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−1higher 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−1for this conformational change with that of 17.7 kcal mol−1previously obtained for di-o-thymotide (41) suggests that cis-thioester linkages are subject to more resonance stabilisation than are cis-ester linkages.
|Original language||English (US)|
|Number of pages||12|
|Journal||Journal of the Chemical Society, Perkin Transactions 1|
|State||Published - 1982|
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