Diols, half-esterification

Mixed esters of oxalate containing one benzylic substituent are reduced in good yield using in situ trans-esterification to the corresponding hydrocarbon and oxalate half-ester [72]. Esters of vicinal diols (including cyclic carbonate esters) [73] and -halo esters [74] are often reduced to the corresponding olefin [72,75]. 

Utilization of enzymes in organic synthesis to prepare chiral compounds of synthetic value is well documented.For instance, porcine pancreatic hpase (PPL, E.C. 3.1.1.3.) which is an inexpensive commercially available enzyme, specifically catalyzes the hydrolysis of esters of racemic alcohols and wc50-diols. Thus, on a preparative scale (0.25 mol) dimethyl 2-methyl-butanedioate, on treatment with PPL in buffered water at pH 7.2, underwent regio- and en-antioselective hydrolysis. Extraction with diethyl ether gave the unhydrolyzed dimethyl (/ )-2-methylbutanedioate (93%), while acidification of the aqueous phase provided the (5)-half ester,which on esterification with methanol (acidified by thionyl chloride) gave the dimethyl (5 )-2-methylbutanedioate (76%). 

Enzymes can be used to selectively hydrolyze only one of two ester groups in a molecule. An esterase from Pseudomonas putida was used to hydrolyze dimethyl adipate to more that 99% half ester.198 Pig liver esterase was used to hydrolyze dialkyl phthalates to the monoesters in 84-93% yields in 1-13 h.199 A porcine pancreatic lipase was used to hydrolyze linear diol diacetates to the monoacetates in 79-95% yields.200 Esters have been prepared from 12-hydroxystearic acid and C8-C18 alcohols in 82-90% yields without esterification of the 12-hydroxy group, by using an immobilized lipase from Rhizomucor miehei.201 Ricinoleic acid, which differs only by having a cis 9 double bond, formed oligomers with an immobilized lipase from C. rugosa.202 These were much less colored than the ones made by the commercial process, which is done at 200 C. 

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