LIOH PROMOTED ALLOMERIZATION OF PYROPHEOPHORBIDE A - A CONVENIENT SYNTHESIS OF 13(2)-OXOPYROPHEOPHORBIDE A AND ITS UNUSUAL ENOLIZATION

Treatment of pyropheophorbide a with aqueous LiOH-THF gave the corresp onding enolate, which on in situ oxidation produced the corresponding 13(2)-oxo derivative in 79% yield; prolonged autoxidation gave purpuri n-18 and purpurin-5 as major products and a mechanism for the formatio n of 12-formyl derivative from 13(2)-oxopyropheophorbide a under unusu al enolization is discussed; the electronic absorption spectroscopy da ta of the enolic intermediates showed a considerable perturbation in t he pi-systems of the macrocycle.