Thioglycosides armed-disarmed glycosylations

An additional useful controlling element in thioglycosides-based glycosylation, in close analogy to that obtained from armed-disarmed glycosyl donors, has been nicely introduced by the group of Ley [80, 81]. An exemplary application of conformationally locked dispiroketal ( dispoke acetal ) has been used toward the synthesis of the pentasaccharide unit common to the surface glycoprotein of Trypanosoma brucei [81, 82]. 

Transfer of the Armed-Disarmed Concept to Thioglycoside Glycosylation... 

Armed thioglycoside donor 6 was selectively activated over disarmed glycosyl acceptor 7 in the presence of IDCP to provide the disarmed l,2-c -linked disaccharide (8) in 91% yield. The latter was then subjected to a two-step debenzoyla-tion-benzylation sequence, whereupon the resulting disaccharide donor (9) was glycosidated with disarmed acceptor 7, to afford the cis-cis-linked trisaccharide (10) in 72% yield. The conversion of the second generation glycosyl donor 8 into the armed state 9 also allowed for the second coupling step to be performed with the mild promoter IDCP. 

Diol 159 was chosen as a model for probing regioselective glycosylation with armed and disarmed thioglycosides 160 and 163, and NPOE 108. The results obtained were not anticipated (Scheme 5.35b-d). 

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