Reactivity of ring systems with two hetero atoms. I. On the reaction of 1,4-dithiins with n -butyllithium

The reaction of 1,4-dithiin (1) with n-butyllithium has been examined. At −110° in tetra-hydrofuran 2-lithio-1,4-dithiin (2) is sufficiently stable to allow of the preparation of derivatives. Anion (2) undergoes ring opening by a 1,2-elimination at −60°; subsequent methylation and hydrolytic work-up gives ZHC≡CS-CHCHSMe (6), or Z−MeC≡C−S−CHCH−SMe (7). At about −5° butyllithium attacks sulfur in the intermediary Z−LiC≡C−S−CHCH−SLi (4), leading to Z−BuS−CHCH−SLi (8), and LiC≡CLi. These results are not in agreement with the mechanism proposed by Parham et al. for benzo-1,4-dithiin and 2,5-diphenyl-1,4-dithiin. We studied also the reactivity of 2,6-dimethyl-1,4-dithiin (13), and 2,6- di-tert-butyl-1,4-dithiin (25), towards butyllithium. Starting from 13 the otherwise, difficultly accessible tri- and tetramethyl-1,4-dithiins could be prepared in high yield. The ring opening provides a valuable method for the synthesis of stereochemically pure Z-1-(alkynylthio)-2-(alkylthio)alkenes and Z-1-(butylthio)-2-(alkylthio)alkenes.