Chlorins with an Exocyclic δ‐Lactone Ring and Their Derivatives.

Two novel reactions are described in the series of di- and tetra-hydroporphyrins possessing an exocyclic six-membered anhydride ring. In the first reaction, the partial reduction of the anhydride ring of bacteriopurpurin and of two purpurins with sodium borohydride gave δ-lactones, which had considerable stability towards acidic and basic cleavage. The partial reduction showed regioselectivity, which was especially pronounced with bacteriopurpurin. In the second reaction, the attempted dehydrogenation with DDQ of chlorins possessing a free propionic acid residue at C-17 (purpurin 18 and its 3-acetyl-3-devinyl analogue) led to intramolecular cyclisation to give products with a δ-lactone ring fused to the β-face of ring D. In this case the lactone ring could be opened to give the 18-hydroxychlorins, the dehydration of which yielded the corresponding porphyrins (purpuroporphyrins, retaining the exocyclic anhydride ring).