While working on thieno[3,2-d]thiazole 11, we encountered an unexpected reaction. Treatment of 1 with butyllithium at -100°C and addition of N-chlorosuccinimide yielded a product containing a butyl group and an extra chlorine atom. This unknown compound was then oxidized to the sulfone 4, which was identified by X-ray crystallography (scheme 1 and figure 1). This addition product could be explained by the addition reaction of the butyl group to the sulfur atom of ...