Transformations of β-aryl-N-Cbz-α,β-didehydro-α-amino esters with hydrazine hydrate

Abstract

Cyclizations of Cbz-protected α,β-didehydro-β-arylalanine esters 1 with excess hydrazine hydrate afforded mixtures of the expected 3-pyrazolidinones 2 and the unexpected 1-amino-5-benzylidenehydantoins 6 and N-Cbz-β-arylalanine hydrazides 7. Presumably, the pyrazolidinones 2 and hydantoins 6 are formed as primary products via competitive 1,2- and 1,4-addition of hydrazine hydrate followed by cyclization, whereas β-arylalanine hydrazides 7 are formed as secondary products via reductive cleavage of the C(5)–N(1) bond in pyrazolidinones 2. The overall selectivity depends on the reaction time and on the β-substituent in the starting dehydroalanine ester 1.