O- -Toluenesulfonyl L-Lactic Acid Derivatives

Tosyl lactates 120 are readily prepared by treating the corresponding L-lactic acid ester with / -toluenesulfonyl chloride in the presence of either pyridine [45] or triethylamine [30]. Ethyl ester 120b has been prepared on a multi-kilogram scale in nearly 97% yield [46]. Careful hydrolysis of the ester furnishes ( S)-( — )-2-tosyloxypropionic acid 121. 

Lithium di- -butylcuprate reacts with 121 to produce (5)-( + )-2-methylhexanoic acid (130) with 98% inversion of the stereo center. Similarly, lithium diphenylcuprate gives (5)-( + )-2-phenylpropionic acid (131) with 95% inversion. Yields for this process are generally low, however [47]. 

Reduction of tosyl lactate 120b with sodium borodeuteride at 100 °C (48 h) in the absence of any solvent furnishes deuteropropionate 132 [50]. 

Displacement of the tosyl function of 120a with potassium thiolacetate in boiling acetone produces (R)-2-acetylthiopropionic acid methyl ester (133) with an optical purity of 92% [45]. Alternately, 133 may be obtained with higher optical purity from an identical reaction with 

Chiral allylthioether 137 has been prepared in order to study stereoselectivity in the ketene Claisen rearrangement [51]. It is accessible from 120a in 5 steps. The first step, 8 2 displacement of the tosylate, occurs with nearly complete inversion of the asymmetric center (98% ee). Reduction, Wittig reaction, and subsequent protection furnishes 137 with an ee of 

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