Photocyclization reactions of 1-benzyl-1-pyrrolinium salts by diradical and diradical cation pathways. Novel photochemical Pictet-Spengler cyclizations

Concise Photochemical Synthesis of the Antimalarial Indole Alkaloid Decursivine

“…[18] An intuitive assignment of the positive charge of the indole radical cation to positions stabilized by N1 (C2, C5, and C7) leads to the conclusion that the odd electron will preferentially reside at C3, C4, and C6, and this has been substantiated by computational modeling, which predicts a higher spin density at C4 than the alternative cyclization site C2 at all levels of theory. [20] Finally, we note that the step which determines the absolute configuration in this synthesis is the protonation of 11 to give 12. The elimination of HCl from 10 is a reaction that is precedented in the photolysis of the 2,2-dichloro-3-methylbutanamide of tryptophan methyl ester, [10] leading to an isopropylidene-substituted lactam analogous to 11.…”

“…For example, a photochemical ring closure of a pendant iminium function onto an electron-rich aromatic ring has been described and was likewise shown to proceed through a diradical cation pathway. [20] Finally, we note that the step which determines the absolute configuration in this synthesis is the protonation of 11 to give 12. The absolute configuration of natural (+)-1 has not been reported, but the configuration of the nearly identical (À)-2 is known to be R,R.…”

Electron‐transfer Organic and Bioorganic Photochemistry

“…bicyclic pyrrolidines, benzoindolizidines, and benzopyrrolidines (Tu et al, 1987;Lan et al, 1987). The photocyclization of l-benzylpyrroliniutn cation proceeds by way of intramolecular photoelectron transfer from the benzyl group to the pyrrolinium chromophore followed by diradical and diradical-cation couplings (Cho and Mariano, 1988). This method was successfully applied to protoberbelin alkaloid synthesis (Ginny and Mariano, 1987).…”