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Lactams part 14 cis trans isomerization in the 5 ethyl 2 oxo 4 piperidine acetic acid system under fischer speier esterification conditions


, : Lactams part 14 cis trans isomerization in the 5 ethyl 2 oxo 4 piperidine acetic acid system under fischer speier esterification conditions. Chemical and Pharmaceutical Bulletin (Tokyo) 26(7): 2253-2257

It was found that the reaction of the trans- (2a or 3a) or the cis-isomer (2b or 3b) of 5-ethyl-2-oxo-4-piperidineacetic acid (2) or its ethyl ester (3) with 10% EtOH-HCl at reflux for 20-27 h gave an equilibrated 70:30 mixture of the trans- (3a) and the cis-isomer (3b) of the ester (3) in a good yield. Such a cis-trans isomerization did not occur at all when the trans- (2a) or the cis-lactam acid (2b) was similarly esterified but at 15.degree. C for 16 h, whereas at 32.degree. C it did occur to a slight extent. In contrast to the N-unsubstituted lactam acids (2a, b), trans- (5a) and cis-1-benzyl-5-ethyl-2-oxo-4-piperidineacetic acid (5b) could be converted into the corresponding ethyl esters by similar esterifications even at 32.degree. with complete retention of their original stereochemistry. The cis-trans isomerization of the methyl esters of 2a, b in 10% MeOH-HCl seemed somewhat easier than that of the ethyl esters (3a, b) in 10% EtOH-HCl. On the other hand, the N-benzylated methyl esters did not isomerize in 10% MeOH-HCl even after 5 h reflux.

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