Recent advances in the synthesis of hydroporphyrins

The various approaches to the synthesis of hydroporphyrins are summarized. Transformations of synthetic porphyrins have been extensively studied in the last decade due to their easy availability. Many two and three step reaction sequences have been developed, which have allowed access to a broad variety of structures including not only chlorins but also benzochlorins, secochlorins etc. The fact that porphyrins can act as dienophiles or dipolarophiles has been broadly investigated and utilized. On the other hand 'total syntheses' of chlorins from pyrrole and other simple starting materials have been pursued by only a few research groups. Generally a [2+2] approach has been investigated and usually chlorins possessing two geminal methyl groups on the reduced pyrrole ring ('locked' chlorins) were the targets of the studies. The synthesis of tetrahydrodipyrrin derivatives as popular building blocks was the subject of intense investigation. Overall, a few interesting and ingenious approaches toward 'total synthesis' of chlorins were proposed, reaching total yields on the level of 1-5%. Some derivatives with auxochromic groups were prepared, which allowed the study of the relationship between structure and spectroscopic properties. Bacteriochlorin synthesis, probably due to their limited stability, has been studied less extensively.