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6.1 Introduction 137

which results in yields of not more than 30–40% [26]. Recent progress has been
made to overcome the shortcomings of glycosidases. One tool for enhancing trans-
glycosylation with endo- and exohexosaminidases is the use of transition state
analogs – oxazoline activated donors – which have proven to be more reactive, lead-
ing to higher transglycosylation rates [56–58]. Also microwave assisted catalysis
has been employed successfully to avoid product hydrolysis in the synthesis of
UDP-activated oligosaccharides [46].
A protein engineering approach for retaining glycosidases was introduced by
Withers and coworkers [59]. Exchange of the catalytic nucleophile in the active center
by nonnucleophilic amino acids (Ala, Gly) creates glycosynthases that lack hydrolytic
activities. Glycosyl fluorides [60] or glycosylazides [61] of the opposite anomeric con-
figuration to the acceptor have shown to be suitable donors to generate high product
yields [27]. Up to now, there is a broad range of glycosynthases from a host of differ-
ent sources described in the literature; among them are both retaining and inverting
glycosidases, suitable for numerous acceptor glycosides [23, 26, 29, 62–66].
The biocatalytic toolbox discussed here includes a variety of GTs, glycosidases,
and glycosynthases, each with their own advantages and disadvantages. In concert
with established chemical synthesis steps, they enable tailor-made strategies for
the synthesis of glycoconjugates. In the following subsections, we divide these
approaches into three categories: sequential, one-pot, and convergent synthesis.

6.1.3
Definition of Cascade Reactions

Multistep syntheses or reactions are known in chemical catalysis as well as
biocatalysis [24, 67–70]. In particular, building up complex glycan structures is a
challenging process involving multiple enzymatic steps as known from nature. The
separation of the reactions yielding complex glycans in different compartments
within the cell is striking. Here, the question arises if this naturally occurring
change of the ‘‘reaction vessels’’, and therefore the reaction conditions, may
be overcome or can be mimicked in vitro [71, 72]. This would lead to a more
facile production of glycans by the combination of single enzymes from different
sources creating novel in vitro pathways with improved biocatalytic performance.
In addition, combination of chemical and enzymatic catalysis even broadens the
set of glycoconjugates as chemically modified glycans are sought after for various
applications [73–77]. The term cascade reactions is widely used; however, there are
multiple ways of classification [78–80]. Within this chapter, we focus on three
different types of cascade reactions for the synthesis of various glycoconjugates as
depicted in Scheme 6.3.
The first type of cascade reaction is sequential synthesis which describes the usage
of multiple consecutive catalytic steps for building up complex structures. Each step
may be performed under different reaction conditions; however, no purification
of the intermediate products is necessary. Herein chemical catalytic steps may be
combined with enzymatically catalyzed ones or the cascade may be operated with
enzymes solely.