Stereochemistry of the Formation and Hydrolysis of a Dithioether Dication

Naphtho[l,8-6c]-l,5-dithiocin 1-oxide (7) was prepared by oxidation of naphtho[l,8-5c]-l,5-dithiocin with sodium metaperiodate in aqueous methanol. The structure and conformation of sulfoxide 7 in the solid state was unequivocally established to be boat 7A with equatorial sulfoxide by X-ray crystallographic analysis. Sulfoxide 7 crystallizes in the triclinic space group PĨ (no. 2) with a = 8.959 (2), b = 11.313(3), and c = 12.975 (3) A, a = 64.98 (2)°; β = 81.18 (2)°, γ = 78.67 (2)°, and Z = 4. The structure was solved by direct methods. Full-matrix least-squares refinement led to a conventional R factor of 0.040 after several cycles of anisotropic refinement. Dissolution of sulfoxide 7 in concentrated sulfuric acid produces disulfide dication 8, which on hydrolysis regenerates the sulfoxide. Deprotonation of sulfoxide 7 with methyllithium followed by treatment with D₂0 stereoselectively gives monodeuterated sulfoxide 7 (X = D). ¹H NMR spectroscopic analysis of sulfoxide 7 (X = H) revealed that it is predominantly in chair conformation 7B with equatorial sulfoxide and the deuterium atom in the mono-deuterated derivative is predominantly axial (7B, H_a = D). Disulfide dication 8 is deduced to be predominantly in boat conformation 8A by ¹H NMR spectroscopic analysis and that derived from monodeuterated sulfoxide 7 has the deuterium predominantly axial (8A, H_b = D). Hydrolysis of the deuterated disulfide dication regenerates sulfoxide 7 with the deuterium axial. These surprising stereochemical results require retention of configuration at the sulfoxide sulfur both in the formation of disulfide dication 8 and its hydrolysis.