Conformational Analysis. XIX properties and reactions of 1,3‐oxathianes VIII A 1H NMR conformational study of methyl‐substituted derivatives

The addition of thioacetic acid to unsaturated alcohols or acids was utilized to obtain mercaptoalkanols which were condensed with suitable carybonyl compounds to prepare 24 methyl‐substituted 1,3‐oxathianes. The 1H NMR spectra of the 1,3‐oxathiane products were recorded at 60, 100 and/or 300 MHz and fully analysed. The results are best explained by a chair form which is completely staggered in the C‐4C‐5C‐6 moiety ψ45 or (ψ56=60±1°). 1,3‐Oxathianes having syn‐axial 2,4‐ (and/or 2,6‐) methyl‐methyl interactions exist appreciably, if not exclusively, in twist forms. The vicinal coupling constants lead to the conformational free energies of axial methyl groups at C‐4, ΔG°=7.4±0.4 kJ mol−1, and at C‐5, ΔG°=3.7±0.3 kJ mol−1, in good agreement with previous estimates. They also show that both r‐4,cis‐5,trans‐6‐ and r‐4,trans‐5,trans‐6‐ trimethyl‐1,3‐oxathianes greatly favour the chiar form where the methyl group at C‐4 is axial. The chair‐twist energy parameters are reestimated at ΔH°CT 27.0 kJ mol−1, ΔS°CT 11.6J mol−1K−1, and ΔG°CT(298) 23.5 kJ mol−1 for a 2,5‐twist form.