An efficient synthesis and regioselective hydrogenolysis of dioxolane-type of carbohydrates

Abstract

Carbohydrates, inexpensive chiral sources, are very useful for many syntheses of chiral bioactive compounds. We have chosen to synthesize benzyl α-d-mannofuranosides and d-lyxofuranosides with dioxolane-type, 2,3-O-isopentylidene and 2,3-O-benzylidene acetals from d-mannose. Interestingly, we have discovered new knowledge about the cleavage of dioxolane-type using two catalysts, Cu(OTf)2/BH3·THF and TiCl4/Et3SiH. The results showed that the regioselectivity of the benzylidene acetal ring opening was affected by the adjacent hydroxy (OH) group and was not directed by the stereochemistry of the acetal center. The substrates containing OH groups at position 5 can be cleaved by Cu(OTf)2/BH3·THF in tetrahydrofuran to exclusively provide the 2-O-benzyl-3-hydroxy compounds. When the substrates bore the OH group at position 6, using TiCl4/Et3SiH in dichloromethane gave a similar pattern. If the substrates lacked the OH group at position 5 or 6, the reactions did not proceed or gave low selectivity. Our discovery is very useful for dioxolane-type ring opening of carbohydrate synthesis.