Alkylation, photochemical heterocycles

In an extension of atom-transfer radical reactions to heterocyclic systems, Byers has introduced a novel methodology for the addition of electron-deficient radicals to unprotected pyrroles and indoles in a stannane-fi ee, non-oxidative process <99TL2677>. For exanqrle, photochemical reaction of pyrrole (33) with etl l iodoacetate (34) in presence of thiosulfiite as an iodine reductant, phase transfer catalyst and propylene oxide led to high yields of the 2-alkylated pyrrole 35 <99TL2677>. 

Ultraviolet (UV) spectroscopy does not tend to be the method of choice for structure determination, but a list of UV absorptions was given in the review by Knowles <1996CHEC-II(7)489>. Fluorescence properties and triplet yields of [l,2,3]triazolo[4,5-r/ pyridazines in various solvents have been reported <2002JPH83>. These heterocyclic systems were found to be photochemically very stable. In a recent paper, Wierzchowski et al. studied the fluorescence emission properties of 8-azaxanthine ([l,2,3]triazolo[4,5-r/ pyrimidine-5,7-dione) and its A -alkyl derivatives at various pH s <2006JPH276>. For the 8-azaxanthines, an important characteristic of emission spectra in aqueous solutions was the unusually large Stokes shift. Since 8-azaxanthine is a substrate for purine nucleoside phosphorylase II from Escherichia coli, the reaction is now monitored fluorimetrically. The fluorescence properties of [l,2,3]triazolo[4,5-r/ -pyrimidine ribonucleosides were earlier described by Seela et al. <2005HCA751>. 

A study of die addition reactions of radicals to fiillerenes (C60/C70) by EPR has appeared and die dynamic effects in the EPR spectra of fidlerenyl radicals due to hindered rotation and the multi-addition of radicals to fidlerenes are described.9 Other review articles which have appeared this year include recent advances in the radical substitution reactions of alkyl, aryl, and vinyl halides10 and the substitution and photochemical reactions of heterocyclic A -oxidcs.11 The mechanisms for the oxidation of hydrocarbons, lipids, and low-density lipoproteins have been reviewed.12... 

A new type of photoreduction has been reported recently 46, 47). Irradiation of quinine, quinidine, cinchonine, and cinchonidine in aqueous acidic solution gave the corresponding 9-deoxy derivatives 182. The postulated mechanism of Stenberg 47) proceeds via the triplet state (T ) and in the initial stages is very similar to the mechanism proposed for the photochemical alkylation of aromatic nitrogen heterocycles in alcoholic solutions 48). 

E. Kozlowska-Gramsz and G. Descotes, Photochemical addition of alkyl azidoformiate to 2-methylenetetrahydropyran and 2-methoxy-5,6-dihydro-Y-pyran, J. Heterocycl. Chem., 20 (1983) 671-672. 

Four-membered Ring Nitrogen Heterocycles.— The synthesis of azetidine and its derivatives by akylation of an external or an internal amine is well known, and two more examples were reported in 1980. Interest in simple alkyl substituted azetidines now lies in studies on their stereo-preferences, and conversions to other important systems. That said, two photochemical investigations surprisingly turned up such systems as unexpected major products. Pete et while extending their work on 2-amino-cyclohex-2-enones examined the consequence of N-sulphonylation on their photochemical behaviour, and instead of getting aziridines as the principal products, they obtained azetidines (Scheme 35). These reactions were not as clean as the earlier ones, and aryl migration... 

The priority of vinyl azides over alkyl azides in light absorption and photochemical transformation was demonstrated by the irradiation of diazide 192, which led to the strained heterocycle 193 without degradation of the second azido group. The... 

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