Transesterification titanium-mediated

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). 

Protection of Alcohols.—The acylation of alcohols by polymers (48) of 3-acyl-2-oxazolone has been reported using fluoride ion as the catalyst, selective acylation of primary alcohols in the presence of secondary alcohols has been observed. Transesterification, mediated by titanium alkoxides, has been investigated and shown to be a selective, mild, and inexpensive technique for example, for the deacylation of alcohols. ... 

Phenylsulfonyl)-3-phenyloxaziridine (1S6. Davis reagent. Figure 11.61) has been shown to react with sodium enolates of chiral carboximides of the Evans-Oppolzer type, providing a-hydroxyacyl derivatives with diastereoselectivities of 90-98% d. g 92,133 Subsequent titanium(IV) isopropoxide-mediated transesterification with benzyl alcohol and hydrogenolytic debenzylation releases the free a-hydroxy acids in enantiomerically pure form and in yields of 65-75%. This procedure was applied... 

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