A series of 2-substituted mesomeric betaines containing a cyclic azomethine imine unit were synthesized and their cycloaddition with acetylenic dipolarophiles were examined. Unexpectedly, the cycloaddition of the betaines with electron-deficient dipolarophiles gave ring-expanding adducts having a triazocinone structure. With electron-rich dipolarophiles such as ynamines, the reactions proceeded more readily leading regioselectively to the same type of triazocinones in almost quantitative yields. In particular, in the case of an imidazobetaine and an ynamine, the cycloaddition took place at room temperature to afford the initial cycloadducts exclusively and in excellent yields. The isolated tricyclic precursors were found to rearrange quantitatively to the final products on heating to 60 °C.
- Medium-ring heterocycles
- Mesoionic compounds