Polarized ketene dithioacetals. Part 41. Studies on base-catalysed rearrangements of 1,l-bis(alkylthio)-2-aroylpenta-1,4-dienes to novel 1,5-bis(alkylthio)-2-aroylpenta-1,3-dienes via a 1,5-alkylthio shift

Abstract

1,1-Bis(alkylthio)-2-aroylpenta-1,4-dienes (7a–f) undergo a facile rearrangement in the presence of sodium hydride and dimethylformamide–benzene to give novel (1Z,3E)-1,5-bis(alkylthio)-2-aroylpenta-1,3-dienes (9a–f), respectively. A probable mechanism involving the mobile oxopentadienyl intermediate (12), which on subsequent 1,5-alkylthio shift affords the dienes (9), has been suggested for this highly stereoselective rearrangement. Mechanistic studies have shown that the 1,5-alkylthio shift in this transformation is an intermolecular rather than a concerted process and involves the solvent-equilibriated enolate anion (17) formed by Michael addition of thiolate anion to (12). The synthesis of the α-allyloxoketene dithioacetals (7a–f) is also described