Singlet Oxygen Oxidation of Substituted Furans to 5-Hydroxy-2(5H)-furanone

Article abstract of DOI:10.1021/jo00025a012

The conditions for the regiospecific singlet oxygen oxidation of various 2,4-disubstituted furans 9 to 4-substituted-2(5H)-furanones 3 are developed.The presence of a C-2 substituent (e.g., trimethylsilyl, tert-butyldimethylsilyl, or tributylstannyl) in 9 is an absolute requirement for the formation of the 4-substituted-5-hydroxy-2(5H)-furanone regioisomer 3.When the C-2 substituent is triethylsilyl (TES) or TBDMS, however, apart from 3, the corresponding 5-trialkylsiloxy derivative 11 is also isolated in a significant amount.These silyl acetals are unexpectedly stable but can be hydrolyzed back to 3 on stirring with dilute acid.The formation of silyl acetals, to our knowledge, has never been reported in the singlet oxygen oxidation of (trialkylsilyl)furan.A plausible mechanism for their formation is proposed.The presence of a catalytic amount of water in the oxidation of 2-(trialkylsilyl)-4-substituted-furans not only eliminates the formation of the silyl acetals but also speeds up the rate of the oxidation process.Moreover, the oxidation can then be carried out at 0 deg C instead of at -78 deg C.Oxidation of 2-(1-hydroxyalkyl)-4-substituted-furans in the absence of a reducing agent gives little or no sign of 2,5-disubstituted-6-hydroxy-3(2H)-pyranone 23 but instead 26 selectively.Thus, the (1-hydroxy)alkyl group can be utilized as the trialkylsilyl or trialkylstannyl group in dictating the regioselectivity in the singlet oxygen oxidation of substituted furans.