Some Aspects of Selectivity in the Reaction of Glycosyl Donors*

Reciprocal Donor–Acceptor Selectivity: the Influence of the Donor O‐2 Substituent in the Regioselective Mannosylation of myo‐Inositol Orthopentanoate

“…[5][6][7] The NPOE has, yet again, displayed a higher regioselectivity than the armed donor in all couplings. The observed regioselectivity in the coupling of triol 13 with 15, coincides with that observed in the acylation (RCOCl, pyridine) of orthoester myo-inositol derivatives, as reported by Ozaki's [12] and Chiara's [13] groups.…”

“…The higher regioselectivity displayed by NPOE 15 when compared with armed donor 16 towards triol 13 is in agreement with previous results from our research groups. [6,7,18] The observed regioselectivity displayed by NPOE 15 is remarkable because there is just one equatorial OH group compared with two enantiotopic axial OH groups in the acceptor. Blocking of one of the axial OH groups in compound 14 resulted in a considerable enhancement of the glycosylation regioselectivity towards the equatorial OH group.…”

“…Thus, the reaction of diol 2 with two glycosyl donors, differing in the protecting group at O-2, e.g., 1 and 4 (Scheme 1 a, b), might give raise to regioisomeric disaccharides, e.g., 3 and 5. These O-2 substituent-induced regiopreferences have been found in several pairs of secondary OH acceptors, [5,6] and even in primarysecondary-OH pairs. [7] Furthermore, we have noticed several instances where an acceptor-diol displays comparable regioselectivies towards glycosyl donors and protecting reagents.…”

New Principles for Glycoside‐Bond Formation

“…[118] Even more interestingly, NPOE donors often exhibit much better regioselectivity than other glycosyl donors and sometimes even opposite regioselectivity in the glycosidation of a particular diol acceptor. [119] The cyclic-ketal-protected glucose diol 49 was glycosylated regioselectively with the mannosyl NPOE donor 48 to give only the 6-O-linked disaccharide 50 in reasonable yield (Scheme 7 c), whereas coupling with the corresponding phenylthioglycoside donor under the same conditions gave a mixture of 6-O-and 3-O-linked disaccharides, whereby the 6-O-linked isomer was produced in much lower yield.…”

Experimental observations on the regioselectivity of glycosylation of a 4,6-diol system in the β-d-mannopyranosyl unit of a N-glycan pentasaccharide core structure

“…Besides, the complexity of the system further complicates attempts of rationalization. Fraser-Reid et al, who have addressed more systematically the regioselective glycosylation of diol systems, [31][32][33][34][35][36] has suggested that primary hydroxyl groups can only be considered more reactive than secondary hydroxyl groups in reaction with disarmed glycosyl donors and orthoesters and that armed glycosyl donors can favour glycosylation at the secondary rather than the primary position. 33 Our present data do not permit any interpretation of the obtained results on the basis of this suggestion.…”