We describe experiments here which show that chemical esterification of 5′-adenylic acid (5′-AMP) with N-acetyl D-or L-phenylalanine (Ac-D- or Ac-L-Phe) imidazolide occurs principally, if not exclusively, at the 2′ position. Furthermore, in experiments with the formation of the 2′-3′ diester with butyric acid and N-acetyl glycine (Ac-Gly), we found the second esterification was also predominantly at the 2′ position. This means that mixed diesters can be predictably prepared with the positions of the substituents known. The results are consistent with a model for the preferential catalytic synthesis of L-based peptides via a 2′-3′ diester intermediate of purine monoribonucleotides. © 1990 Springer-Verlag New York Inc.