LiOH promoted allomerization of pyropheophorbide a. A convenient synthesis of 13²-oxopyropheophorbide a and its unusual enolization

Treatment of pyropheophorbide a with aqueous LiOH-THF gave the corresponding enolate, which on in situ oxidation produced the corresponding 13²-oxo derivative in 79% yield; prolonged autoxidation gave purpurin-18 and purpurin-5 as major products and a mechanism for the formation of 12-formyl derivative from 13²-oxopyropheophorbide a under unusual enolization is discussed; the electronic absorption spectroscopy data of the enolic intermediates showed a considerable perturbation in the π-systems of the macrocycle.