CONFORMATIONAL-ANALYSIS OF METHYLTHIAZANES - THE PROBLEM OF THE ME-C-N-ME GAUCHE INTERACTION

The position of conformational equilibria in 2- and 3-methyl-1,4-thiazanes and cis- and trans-2,3-dimethyl-1,4-thiazanes, their N-methyl derivatives, and several of the corresponding sulfoxides and sulfones have been measured. The DELTAG-degrees values for the methyl groups are generally in agreement with what one would expect on the basis of known conformational equilibria in methylthianes, methylpiperidines, and N-methylated homologs of the latter; however, the differences in DELTAG-degrees between the 2-methyl and N,2-dimethyl homologs are even larger than in the piperidine series. An explanation for these differences is based on MM3 force field calculations of molecular geometry. The sulfoxide and sulfone data throw light on SO/H and SO/Me syn-axial as well as SO/Me gauche interactions.