Chemistry Reference
In-Depth Information
linkages can be constructed using a participating group in the 2- position. Ideally,
the natural acetamido function could be used, but with this type of donor, quite
often the oxazoline, formed by the loss of a proton from the initial intermediate,
is found to be the product instead of the glycoside (Figure 3.7). The oxazoline is
more stable than the corresponding orthoester, but can be transformed into glyco-
sides using strong acids and excess of a reactive acceptor. However, with other
types of acceptors alternative participating
N
-protection groups have to be used. A
variety of such groups have been developed, and used with high yields and com-
plete
-selectivity in the glycosylation reaction [2], the most commonly used ones
being the phthalimido ( Phth ) and the trichloroethylcarbamate ( Troc ) groups, but
all still with the drawback of additional protection, deprotection and acetylation
reaction steps required to give the target acetamido structures.
β
Figure 3.6
Synthesis of 1,2 -
trans
-linkages using neighboring group participation.
Figure 3.7
Competing formation between an oxazoline or a 1,2-
trans
- glycoside from a 2 - acet-
amido - 2 - deoxy donor.
3.5.2
Formation of 1,2-
Cis
- Linkages
As discussed above, if no special technique is used, other than trying to optimize
the reaction conditions (solvents, promoter, temperature), a glycosylation using a
