Efficient Synthesis of Conformationally Constrained Peptidomimetics Containing 2-Oxopiperazines1

The enantioselective synthesis of a series of peptides containing the 3-substituted 2-oxopiperazine system is reported. The method involves a direct diastereoselective alkylation of the N-(hydroxyalkyl)-2-oxopiperazine 3, prepared in three steps from methyl l-leucinate in 38% yield, followed by oxidation of the hydroxyl function. Esterification of the resulting acids 11 and then deprotection and acylation of N-4 afforded tripeptide analogues 16 substituted by a large variety of alkyl side chains at the 3-position of the 2-oxopiperazine.