Chemical and Pharmaceutical Bulletin p. 1525 - 1531 (1999)
Update date:2022-08-03
Topics:
Akasaka, Kozo
Akamatsu, Hiroshi
Kimoto, Yuichi
Komatsu, Yuki
Shimizu, Toshikazu
Shimomura, Naoyuki
Tagami, Katsuya
Negi, Shigeto
Diastereoselective methylation of the enolate generated from various protected 6-oxopipecolic acid esters (3aa-3cd) was studied. The protecting groups on the carboxylic acid and amino groups significantly influenced the trans/cis selectivity in the methylation reaction. The optimal substrate (3ca), bearing benzhydryl ester and carbobenzyloxy moieties gave a trans/cis isomer ratio of ca. 4:1. Investigation of the reaction conditions revealed that the reaction solvent, alkylating reagent, and base employed to generate the enolate, were decisive factors for diastereoselectivity. Further optimization of reaction conditions, including the amounts of the reagents and their addition sequence enabled maximization of reaction conversion and minimization of by-products to produce the trans rich 5-methyl-6-oxopipecolic acid ester (4ca) on a large scale.
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