Homo-Diels-Alder reactions of 2,4,6-tri-tert-butyl-1,3,5-triphospha-Dewar-benzene

Homo-Diels-Alder reactions take place under very mild conditions when the 1,3,5-triphospha-Dewar-benzene derivative 4 is allowed to react with electron-deficient alkynes, such as dimethyl acetylenedicarboxylate or methyl propyonate, or with tert-butylphosphaacetylene. The resultant [2 + 2 + 2]-cycloadducts 5, 6, and 8 can be isolated in excellent yields and are formed regioselectively. Moreover, the first example of a [4 + 2 + 2]-cycloaddition has been realized by the reaction of 4 with tri-tert-butylazacyclobutadiene 10 which yields the two exo-cycloadducts 11 and 12. The cycloadduct 12 is unstable under the conditions of column chromatography on silica gel and undergoes rearrangement to the isomer 13.