the crude

Scheme 19

29

bromohydrin mixture waa treatedwit h TBTH/AIBN in refluxing benzene. The resulting product mixture consisted of three compounds wit h spectral properties consistent with the structures (4 5 ), (5 1 ) and (52) and an unstable fourth comp o und , which we were unable to characterise.

The major product (48%) was identical with (4 5 ). which was obtained previously as in Scheme 18.This indicated that the bromineatom of the bromohydrin could indeed be removedby TPTHto form intermediate (4 9 )with a radical atC-S. but that resulting radicaldid not undergo transannular addition to the a,p- unsa t ur a t e d ketone to form the desired la,sa -cyciosteroid (SO)•

It is possible, of course, that the a,p-uns atur a t ed ketone was reduced first so that no transannular Michael addition would occur. However, as (51) and (5 2 ) were also isolated. this result could imply that radical formation occurred at C-5 from bromohydrin (46) and at C-6 from brcrochydrin (U ), while thea,p-uns a t u r a t e d ketonewasst i ll present. Since compound (51) had a trans AlB ring junction , thisresult suggests that the radical fonned at C-5 (49) was pyramidalized in the a- f a c e which is required for the cyclizationto occur, butth~tcyclization did not occurunde r these conditions. Here again the transannular C-5 to C-l

cycl i zation is a disfavour e d 5-endo-trig type cycl izat i o n acco rdingto Baldwin'srules. Thus, hydrogenab s tractionfrom TP'I'Hby the C-5 radical was likely toha ve be e n fasterthan thecycliz"'t i o n .

Compound (52) wa s found to be inert to PeC oxidation . confi rmingthe pr e s e nc e of a tert i ary hydroxy gr oup .

It is inte rest ing to note that whe n the Sp,6p-epoxid e gr o up was present as in (24), the major product of TPTH reduct i on WdSthat in wh i c h th eC-3 c",rbony lgroup wa s reduced (3) (Scheme 18).Withou t theepnx.ide group prese nt , howeve r, norma l re duc t i on of the ca r bon- c arbo n double bond WlJlS

pr e f e r r e d ( 5) (Schem e 19).

In 1988, Nugent and RajanBlJlBuu repo r ted that bis (cyclopentadienyl)ti tani urn(I II) chloride promotes red uc t i ve cle av a ge of the C-O bon d of anepoxide ri ng to ge n erate a rad ical, wh ich afte r cycliza t i o n cou ld be effici entl y scavengedbya second equ i val ent of ti t an i umlIII l to yield a cyc l i z ed product. Scheme20^{l l}showsan example of this type of epoxyolefin cycliza tion . Bisjcyclope ntad i e ny ll titanium (III) chloride is prep ared by zinc red uctio n of commerc i ally avai l a bl e bis(r:yc l opentadienyl)titanium( IV) dichloride .

31

l:P2lC11'l'10- c r _ ~{Cll'!'10~'

Scheme 20

Itwasth e n of interest to usi fthi s proce sscouldoccur withoursystem.As depictedinScheme 21,itwa s hop edtha t, af t er reductive cleav ag e of the tertia ry c-o bond.the radi ca l formedatC-5in int e rme dia t e(53) wo uldbe suffici e n tly10n9- lived toundergoa disfavoured5-en do-trig ty pe cycli zationto gi veIn,sa- cyc::los ter o i d (2 Sa l.

When epox i d e (2 4.) tr e a t e d with bis(cyclopenead i enyll ti t a nium !III)chloride , th e onlyproduct thatwa s se pa ra t ed besides unreactedepoxide (24.) was dienone (31 1 .Elimin at ionof th eepoxyoxygen obviou slydidoccur,but

cyclization did not.

scheme 21

The reactions shown in Scheme 22 can account for the observed product. In this scheme c-o bond cleavage indeed occurred with radical fo rma t i on at C-5 (53). Instead of cyclization, however, the radical was scevenced by a second equivalentof the titanium(III }reagent to form intermediate (5.).Aftersynelimination from (5.) dienone (U) was formed as th efina l product.

Pyramidalizationof the intermediate radical can again be invokedto rationalizethe observation.The radicalformed at C-5would havebeenmainly~yramidalizedin the~- face (5341.

33

which woul d not be suitablyor i ente d for the cyclization to oc cur. Alternatively, even if a-py r a mi daliza t i o n (S3b) di d occur, the5-endo-trigtype cyc Liza t.Lon from c-sto c-fcould be cons id e r ed as too unf a vour ed and as a resul t the C-5 radicalwou ld re a c t with asecondequivalentoftitaniurn(III) reagent to form(5 4.)•

..w

^~

^;;;0

!"'T'"

~~,

^~

~ "'"

(5)) In a)

scbeee22

15Sb)

J4

CHAPTER'l'HRBB

OTHERSYNT HE T I C APPROACHBS TOWARDS la ,5a~ C YCLOS'l'ERDZDS

Since eff o r t s us ing 513,6~-epoxide (24 ) as a synthetic precu rsor for la, sa- cy c l o s t e r o i d s were unsuccessful, we evaluated Sa,6a -e poxide (32) as an alternati ve starting compound.

Ep o xi de (3:2)reactedwith lithium inliqu i d ammonia at -7 80(: toyieldSa,6a.-e po xy - 4 , 4-di met hy l ch o l es t a n-3-o ne(5 5 1as themajorpr oduc t. Thus,althoughthea,p-uns a t ur a t e d carbonyl system could bereduced, presuma blyviathe dicarbanion(56), cyclization vi a trans annularattackof the epcxi.de didno t occur (Scheme 23).

He r e again the ca r banion forme d at C-l will prefe rentia llypyrarnida li ze in thep~face (S6a )asopposedto thea-fa c e(S6b)by the sameargumen tusedpreviously .However inthi s case the geome t ry wassui t a blefor cyclization, and cycliza t io n fromc- f to C- Swas a 3 (or 5)-exe- eee type bo t h favoured according to Baldwin' s rul e s . However. cyc.tt aetIon stil l di dno t occur. I tseemsto us tha t themainreas onfor th efailureofcyclization .we s tha t theca rb anionat c-f was sufficient l y lonq-lived to unde r go the desire d cycliza t ion

35 before beingprotonatedby the anunonia.

Scheme23

The reactions of (32) with TPTH and TBTH were also investigated (see Scheme 24) .

Sch e me24

36

Again, as inthe correspondingreactionwith (2 ..) (see Scheme 18) . the radical fo rme d atc-a was not sufficiently long-lived to effect cycli zation and instead abstracted hydrogen fr om the TPTH or TBTH more rapidly.

Alike l yproductfr omth e TPTHre d u c t i on of Sex,6 a-epoxide (3.2) would be (57 ). Anattemp t to synth e size(57 ) by direct NaBHt re du c t i o n of (32) gave at least. four productsby c Ic . Insufficientquantities wer e obt.ained to characterize fully the s e products. The epox ide (57) was labile and decomposed either during the react ionor upon work-up.

(57)

In order to ascertainwhethe rradical-inducedcyclization wo uld be eff ected witha diffe rent substrate, compound (31) was reac tedwith TPTH (or TBTH). As depicted inScheme25.

4, 4 - d im e t h yl ch o l e s t- 5 - e n-3- on e(30)and4,4-d i meth y l c ho l e s t a- l,5-d i en- 3p- o l (58) were obtained in 71\ and 12\ yields,

37

respectively. The structure of (58) was confirmed by comparison with the product obtainedfrom NaBH. reduction of (31). Thus , as seen previous ly , the a,jl-unsat urated ketone cou l d be reduced without anycycl i z a tio n occurring.

Scheme 25

4,4-Dimethylcholesta-l ,s-erene-a,7-diane (59) was also rea cted with TPTH and TBTH (Scheme 26). The only product obtained besides unreacted st a r t i ng material was 4,4- di me t hylc h o l e s t - S- e n e - 3 ,7-dianet60 I.This suggested that the tin enolate (61 ) could have been formed but as seen in the previous examples, hydrogen abs t r a c t i on couldhave occurred faster than any Michael-typeintramo lecular addi t i o n to the C- 5 - C·7 a,j!- un s a t u ra t e d keto ne. This would be especially

38

likelyi fthe radicalat c-L were preferent iallypyramidalized in the ~- face (&la), and thus could no t undergo the intramolecularcyc Ldaat.Lon, Ou r resultse Leo imply that the reductionof the A-ring a,~-unsaturat edketone is fas terthan that of the B-ri ng(X,p-uns at ur a t edketone.

Scheme 26

39

CHAPTBR POOR SYN'l'HBSrSAND CHB1lIS'l'RYOF 4,4-DINZ'l'HYL-1.a ,5a-CrCLOCHOLIIS'l'A-3,7-DI ONB (62)

In198 9Wenkert and Moeller^JCreported an intramolecula r reductivecoupling of thebis-a,p -u nsaturatedketone (63) to give (64) in97%yield (Sch eme 27).

(63)

LiINB3

Scheme 27

(64)

It was described earlier (Scheme 26) tha t thebis-a,/3- unsaturatedketone (59)was used to evaluatethe potentialfor a radical - inducedcyc!ization withTPTH (or TBTH). I twas of in t e r e st todetermi n e whether (59) byanalogy with Wenkertand Moeller's system, could undergo a similar intramolecular reduc tivecoupling to give (62) (Sc he me 28).

40

(59)

Scheme28

(62)

Dienedi one (59) was pre~aredby photo-oxidationl~of the dieno n e (31 ) in dioxane with N-bromo 5u c cinimide (NBS). A simila rallylic oxida tion oc c u rre d when4,4-dlmet hy lcho lest-S - en-a-one(3 0 )was tr e at edunde r thesame condi t i on s toyi e l d 4,4-dime thylcholest -S -e ne-3 . 7-d i one (60) (Sc h eme29).

41

o4i:

<311

(3 0 )

d.1~••H:zO.l1ll1l1t

Scheme 29

(60) (10%)

The mechanism of the reactionis believed to involve a brominat ionfollowedbyhydrolysis. as depicted in pathwayI orI IinScheme30.

42

$~

II) ---!!!!...-

Light. : :

(31)

I

',e

c

¢ . _{- .~.}

(51)

(II )

_O~ ~

^---.!!!!.-

~

Light.

~k

(31)

I _',.

~.~~ ..

Uti Scheme 30

43

When(59) wa s reactedwi t h lithiumin liquid arrmonia, a mixtu r e of several products was obtained. The mixture was si mpli f i ed considerablyby oxidationusin g Pee to afford a singleproductin 72\ yield.

(fa l172'l11

Scheme] l

Th i s prod uct showe d two carbonyl ab s o r pt i on s in its IR spec t rum, at 1,142 em-I and 1, 713em"I, whi chare cons i s tent

44

with a cyclopentanone andacyclohexanone,respectively.These data, and the mass spectrumare inagreemeut.withthe 1a,5a- cyc!ostero idstructureCUI.However ,therewas no cyclopropyl C- H infraredabso rpt i ondiscernablein the 3. 040em-'region . Furthermore, the 'H-NMR spectrum was not unambiguous. An ant i cip at e d high fieldsignalcorrespondingtothe cyc.lopropyl, proton^{l 6}cn C-l (H- 1) was no t evident (Fi g u r e 2).

PPM Figure2. 300MHz'H- NMR spectrum of (62) .

45

Although (62) was crystalline and a data set was collectedbyX-ray diffraction, there werein s u f fi c i e nt data points to solve a structure since two crystallographically distinct steroid molecules were found to be present in the unit cell. Therefore in orderto obtain more suitable crystals for X-ray diffraction, derivatives of (62) were prepared.

Selectivereduction of (62 1using NaEK. reduced the sterica lly less hinde redC-7 ketone to givethe 7p-hydroxy compound(65 a) as the major product, and the 7(X-epimer (6 5b ) as the minor product.

lU I us..)(1"1 (UbIC""'l

1'5_ )

Scheme 33

(" cl(10' ,

46

The IR spe c t rum of (6 5. ) showedonl y a single carbonyl absorptionat 1,739 ca-'and a sharpabs o rp t i onee J.620 em-I corresponding to the hydroxyl gro up . The IH_NMR spectrum IFi gue3)revee.iedan unexpecteddouble triplet centredat

a

0. 52 ppm, withth e C-18 methyl signalbeinQ at 60. 7 1 ppm.

COSY indicated that this signa l unexpectedlybe longed to H·9 and confirmedtha t itwasno t du etothecyc l o propylprotonH·

1. In fact. H-1was located asa dou b l e t at

a

^{0.93 ppm.}

Pigure3. 300 MHzlH-NMRspectrumof (6Sa).

47

HE'I'COR, APT and NOED experiments (Figure 4, also Table I in Appendix for detail) were in agreement with the assigrunents given to th e IH_NMR spectrum of (65a). For example, saturation of the signalsduetoH-2a at

a

2.7 4 ppm, and also ofth e H-9 signals enhancedthe H-1 doublet. The doubletripletdueto H-9 isno t as clearlyevidentin the IH_

NMR spe c trum of (iSb) but is shifted downfield into the envelope region.and could not be clearlydiscernedin the 300 MHzspectrum. Saturationof thesignaldue toH-1 enhanced the H-2a andH-9 signals.

Figure (.SelectedNOEcorrelations for (6 5e).

48

The X-ray dif f ra.ction analy si s of (5 Sa) con f i rmed the re. Sa-cyclos t eroidal str u ctur e (Fi g u r e 5). Ring Bis in a clearly definedboat conforma tion , whi c h is co n s ist e n t with theNOED data.

Pigure 5. X-Ray cryst a l struct ure of (6 58).

49

Lithium aluminium hydridewas needed to reduce the C-3 carbonyl of (6S&) to produce diol(66&) (Scheme 32).A small amount of the C-3 epimeric diol (66e) was also obtained.The lH_NMRspectrum of (668) showed thedoubletriplet. although it. was shifted further up field to

a

0.3 7 ppm.COSy confirmed th a t this signalwa s due to 8-9. The H-l doublet was located at 0 0.72ppm. HETeOR, APTand NOED epeccra (Fi g ure 6, also see Table I in Appendix for detail) of (6 6a) were also in agreement with this structural assignment. For instance.

saturationofth e signals due to H-2 0:at

a

^{2.4B ppm, and al s o}

of th e H-9 signal. enhanced the H-1 doublet. Correspondingly, saturation oE signal due to H-l enhanced the H-9 signal (Fi gu re 6).

Figure 6. SelectedNOE correlations for (66&).

50

When compound (621 itself was reduced with lit h i um aluminiumhydride, a 2: 1 mixtureof (66a)and it sC-7u- e o t me r (66:b) was obtained. The ch emi c a l shifts of H-9 in (66a) and (66c)were shifted further upHeld by 0.15 ppm and 0. 2 1 ppm relative to that of H-9 in (65a) .

~ ^\

uo co

(" .J ^{(66bl (Uel}

Thehigh-field IH_NMR signalswhichwereobservedfo r H-9 at

a

^{0.52 ppm for (65a,) and at}

a

^{0.37 ppm fo r (668),}

respect ively, are unpr ecedentedfor such ameth i ne pr o t o n in a steroidmolecule. Mol ec ula r models of (65a) ind icatethat the cyc!opropyl ring partially eclipses H- 9 . This is es peciallysowhe n ringB isinthe boat conformationthat is observe d in the cz-yat a I struct ure of (6Sa) and that is sugges t ed by the NOED experiments on these compoun ds.

sinceX-r ay crystallographic data for the other la,Sa- cyclosteroids that we r e synthesized we r e not available, we conducted molecularmode lli ng ce l cul.ations,l' The calculated

51

torsionangles ofIn). (6 5a). (6Sb)and (66 11.) indicate that the cyclopropyl ring partiallyec lip s e s H-9 in all cases, but withi na very narrow range.The H,-C,-C1Q-C1andco rre s po nd i ng H~..-C6-C~ - C ltorsion angles obtaineddirectly from the crystal structure of (6Sa)are_12⁰and_7°(Figure7). respectively, and from the modelling calculationsthese were-7.rand _7.0°, respectively. The two torsion angl es of relevance are tabulatedin Table II (s e e Appendix).

Figure 7. Plutodra ...ings showing thetorsio nangl e s H",- Ci-CS-C1 (l eft) andH,-C,-C\O-c\(ri g h t)in(6 5a).

I tis possible that the unusual signals observed fo r H-9 in flu encedby an anisotropic cyclopropyl ring current effect. Thiseffect isin tu r n obviously stro ngly influenced by the nat ureof the functionalgroups at C-3 and C-6.

With samples of compounds (6:1), (6 5 11 ~ b) and 166a-c) is o l a ted and characterized, the lithium-liquid anunonia red uc ti o n of (59) was re-investigatedand the crudemixture obtained directly from the reaction was chromatographed. The pr od uc t s obtained consisted of (6:1). the epimeric mono- al c oho l s (6Sa - b ). and the epimeric diols (66a~c) (Scheme31). The combined yieldof theseproductsamounted to an overall yi eldof 90% of the la,Sct-cyclosteroid. sec oxidatio nof the mixtureto(62)resulted in a decreas ein the yield.Reduction of (59) usingytterbium^H in liquid ammonia, followedby PeC oxi dat ion of the crude reduction mixture gavethe same overall yield of the diketone(62 ) .

Th ere are sever a l mechanistic alterna tives that can account fo r this Michae l add i tion of one metal -eno late carba n ionto the othe r a,li-unsa t u r a t e d ketone .

If the eno l a t e carban ion was formed at c-a the transa nnularcyclizationis a favoured)-exo-tri gand / ora5- ex o-tri g ring closure ac c o r d i ng to Ba l dwin ' s rules. On the

53

other hand, i f the C-l carbanion was preferenti a lly pyramida !ized inthep-f a ceof the molecu le(67) it wouldnot possess t.he correct.stereochemi s tryforintramo l e cularMichael addition to the a.li-unsat ura t e d ke t.o ne at C-S - C-7. When dien e dione(59) underwe nt react i on with TPTH or TBTH (Scheme 26). theC-I-C-2doubl ebond W4 Sprefer ent i a llyred u ced wit.h no cor re s pon d i ngre du ctionof the C-5- C·6doub lebond, whic h sugg es t s tha t cercan r cn forma t ion at c-f is more favoured .

~ .

»r-:«

tn)

(",

Alternat ivel y, i f the ca r banion wa s generated more rapi dlyat C- S, analysis ofth eo-framework~'sugges t s thatit is more likely tobe pyrami da li ze d pre domi na nt l y inthe a-fa c e (681 of the molecule. Thi s wo uld fa vour the int r amol e cu lar Mi ch aelcyc:lizati onfromthisdir ect i on ev e n thoughthi s mode would involv ea3·exo-t r i gand/ora S·endo-t rig(d i:!!Jfa vouredl modeof cycliza ti on.

54

The introductionof a suitableleaving functional group (L) at the C-6 position would produce a systemtha t might und e rg o a cyclopropylcarb inol-homoa llylicrearrangement with a suitablenucleophileto give a C-l-substitutedsteroid as shown in Scheme 33.

Sch8lll8 33

Introductionof a bromineat o m at C-6 was accomplished by photocatalysed NBS reaction of (62) to afford the 6a-bromide (69 ).This product was not stable enough tobe purified. When the crude reaction mixture was flash chromatographed. (69) was isolatedin only 8% overall yield.The majorpr od u c t was (59). which could result from enolization of (69) and subsequen t transannulareliminat ionof hydrogen bromide(Sc heme 34).This approach was not investigated any furt her due to time constraints.

55

In)

~--.~

Scheme34

(n)

wenke rt and Mo e lleru had no t ed that exposure of thei r tetracyclic produe t (,,) tome t han o l in 25\ sulphu ric acid re s u l t ed inarearranllementto af fo rda spirodike tone(70)via the enol (71) (Sch eme 35) . We were ehe re f or e int e r e s ted in examining the re act i vity of ou r cyclosteroid (62) using similar reaction conditions.It wasanti cipatedthatif thec- 1-C-IO bond were tocleave as sho wn in (72a). that.a sp i r o compoun d (73) would be expected. If however, th ec-f ~C-5 bond were to break as shown in (7:lb). a cyclopropanol (7.) coul d be formed but it would not be expectedto survive the acidic co ndi tions. and would rearrange to form (75) and/or

56

(7 6 1.Ofcourse,thefo rme r would be producedi fthe C-S -C-7 bond cleaves,andt.he latter would be producedi fthe C-6 -

c-

7 bond cleaves. When (62) was subj ected to Wenkert and Moeller's reaction conditions however,no change was observed.

even when the mixture was refluxed for several hours.

I

.~.- ~ -.@. -,., ~

Schema 35

57

CHAPTER P'IVI:

O'l'HBR SYNTHrz'ICINVBS'l'I GA'l'IO NS

Theapproacheswhich we hadinvest igated up tothis point all us ed cholesterol derivatives that ha d gemi nal methyl gro ups at theC-4 pos ition. Cholecalcif ero ls, of course, do not cont ai n thesemethyl gro up s at C-4. Howev e r since this posi ti on is very la b i l e, the methyl groups in this st udy sho uld be considered toha ve been readilyac c es sible"blocking groups· forC-4.The methy l groupsarenottobe consideredas be i ng typical ·pro t e c t ing group'sinceonc e introduced, these methyl gro up sca nno tea s i l y beremoved .A dithianewouldha ve serve d as possiblya better protecting group for C-4 as it wouldmore easilybe removedafter completionof thede s i r e d tra nsfo rmat i o n. However,besidesbeing ea siertosyn t hes i z e , 4,4-dimethylsteroidshad anotheradvantagein that th e r e are many tha t have been repo r t e d in the lite rat ur e, potentially makingcharac terizati on of our productseasier.

sincewe hadsu c c e e ded informing the 4,4-dimethyl-la,5- cyc l os t e r o i d (6 2 ) we were interestedin determining whether thesynthe s is the correspondingla,5a-cyclosteroid (7 7)whic h doe s not contai n the geminal dimethyl group. Scheme 36 out l i ne s the reactionse que n c e examined.

58

M OW

phot o - oxidation..

MO&

(181 (791

l~ly·i.

~

^•old-da tioo

00&

(81) 1801

!...

~

^{toi,-noeHK] I}

JtX

(83) (71 1

r"'to-oxidati~

oW

(8 3 )

Scheme 36

59

Directphotooxidaeionof cncfesna- t, 5-dien-3-one (83)or cho l e ste ryl acetate(78) usingFinucane ' smeth od " (Sc h eme 29J produc e d complex mi xt u r e s who s e compon ent s coul d not be se pa r a t e d . However,3p,-acetoxycholest-S-en -7-one(79 )coul d be obt a i ne d in SB% yieldby HgBrl-cat alyzed^Jl uv-i rra d iat i on of (78). Hydrolysiswith potass i um carbon a t e converted (79) to its co r r e s po n di n g alc ohol (80) . Attemptedoxidation of (80) using several different reagen t s fai led to give useful quantities of the de sired product. cho l e s t-5- e ne - 3 . 7-dione (81).

pe eandpyridinumdichromate (POC) oxidationof compound (BO)allproduced comp l exmi xtures whose componentscould no t be separa ted and iden tified . Oxidation usi ng Swern¹⁹ condi t i onswasthenevaluatedbyco nd uc t i ng model experiments on choles terol (28). Choles t-4-en -3 -one (29) was easily obt a i ne d as a majorproduc t. Howe v e r, when oxidation of (80) using the same condit ionswasattempted, the reaction failed togotocompletion . In fac t only 10% of (80)was ox i d ize d to the des ired cho l es t- S-ene-].7-d i on e (81 ), with most of th e startingmaterial (80) remaining unc ha ng ed.

Vario us modificati ons to th e conditionswere undertaken to enhance the y LeLdto acceptable levels. At temperatures below -78"C no reaction occurred . The most suitable

60

te mper a ture was·60°C.When therea ct io nwas carrie doutabo ve o"e lllOr e und es ired sid e prod ucts were produ c e d . When two equivale n ts of activat e dDHSOwereused, the yield did not incr e a s e. If 4 lar;e exce s s of OMSOwere used however. the rea ction prod uced a compl e x mixtu r e. which could not be se pa r a t e dand characterized.

Since(81 )coul d notbeobtai n ed in4 reasonableyie ld . thisapproached wasabandoned.

We al so conduc ted a preliminary investi ga t i on of the react i on seque n ce sho wninSch eme 31. It was hoped tha t with th e absence of thege mina l methy lgr o up s at C-4. thatsteric hi n dr a nce would be reduced towards the in tramo lec u l a r nucleop h i li c epoxide openine)' of the 5~ .6p- epoxide lUI or Sa.6a -epo xide (85). as shown in (S61{Sch eme 371.

--"'-"'-----:11 ~ -

Scheme 37

61

Epoxidation of ch olests-l .5-dien-3-one (83) withrnCPBAin reflux i nq dichloromethane solutio n for fo ur hours gave a mixtu r... of the sa,6a- epoxycholest-5-en -)-one (851. 613- hy d ro xy chol e s ta-l,4 · di e n-)-on e (88 ) and 6a -hyd r o xy chol es ea - 1. 4-d i en- J-on e (89 ) . Theyieldof Sa,6a-epoxide (85)wasonly 14 \. withnone of the Sp.6p-epox ide (8Ube i ng obtained. As expecteditappearedthatthese epoxidesweremorelabile than th e i r correspondino dimethyl analogs (24) and (32). Th e epoxide ringope ni nqse qu e nc e shown inscheme 38 account sfor the formation of (SS) and 189) fr om (84) and (85).

respecti vely.

ScheJle 38

62

Since we only had 14 mg of (8 5) and due to time constraints its reactionwithlith i umin liquid ersecme was not examined.

Halsall ee az;" have reported obta i ning a a-nc c-kee c- aldehyde (9 0) when SIl,6p-epoxy-4 ,4-dimethylcholestan-)-one (91)wasreacte dwith BFI-etherate .They were unebte to "'55igo the stereochemistry tothe aldehyde at

c-s

in (90). Si nc e we ha d theana logo us

Sp.

6~-epoxide (2')on hand.we examinedits reactionwithBFI-etherate.The reaction produceda mixture of seve ralproducts,the major one(77 %) being(921 (Scheme 39).

This compound and the analogous (90) obtained by Halsall et a.z. can be fonned vi a a pinaccLet.ype rearrangement . Mechanistically ,the C-S aldehyde groupin bothproductscould be expectedto be "13 ", orcisto the C-19methyl group.

NOED experiments on (92) were undertaken. The IH_NMR signals due to the a- and ~-C-4 methylgroups and the C-19 methyl groups were loca ted at 0 1.1 7. 1.04 and 1. 04 ppm.

respectively. Saturatio n of the signa l due toth e aldehy d e proton at 0 9.57ppm enhanced th esig nals due toboth th eC-19 methyl and the~-C-4methylgroupsby 1%.Thisevi denceruled out the possibility of ana-configuration for the aldehyde group since in this case only the 4a.-methy1cou l d be expected to be positively enhanced. However, these results were no t

63

neccessarily unequ ivoc al since the C- 19 and ll-c-4 methyl signals over la pped and only a relati vely small NOEDwas ob s e r v e d. Suitablecrystal sof (921 forX-raycrystallography co uldnot be obtained.therefore(93)wa sselectivelyreduced with NaBH4to the correspondingketo-carbinol (93).

" ~

^B'3lorAIel))

o4?

1::t&1 Ilt2} (7 '1%)

o~

^BI']

o4?

(91) (901

Scheme 39

64

(93)

NOED experime nts on (93) confirmed the structure . Ir r adi a t io n ofthesiQnal dueto the methylene protons of t.he carb i no l gr ou p at

a

3.71 ppm enhanc edthe sig nalsdueto the C-19 me thyl groupat 81.21 ppm andth eP-C-4methyl group at cS 1.09 ppm by 1.n and 2.3\ , re s p e ct i ve ly. The likely mechanism for thefo rma t i o nof (921is rrostlikelyas sho wn in Scheme 40 . Reaction of (~fo)wi t h AIel,q4v eessentially the same results.

(2. ' (131

0 $ _, ^@

",

Sch6lllefoO

65

Byextension of these arguments therefor e.the product of Halsall eeal.. (901,most likelyalso had the aldehyde group at C-S in theP-pos t i on, cis to the C-19 methyl group.

66

Me l e i ng points (mp ] were determined on a Fisher-Johns appar atus and are uncorrected. Infrared (irj spectra were recorded on a Mattson Pola ris FT instrument. Mass spectral (ms ) data were from a V.G.Mi c r omas s 707 0HSinstrument.IH_NMR spectra were recorded with a GE GN-300NBSpectrometerat 300 MHz.uC_NMRspe c t r a were recorded with the same instrument at 75. 47 MHz. The solvents used are noted in the experimental details. Proton nuclear Overhauser ef rec'; difference (NOED) spectra we r e obtained from zero-fi lled32K data tables to wh i ch a 1- 2Hz exponentia l line-broadeningfunct ionhad bee n applied. A set of four "dummy- scans wa s employed to equi libra t e the spins prior todata acquisi tion.No relaxa tion de l ay was applied between successive scans of a give n fr e qu en cy . Ultraviolet (uv) spectra were determined on a Uniearn sa.BOOUltraviolet spectrophotometer. Microanalyses were perfonned by Canadian Microanalyti c al Servi ce Ltd., Delta, B.C.

Preparat ionofSIl, 6l3-Epo>ey-4,4-dirrrethylcholese-l-en-3-one(24) an dSa,6a-epoxy-4, 4- di rnethy l c hol e s t- l- en- J - one (3:1) .

A solution of dienone (31)1' (l050mg, 2.55l'mIOl) in20 ml CHI C l }wasre fl ux e d under argon. m-Chloroperoxybenzoicacid

67

(5 0 - 60 \ . 865mq. 2.76mrnol) in20 ml CH2C12was added dropwise over 0.5 h. The mi xt ur e was st i rre d at re flux for 4 h. The soluti on was extra cted with ether (x4l and the combined organic lay e r s werewash ed with 10% NaKCD) Ix 3 )an dsat u ra t e d NaC l (x 4) .dried IMgSO,)and concentrated .Chroma t ography(10%

ben z en e inhexaneover alwniniwn ox i d e(Gr a d eII I) yieldedth e lact on e (33) (14 2mg, 12\ ), Sp,6p-epoxide (2") (77 6mg. n\) andSa,6a-epo x ide(32) (99 mg, 9%1. For(2 4 1.m.p•101 -1 02"c (Found : C. 81.79 ; H, 10.4 4%. C19HU01 requires C. 81.63 ; H, 10.87 ; 0, 7.50%); v....fCCl,l/em·^l 1687 (a, p-uns atu rat e d keton e); OM (300MHz;CDCll) 0. 67 (3 H, a,IS-Me) ,0.86 (3R , d.

J:::6.6Hz, 26~Me). 0.86 (3R, d, J:::6.6Hz, 27-Me). 0.90 {3R, d, J=6.5Hz, 21-Mel . 1.77 -1.90

uu.

mrn), 0.94 (3H, S, 4a-Me) , 1.27 (3K,S, 19-Hel, 1.33(3H, 5,4j}-MeJ , 2.01(l H, dt,J=3 .4 , 12.7Hz),2.14 (lH, ddd,J=2.2 ,4.3,14. 9Hz, 7- H) ,3.32 (lH, e, 6-H) , 5. 9 3 (l H, d, J=10.5Hz, 2-H), 6.90(lH,d , J= 10.5Hz, 1- H); m/z (%): 426(65 , WI, 398(2 5) , 38 3(1 5 ), 356 (1 0 ), 339( 4) , 295 (5 ) , 247(17), 161(19), 136(82 1, 107(58), 81(61 ), 43 (1 00 ) . For (32), m.p. 132.0-13 5.0 -c (Fo u n d: C, 81.69 ; H, 10.79%.

C2oH4,02requires C, 81.63 ; H, 10.87; 0, 7.50 %) ; V.... (CCl, )/crn·^l 16 8 1 (a, ~- unsa tura t edke tone); lSK(3 00MHz;CDC1)} 0.6 5 (3K, e , 18-Me),0.8 6 (3H, d ,J=6.6Hz, 26-Me), 0.86(3H, d, J=6.6Hz, 27-Mel, 0.90 (3H, d, J=6 . 6Hz, 21-MeJ, 0.9 3 (3R, a, 4a-Mel.

1. 31 (3K, 5, 19- Me) . 1.3 6 (3H, 5, 4j3-Me), 3.10 (l H, d, J=3.6Hz, 6-H), 5.99 ua, d. J=10. 3Hz, 2-K), 7.11 (lK, d,

68

J ...IO.3Hz, I-H);m!z(%): 42 6 ( 2 1 . M"l, 411 ( 9) ,]8](13),365(4).

343(3). 30 D(] ), 275(10).247(1 5). 43(100).

X-r ay sc:ructuredeterminationof(32).

naee collect ion wa s on a RigakuMC6Sdiffractome ter wi t h graphit e monochromatedCuKCl ra d i a t i on {A. '"1.54178 AIand a

2KW sealed tube generator. Crys tallog raphic data are summarized inTa bl e III. Ce lldime nsionsweredeterminedby least-squa res refinement using the set ting angles of 18 care ful lyce ntredreflectio nsinthe range48.34<29<49.61° . Data we re collected at a tempe ratureof 25 t. 1°C using the w26scan techni qu eto a maximumof29va lueof 120.1°.Omega scans of severa l intense reflec tions. made prior to data collection. had an averagewidth at half- heightof0.]5'" with a take-off angl e of 6.0". Scans of (1.84+0.3 tan 9)"were made at a spe ed of 16.0"/mi n {i n omega}.The weak reflect ions II <10.00'( 1)) wereres ca nned (maxi mum of 2 rescans) and the counts wereaccum ul t e d to assur e good countingstatistics. Stationary backg roundcountswe r e recorde doneach side of the refl ection. The rat i o of peak count ing time to background counti ng ti me was 2:1. The di ame ter of the inci de nt beam colli mato r was 0.5mm and the crys t a l -to-detectordistance was 400.0 mm. Reference reflec tions mea sured during data collect ion showed no decrease in int ens ity. An empi r i c a l absorptioncor r ectionwas appliedusing the DI FABS⁴⁰prog r am,

69

and res ulted intransmi ss i on fact ors ranging from 0.74to 1.1B. Co r r ections were applied for Lo r entz and polarization effe c ts. Acorrec tio n for sec ondary ext incti onwas applied (coeffi ci ent ".0. 7 60 59 E-06 ) . The st r uc t u r e was solved by direct met hodsu,^1Z Non - H atoms refined eit her ani s otropically or is o tropica lly. Full-matri x lea st-square s refinements onFconve rgedto R=O.lOO, R.."0.013 , G.o.F..2,80. Weights were based on coun t i n g stat is t ic s and include d a factor (p =O. Ol) to downweigh t the intense re flec tions. The la r gestpeaks in th e final difference Four i e rmap were +0 .31 and -0.32e/A)respectively.Neutral atomscatt e ri ng factors we re take n fromCromer and Waber u. Anomalous disp e r s ion eff ec t s were includedin Fc a lc^H•Val u esfor.6.f' and .6.f"were taken from cromer^t5• All calcula tions were made with the TEXSAN'~crys t allog raphicsoftwa r e . Figure1wa spreparedfr om theoutputof PLUTQt7 .

Format ionof 5~,6f!-epoxy -4 ,4-dimet hyl-4-oxa-A-homoch olest - l - en-a -cnela c t one (33 ).

Asol ut ionof dienone (31 ) (51mg, 0.12 lMI01) in 10ml CHlCI1was re fl uxe d unde r Ar .m- Ch1o r o p e r oxy b enzoi c acid (SO- 60\, 300mg, 0.96mmo11in10m1CH1Cl1was added dropwise over 0.5 h. The mi x ture was st irred at reflux for 12 h. The solution wa s extract ed wi!:h ether (x4 1 and the combi ned or gani c lay e r s werewashe dwith 10%NaHCOdX3 ) andsaturated

70

NaCl (X4 ) , dried (Mq SO,l and concentrated.Chromatography(1 0 % benzene in hexane over o5lurninum oxide) yielded the la c t o ne (3 3) (27 mg, 48 %). and a mixture containing Sa,6a-epoxide (32)j6mg,12%).For lactone(33). m.p .177.5 -178.5"C(Fo un d : C, 78.24: H, 10.09%.CnHuO)requires C. 78.68 : H, 10.47; 0, 10.841);V"&Jl(CCI,)/c m' 1754te-LececnetraN(JOOMHz; CDCI_{l )} 0.64

(3 H, S. IS-Me). 0.86 (3H. d, J=-6.6Hz., 26-Me l. 0.8 6 13H, d, J=6.6Ht.. 27-Mel. 0, 89 (3H, d. J:6.6Kz. 21-MeJ. 1,19 (3H. s, Me) ,1.3 1 (3H, s,Me), 1.39 (3H, s,He), 1.77-1.90 (lH, mm) , 1.9 8

ua.

dt,J=3.4,12.6Hz), 2.13 (lH. ae , J:3.2,14.0Hz ,7- Hl , 3.2 0 (l H, br d.J=1.9Hz.6-H), 5.19 (lH,d, J:7.3Hz,2-Hl.

6.26 (lR, d , J=7 .3Hz, I-H): m/z(%): 442(1 , WI. 427 (3 ).

399(13 ). 371(10 ), 353(2), 329(3), 287(3), 262(7), 175(6 1 . 153( 25),12 3(4 0),43(10 0 ).

Preparation ofSp,6~-epoxy-4, 4-d i me t hy l cholestan-3 -one (J4). Freshly condensed ammonia (30 ml, dried over sodium at -7 8 eel was allowed to distil in t o a stirring suspens ion of lithium (20 mg, 2.9rrmoll in anhydrousTHF (5 mll at -78°C an d the stirring continued until the metal had dissolved . A solution of 5~,6~·epoxide (2 4) (1 9 0 mg, 0.45 mmol ) in an hy d r ou s THF (5 ml) was added dropwise over 1 h and th e solutionstirred at -78°Cfor a.nother 1 h. Enough NH~Clwas addedto discharge the blue colour and the atnnloniawas all owed to evaporate.Upon the addition of 30 ml of ether and 20ml of