Copper(II) in organic synthesis. IV. Reaction of the copper(II) acetate complex of isatin-3-arylhydrazones with dimethyl acetylenedicarboxylate

The copper(II) acetate complex of isatin-3-phenylhy-drazone (2a) reacted with dimethyl acetylenedicarboxylate (DMAD) by two competitive pathways: a Cu++ oxidation of the ligand and a [2+2] cycloaddition. The former reaction gave 3-car- benindolin-2-one (13) which reacted with DMAD in a l,3-dipolar cycloaddition, to give 3a. This was synthesized by an independent route. The phenyl radical, generated in the same process, was trapped by three DMAD and gave 4a. The [2+2] cycloaddition gave a spiro adduct 10 which, by electrocyclic ring opening and intramolecular cyclization, allowed isolation of a pyridazino [3,4-b] indole (|5a). The reaction was performed on p-chlorophe-nylhydrazone 1b and the structure of 5b was demonstrated by X-ray analysis. A rationalization of the reactivity was attempted in terms of MO interactions of the reactants.