INTERGLYCOSIDIC ACETALS IV - PREPARATION AND REGIOSELECTIVE CLEAVAGE OF PHENYL 2,2' 6'-TRI-O-BENZYLIDENE-1-THIO-BETA-LAMINARIBIOSIDES/

A(+)-10-Camphorsulfonic acid-catalysed acetal exchange reaction of phe nyl 1-thio-beta-laminaribioside using 3.5 molar equivalents of alpha,a lpha-dimethoxytoluene gave a tris(benzylidene acetal), which was isola ted and characterized as phenyl 3'-O-acetyl-2,2' ,6'-tri-O-benzylidene -1-thio-beta-laminaribioside, and the corresponding 3'-O-benzyl deriva tive 6. Upon a treatment with pyridinium p-toluenesulfonate, the inter glycosidic 2,2'-acetal in 6 underwent selective cleavage to give the 2 ,2'-diol. Additionally, a reductive ring-opening reaction of 6 with li thium aluminium hydride/anhydrous aluminium chloride, followed by O-ac etylation, gave the 2',6,6'-O-acetyl-4,2',3',4'-tetra-O-benzyl derivat ive in 73% yield. A different regioselectivity was observed in the red uction of 6 with borane-trimethylamine adduct/anhydrous aluminium chlo ride or sodium cyanotrihydroborate/methanesulfonic acid, giving the co rresponding 2, 4,4'-triol as the major product.