A new general synthesis of 3- and 3,4-substituted 4a–q, 3,4-annelated 7a–d and condensed tricyclic 9a–e thiophenes has been developed through Simmons-Smith reaction on the respective α-oxoketene dithioacetals. Extension of the reaction to α-cinnamoylketene dithioacetals 10a–e and its higher enyl analogs 13a–d, 15a–c gave the corresponding 3-styryl 11a–e and 3-di- and trienyl 14a–d, 16a–c thiophenes
Cycloaromatization of α-Oxoketene Dithioacetals with 5-Lithiomethyl-3-methylisoxazole: A New General Method for the Synthesis of Substituted and Annulated 1,2-Benzisoxazoles
A new route to 6-substituted 4a-e and 5,6-annulated 4f-l 1,2-benzisoxazoles has been developed through regioselective 1,2-nucleophilic addition of 5-lithiomethyl-3-methylisoxazole (2) to a variety of α-oxoketene dithioacetals 1a-l and subsequent cycloaromatization of the resulting hydroxyacetals in the presence of boron trifluoride -diethyl ether complex. The corresponding 4-dethiomethylated benzisoxazoles 6a-d were also synthesized by cyclocondensation of β- methylthio-α,β-unsaturated ketones with 2 under identical conditions. The reaction was further extended for the synthesis of (6-benzisoxazolyl)-substituted ethylenes 9a-e, butadienes 9f-g and hexatriene 9h by subjecting α-cinnamoyl ketene dithioacetals 7a-e and their higher enyl analogs 7f-h to similar transformations.