The preparation of four per-O-benzyl-D-or Lglycero-D-galacto and D-or L-glycero-D-gluco heptopyranosyl sulfoxides and the influence of their side-chain conformations on reactivity and stereoselectivity in glycosylation reactions are described. The side-chain conformation in these donors is determined by the relative configuration of its point of attachment to the pyranoside ring and the two flanking centers in agreement with a recent model. In the D-and L-glycero-D-galacto glycosyl donors, the D-glycero-D-galacto isomer with the more electronwithdrawing trans,gauche conformation of its side chain was the more equatorially selective isomer. In the D-and L-glycero-D-gluco glycosyl donors, the L-glycero-D-gluco isomer with the least disarming gauche,gauche side-chain conformation was the most equatorially selective donor. Variable temperature NMR studies, while supporting the formation of intermediate glycosyl triflates at −80 °C in all cases, were inconclusive owing to a change in the decomposition mechanism with the change in configuration. It is suggested that the equatorial selectivity of the L-glycero-D-gluco isomer arises from H-bonding between the glycosyl acceptor and O6 of the donor, which is poised to deliver the acceptor antiperiplanar to the glycosyl triflate, resulting in a high degree of S N 2 character in the displacement reaction.

Novel heterolayered (“onion peel”) hybrid glycodendrimers containing optimised sugar head groups with galactoside and mannoside units with affinities for two different lectins.

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