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138 6 Chemo-Enzymatic Cascade Reactions for the Synthesis of Glycoconjugates

Sequential One-pot Convergent

Chemocatalysis
Biocatalysis Chemocatalysis
Biocatalysis Chemocatalysis
Chemocatalysis
Biocatalysis
Biocatalysis

Biocatalysis

Glycoconjugate Glycoconjugate Glycoconjugate
Scheme 6.3 Cascade reactions in glyco- under same reaction conditions. In conver-
conjugate syntheses. Either one-pot, sequen- gent reactions, separate catalytic pathways
tial,or convergent cascade reactions can yielding different products are joined to the
be applied. Sequential syntheses reactions desired one in a final reaction. The three
employ multiple consecutive catalytic steps, cascade types consist of any desired com-
whereas in one-pot syntheses multiple catalytic bination of chemical, enzymatic, or chemo-
steps are combined in one reaction vessel enzymatic reactions.

One-pot synthesis focuses on the combination of multiple catalytic steps in one
reaction vessel under the same reaction conditions. Optimization of the reaction
conditions to suit all involved catalysts is crucial to reach shorter reactions times
than for the sequential reaction type. Again, here the combination of chemical
steps and enzymatic steps in one-pot is possible.
The last reaction type covers convergent reactions. They involve separate catalytic
pathways yielding different products that are finally combined to the desired one.
Each of the pathways may include cascade reactions in themselves.
All these types of cascade reactions have advantages and include some obstacles
that have to be overcome for the production of complex glycoconjugates. Sequential
syntheses may be rather simple in terms of reaction optimization, and may be
the starting point of more sophisticated synthesis approaches. One-pot reactions in
contrast show a high sensitivity to varying reaction conditions. Product distribution
may be highly affected by temperature, pH-value, or catalyst ratios. Finally convergent
reactions in chemo-enzymatic glycoconjugate synthesis are rather new combining
two or more complex synthesis routes.
In summary, cascade reactions are of great interest for the synthesis of glycocon-
jugates combining in depth knowledge of biotechnology, reaction technology, and
chemistry. Some of them are already established with multiple striking examples;
others open new pathways for future applications for glycoconjugate synthesis.
The best cascade combinations should then lead to facile, cost efficient, and fast
reaction-yielding complex glycans for a wide range of biomedical applications.
In this chapter, we take a look at the synthesis, modification, and regeneration of
activated sugar donors. Further examples like state of the art synthesis of epitopes
involving multiple enzymatic and chemical steps are presented. Here, mainly two