Synthesis of mesomeric betaines containing a pyrrolo- or imidazotriaziniumolate system and their cycloaddition with acetylenic dipolarophiles leadingto triazocinone derivatives

A series of 2-substituted mesomeric betaines containing a cyclic azomethine imine unit were synthesized and their cycloaddition with acetylenic dipola rophiles were examined. Unexpectedly, the cycloaddition of the betaines wit h electron-deficient dipolarophiles gave ring-expanding adducts having a tr iazocinone structure. With electron-rich dipolarophiles such as ynamines, t he reactions proceeded more readily leading regioselectively to the same ty pe of triazocinones in almost quantitative yields. In particular, in the ca se of an imidazobetaine and an ynamine, the cycloaddition took place at roo m temperature to afford the initial cycloadducts exclusively and in excelle nt yields. The isolated tricyclic precursors were found to rearrange quanti tatively to the final products on heating to 60 degrees C. (C) 1999 Elsevie r Science Ltd. All rights reserved.