Prochiral sulfides described

The Orsay group found serendipitously that methyl p-tolyl sulfide was oxidized to methyl p-toly 1 sulfoxide with high enantiomeric purity (80-90% ee) when the Sharpless reagent was modified by addition of 1 mole equiv. of water [16,17]. The story of this discovery was described in a review [19], Sharpless conditions gave racemic sulfoxide and sulfone. Careful optimization of the stoichiometry of the titanium complex in the oxidation of p-tolyl sulfide led to the selection of Ti(0iPr)4/(7 ,7 )-DET/H20 (1 2 1) combination as the standard system [ 17]. In the beginning of their investigations, the standard conditions implied a stoichiometric amount of the chiral titanium complex with respect to the prochiral sulfide [16,17,20-23]. Later, proper conditions were found, which decreased the amount of the titanium complex without too much alteration of the enantioselectivity [24,25],... 

Cotton etal. [14] described an asymmetric synthesis of esomeprazole. Esomeprazole, the (S)-enantiomer of omeprazole, was synthesized via asymmetric oxidation of prochiral sulfide 5-methoxy-2-[[(4-methoxy-3,5-dimethyl pyridin-2-yl)methyl]thio]-lH-benzimidazole 1. The asymmetric oxidation was achieved by titanium-mediated oxidation with cumene hydroperoxide in the presence of (S,S)-diethyl tartarate (DET). The enan-tioselectivity was provided by preparing the titanium complex in the presence of sulfide 1 at an elevated temperature and/or during a prolonged preparation time and by performing the oxidation of sulfide 1 in the presence of amine. An enantioselectivity of 94% ee was obtained using this method. 

The catalytic asymmetric oxidation of prochiral sulfides by chemical means is a difficult task. While a number of workers have been active in this area during the past few years, few systems simultaneously show good induction of chirality and good catalytic activity. The most common catalysts involve transition-metal complexes (homogeneous or supported) as well as chiral electrodes. These approaches are described successively below. 

Several approaches have been described for the preparation of optically active sulfoxides [5-7]. The three main routes to obtain these compounds are as follows (i) the asymmetric sulfoxidation of prochiral sulfides, (ii) nucleophilic substitution using a chiral sulfur precursor, and (iii) the kinetic resolution of racemic sulfoxides. The first of tiiese methods involves the use of various oxidants and catalysts and has been the most extensively employed. There are many examples in the scientific literature and reviews are available on this approach. In recent years, much attention has been focused on the synthesis of organic sulfoxides by emplo5dng conditions compatible with the green chemistry procedures [8-10]. For this reason, mild oxidants such as molecular oxygen or hydrogen peroxide are considered in combination with novel catalysts in order to develop a mild and environmentally friendly process. 

In the present paper we describe the mesophase behavior of a new series of chiral poly(ester 0-sulfide)s I-n and report on the asymmetric transformation of another class of suitably prochiral poly(ester 0-sulfi-... 

The methods described in this chapter are based on the enantioselective deprotonation of alkyl groups in - usually achiral - phosphine derivatives, which provide highly enantioenriched a-carbanions that are versatile precursors of a variety of mono- and diphosphines. This method was developed by combining two known facts firstly, that the methyl group in methylphosphines and their oxides, boranes and sulfides can be deprotonated with strong bases and secondly common organolithium reagents ( -, s- or r-BuLi) can exert enantioselective deprotonations in prochiral substrates in the presence of certain chiral auxiliaries. ... 

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