Stereoselective Synthesis of Dipeptide β-Turn Mimetics: 7-Benzyl and 8-Phenyl Substituted Azabicyclo[4.3.0]nonane Amino Acid Esters

A stereoselective method has been developed for the synthesis of 7- and 8-substituted dipeptide beta-turn mimetic azabicyclo[4.3.0]nonane amino acid esters. The allyl groups were introduced in high diastereoselectivity, controlled by 3-phenyl or 4-benzyl groups in pyroglutamic acid derivatives 3 or 9, respectively. The precursors, dehydroamino acids 7 and 13 derived from 5 or 11, underwent asymmetric hydrogenations with Burk's DuPHOS Rh(I)-based catalysts to furnish alpha-amino acid derivatives in high stereoselectivity. The resulting amino acids 8 and 14 were converted to the beta-turn mimetics 6,5-bicyclic lactams 1a-d in high yields.