Bausteine von Oligosacchariden, LVIII. Regioselektive Glycosylierung an 3′‐OH oder 4′‐OH der Lactose durch Einsatz unterschiedlicher Katalysator‐Systeme

Abstract

Building Units of Oligosaccbarides, LVIIIl). -Regioselective Glycosylation at 3'-OH or 4'-OH of Lactose Using Different Catalyst SystemsThe lactose derivative 4 with unsubstituted 3'-OH and 4'-OH groups makes regioselective glycosylation reactions possible depending on the chosen type of catalyst system. In the presence of silver triflate, 4 reacts in homogenic phase with the phthalimido bromide 1 to yield the b(1 -P 3)-glycosidically linked trisaccharide 5 . The same compounds give in heterogenic phase with silver silicate regioselectively the p(1 -~4)-glycosidically linked trisaccharide 7. In the presence of silver silicate, 4 reacts with the azido halide 2 to yield in heterogenic phase regioselectively the p(1 -+ 4)-glycosidically linked product 17. In homogenic phase with silver perchlorate, however, the azido halides 2 and 3 yield the a-glycosidically linked products, following the in siru anomerization procedure.

Acceptor reactivity in glycosylation reactions

“…Diol 120 can be regioselectively glycosylated at the equatorial hydroxyl with pentenyl orthoester donor 121, while the axial alcohol in 120 reacts selectively with pentenyl mannoside 123 (Scheme 2). [42][43][44][45] Building on Paulsen notion that the reactivity of both reaction partners should be ''matched'' for an optimal glycosylation reaction, [46][47][48][49] Fraser-Reid coined the concept of reciprocal donor acceptor selectivity (RDAS), to account for these observations. [50][51][52][53] Although the concept still awaits a proper mechanistic explanation, the counter-intuitive outcome of several recent reactions have been related to this phenomenon.…”

Controlling the stereoselectivity of glycosylation via solvent effects

“…30,44 In the absence of participating group at C-2, the reaction leads to a mixture of α and β anomers, and in this case effect of solvent should be remarkable. 45 Although the formation of dioxolenium ion drives the reaction to give predominantly βselectivity, formation of cis-isomers (α) has also been observed occasionally. This could be due to the reaction going through pathways involving either a reactive glycosyl cation or resonance stabilized oxocarbenium ion ( Figure 2).…”

Synthesis of oligosaccharides of 2-amino-2-deoxy sugars

“…When the same reaction was effected with silver silicate as the promoter and molecular sieves 4A under heterogenous catalysis mode, the glycosylation occurred mainly on the axial group HO-4', affording the /3-(l-*-4)linked trisaccharide (86% yield), together with the d-(l-*-3)-linked isomer in 8% yield. 237 Glycosylation of the same diol containing acceptor with hexa-0-acetyl-2-deoxy-2-phthalimido-/3-D-lactosaminyl bromide under homogeneous catalysis (silver triflate) afforded the /3-(l-*-3)-linked tetrasaccharide in 65% yield, whereas heterogeneous catalysis (silver silicate) forded mainly the /3f l-*-4) tetrasaccharide.238 When 4,6-di-0-acetyl-3-0-(2,3,4,6-tetra-0-acetyl-/8-Dgalactopyranosyl)-2-deoxy-2-phthalimido-D-galactopyranosyl bromide was reacted with the same diol acceptor under homogeneous catalysis using mercuric cyanide and mercuric bromide, the corresponding /3-(l-^3)linked trisaccharide was obtained in 25% yield.236 239 When 3,4,6-tri-0-acetyl-2-deoxy-2-phthalimido-D-galactopyranosyl bromide was used as the donor, good yields but a poor regioselective glycosylation trend were observed.239 Also, glycosylation of the HO-3' of the diol acceptor methyl 3,6-di-0-acetyl-4-0-( 2 in 64% yield. 240 The latter was further transformed into glycopyranosyl chloride and condensed again with the same acceptor to give the /3-(l-*-3)-linked hexasaccharide.…”