C-5 Modified S-Benzoxazolyl Sialyl Donors: Towards More Efficient Selective Sialylations

We previously reported that the selective activation of an S-benzoxazolyl (SBox) sialyl donor over a galactosyl acceptor equipped with a thioethyl anomeric moiety can be performed in high yield but with poor stereoselectivity. To optimize this strategy, which allows the synthesis of complex carbohydrates to be shortened, a range of SBox sialyl donors modified at C-5 were synthesized and tested. In particular, the combination of the SBox leaving group and an oxazolidinone at C-4,5 allowed superior stereocontrol in various solvents and at different temperatures. Nearly complete stereoselectivity was obtained in the presence of bismuth(III) triflate in a dichloromethane/tetrahydrofuran (1:1) solvent system.