Ketomethylenebestatin: synthesis and aminopeptidase inhibition.

The synthesis of (6R,5S,2RS)-6-amino-5-hydroxy-2-isobutyl-4-oxo-7- phenylheptanoic acid (9), a carbaanalogue of the aminopeptidase (AP) inhibitor bestatin (1) is described. This synthesis was carried out by a malonic ester alkylation with the suitably protected halomethyl ketone of (2S,3R)-AHPBA*), followed by a second alkylation with isobutyl bromide of the resulting 4-ketodiester, and subsequent decarboxylation and deprotection. The inhibitory potencies of the 1:1 diastereomeric mixture 9 against AP-B, AP-M and Leu-AP were approximately 10-fold lower than those of bestatin.