SINGLET OXYGEN OXIDATION OF SUBSTITUTED FURANS TO 5-HYDROXY-2(5H)-FURANONE

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-degrees-C instead of at -78-degrees-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.