CONFORMATIONAL-ANALYSIS 29 - PREPARATION AND H-1 AND C-13 NMR, FTIR, MS, AND CRYSTALLOGRAPHIC CONFORMATIONAL AND CONFIGURATIONAL STUDY OF 3-OXO-1,3-OXATHIANE AND ITS MONOMETHYL DERIVATIVES

3-Oxo-1,3-oxathiane (1) and its monomethyl derivatives were prepared b y oxidation of the corresponding 1,3-oxathianes. The structural analys is was carried out by H-1 and C-13 NMR, FTIR, and mass spectrometry. A t 298 K compound 1 was a 1 : 1 (at 173 K a 3 : 1) mixture of the SO(ax ) and SO(eq) chair forms. The major oxidation products of methyl 1,3-o xathianes attained exclusively the SO(ax), Me(eq) chair forms except t hat of the 5-methyl derivative, which consisted of 7% of the SO(eq), M e(ax) chair conformation in CDCl3 solution. The minor products of oxid ation existed in anancomeric SO(eq), Me(eq) chair conformations. The o xidation of 2-methyl-1,3-oxathiane, however, led to 3,3-dioxo derivati ve (6) in addition to the trans [SO(eq)] monoxide. The crystal structu res of 6 and trans-3-oxo-5-methyl-1,3-oxathiane were solved by X-ray d iffractometry.