Synthesis of L-fucose

Abstract

L-Fucose is a sugar of considerable biological interest and wide natural occurrence. Its isolation from the Fucus species of seaweed is not difficultl. However, it is a reIativeIy expensive sugar and the only synthesis described afforded an overall yield of 1% from L-arabinose2. Our approach, which affords a good yield of L-fucose from the readily available D-galactose, is also of theoretical interest in that it involves the formal inversion of the D-galactose molecule to produce derivatives of L-galactose. Furthermore, the open-chain intermediates prepared may be utilized in the synthesis of disaccharides of L-fucose which we are investigating at present. The secondary hydroxyl groups in acyclic derivatives are expected to be more reactive than the more sterically hindered hydroxyl groups attached to a pyran ring. The conversion of D-galactose into L-fucitol pentaacetate3 and 2,3:4,5-diO-isopropylidene-D-galactose diethyl dithioacetal” (1) have been described. By adaptations of these methods, 2,3:4,5-di-CLisopropylidene+fucitol(2) was prepared, in 29% yield from D-galactose, as a crystalline product which gave a crystalline p-toluenesulfonate (3) in good yieId. Oxidation of 2 with dimethyl sulfoxidephosphoric acid-dicyclohexylcarbodiimide’ gave a mixture which was hydrolyzed directly with acetic acid. After column chromatography, r_-fucose (4), was obtained from the hydrolyzate in 15% overall yield from D-galactose. A small amount of unoxidized L-fucitol (1% yield based on D-galactose) was also obtained.