CYCLOAROMATIZATION OF ALPHA-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-I 1,2-benzisoxazoles has been developed through regioselective 1,2-nucleophilic addition of 5-lithiomethyl-3-methylisoxazole (2) to a variety of alpha-oxoketene dithioacetals 1a-1 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 beta-methylthio-alpha,beta-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 alpha-cinnamoyl ketene dithioacetals 7a-e and their higher enyl analogs 7f-h to similar transformations.