α,β-Didehydroglutamates have been diastereoselectively transformed into 6-oxoperhydropyridazine-3-carboxylic acid derivatives (OPCAs), which constitute a new class of cyclic amino acid derivatives.Acylation at N-1 renders dipeptides which show considerable conformational rigidity. Semiempiricalcalculations suggest that OPCAs might force peptide turns with different amplitudes dependingon the substitution pattern and relative stereochemistry of the substituents of the pyridazinonering.