The influence of the ring size of thiolactams in the Eschenmoser coupling reaction in presence of DBU. Formation of bicyclic thiazolidinones or thioimines

The different behaviors of pyrrolidin-2-thione and piperidin-2-thione under a modified Eschenmoser sulfur contraction reaction protocol using DBU as base was observed. The pyrrolidin-2-thione 1b follows the expected reaction course, leading to thioimines 5a–d, which can be transformed subsequently into the respective by action of a thiophile, while the piperidin-2-thione leads to the formation of bicyclic thiazolidinones 4b–d in moderate to good yields. The β-enaminocarbonyl compound 11 was hydrogenated to afford the respective five-membered analogue of methylphenidate 12.