1,4-Dipolar Cycloadditions of Cyclopent[cd]azulene

Cycloadditions of nonbenzenoid carbo- and heterocyclic conjugated π-electron systems with suitable reactants provide a facile entry to ring systems that were hitherto difficult to prepare, if at all[¹]. For instance, cyclopent[cd]azulene (1)I²] which is now readily accessible reacts with dimethyl acetylenedicarboxylate even at room temperature to give 75 % of the 1:1 adduct (2).