3-Oxo-1,3-oxathiolanes-synthesis and stereochemistry.

The compound 3-oxo-1,3-oxathiolane (6) and its cis - and trans - 2-methyl (7,8), 4-methyl (9,10), 5-methyl (11,12) and 2-p-nitrophenyl (13,14) derivatives were prepared by oxidizing the corresponding 1,3-oxathiolanes (1-5) with NaIO4 in water. The oxides were purified and their isomers separated using thin layer chromatography. The structural characterization was carried out with 1H NMR spectroscopy and molecular modeling. Compounds 8-10, 12, and 14 attain the half-chair type conformation with O1 above and C5 below the plane (HC1), but 6, 7, 11, and 13 are, for steric reasons, the mixtures with the alternative half-chair (HC2), where O1 is below and C5 above this plane. Accordingly, 6 appears based on the values of experimental coupling constants as an 81:19, 7 as a 37:63, 11 as a 33:67 and 13 as a 54:46 mixture of HC1 and HC2, respectively. The relative energies of these conformations, the values of the vicinal H,H-coupling constants and 1H chemical shifts were estimated for compounds 6-12 also by computational methods and they support nicely the conclusions based on experimental data.