Studies of spiro‐chlorin formation using porphyrin amides

Abstract

Syntheses of 7‐(3‐ethoxycarbonylpropyl)‐2,3,5‐triethyl‐1,4,6,8‐tetramethylporphyrin (8) (via the MacDonald dipyrrylmethane route) and 2‐(2‐chloroethyl)‐4‐ethyl‐6‐methoxycarbonyl‐7‐(3‐methoxy‐carbonylpropyl)‐1,3,5,8‐tetramethylporphyrin (22) (via the tripyrrene route) are described. The corresponding pyrrolidides of these compounds were cyclized in high yield to furnish the spiro imines (20 and 30 respectively), but attempts to hydrolyze these imines to the appropriate spiroketochlorins were largely unsuccessful. A small amount of the spiroketochlorin 9 (from 8) was obtained, and this was successfully hydrogenated to give the dihydro derivative 21. The major problem in imine hydrolysis was observed to be reversion to the parent porphyrin through simple bond migration and spiro ring cleavage. A successful transformation of protoporphyrin‐IX dimethyl ester (31) into mesoporphyrin‐IX dimethyl ester (32) using di‐imide generated from dipotassium azodicarboxylate is described.

Synthetic Chlorins, Possible Surrogates for Chlorophylls, Prepared by Derivatization of Porphyrins

“…A more in-depth study of spirocyclization with porphyrin–amides was carried out by Smith and co-workers . A porphyrin bearing a butyramide group ( 8-H 2 4 ) underwent spirocyclization upon treatment with POCl 3 and pyridine (Scheme ).…”

“…The reduction was reported to afford the trans configuration (17-methyl with respect to 18-keto); hence, the pair of chlorin enantiomers ( 8-H 2 8 ) was obtained rather than all four possible stereoisomers. An analogous spiroketo–chlorin was subjected to mild reduction of the exocyclic methylene group using the diimide source PADA …”

“…A more in-depth study of spirocyclization with porphyrin− amides was carried out by Smith and co-workers. 259 A porphyrin bearing a butyramide group (8-H 2 4) underwent spirocyclization upon treatment with POCl 3 and pyridine (Scheme 57). The resulting imine 8-H 2 5 is at the same oxidation state as that of the porphyrin; hydrolysis could afford the porphyrin bearing a butyric acid group (8-H 2 6) or the spiroketo−chlorin bearing an exocyclic methylene group (8-H 2 7).…”

“…An analogous spiroketo−chlorin was subjected to mild reduction of the exocyclic methylene group using the diimide source PADA. 259…”

New synthetic pathway leading to oxospirochlorins

“… 5 Smith and colleagues described that spiroketochlorins can be obtained with high yields from porphyrin bearing a butyramide group. 6 Kai and Suzuki prepared a spirochlorin by the reaction of tetracyanoetylene with bromovinylheptaetylporphyrin. 7 Spirolactones can be obtained in reaction of vinyl porphyrins with hydrobromic acid and subsequent five-step procedure striving to intermolecular cyclization of propionate side chain.…”