Synthesis of beta-lactams and cyclo-beta-dipeptides from beta-amino acids:experimental observations and theoretical analysis

The cyclization of beta -amino acids by means of activating agents is one o f the most useful approaches for the construction of beta -lactams; however , we found that when PhP(O)Cl-2 (in Et3N) is employed as the activating age nt, cyclization of the derived 'active ester' affords varying amounts of cy clo-beta -dipeptides, depending on reactions conditions (solvent, temperatu re, and concentration), as well as on the substitution pattern in the start ing beta -amino acid. Theoretical rationalization of the experimental resul ts was achieved by modeling studies of the presumed intermediates, both at semi-empirical (MNDO, AMI, and PM3) and at ab initio (HF/3-21G) levels. In the latter calculations, simulation of solvents was accomplished by means o f self-consistent reaction field theory (Onsager's method). (C) 2001 Elsevi er Science Ltd. All rights reserved.