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Scheme 20

Ogawa and coworkers introduced C3 phenylselenyl and phenylthio substituents as stereocontrolling auxiliaries [24,25]. These directing groups had already been developed in the context of stereoselective syntheses of 2-deoxyglycosides by several groups [26]. Phenylthio and phenylselenyl neighboring groups were anticipated to provide better charge stabilization than a C3 hydroxyl substituent because of the increased polarizability of sulfur and selenium [26]. Initial glycosylation attempts with the phenylselenyl substituent proceeded with good stereoselectivity, although yields were low owing to elimination of a cationic selenium species to provide the corresponding 2,3-dehydro NeuAc derivative as the major product of the reaction [24]. The less polarizable phenylsulfide auxiliary was found to circumvent his problem [25]. Mercury-promoted glycosylation of the acceptor 73 with the donor 72 afforded the GM3 precursor 74 in good yield with no trace of the /3 isomer (Scheme 22). An additional benefit of using a sulfur auxiliary over a hydroxyl was its relative ease of removal in one step by a radical reduction. Treatment of 74 with Bu3SnH and AIBN provided the trisaccharide 75 in 75% yield. The phenylsulfide auxiliary was also used to synthesize an a-2,8 NeuAc dimer [25b]. The Ogawa group immobilized these types of sialyl donor on solid support resins and demonstrated their ability to act as donors in solid phase glycosylations [25c]. Thioglycoside donors combined with C3 phenylthio directing groups have been effectively employed by the Magnusson laboratory [27].

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-

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