THEORETICAL-STUDY OF DEGENERATE BOULTON-KATRITZKY REARRANGEMENTS - SEMIEMPIRICAL AND AB-INITIO PROCEDURES

A theoretical study of degenerate Boulton-Katritzky rearrangements con cerning the anions of the 3-formylamino-1,2,4-oxadiazole, 3-formylmeth yl-isoxazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carri ed out by using semiempirical MNDO and ab initio Hartree-Fock procedur es. Different transition structures and reactive pathways were obtaine d in the two cases. Semiempirical treatment shows asymmetrical transit ion states and non-concerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous p rocesses involving symmetrically-located transition states. Some comme nts and criticisms on the theoretical treatment of these typical rearr angements of five-membered heterocycles are reported. (C) 1998 Elsevie r Science B.V. All rights reserved.