Regio- and stereo-specificity of cycloaddition reactions of sulphines with diphenylnitrilimine

Sulphines (thione S-oxides) undergo cycloaddition reactions with diphenylnitrilimine to yield in a regiospecific manner 1,3,4-thiadiazoline 1-oxides, with the exception of thiofluorene S-oxide, which gave the other regioisomer (a 1,2,3-thiadiazoline 1-oxide) as a by-product. The products obtained from cyclisation with geometrically isomeric sulphines indicate that steric integrity is lost during the reaction. This stereomutation does not come about by sulphine isomerization prior to reaction but is caused by product equilibration afterwards. It is shown that the steric integrity is lost by ring opening–ring closure of the cycloaddition product. A ring expansion of the addition product similar to the penam–cepham conversion is described.