Lichtinduzierter Elektronentransfer an isoprenoidenPolyalken-1,1-Dicarbonitrilen : Zum Mechanismus photochemisch ausgelöster radikalischer Cyclisierungen
Radical cyclizations of isoprenoid polyalkene-1,1-dicarbonitriles are efficiently initiated upon photoinduced electron transfer from the alkene to the electronically excited states of cyanoarenes or 2,4,6-triaryl pyrylium salts. In the course of these reactions, a radical cation, formed at the w?terminal site of the alkene is in situ trapped by anti-Markovnikow addition of a nucleophile. The radical thus formed initiates a sequence of 6?endo?trig and 5?exo?trig ring closures, resulting in a resonance-stabilized radical. Subsequent reduction to the corresponding carbanion, followed by protonation furnishes mono- and polycyclic products in good yields. The mechanism of these synthetically useful biomimetic cyclizations was supported using time-resolved UV-vis spectroscopy, conductivity and molecular modelling studies.