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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 .

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