Studies on Diels-Alder cycloadditions of vinylketene dithioacetals

Abstract

The Diels-Alder cycloadditions of the acyclic and cyclic vinylketene dithloacetals 4 prepared from the corresponding oxoketene dithioacetals 2 either by 1,2-addition of methylmagnesium iodide and subsequent dehydration or by Wlttig reaction, were studied. Thus 4a-e and 4h-i underwent facile [4+2] cycloaddition with maleic anhydride followed by elimination of methylmercaptan and subsequent aromatization through dehydrogenation to give 5-aryl/methyl -3-nethylthiophthalic anhydrides (9a-e ), the corresponding tetrahydronaphthalene (13 ) and dihydrophenanthrene (14 ) derivatives in good yields. The vinylketene dithioacetals 4f and 4g derived from higher alkylketones, on the otherhand, underwent sequential [4+2] cycloadditions with two moles of maleic anhydride to afford bicyclic adducts 10a and 10b respectively,while the cycloadditions of dithioacetals 4a and 4f with dimethylacetylene dicarboxylate afforded the corresponding substituted phthalates 12a and 12b in good yields. The dienes 4a-i were found to be unreactive towards weaker dienophiles like acrylonitrile,ethylacrylate and methylvinyl ketones.