Glucosyl acceptor

Ziegler and coworkers prearranged a glycoside by employing a succinyl tether between C-6 of a mannosyl donor and C-3 of glucosyl acceptor [151,152]. They found that the nature of the glycosyl acceptor and the length of the tether affected the anomeric selectivity of the intramolecular mannosylation (Scheme 4.4d) [153]. 

Silylation of the glucosyl acceptor by Et3SiOTf has been observed only in some cases when the donor or the acceptor contained an N-acetate. 

Uridine 5 -(a-D-glucopyranosyl pyrophosphate) -I- acceptor — uridine 5 -pyrophosphate -I- a-D-(1 4)-glucosyl-acceptor. 

Thus, sucrose, ubiquitous in everyday life, is still an elusive target substance in the chemical laboratory. Perhaps acyl-protecting groups in the glucosyl acceptor could stabilise the acetal and the anomeric linkage enough to enable sucrose to be finally produced in a simple procedure. 

The synthesis of BOA-N-P-D-glucoside starting from BOA, a very unreactive glucosyl acceptor, was only achieved by using a modified Vorbriiggen protocol [190]. BOA was silylated with N,0-bis-trimethylsilyl-trifluoroacetamide at 110 °C for lactam activation. 

The initial glycosidation of the 3-hydroxy derivative 21 with glucosyl acceptor 22 in the presence of AgOTf and NaHP04 in toluene afforded Neu5Ac-a-(2-6)-Glc disaccharide 23 in 63% yield accompanied with a 8-isomer (16%) (O Scheme 6) [29,30]. 

The initial purpose of the acetylation of the C5 acetamido group is to prevent M-methy-lation in the course of MeOTf-promoted glycosidation using methylthioglycoside of Neu-5Ac-a-(2-3)-Gal as a glycosyl donor. Without the acetyl protection, the glycosidation with a glucosyl acceptor produced Af-acetyl-Al-methyl trisaccharide in 70% yield. AW-Diacetyl... 

As depicted in O Scheme 36 [106], the results of glycosylations of the primary hydroxyl of glucosyl acceptor 22 with A-Troc, A-TFAc, and AA -diacetyl sialyl donors (104,102, and 94) promoted by NIS-TfOH in MeCN at —35 °C also showed a similar hierarchy of the a-sialyla-tion efficiency between these donors A-Troc sialoside 119 was obtained in 91% (a/)3 = 89/11), A-TFAc sialoside 107 in 92% (aip = 92IS), and AA -diacetyl sialoside 134 in 65% yield (a/p = 62/38), respectively. On the basis of these results, it can be concluded that the efficiency of a-sialylation with a AA -diacetyl sialyl donor is less constant than the corresponding A-Troc and A-TFAc derivatives, depending on the reaction system. 

Enzyme Glucosyl donor Glucosyl acceptor Reversibility Optimum pH References... 

Scheme 10-77 Tethering the glucosyl acceptor through an acetal to the 2-OH of the mannosyl donor affords exclusively the j3-l,4-linked mannoglucoside.

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