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

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.