Chemoselectivity glycosylation using glycosyl

Glycosylation Strategies Using Thioglycosides 4.1.6.1 Chemoselective Glycosylations... 

Scheme 4.23 Chemoselective glycosylation by preactivation strategy using Ph2S0/Tf20 promoter.

Codee JDC, Litjens REJN, den Heeten R, Overkleeft HS, van Boom JH, van der Marel GA. Ph2S0/Tf20 a powerful promoter system in chemoselective glycosylations using thioglycosides. Org. Lett. 2003 5 1519-1522. 

Scheme 3, Chemoselective glycosylation using gfycosyl phosphates.

From the above can be concluded that BSP/Tf20 mediated chemoselective glycosylations are restricted to the use of a set of donors and acceptors, in which the donor is more reactive toward the sulfonium activator than the acceptor. Next, attention was focused on the elongation of the obtained thio dimers with the more potent DPS/Tf20 combination. An example, the assembly of the fully protected a-Gal epitope 55, is portrayed in Scheme 9. The second DPS/Tf20... 

The armed/disarmed chemoselectivity, although useful and serviceable, required further investigation to determine the limits of its effectiveness. From the earliest stages it had been evident that a disarmed donor was not inert, because it could be activated, probably under different conditions. Thus the disarmed partner, 10, on its own could serve as a glycosyl donor as will be exemplified in Scheme 6 [25]. However, this role was subjugated when it was forced to compete with the armed counterpart 9a in Scheme 3. 

The results discussed in this chapter clearly indicated that NIS/TfOH is a valuable promoter system for the chemoselective glycosylation of latent acceptor with active thioglycosyl donors. Furthermore, this study revealed that the amount of triflic acid used was crucial for the selective activation of active thioglycosides over relatively latent thioglycosides. 

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