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.