An Efficient Synthesis of Methyl Esters of Heterocyclic α,β-Didehydro-α-amino Acid Derivatives

N-Benzoyl-protected α,β-didehydro-α-amino acid (DDAA) derivatives containing a pyrazole or an isoxazole ring at the β-position were transformed with diazomethane into the corresponding methyl esters.Applying this method, the double C=C bond of the DDAA derivatives was not affected, thus selectively giving the corresponding esters in good-to-excellent yields.