Furan with singlet oxygen

Mass spectrometry has been used to investigate the mechanism of ozonolysis <79JOC3i85,80JA4763) and to study the gas-phase reaction of furans with singlet oxygen in which endoperoxides are intermediates. 

Adam, W. and Schuhmann, R.M., Selective oxyfuncionalization of (tricarbonylcyclohexadiene) iron-substituted furans with singlet oxygen and dimethyldioxirane, Liebigs Ann., 635, 1996. 

A palladium catalyst is used in the transformation of a siloxyfuran to a phenyl substituted furanone <00JCS(P1)3350>. Similar products, furan-2(5//)-one derivatives, are afforded through the reaction of tetra-n-butylammonium fluoride with the corresponding substituted 2-siloxyfuran <00S1878>, as well as the oxidation of 3,4-disubstituted furans by singlet oxygen . 

Detailed accounts on the photooxygenation of 1,3-diphenyl-benzo[c]furan (and other benzo [cjfurans " ) reveal a dual mechanistic process. In the high concentration region 138 reacts in its ground state with singlet oxygen (generated either by dye sensitization [Scheme 15,... 

A six-step synthesis of hexahydrofuro[3,2-3]oxepin-2(3/7)-one involves cyclization with the formation of an ethereal C-0 bond (Scheme 13). However, its specific peculiarities are that the product skeleton is built from tetrahydrofuran (THF) and furan, the key reactions being the oxidation of a furan ring with singlet oxygen, followed by an intramolecular Michael addition to 2(5/7)-furanone C=C bond <2003TL4467, 2004TL5207>. 

It is known that thiophenes react with singlet oxygen forming endoperoxides, which are thiaozonides, and whose thermal reactions have been investigated. A novel reaction of acylthiophene endoperoxides 213 (photosensitized oxidation with tetraphenylporphyrine at —30 °C) involves their treatment with triphenylphosphine. The nucleophilic attack finally leads to a furan derivative 214 by elimination of O and S atoms bound to phosphorus (Scheme 71) <1995TL7431, 1998JA8914>. 

Furans also form cyclic peroxides on reaction with singlet oxygen these undergo some interesting rearrangements as shown by the formation of the 2-aroyl enol esters 226 from the peroxides derived from 2-arylfurans. 

The Z-alkene in the other half 257 was made from a furan 262. Cycloaddition with singlet oxygen (rose Bengal is a dye that sensitises the conversion of normal triplet oxygen to the singlet) leaves one necessarily Z-alkene in the five-membered ring 264. One possible mechanism for the decarboxylation of the cycloadduct 263 is given 265. 

The oxazoles also display a number of characteristics that are typical of the furans and are explained by the structural similarity of these heterocyclic systems. The ease with which they undergo Diels-Alder reactions with dienophiles and autooxidation with singlet oxygen (see Sections IV, D and E) clearly demonstrates that oxazoles are not fully aromatic. This fact and ultraviolet data (Section III, E) suggest that oxazoles should be considered partly as conjugated dienes. 

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