A Highly Efficient Triplet Analogue of a Thermal Biradical Cyclization—The Photochemical C2–C6 Cyclization of Enyne-Heteroallenes

Abstract

In einer mit hohen Ausbeuten verlaufenden Photoreaktion (90–99%) cyclisieren Enin‐Carbodiimide (X=N; siehe Schema) innerhalb von wenigen Minuten zu Indolochinolinen. Analog können Enin‐Ketenimine (X=CAr) zu Benzocarbazolen photocyclisiert werden, wodurch der Zugang zu photoaktiven Prodrugs eröffnet wird.

Isolation and Synthesis of Biologically Active Carbazole Alkaloids

“…This photolytic reaction proceeds via the triplet analogues to the thermal diradical cyclization. The photolysis of the enyne ketenimines 729 in degassed toluene in a Rayonet RPR-100 photochemical reactor at 254 nm afforded the 5 H -benzo[ b ]carbazoles 730 (Scheme ) . The low yield of the 5 H -benzo[ b ]carbazoles 730 was ascribed to the formation of polymeric material.…”

Synthesis and Photochemical Cyclization of a Novel Enyne-Carbodiimide

“…In contrast, when 4 was irradiated without triplet sensitization (toluene, λ = 300 nm) no reaction was observed after 90 minutes. Previous mechanistic results [7] exclude a singlet reaction and suggest a pathway via the triplet excited state of the carbodiimide unit [12] which then triggers ring closure to the biradical 6, formally by an 5-exodig cyclization. 6 can be understood as a triplet analogue of the biradical that is formed in the thermal C 2 -C 6 cyclization of enyne-(hetero)allenes [4,6].…”

“…C 2 -C 6 cyclization). Recently, it was demonstrated that the thermal C 2 -C 6 cyclization of enyne-carbodiimides and enyneketenimines [6] can be complemented by a very efficient triplet sensitized photochemical route [7]. As the latter reaction has important consequences with regards to the development of photoactive prodrugs, e.g.…”