L-Fucose (4), a sugar widespread in Nature ', has lately acquired special significance as a constituent of several families of blood-group antigens'. L-Fucose is either the immunodominant sugar of complex carbohydrate antigens, or its presence may increase the strength of the antigenic response. Its preparation by acid hydrolysis of the Fucus species of seaweed3 is cumbersome, and a synthetic method appears to be preferable. D-Fucose is readily synthesized from D-g&CtOSe', but the same methods are not practical for L-fucose, because L-galactose is not readily available. Several syntheses from common sugars have recently been published4-', but they all involve numerous steps, and the overall yields are low (l-24%). We now describe a synthesis, from the readily available methyl cu+rhamnopyranoside', that requires five steps, all except the last of which proceed in yields of >90%. The last step, the conversion of 6-deoxy-L-talose into L-fucose, is a reaction leading to an equilibrium mixture; the proportion of L-fucose therein is 84%, and the unchanged 6-deoxy-L-talose can be recovered and recycled. In this synthesis, there is no need to purify any of the intermediate products.
6-Deoxy-L-talose (3) was prepared by the method described by Collins and Overend', but the yield and the convenience of handling in each step have been improved by the use of recently described procedures.
Methyl a-L-rhamnopyranoside' was converted, in 30 min, into its 2,3-O-isopropylidene derivative (1) according to Liptak et ~1. lo. Oxidation of the free hydroxyl group on C-4 was performed with pyridinium dichromate" in the presence of molecular sieves in dichloromethane solutioni2, to yield methyl 6-deoxy-2,3-Gisopropylidene-cw-L_lynohexopyranosid-4-ulose
(2). Collins and Overend used chromium trioxide in pyridine for this oxidation'. Borohydride reduction of this hexulose should be