3-Acyl-l,3-oxazolidine-2-thiones

Acyl-l,3-oxazolidine-2-thiones, chiral (1). Nagao and co-workers1 have prepared the chiral 3-acetyl-l,3-oxazolidine-2-thiones (la and lb) and used them to effect diastereoselective aldol reactions. The two chiral auxiliaries show, as expected, opposite diastereoselectivities, but contrast with the diastereoselectivities observed with chiral 4-alkyl-2-oxazolidones (11, 379-381). This aldol reaction has been used to prepare the chiral azetidinone 4 (equation I) and (-I- )-Prelog-Djerassi lactone. 

Among chiral auxiliaries, l,3-oxazolidine-2-thiones (OZTs) have attracted much interest for their various applications in different synthetic transformations.2 Such simple structures, directly related to far better known chiral oxazolidinones,11,12,57 have been explored in asymmetric Diels-Alder reactions and asymmetric alkylations, but mainly in condensation of their /V-acyl derivatives with aldehydes. Chiral OZTs have shown interesting characteristics in anti-selective aldol reactions58 or combined asymmetric addition. 

As in other applications of 7V-acyl-l,3-oxazolidin-2-ones, 2-thiones, and sthiazolidine-2-thiones, chelation of the Lewis acid center for restricted rotation is considered decisive for the reactions occurring under the influence of the Ti-TADDOLates. Generally, the attack of the nucleophilic component (diene or ene) on the chelated electrophile occurs from the bottom face if the chelate ring is drawn as shown in structures (17) and (18), (19), and (lO). For the oxazolidinones, this means that the trigonal a-carbonyl center is approached from the (/ e)-face when an (/ ,/ )-Ti-TADDOLate is used (rel. topicity like) the mechanism of this reaction has been discussed. ... 

Yamada and Ohe [38] reported the resolution of secondary alcohols by axially chiral twisted amides. (S)-4-Alkyl-3-pivaloyl-l,3-thiazoHdine-2-thiones 4 and the oxazolidine analogue 5, readily prepared from the corresponding i-amino alcohols, were used as acylating agents for secondary alcohols (5 equiv). The corresponding pivalates were obtained in 26-84% ee. 

Acyl-1,3-thiazolidines-2-ones 1.123 (X = S, R = COOMe), obtained from cysteine methyl ether [261], have been introduced by Mukaiyama and coworkers for use in asymmetric aldol reactions [261, 433, 434, 435], In reactions of related //-acyl-1,3-oxazolidines-2-thiones 1.123 (X = O, R = COOMe), each enantiomer can be obtained either from L- or D-serine [434] and the auxiliaries can easily be recovered by methanolysis. Similarly, //-acyl derivatives of 1.121 (X = S) have been used in asymmetric aldol reactions [429, 436], and //-acyl- 1,3-thiazo-lidinethiones 1.123 (X = S, R = r -Pr) are useful in asymmetric acylation [437] and aldol and related reactions [437, 438], Cleavage of the chiral auxiliary is accomplished by aminolysis with O-benzylhydroxylamine or by reduction with LiAlH.,. ... 

Comparison of the use of 3-acyl-(4S)-methoxycarbonyl-l,3-oxazolidine-2-thione [(4S)-AMCOT] (18) with that of (4/ )-AMCTT (14) showed the superiority of the latter over the former. 

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