674
3
Chemical Glycosylation Reactions
⊡ Scheme 26
⊡ Scheme 27
⊡ Scheme 28
toxy)trimethylsilane (Me 3 SiOC(S)Me) gave good β-selectivity, especially with the poorer
OMe leaving group (as compared to OAc).
Under some conditions the formation of the intermediate allylic cation can be prevented and
direct addition to the double bond is the dominant reaction to give 2-deoxy thioglycosides.
The treatment of tri-O-acetylglucal (and -galactal and -rhamnal) with 2-mercaptopyridine
and p-toluenesulfonic acid resulted in an addition reaction and gave the 2-deoxythioglycoside in good yield ( > Scheme 28) [105]. Use of BF 3 -etherate as catalyst, however, gave
the 2,3-dideoxy-1-thio-α-D-glycoside. Recently, use of CAN, a Re(V)-oxo complex, or
GaCl 3 were reported to give good selectivity for the 2-deoxythioglycoside ( > Scheme 29,
> Scheme 30 and > Scheme 31) [106,107,108], although, as expected, the outcome is also
dependent on the glycal and the thiol used.
Thioglycosides from glycals can also be obtained in a two-step procedure via a 1,2-anhydro
intermediate ( > Sect. 2.1.7).
3
Chemical Glycosylation Reactions
⊡ Scheme 26
⊡ Scheme 27
⊡ Scheme 28
toxy)trimethylsilane (Me 3 SiOC(S)Me) gave good β-selectivity, especially with the poorer
OMe leaving group (as compared to OAc).
Under some conditions the formation of the intermediate allylic cation can be prevented and
direct addition to the double bond is the dominant reaction to give 2-deoxy thioglycosides.
The treatment of tri-O-acetylglucal (and -galactal and -rhamnal) with 2-mercaptopyridine
and p-toluenesulfonic acid resulted in an addition reaction and gave the 2-deoxythioglycoside in good yield ( > Scheme 28) [105]. Use of BF 3 -etherate as catalyst, however, gave
the 2,3-dideoxy-1-thio-α-D-glycoside. Recently, use of CAN, a Re(V)-oxo complex, or
GaCl 3 were reported to give good selectivity for the 2-deoxythioglycoside ( > Scheme 29,
> Scheme 30 and > Scheme 31) [106,107,108], although, as expected, the outcome is also
dependent on the glycal and the thiol used.
Thioglycosides from glycals can also be obtained in a two-step procedure via a 1,2-anhydro
intermediate ( > Sect. 2.1.7).