The Formation of Furan Compounds from Hexoses

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This chapter discusses the formation of furan compounds from hexoses. The sequence of reactions in which three molecules of water are eliminated from a hexose molecule, giving rise to the furan ring, remains unknown. The presence of methoxyl groups in a ketohexose does not impede the degradation to a furan body and similarly, the methoxyl groups do not block furfural formation when methylated pentoses are heated with mineral acids. The presence of methoxyl groups may facilitate the degradation of fructose to the furan compound because higher temperatures are necessary to bring about the degradation of fructose itself. The ease with which the hydroxyl group is replaced by a halogen atom is not a usual feature of primary alcohols and is caused by an electronic effect of the furan ring. Reduction of the furan ring also occurs at normal temperatures with sodium amalgam.