Conformationai analysis of naphtho[1,8- [formula omitted], [formula omitted]]-1,5-dithiocin and its s-oxides in solution

Abstract

The conformation and conformational barriers In dithioether 1: naphtho(1,8- b, c]-1,5-dithiocin were determined in solution by 1H and 13C NMR spectroscopic analysis and AMl semiempirical computations. In solution the chair conformer of dithloether 1 is 0.6 kcal/mol lower in energy than the boat conformer. The barriers for chair-to-chair ring inversion and chair-to-boat conversion in this compound are 8.9 kcal/mol and 7.9 kcal/mol, respectively. Monosulfone 3, disulfoxide 4, and sulfoxide-sulfone 5, adopt the chair conformation in solution in contrast to their boat conformation in the solid state. In solution the chair conformation for 3–5 with equatorial sulfoxides is more stable than either the chair conformation with axial sulfoxides or the boat conformers by at least 1 kcal/mol. The barrier for chair-to-chair ring inversion for monosulfone 3 has been determined to be 10.8 kcal/mol. In solution, disulfone 6 appears to adopt either a boat or chair conformation in contrast to the twist conformation in the solid state. The maximum barrier for ring inversion of 6 is 7 kcal/mol. New evidence for the geometry of the transition state for the ring inversion is presented.