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The [2+2] photocycloaddition mediated by 1-butyl-3-methyl-7,8-dimethoxyalloxazine ( 1) has been found to be an effective tool for cyclising ω-phenyl- and ω,ω′-diphenyl-4-aza-1,6-heptadienes, in which the nitrogen atom is protected by acylation or quaternisation, towards the synthesis of a variety of phenyl- and diphenyl-3-azabicyclo[3.2.0]heptanes and their corresponding quaternary salts. Thia derivatives, with the sulfur atom in the form of a sulfone group, underwent an analogous cyclisation. Advantageously, visible light (400 nm) was used for the cycloadditions in the presence of 1, in contrast to the previously described procedures affording azabicyclo[3.2.0]heptanes by using UV irradiation. Practical applications are demonstrated through the synthesis of bicyclic quaternary ammonium salts, 6-phenyl-azabicyclo[3.2.0]heptanes known to exhibit biological activity or chiral spiro ammonium salts. Flavin 1 was also found to promote the efficient E→ Z isomerisation of electron-rich cinnamyl derivatives to produce mixtures enriched with the Z isomer (with Z/E ratios of up to 77:23). [ABSTRACT FROM AUTHOR]

Alkyl- and aryl vinyl sulfones were obtained by eosin Y (EY)-mediated visible-light photooxidation of sulfinate salts and the reaction of the resulting S-centered radicals with alkenes. Optimized reaction conditions, the sulfinate and alkene scope, and X-ray structural analyses of several reaction products are provided. A detailed spectroscopic study explains the reaction mechanism, which proceeds through the EY radical cation as key intermediate oxidizing the sulfinate salts. [ABSTRACT FROM AUTHOR]

Abstract: New photocatalysts from the flavin family were found to mediate the [2+2] photocycloaddition reaction. 3‐Butyl‐10‐methyl‐5‐deazaflavin (3 a) and 1‐butyl‐7,8‐dimethoxy‐3‐methylalloxazine (2 e), if irradiated by visible light, were shown to allow an efficient (Φ≈3–10 %) intramolecular cyclisation of various types of substrates including substituted styrene dienes and bis(aryl enones), considered as electron‐rich and electron‐poor substrates, respectively, without any additional reagent. The versatility of the procedure was demonstrated by the cyclisation of photosensitive cinnamyl (E)‐3‐iodoallyl ether. Structure–activity studies found alloxazine 2 e was more active than 7‐monosubstituted (R=Cl, Br and MeO) alloxazines. The introduction of chlorine and bromine atom on the deazaflavin skeleton did not enhance the catalytic efficiency of 3 a. A detailed electrochemical and spectroscopic study explains the reaction mechanism proceeding through energy transfer from the flavin excited triplet state to the diene followed by its cyclisation. [ABSTRACT FROM AUTHOR]

A new application of flavin derivatives in visible light photocatalysis was found. 1-Butyl-7,8-dimethoxy-3-methylalloxazine, when irradiated by visible light, was shown to allow an efficient cyclobutane ring formation via an intramolecular [2+2] cycloaddition of both styrene dienes, considered as electron-rich substrates, and electron-poor bis(arylenones), presumably proceeding via an energy transfer mechanism. [ABSTRACT FROM AUTHOR]