2H-naphtho[1,8-bc]thiophen and 2-methyl-2H-naphtho[1,8-bc]thiophen 1-oxides: synthesis, configurational assignments, and stereoselective interconversions

Abstract

The carbonyl function of a thiolactone was reduced to obtain the 2H-naphtho[1,8-bc]thiophen, from which the 1-oxide (1) and the diastereoisomeric 2-methyl 1-oxides (2) and (3) were derived. The resonances of the diastereotopic methylene protons of (1) and the configurations of (2) and (3) were assigned through a study of solvent- and Eu(dpm)₃-induced shifts in their ¹H n.m.r. spectra. The interconversion of the sulphoxides (2) and (3) through the corresponding alkoxysulphonium salts was stereospecific when a weak base was used, but not in aqueous sodium hydroxide. In the latter case, the base was found to isomerize the initially formed sulphoxides (2) and (3)via an α-sulphinyl carbanion, and always to yield a final mixture containing >95%cis-sulphoxide (3). The composition of this equilibrium mixture was shown to be controlled by solubility factors and not by the relative thermodynamic stabilities of the two isomers.