Conformationally constrained dipeptides. Obtention of enantiomerically pure 6-acetamido-5-oxo-1,2,3,5,6,7-hexahydro-3-indolizine carboxylic acid

The separation of a stereoisomeric mixture of esters 6, and the synthesis o f the new enantiomerically pure unsaturated 6,5-fused bicyclic lactam 2 are described. The key-step involves trimethylsilyl iodide cleavage, without r acemization, of a vinylogous carbamate. The cis N-acetylamino acid 2 is a n ew scaffold for the synthesis of rigid beta -turn mimetics.