2-Methoxyfuran, irradiation

The sensitized irradiation of 2-methoxyfuran gave a product (34) deriving from a transposition reaction of the methyl group (Scheme 13) (69TL2767). The 5-methyl derivative 35 was obtained starting from 2-acetoxyfuran (Scheme 13). 

The oxaziridine ring system has been formed by the oxidation of C=N double bond . The two-step synthesis of iV-phosphinoyloxaziridines 16 from oximes 15 was described (equation 7) . Irradiation of hydroperoxynitrones 18, prepared by dye-sensitized photooxygenation of 2-methoxyfuran in the presence of oximes 17, led to fraw -oxaziridines 19 in yields up to 89% (equation 8) ". ... 

The presence of functional groups in the heterocycle frequently determines the nature of the photoreaction observed. The major product of irradiation of 2-methoxyfuran (191) in the gas phase or in solution is the lactone (192).157 Similarly, 2-nitrofuran (193) undergoes a photoreaction typical of a,f -unsaturated nitro compounds to give the oxime (194) by the pathway shown in Scheme 13.158 A different process is observed, however, on irradiation of thenitrovinylbenzo[6]furan(195) to give the 6-hydroxy-l,2-oxazine(196)159 this transformation is viewed as proceeding via an electrocyclization pathway (Scheme 14) for which there is a precedent in the known photochromism of nitrostyrenes. 

Examples of related photochemical (hetero)arylations are the pentafluoralkyla-tion of pyrroles and indoles by irradiation in the presence of pentafluoroiodo-benzene or of pentafluorophenyl perfluoroalkanesulfonates [97], and presumably also the acetone-sensitized photocoupling of 5-bromo-l,3-dimethyluracil (47) with indoles. The latter reactions apparently follow an ET path, rather than the homo-lytic debromination which occurs on direct irradiation (Scheme 32) [98]. A different arylation leads to 2,5-dihydro-2-(4 cyanophenyl)-5-methoxyfuran on irradiation with 1,4-dicyanobenzene in methanol [99]. 

The location of the oxime group in the product in Scheme 4 depends upon where the methyl group is located, but the yields can be high (79%). Irradiation of 2-methoxyfuran also induces side-chain dissociation and the formation of lactones (i.e., methylbutenolides).21 Here the abundant formation of ethane testifies to the participation by methyl radicals. Such reactions are too complex for synthetic purposes. This is true also of 2-acetoxyfuran photochemistry in which the photo-Fries reaction is not observed.21 Curiously, furan itself sensitizes the photo-Fries reaction in benzenoid esters.22... 

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