electrochemical methods, to furnish 38-52% yields of a blue-green intermediate 6. The optical spectrum [A, 303 (e 15 300), 380 (45 300), 646 (inf; SOOO), 704 nm (9300)] was almost identical with that of the previous intermediate; the greatly simplified proton NMR spectrum showed three methine peaks (6.26, 5.35, 5.00 ppm), two NHs (13.84, 13.22 ppm), nine methyl resonances (2.03-1.77, 1.40 ppm), and an AB quartet [2.98, 2.52 ppm (each d, JAe = 15.3 Hz)] (Figure 3). Insert A in Figure 3 shows the methine protons of the intermediate from the unsymmetrical a,c-biladiene 3 and demonstrates the presence of unequal amounts of two isomeric structures depending upon which of the two terminal methyls in 3 forms the macrocyclic bridging carbon. Irradiation of the methyl singlet in 6 at 1.40 ppm gave a nuclear Overhauser enhancement at the upfield doublet (2.52 ppm) and also at a methyl resonance (1.77 ppm). On the basis of this evidence, we propose structure 6 for the intermediate, with proton NMR assignments as annotated. High resolution FAB mass spectrometryI6 confirmed the expected molecular weight. The mechanism shown in Scheme I is proposed for the decamethyl-apbiladiene 5 electrocyclization; following deprotonationI7 the conjugated tetrapyrrole suffers two-electron oxidation and macrocyclization to give the intermediate 6. Nucleophilic attack,18 presumably by the electrolyte, causes formation of the phlorin 7 which undergoes spontaneous oxidation19 to give porphyrin. Thin-layer spectroelectrochemistry (not shown) indicates that the order of the nucleophilic attack/oxidation steps may be reversed in the electrochemical conversion of 6 into porphyrin.