Highly regioselective alkylation of sugar hydroxyl groups has always been an important challenge in carbohydrate chemistry, especially for the selective alkylation of trans diols, there is no direct and efficient catalytic method so far. A chiral copper catalyzed universal highly site-selective alkylation of trans-diols method is realized. This reaction is performed under mild conditions and has broad
acetylation of vicinal and 1,3-diols is presented. Herein, the acetylation of the hydroxyl group with acetic anhydride can be activated by the formation of H-bonds between the hydroxyl group and anions. The reaction exhibits high regioselectivity when a catalytic amount of tetrabutylammonium acetate is employed. Mechanisticstudies indicated that acetate anion forms dual H-bonding complexes with the diol
approach to regioselectivebenzylation of carbohydrate trans-diols was developed, where 0.1 equiv. of Bu2SnCl2 and 0.1 equiv. of TBABr were used as the catalysts and 2.0 equiv. of BnCl was used as the benzylationreagent. In most cases, similar or better benzylationregioselectivities and isolated yields were obtained by using catalyticamounts of Bu2SnCl2, rather than stoichiometric amounts of organotin
In the present study, the mechanistic origin of the promoted organotin-mediated carbohydrate benzylation by halides was explored by the comparison of the activation ability of halides on benzylation of methyl beta-D-galactoside. It was demonstrated that the improvement of benzylation in terms of speed, yield and general convenience in the presence of halides was due to the formation of an activated oxide species by the coordination of halide anions to the tetracoordinate tin atoms. The halide, being able to form a stronger bond with tin, showed stronger activation ability. The catalytic amount of bromide should be preferentially chosen as an activation additive in the reaction with consideration of economy, convenience and moderate activation ability. The results were further applied to mono- and multibenzylation of additional carbohydrate structures. A range of prototype carbohydrate structures were efficiently prepared with the guidance of the principle. In light of the previous studies and the present experimental results, the general rules for organotin-mediated benzylation of carbohydrates have been summarized. (C) 2013 Elsevier Ltd. All rights reserved.