Pentamethylbenzyl esters of α-hydroxy acids and their use in depsipeptide synthesis

Pentamethylbenzyl esters of several a-hydroxy acids have been prepared as possible intermediates in depsipeptide synthesis. Potentially useful features of pentamethylbenzyl as a carboxyl-protecting group are the crystalline nature of the esters, and the rapid cleavage of the group under mild acidic conditions. The glycollate, lactate, and mandelate were coupled with various protected amino acids to form crystalline aminoacyl hydroxy acid derivatives. Selective cleavage of the ester group in these compounds was readily effected with cold trifluoroacetic acid, and the products were used in the synthesis of some representative protected depsipeptides.