1.2- frans-Glycosylation

M. Ribeirao, V. L. Pereira-Chioccola, D. Eichinger, M. M. Rodrigues, and S. Schenkman, Temperature differences for frans-glycosylation and hydrolysis reaction reveal an acceptor binding site in the catalytic mechanism of Trypanosoma cruzi trans-sialidase, Glycobiology, 1 (1997) 1237-1246. 

Synthesis of 1,2-frans-O-Linked Glycosyl Hydroxylysine via the Epoxide... 

Glycosyl halides (7a-e) were stereoselectively transformed into 1,2-frans-thio-glycoses by i) (8a-d, 8j) a two-step procedure via the pseudothiourea derivatives [9,10a] the substitution of halide by thiourea is mostly a SNl-type reaction since acetylated 1-thio-a-D-mannose (8b) was obtained from acetobromoman-nose (7b) [9c] ii) (8e-i) using thiolates in protic and aprotic solvents [10], or under phase transfer catalysis conditions [11], Another approach involved the reaction of thioacetic acid with 1,2-frans-per-O-acetylated glycoses catalyzed with zirconium chloride [12]. The 1,2-frans-peracetylated 1-thioglycoses (8e-h) were obtained in high yield. No anomerized products could be detected in these reactions (Fig. 1). 

Add glycosyl donor, 1,2-frans-peracetylated monosaccharide (1 eq) and stir overnight at room temperature (see Note 7). 

FIG. 12. The / -mandelic acid (in gold)-based chiral auxiliary at 0-2 can form a more-stable 6-membered dioxolenium ion, frans-decalm-like (32.9 kJ mol-1 relative to its BO), whereas its epimer can lead to a cw-decalin type of dioxolenium ion (72.8 kJ mol-1), ADF-DFT (DPZ frozen core). The trans-decalin leads to a glycosides, whereas the cw-decalin leads to / glycosides. Both 6-membered-ring dioxolenium ions are more stable then any dioxolenium ion based on participation from 0-3 (R 43.9 kJ mol-1 and 5 46.7 kJ mol-1). If R = allyl, then it is hypothesized that non-facially selective glycosylation takes place through the oxacarbenium ion (BO or Bl). Note that the experiment used O-benzyl and not the O-methyl, which was used for the calculation to avoid complications of multiple minima. 

In contrast, selective protection of 1,2-diequatorial diols in sugars, in the presence of other hydroxyl group combinations, had proved to be an especially difficult task. Until a few years ago, the protection could only be achieved using disilox-anylidene acetals [24]. However, this type of protecting group is frequently not sufficiently stable as to withstand normal glycosylation conditions. The final solution to the protection of frans-l,2-diols [25] was provided in the 1990s by Ley and co-workers with the development of 1,2-diacetals (Fig. 1). 

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