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 ZHC≡CS‐CHCHSMe (6), or Z−MeC≡C−S−CHCH−SMe (7). At about −5° butyllithium attacks sulfur in the intermediary Z−LiC≡C−S−CHCH−SLi (4), leading to Z−BuS−CHCH−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.