Chemo-, stereo- and regioselective hydrogenolysis of carbohydrate benzylidene acetals. Synthesis of benzyl ethers of benzyl α-d-, methyl β-D-mannopyranosides and benzyl α-D-rhamnopyranoside by ring cleavage of benzylidene derivatives with the LiAlH4-AlCl3 reagent

Abstract Treatment of benzyl α-( 1 ) and methyl β- d -mannopyranoside ( 2 ) with α,α-dimethoxytoluene gave the exo and endo isomers ( 3,5 and 4,6 ) of the dibenzylidene derivatives of 1 and 2 . Hydrogenolysis of the exo isomers ( 3 and 5 ) with a molar equivalent of AlH 2 Cl gave the 3-0-benzyl-4,6-0-benzylidene derivatives ( 7 and 21 ), whereas the endo isomers ( 4 and 6 ) gave the 2-0-benzyl-4,6-0-benzylidene compounds ( 8 and 22 ). The 2-0-allyl ether 9 of 7 , the 3-0-allyl derivative ( 10 ) of 8 and compounds 21 and 22 were treated with an additional molar equivalent of AlH 2 Cl at reflux and the products were the 4-0-benzyl-6-hydroxyl derivatives ( 11, 12, 23 and 24 ), whereas in the case of 22 the 6-0-benzyl-4-hydroxyl isomer ( 25 ) was also isolated. By deallylation of 11 and 12 , 3,4-( 13 ) and 2,4-di-0-benzyl ( 14 ) ethers of 1 were prepared. Tosylation of 11 and 12 , and subsequent reduction of the products ( 15 and 16 ) made possible the preparation of the partially protected benzyl α- d -rhamnopyranoside derivatives ( 17–20 ). The structures of the compounds synthesized were characterized by 1 H and 13 C NMR spectroscopic investigation and by chemical methods.