O- Methanesulfonyl L-Lactic Acid Derivatives

Mesyl lactates 156 are readily prepared by treating the corresponding L-lactic acid esters with methanesulfonyl chloride in the presence of triethylamine [56,57]. Hydrolysis of the car-boxylate ester furnishes (5)-( — )-2-mesyloxypropionic acid (157) [46,58]. 

The cesium salt of thioacetic acid undergoes clean Sn2 substitution with 156b to afford (/ )-2-acetylthiopropionic acid ethyl ester (168) in nearly quantitative yield and with 100% ee [63]. Acid hydrolysis furnishes ethyl (7 )-2-mercaptopropionate (169) (92% ee). 

The properties of cesium salts can be taken advantage of to convert (5)-ethyl lactate to its / -enantiomer without racemization [63]. Treatment of mesylate 156b with cesium propionate gives (7 )-(acyloxy)propionate (170) in good yield [72]. Titanium-mediated transesterification of the ( S)-enantiomer of 170 under essentially neutral conditions has been reported to give ethyl L-lactate (60% yield) with no racemization [64]. Application of this methodology to (/ )-170 should give ethyl D-lactate (171). 

Reaction of o-methylaminothiophenol with 156b in the presence of base at 0 °C for 1.5 h and then at room temperature for 2 days furnishes benzothiazinone 172 in nearly quantitative 

Analogously, treatment of 156b with thiophenol affords ethyl (i )-2-(phenylthio)propionate (174). Acidic hydrolysis gives (i )-2-(phenylthio)propionic acid (175). In this case, oxidation of the sulfur produces a near statistical mixture of diastereomeric sulfoxides 176, which can be separated by fractional crystallization. 

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