Ring fragmentations of 2-alkenyl- and 2-benzyl-1,3-dithiolanes induced by bases. A novel method for the preparation of 1,1-bisalkylthio- or 1-alkylthioalk-1-enes and -alka-1,3-dienes

Abstract

The reaction of 2-alkenyl- and 2-benzyl-1,3-dithiolanes with several bases and successive alkylation with alkyl halides has been investigated. In the reaction of 2-alk-1-enyl-1,3-dithiolanes with organolithium reagents and alkyl halides, deprotonation at C-2 and subsequent cycloelimination occurs to give α,β-unsaturated dithiocarboxylate anions; these readily undergo conjugate addition and/or enolation with bases followed by alkylation to afford 1,1-bisalkylthioalk-1-enes and/or 1,1-bisalkylthioalka-1,3-dienes, respectively. In contrast, when 2-alk-2-enyl-1,3-dithiolanes are submitted to an identical procedure, the reaction proceeds via ring-opening by attack of organolithium reagents at C-4 and subsequent elimination to afford 1-alkylthioalka-1,3-dienes. In the reaction of 2-benzyl-1,3-dithiolane with bases and methyl iodide in diethyl ether, β,β-bismethylthiostyrene is the major product; this is obtained via deprotonation at C-2 and subsequent cycloelimination. In contrast, in the same reaction sequence in tetrahydrofuran, base attack at C-4 gives ring-opening and 1-methylthiophenethyl vinyl sulphide and/or β-methylthiostyrene are produced preferentially. Since each of the above reaction courses is dependent both on the C-2 substituent and the reaction conditions (mainly the reaction medium) each of the products may be prepared selectively.