enamine intermediate (derived from a Schiff-base between Pro-1 and the aldehyde
donor), a reminiscent to the organocatalytic analog. With acetaldehyde as donor, e.e.s of
up to 98% were achieved, the stereoselectivities were lower with propanal [1774–1776].
2.6 Transfer Reactions
2.6.1 Glycosyl Transfer Reactions
Glycosidic molecules in the form of oligo- or polysaccharides represent about
two-thirds of the carbon found in the biosphere, largely in the form of (hemi)
cellulose and chitin.
49 While β-linked polysaccharides provide structural support,
α-linked glycans, such as starch and glycogen, are more easily cleaved and serve as
energy storage [1777]. As a consequence, glycosyl transfer is certainly one of the
most important biochemical reactions [1778].
While polysaccharides are the basis for material sciences [1779], oligo-sugars
play a vital role in intracellular migration and secretion of glycoproteins, cell–cell
interactions, oncogenesis, and interaction of cell surfaces with pathogens [1780–
1782]. The building blocks are monosaccharides which (theoretically) occur in an
enormous number of stereoisomers, which results in a structural diversity far
greater than that possible with peptides of comparable size [1783].
50 Fortunately,
Nature is using almost exclusively pentoses and hexoses for in vivo synthesis.
The ready availability of such oligosaccharides of well-defined structure is
critical for the synthesis of drug candidates. Isolation of these materials from
natural sources is a complex task and is not economical on a large scale due to
their low concentration in carbohydrate mixtures obtained from natural sources.
Chemical synthesis of complex oligosaccharides is one of the greatest challenges
facing synthetic organic chemistry since it requires many protection and
deprotection steps which result in low overall yields [1784]. In this context,
biocatalysts are attractive as they allow the regio- and stereospecific synthesis of
oligosaccharides with a minimum of protection and deprotection steps [1504–1506,
1785–1793]. There are four groups of enzymes which can be used for the synthesis
of oligosaccharides
51 (Scheme 2.212) [1794]. However, differences are not always
clear-cut and mixed activities are sometimes observed.
Glycosyl transferases are responsible for the biosynthesis of oligosaccharides
in vivo. They require that the sugar donor is activated on the anomeric center by
49 Cellulose is the most abundant organic carbon in the ecosphere and its global standing crop has
been estimated as 9.2 Â 10
11 tons, with an annual production of 0.85 Â 10
11 tons; the annual
production of marine chitin was estimated as 2.3 Â 10
9 tons.
50 The possible number of linear and branched oligosaccharide isomers for a reducing
hexasaccharide was calculated to encompass 1.05 Â 10
12 structures, see [1782].
51 http://www.cazy.org.
2.6 Transfer Reactions
235
donor), a reminiscent to the organocatalytic analog. With acetaldehyde as donor, e.e.s of
up to 98% were achieved, the stereoselectivities were lower with propanal [1774–1776].
2.6 Transfer Reactions
2.6.1 Glycosyl Transfer Reactions
Glycosidic molecules in the form of oligo- or polysaccharides represent about
two-thirds of the carbon found in the biosphere, largely in the form of (hemi)
cellulose and chitin.
49 While β-linked polysaccharides provide structural support,
α-linked glycans, such as starch and glycogen, are more easily cleaved and serve as
energy storage [1777]. As a consequence, glycosyl transfer is certainly one of the
most important biochemical reactions [1778].
While polysaccharides are the basis for material sciences [1779], oligo-sugars
play a vital role in intracellular migration and secretion of glycoproteins, cell–cell
interactions, oncogenesis, and interaction of cell surfaces with pathogens [1780–
1782]. The building blocks are monosaccharides which (theoretically) occur in an
enormous number of stereoisomers, which results in a structural diversity far
greater than that possible with peptides of comparable size [1783].
50 Fortunately,
Nature is using almost exclusively pentoses and hexoses for in vivo synthesis.
The ready availability of such oligosaccharides of well-defined structure is
critical for the synthesis of drug candidates. Isolation of these materials from
natural sources is a complex task and is not economical on a large scale due to
their low concentration in carbohydrate mixtures obtained from natural sources.
Chemical synthesis of complex oligosaccharides is one of the greatest challenges
facing synthetic organic chemistry since it requires many protection and
deprotection steps which result in low overall yields [1784]. In this context,
biocatalysts are attractive as they allow the regio- and stereospecific synthesis of
oligosaccharides with a minimum of protection and deprotection steps [1504–1506,
1785–1793]. There are four groups of enzymes which can be used for the synthesis
of oligosaccharides
51 (Scheme 2.212) [1794]. However, differences are not always
clear-cut and mixed activities are sometimes observed.
Glycosyl transferases are responsible for the biosynthesis of oligosaccharides
in vivo. They require that the sugar donor is activated on the anomeric center by
49 Cellulose is the most abundant organic carbon in the ecosphere and its global standing crop has
been estimated as 9.2 Â 10
11 tons, with an annual production of 0.85 Â 10
11 tons; the annual
production of marine chitin was estimated as 2.3 Â 10
9 tons.
50 The possible number of linear and branched oligosaccharide isomers for a reducing
hexasaccharide was calculated to encompass 1.05 Â 10
12 structures, see [1782].
51 http://www.cazy.org.
2.6 Transfer Reactions
235