Conformational behaviour of medium-sized rings. Part 10. Dithiosalicylides and trithiosalicylides

Abstract

The trithiosalicylide derivatives (8)–(11) have been synthesised and shown by temperature-dependent ¹H n.m.r. spectroscopy to exist in solution as ring inverting (35a)⇌(35b) enantiomeric helical conformations with trans-thioester linkages. The free energies of activation for these conformational changes are ca. 10 kcal mol^–1 higher than those for the similar process in the corresponding trisalicylides. In contrast with the trisalicylides, the trithiosalicylides can only ring invert between enantiomeric helical conformations via intermediates containing a cis-thioester linkage. The dithiosalicylide derivatives (3)–(7) have been synthesised; the temperature dependence of the ¹H n.m.r. spectrum of di-o-thiothymotide (7) has been interpreted in terms of ring inversion (40a)⇌(40b) between enantiomeric boat conformations. Comparison of the ΔG^‡ value of 24.6 kcal mol^–1 for this conformational change with that of 17.7 kcal mol^–1 previously obtained for di-o-thymotide (41) suggests that cis-thioester linkages are subject to more resonance stabilisation than are cis-ester linkages.