3.6.Case of the methyl derivatives: short side chain effects

II-3.6.a.Resolution of rac-(Me)₂-DMQA 1b

As mentioned, to form water soluble compounds, another possibility was the introduction of voluntarily short alkyl chains. We thought then of derivatives from methylamine addition 1b since it is the smallest possible alkyl side chain which would still allow the same biological properties without the need for protection or any of the drawbacks of the purifications observed before. The possible synthesis of the compound in only one step from the (DMP)3C⁺BF4 salt 6 would also be straightforward and time saving.

The racemic cation was thus obtained in a very good 80% yield (Scheme II-7, 1b, vide supra) and the purification process was fast, allowing the precipitation of the product by simply adding water, which afforded the isolation of the desired salt [Me2-DMQA-1b][BF4] as a dark green solid. The “sulfoxide” addition protocol was then carried out on the racemic 1b to form the two possible diastereoisomers for this C2-symmetrical molecule. The immediate discoloration upon the addition of the anion on the cation, a qualitative indication of the full conversion, was lost after a few minutes (Scheme II-12).

  Scheme II-12: Synthesis and resolution of Me2-DMQA 1b

To prevent such an issue, we decided to quench the reaction immediately upon the final addition of the anion on the suspension of DMQA in THF. This provided less degradation or starting material regeneration and was chosen as a rule for the future resolution protocols. The TLC plates showed in this case, only two UV-detectable spots which gave upon heating respectively pink and green colour (Figure II-7).

Figure II-7: TLC plate (SiO₂, Et₂O / EtOAc 70:30) of the dimethyl sulfoxide adducts with their HPLC separation chromatogram

Previous experiments had led us to identify the pink spot as the sulfoxide adduct of the corresponding DAOTA moiety (§ II-3.4., vide supra). What was then surprising was the obtention of a single green spot. At first, we thought that a single diastereisomer had been possibly obtained but this was inconsistent with the colorless crude reaction that indicated that both enantiomers of the cation had reacted, and hence, both diastereoisomers ought to be present.

O N N

O S

p-Tol O

(R)

38 | P a g e  Then, the only explanation was a lack of difference in the retention factors of the formed diastereomeric adducts on TLC (Figure II-7, vide supra). This was confirmed by the ¹H-NMR and ¹³C-NMR spectra showing two distinct compounds in a 1:1 ratio. This result was further established by HPLC (tR= 37.54 and 42.06 min, Figure II-7, vide supra).

After removal of the DAOTA side product by standard chromatography (SiO₂, Et₂O / EtOAc 70:30), a screening of various eluents was performed which did not affect any changes in the retention behaviour of the diastereoisomers.

Thus, the only way we could find²² to separate them was using a semi-preparative HPLC on a regular achiral nucleosil column (Macherey Nagel, SP 250/10 Nucleosil 50-7) with

nhexane/ⁱPrOH 90:10 as eluent (flow 3 mL/min, 23 °C, P = 39 bars, 200 μL per injection) after the optimal analytical conditions were found on the analytical nucleosil column (CC 250/4 Nucleosil 50-5) with ⁿhexane/ⁱPrOH 97:3 as eluent (1 mL/min, 4 μL per injection).

It is worth mentioning that these adducts were sensitive to light and required protection during the course of manipulations. Several times, we encountered problems with precipitation in the injection vials that required the resolubilisation of the sample in order to reinject it. The compounds were only soluble in pure isopropanol and any traces of hexane afforded immediate precipitation.

Furthermore, the separation was strenuous (the selectivity parameter α being very low, only 42.06/37.54 = 1.12); we could inject on the semi-preparative column a maximum of 300 μL per run from rather dilute solutions, (10 mg of crude in 1.5 mL of ⁱPrOH, c 0.013 M) otherwise no good enough separation could be afforded to isolate enantiopure material. Even with that, some sizeable loss was nevertheless encountered because the cut-off between the two diastereoisomers was not distinct enough on high loading and thus three fractions were recuperated every time: the (-)-(R, M)-10b, the (+)-(R, P)-10b and the mixture of the two that was reinjected at the end. Subsequently, the yields were a bit lower than usual (82% vs 96-98%

for both adducts combined). Nevertheless, we were able to recover (10 to 15 mg per diastereoisomer) enough material for the next steps.

22 Since even the association of the racemic 1b with enantiopure BT afforded poor yields, with a small amount of compound precipitated upon the addition of CHCl3.

We confirmed the purity of the samples by reinjecting each separated fraction in the same conditions employed and the chromatograms obtained (figure II-8) reascertained their diastereomeric purity.

  Figure II-8: HPLC chromatogram confirmation of the diastereomeric purity of the two separated fractions

of (+)-(R, P)-10b (top) and (-)-(R, M)-10b (bottom)

The ¹H-NMR spectra of the separated fractions of 10b in comparison with their initial crude mixture (Figure II-9) showed the distinct set of signals of each diasteroisomer and hence the good separation and purity of the samples. The analysis of the ¹H-NMR of the different fractions allowed us to assign all the signals as shown in figure II-9. Such details will be given later in this chapter (§ II-3.7.5).

40 | P a g e  Figure II-9: ¹H-NMR of the crude and separated fractions of the sulfoxide adducts (-)-(R, M)-10b and

(+)-(R, P)-10b from Me2-DMQA-1b

Care was also taken after separation to carry out the Pummerer fragmentation on the two fractions and measure the ECD spectra in order to certify that the elution order was the same as for the products that were previously rapidly separated on regular silica gel. Fortunately, it was the case, as the first eluted fraction was the helicene with the M configuration followed by the least eluted P derivative (Figure II-10).

Figure II-10: ECD spectra of the two separated enantiomers (-)-M-1b and (+)-P-1b

Nevertheless, we were really surprised that a simple decrease of the length of the side chain would have such an effect on the chromatographic behaviour of the adducts and decided to investigate.

II-3.6.b.Resolution of rac-(ⁿPr)2-DMQA 1a

To verify that nothing fishy was occurring, we synthesised the ⁿPr2-DMQA 1a derivative which is considered the benchmark in this resolution chemistry. The enantiopure sulfoxide addition proceeded as previously described⁹ with excellent conversion and yields (>45% for each diastereomer). The TLC of the crude reaction mixture is indicated in figure II-11 and the two adducts are clearly less polar than the methyl analogues (ΔR_f = 0.3, figure II-11).

Figure II-11: TLC plate and ECD spectra of the sulfoxide fractions 10a derived from ⁿPr-DMQA⁺ 1a.

(a) the (-)-(R,M) fraction and (b) the (+)-(R,P) fraction. 

The analysis of the ¹H-NMR spectra was useful to attribute the different chemical shifts. In the same NMR region as previously shown with 10b (Figure II-9, vide supra), a similar distribution of signals for each of the diastereomeric fractions was obtained in the case of the ⁿPr-DMQA neutral adducts 10a (Figure II-12).

A distinct separation of almost every aromatic proton is observed and discrimination between the two methoxy substituents of the molecule is rendered possible. However, a more thorough analysis will be given later in this chapter, by comparing the different side chain derivatives.

42 | P a g e  Figure II-12: ¹H-NMR of the crude and separated fractions of the sulfoxide adducts (-)-(R, M)-10a and

(+)-(R, P)-10a from ⁿPr2-DMQA-1a

Having confirmed the extremely good separation of the corresponding diastereoisomers generated by sulfoxide addition on ⁿPr-DMQA 1a (ΔRf of 0.3-0.4), this prompted us to synthesise the hybrid unsymmetrical derivative with one propyl and one methyl, to understand the effect that obviously the side chains have on the separation and the polarity of compounds.

II-3.6.c.Resolution of rac-Me-ⁿPr-DMQA 1n

Formally, the synthesis of this unsymmetrical derivative can be afforded through two possible routes, either starting from the methyl acridinium and following up with the propyl amine to give the second ring closure or from the propyl acridinium heated subsequently with methyl amine.

We chose the first method since the methyl acridinium was previously reported by Laursen et

al.¹² and more importantly because the second step requires prolonged heating at 90 °C (4 h) and this was deemed “more difficult” with volatile MeNH2 (Scheme II-13).

Scheme II-13: Synthesis of the unsymmetrical hybrid Me-ⁿPr-DMQA 1n

The reaction of the methyl acridinium with propylamine at 90 °C was monitored by MS (ESI⁺)²³ and after four hours a simple precipitation in 100 mL of a 0.2 M solution of aqueous KPF6

afforded a single product that was analyzed and attributed to the right structure. The resolution of this new cation via the “sulfoxide” protocol was also performed using 1.5 equivalents of chiral resolving agent and quenching the reaction after 5 minutes.

But, in the case of this unsymmetrical derivative, four possible diastereoisomers are obtained due to the creation of a new stereocenter at the site of the nucleophilic addition. It was foreseen that the separation of these four diastereoisomers would be difficult, as previous results in the group had shown that it was not completely trivial to isolate the four adducts of (+)-(R)-methyl-para-tolylsulfoxide or (+)-(R)-9 addition on the ⁿPr-Ph-DMQA⁺ 1o.^14,24

However in this sole previous case, four retention factors were obtained, one for every fraction (Rf (Et2O) = 0.76; 0.65; 0.36; 0.28). As always, the least polar fractions were later separated²⁵ and identified as the sulfoxide adducts bearing the M helicity whereas the ones of more polar character were of P helicity. These four adducts were isolated by chromatotron with a 1:1 mixture of Et2O/pentane in respectively 23%, 16%, 27% and 10% yield for the respectively eluted (-)-(R, M1)-10o, (-)-(R, M2)-10o, (+)-(R, P3)-10o, (+)-(R, P4)-10o, the four identified structures being shown below (Scheme II-14).

23 Product peak detected at 385.5 m/z.

24 Nicolas, C. Ph.D. Thesis, Univ. Geneva 2008, 3974 (EN); 

25 By radial chromatography or chromatotron (Et2O / pentane (1:1) as eluent, 5 mm thin layer plate, ø 21 cm for a 0.40 mmol scale)

44 | P a g e  Scheme II-14: Synthesis of the four sulfoxide adducts derived from the Ph-ⁿPr-DMQA⁺ 1o

With only one example of an unsymmetrical derivative resolution described so far, we thought some similarities to this case should be seen but also probably some divergence since the two side chains in 1o were an alkyl and an aromatic group whereas in our study (1n) they are both alkyls.