Auxiliary-directed glycosylation

A disadvantage of auxiliary-directed glycosylation is that additional steps are required for the synthesis of the sialyl donor. To address this problem, the Whitesides group developed an expeditious route to a sialyl donor in two steps from the NeuAc glycal 21 (Scheme 23) [28]. Treatment of 21 with 2,4-dimethylbenzenesulfenyl chloride afforded the crystalline intermediate 76 in 85% yield. This compound was quan-... 

Chiacchio et al. (43,44) investigated the synthesis of isoxazolidinylthymines by the use of various C-functionalized chiral nitrones in order to enforce enantioselec-tion in their cycloaddition reactions with vinyl acetate (Scheme 1.3). They found, as in the work of Merino et al. (40), that asymmetric induction is at best partial with dipoles whose chiral auxiliary does not maintain a fixed geometry and so cannot completely direct the addition to the nitrone. After poor results with menthol ester-and methyl lactate-based nitrones, they were able to prepare and separate isoxazo-lidine 8a and its diastereomer 8b in near quantitative yield using the A-glycosyl... 

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