Ab initio study of 2,4-substituted azolidines. I. Tautomerism

Detailed investigation of the tautomerism of oxazolidines, thiazolidines and selenazolidines substituted at positions 2 and 4 by hydroxy and amino groups was carried out by calculations at HF and MP2 levels of theory and NMR spectroscopy. The relative stabilities of the tautomers of the heterocycles were calculated in gas phase and in solvents CHCl3 and DMSO utilizing the PCM method. The ab initio calculations, in agreement with the available experimental data, predict that the azolidines substituted at positions 2 and 4 by a hydroxy and an amino group exist as amino form in solution, while 2,4-diamino-oxazolidine, -thiazolidine and -selenazolidine occur as mixtures of tautomers. HF, MP2 and DFT GIAO C-13 chemical shifts for the thiazolidines were calculated using the 6-311+G(d,p) basis set. Since, the MP2 calculations gave the best agreement with the experimental data, GIAO C-13 chemical shifts for the other compounds were studied at this level. (C) 2004 Elsevier B.V. All rights reserved.