Reactivity of 1-aminoazetidine-2-carboxylic acid during peptide forming procedures: observation of an unusual variant of the hydrazino turn

Peptide formation on the N-terminal of 1-aminoazetidine-2-carboxylic acid is rendered problematic due to a ring opening reaction. However C-terminal development is possible and two diastereomeric mixed hydrazino dipeptides were prepared. Solution-state studies of these compounds suggest the presence of intramolecular hydrogen bonding, consistent with a hydrazino turn, and the crystal structure of one of these compounds shows a horse-shoe conformation, centered around what appears to be a hydrazino turn involving three hydrogen bond acceptors.