Abstract
Axially chiral 5-methyl-2-(o-aryl)imino-3-(o-aryl)-thiazolidine-4-ones have been subjected to aldol reactions with benzaldehyde to produce secondary carbinols which have been found to be separable by HPLC on a chiral stationary phase. Based on the reaction done on a single enantiomer resolved via a chromatographic separation from a racemic mixture of 5-methyl-2-(α-naphthyl)imino-3-(α-naphthyl)-thiazolidine-4-one by HPLC on a chiral stationary phase, the aldol reaction was shown to proceed via an enolate intermediate. The axially chiral enolate of the thiazolidine-4-one was found to shield one face of the heterocyclic ring rendering face selectivity with respect to the enolate. The selectivities observed at C-5 of the ring varied from none to 11.5:1 depending on the size of the ortho substituent. Although the aldol reaction proceeded with a lack of face selectivity with respect to benzaldehyde, recrystallization returned highly diastereomerically enriched products.
Highlights
Synthesis of axially chiral biaryls has received particular attention lately due to the presence of these scaffolds in a large number of natural products including some antibiotics [1]
Curran has shown that [3] several thermal reactions of axially chiral amides and imides are stereoselective because one face of the axially chiral molecule was shielded by the bulky ortho substituent
We report the aldol reactions of 1–5 (Scheme 1) which were found to proceed via enolates possessing an axis of chirality, to produce chiral secondary carbinols separable by HPLC on a chiral sorbent
Summary
Synthesis of axially chiral biaryls has received particular attention lately due to the presence of these scaffolds in a large number of natural products including some antibiotics [1]. 3), controlled one of which can be expected pointed to a face selective reaction where chirality axis of the enolate the configuration to be shielded by the ortho substituent of the aryl group on N3 (Figure 3) rendering face selectivity. The isomer distribution of the aldol products of the single enantiomer of 2 showed the presence of two major (MSS and MSR) and two minor (MRR and MRS) isomers (Figure 2b, Scheme 3). This pointed to a face selective reaction where the chirality axis of the enolate controlled the configuration of the carbon at 5-position of the heterocyclic ring. At C-5 position; b Determined by chiral HPLC at the end of the reaction before purification; c Recrystallized from ethyl acetate-hexane
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