Monosaccharide derivatives as central scaffolds in the synthesis ofglycosylated drugs

Marco Filice* and Jose M. Palomo*

Received 26th July 2011, Accepted 30th November 2011

DOI: 10.1039/c2ra00515h

Carbohydrates are key molecules in nature with multiple roles in biological processes. Specifically the

biological function of the glycosylated part is of great importance in natural products. In many cases

carbohydrate-containing molecules are structurally diverse and this heterogeneity makes the isolation

of sufficient amounts of the organic molecule from biological source extremely difficult. Therefore,

chemical synthesis offers the advantage of producing pure and structurally defined oligosaccharides

for glycoconjugate synthesis for biological investigations. Although the synthesis of these

glycoderivatives is hampered by difficulties associated with the regioselectivity in polyhydroxyl

protection and the stereoselectivity of glycosidic linkages, the preparation of activated regioselective

sugar units, could represent a huge challenge in glycochemistry. The present review discusses some of

the most recent advances in the construction of a set of tailor-made monosaccharide derivatives, key

tools for final glycosylated drugs development.

Introduction

Carbohydrates represent complex and structurally diverse

molecules containing multiple chiral centers which are used in

nature to synthesize useful complex bioactive compounds. They

can exist as oligo- and polysaccharides or included in aglycon

structures as glycoderivatives (peptides, lipids, etc.) with key

roles in a broad range of biological processes including signal

transduction, carcinogenesis and immune responses.1–5

The preparation of these biomolecules is not straightforward—

because of their complex structural diversity6–8—unlike other similar

compounds such as nucleic acids or proteins, where no regio- and

stereochemical issues are involved in the sequential coupling steps for

the construction of phosphate or amide bonds, respectively.

In many cases the biological activity of natural products

derives from the sugar moieties. Changes on the sugar structures

cause deep impact on the biological properties of the parent

Departamento de Biocatalisis. Instituto de Catalisis (CSIC) c/Marie curie2, Cantoblanco, campus UAM, 28049, Madrid, Spain.E-mail: marcof@icp.csic.es; josempalomo@icp.csic.es

Marco Filice obtained his BS inpharmaceutical chemistry and tech-nology at the University of Pavia,Italy, in 2003. He received hisPh.D. degree (summa cum laude)in 2007 at the PharmaceuticalChemistry Department in theUniversity of Pavia. For postdoc-toral research he joined to the groupof Prof. Guisan at BiocatalysisDepartment in Institute ofCatalysis (IPC, CSIC) in Madridin 2008 where currently he works asan Associate Research Scientist with

a JAE-DOC contract. His current research interests include carbohy-drate and nucleoside chemistry with enzymes, applied biocatalysis, site-directed chemical modification of enzymes and asymmetric biotrans-formations in fine and pharmaceutical chemistry.

Jose M. Palomo received his Ph.D.degree (summa cum laude) in 2003in the Autonoma University inMadrid working at the BiocatalysisDepartment in Institute of Catalysis(IPC, CSIC) in Madrid. For post-doctoral research he joined to thegroup of Prof. Waldmann atChemical Biology Department ofMax Planck Institute in Dortmundin 2004. In 2006, he began hisappointment as an AssociateResearch Scientist at BiocatalysisDepartment ICP-CSIC. From2009, he is a Tenured Scientist(Associate Professor) in ICP-CSIC. His current research interestsinclude design of semisyntheticenzymes, asymmetric biotransfor-mations and carbohydrate chemistrywith enzymes.

Marco Filice

Jose M. Palomo

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compounds.9,10 Glycoderivatization of natural products by

changing the sugar moiety on the molecule has been performed

via chemical or biosynthetic pathways.11 Some interesting

improvements in antitumoral activity was found in erythromy-

cin,12 vancomycin,13 or lately digitoxin analogues.14

Furthermore, a way to enhance functional properties of drugs

such as solubility, pharmacokinetic or pharmacodynamic consists

of the incorporation of carbohydrate moieties to the molecules.15–17

For example, tilorone, 2,7-bis[2(diethylamino)-ethoxy]-9H-

fluoren-9-one glycosylated analogues improved its property of

induction IFN production which, in turn, might stimulate

cytostatic effects in the cell in vivo against some cancers18 or

biological activity, and the solubility of taxol was improved after

a non-natural glycosylation.19

Glycosylated steroids have been shown to possess various

types of biological activities. The glycosylation of steroids,

analogues of cardenolides and saponins, enhanced the biological

activity for their potential use in drug delivery systems.20

However, an established chemical approach for oligosaccharide

or glycoderivative synthesis involved multi-step protection and

glycosylation steps. Therefore, the development of strategies to

achieve an efficient and easy glycosylated natural product to

produce new powerful drugs is of great interest.

In this way, the accessibility of new building blocks by an

efficient and simple way represents an outstanding challenge in

glycochemistry. The preparation of selectively protected mono-

saccharide units bearing a single strategically positioned free

hydroxyl group (a nucleophilic acceptor) symbolizes a break-

through in carbohydrate synthesis (Scheme 1) together with the

stereoselective glycosylation.21–26

In the present review, we present the most recent examples in

the preparation of monosaccharide derivatives and their

application as key scaffolds in the synthesis of different

biologically active glycosylated drugs.

1. Chemical strategies to prepare monosaccharidederivatives for biologically active glycosylated drugsynthesis

One of the most interesting strategies to prepare monohydroxy

building blocks is based on the necessity to install suitable

protecting groups on the remaining hydroxyls, with the aim of

tuning the overall electronic properties of the donors and acceptors

so as to ‘match’ the donor–acceptor pair and also for further

deprotection and glycosylation or functional-group modifications.

Hung and co-workers have described a combinatorial and

highly regioselective novel method to protect individual hydroxyl

groups of a monosaccharide in different positions which was quite

useful for the synthesis of influenza virus oligosaccharides.27

This approach permits to install an orthogonal protecting group

pattern in a ‘one-pot’ reaction by Lewis acid catalysis (trimethylsi-

lyl trifluoromethansulfonate (TMSOTf) or Cu(OTf)2), by-passing

isolation and purification of intermediates in the synthesis.

A correct choice of the protecting groups is crucial for the

successful development of this strategy. Substituted and unsub-

stituted benzyl ethers were chosen. They act as orthogonal

protecting groups and can be cleaved advantageously by means

of reagent combination.28 Different protected monosaccharides,

2-alcohols, 3-alcohols, 4-alcohols, and 6-alcohols starting from

2,3,4,6-tetra-O-trimethylsilylated monosaccharides, were obtained

by this methodology (Scheme 2).

Hundreds of building blocks starting from D-glucose—with

different aliphatic, aromatic substituents in anomeric position—

have been efficiently prepared in high overall yields (Table 1).

Some of them were applied as scaffolds in the preparation of

H5N1 avian influenza virus binding trisaccharide 22 and

different analogues 23 and 24.27

The glycosylation reaction between the protected syalic acid

thioderivative 17 and the monohydroxy building block 18 by a

AgOTf mediated activation produced the disaccharide 19

(Scheme 3) which was followed by p-toluenylsulphenyl trifluor-

omethanesulphonate (p-TolSOTf)-promoted coupling with the

alcohols 20, 13, and 21 in a one-pot manner, yielded the expected

linear trisaccharides 22, 23, and 24, respectively.27

Furthermore, another interesting application of this straight-

forward strategy is represented by the construction of branched

oligosaccharides (Scheme 4).

In this case, the utility of the one-pot syntheses of monomeric

building blocks was applied to synthesize the protected trimeric

oligosaccharide by an efficient iterative construction.27

Starting from the monomeric unit 26, a phenylsulfenyl-

promoted assembly with 25 was performed providing exclusively

Scheme 1 Tailor-made glycosylated molecule structures by this approach.

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b-dimeric glycosyl donor 27 in 70% yield. Further activation of

27 and coupling with the acceptor 3 yielded exclusively the

trisaccharide 28 in 66% yield (Scheme 4).

Shao et al.29 describe the synthesis of 1,2-cyclopropaneacety-

lated protected monosaccharides and their use as a new class of

glycosyl donor (Table 2).

The glycosylation reaction was efficient and provides a

method for the stereoselective synthesis of 2-C-acetylmethyl-2-

deoxy-glycosides, oligosaccharides, glycosylamino acids, and

other 2-C-acetylmethylglycosides with interesting applications

in the preparation of glucosidase inhibitors, HIV virus inhibi-

tors, etc.

Despite the multistep pathway30 required for the obtainment

of the cyclopropanated glucosyl and galactosyl donors, this

Lewis-acid catalyzed reaction is noteworthy especially for the

complete control of the stereoselectivity obtained through the

right combination between the donor configuration (gluco- vs.

galacto- structure) and the Lewis-acid catalyst employed. In fact,

the stereoselectivity of the glucosyl donors glycosylation resulted

completely in favor of the b-anomer products (independently

from the added Lewis acid) from good to excellent yields (79–

93%), whereas the stereoselectivity in couplings of a galactosyl

donor with acceptors depended on the catalysts selected:

BF3*OEt2 to predominantly obtain b-anomer and TMSOTf to

predominantly obtain the a-anomer (Table 2).

Another very interesting research work is represented by the

investigation of Than et al.31 By combining systematic geome-

trical diversity with systematic functional diversity,32 this

research group reported the development of a versatile and

practical multistep solid-phase synthetic route applied to two

monosaccharide scaffolds (glucose 41 and allose 42 derivatives,

even if it can be adapted to other pyranose building blocks)

which allowed the generation of easy-to-evaluate libraries of

pyranose based drug-like peptide mimetics (Scheme 5).

This methodology permits the introduction of a wide range of

substituent families in a regio- and stereospecific manner,

furthermore, several synthetical steps have been performed on

solid phase (Wang resin) simplifying and improving the overall

route. Certainly, the entire process is affected by laborious

procedures but undoubtedly its great strength relies in the

possibility to furnish an interesting general alternative in the

creation of a vast pyranose based peptide mimetic library.

Once elaborated, the systematic structurally diverse library

(planned around a tripeptide motif containing two aromatics

and one positive charge) was screened against the somatostatin

receptor subtypes (sst1-5) and the melanin-concentrating hor-

mone receptor 1 (MCH1), examples of class A G-protein

coupled receptors (GPCR) known to bind peptides containing

this tripeptide motif and to directly impact to some physiological

Scheme 2

Table 1 Some of most interesting building blocks prepared by this one-pot method

Entry X Ar Z R R9 Product Yield [%]

1 1 Ph Ph 3 912 1 Ph 4–OMePh 4 943 1 Ph 3,4–diOMePh 5 814 1 4–NO2Ph 3,4–diOMePh 6 815 1 2-naphthyl Ph 7 806 1 Ph Allyl 8 737 2 Ph Bu 9 658 2 Ph Allyl 10 709 2 Ph Ph CH3 11 70

10 2 Ph 2-naphthyl CH3 12 6711 1 Ph Ph Ph 13 6512 1 Ph 2-naphthyl CH3 14 6013 1 2-naphthyl 2-naphthyl CH3 15 6314 1 2-naphthyl 2-naphthyl Ph 16 65

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and pathological processes (i.e. modulation of cell proliferation,

modulation of many endocrine and exocrine secretory processes,

body weight regulation, etc.).33–38 The results were satisfactorily

identifying, among hundreds of molecules produced in the

library, a small set of molecules possessing a submicromolar IC50

against the receptors above mentioned.39

Yang et al.40 describe a unique and rapid approach towards

the synthesis of triazolyl pseudo-glycopeptides by employing

microwave-accelerated Cu(I)-catalyzed Huisgen 1,3-dipolar

cycloaddition (CuAAC or ‘click’ reaction) (Scheme 6). By this

way, a series of triazole-linked serinyl, threoninyl, phenylala-

ninyl and tyrosinyl perbenzylated-1-O-gluco- or galactosides

48–51 have been efficiently and rapidly synthesized in high

yields.

These glycopeptidotriazoles showed a favorable biological

activity as inhibitors toward some elements of the protein tyrosine

phosphatase (PTP), an enzymatic superfamily whose inappropri-

ate regulation of many of its members has been reported to be

causative toward the pathogenesis of major human diseases

involving autoimmune disorders, diabetes and cancer.41

Scheme 3

Scheme 4

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With this approach, by using inexpensive and abundant

natural scaffolds (sugars and amino acids) and via facile and

expeditious synthetic methods (microwave-assisted ‘click’ reac-

tion), a pseudo-glycopeptide-based library has been efficiently

produced, furnishing new perspectives toward the development

of very active PTP inhibitors (Scheme 6).

Together with the synthesis of new bioactive molecules, the

chemical modification of natural compounds with proven

pharmacological activity (in order to improve their properties)

ever resulted as a powerful strategy deeply investigated by many

research groups. Following this promising line, for example, a

wide range of bicyclic sugar-shaped analogs of the natural

iminosugars castanospermine or nojirimycin have been devel-

oped by the Garcia-Fernandez group.42,43 The natural parental

alkaloids possess a potential biological activity as competitive

inhibitors of several glucosidases in the treatment of diseases

such as cancer, viral infections, diabetes or glycosphingolipid

storage disorders. However, their generally low selectivity

together with the poor cell permeability (due to the highly

hydrophilic nature) resulted in disappointing failures in most

clinical trials.

To overcome these drawbacks, the Garcia-Fernandez research

group developed a new family of glycomimetics, incorporating

an endocyclic nitrogen atom (with a high sp2-hybridisation

character: sp2 iminosugar) together with an hydrophobic

modification.44

In this recent publication, an improved stereoselective synth-

esis of stable sp2-iminosugar-type N– (52), S– (53), and C– (54)

glycoside castanospermine analogues bearing a a-configured

pseudoanomeric group (Scheme 7) has been reported.44 The

methodology involves a common pivotal intermediate and it is

well suited for library generation. Biological activity of the new

synthetic castanospermin analogues was tested resulting in low-

to sub-micromolar inhibitors of neutral a-glucosidase exhibiting

much higher enzyme selectivity when compared with the parent

alkaloid castanospermine.

Over the past two decades, the Danishefsky research group

has been notoriously dedicated to the design and de novo

synthesis of carbohydrate and peptide-based antitumor vaccine

constructs that, if properly presented to the immune system,

could stimulate the formation of antibodies which would

selectively bind and eradicate tumor cells overexpressing the

carbohydrate epitopes at issue.

Specially, the main goal of this fascinating research area was

centered on the development of immunostimulating strategies

allowing for enhanced protection against tumor recurrence and

Table 2 Stereocontrolled glycosylation of 1,2-cyclopropaneacetylated gluco- and galacto-derivatives

R Yield (%) Product R Yield (%) Product

86 29 BF3*OEt2: 82(1 : 4) 33TMSOTf: 89(10 : 1) 34

93 30 BF3*OEt2: 71(1 : 8) 35TMSOTf: 76(20 : 1) 36

79 31 BF3*OEt2: 71(1 : 4) 37TMSOTf: 75(4 : 1) 38

88 32 BF3*OEt2: 87(1 : 20) 39TMSOTf: 83(15 : 1) 40

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metastasis following elimination of the primary tumor mass

through the canonical protocols (i.e. surgery, radiation or

chemotherapeutic treatment).

Firstly, this research was approached preparing monovalent

vaccines (a single carbohydrate antigen conjugated to an

immunogenic carrier molecule),45 subsequently, the second

generation studies were focused on the preparation of more

elaborated constructs: multiple repeats of a carbohydrate epitope

(clusters) anchored on a peptide backbone.46 In order to amplify

potency of a broader base, a third generation of carbohydrate-

based antitumor vaccines were synthesized. These new constructs

were elaborated keeping in mind the evidence that any single

tumor cell is characterized by a significant heterogeneity of its

surface in carbohydrate antigens expression. Based on this thesis,

unimolecular multiantigenics vaccine constructs have been

prepared.47

Recently, this research area underwent a further evolution by

considering the possibility that the peptide backbone might also

act not only as an antigenic linker protein but also like a potent

additional antigenic marker. On the basis of these observations,

the Danishefsky group designed a new type of antitumor vaccine

structure featuring both a carbohydrate-based antigen and a

mucin-derived peptide-based marker in an alternating reiterative

pattern (Scheme 8).48

Recurring to a wide set of modern and straightforward

synthetic procedures (glycopeptide solid phase synthesis, mod-

ified and optimized olefin cross metathesis, etc.), the Gb3-

glycosylaminoacyl 55 building block has been prepared.48

Subsequently, cassette 55 was modified in order to install two

N-termini bearing different compatible orthogonal protecting

groups. So that, iterative peptide couplings with the MUC5AC-

based peptide marker (a 8-amino acid epitope—sequence:

TTSTTSAP—belonging to the peptidic mucin antigen

MUC5AC expressed on the ovarian cancer cell surface and

prepared through Fmoc solid-phase synthesis using Novabiochem

proline-TGT resin) followed by selective deprotection of the

Scheme 6

Scheme 5

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desired N-termini permitted to achieve the fully synthetic clustered

Gb3-MUC5AC construct.

Finally, bioconjugation with the immunogenic carrier KLH

(in a 698 : 1 ratio) afforded the vaccine construct Gb3-MUC5AC

cluster KLH conjugate 56 targeting the ovarian carcinoma

(Scheme 9).

The yield of the final product was not very high (,10% for the

entire synthetic pathway) but, in this case, the main purpose was

to obtain a homogenous molecule useful for clinical trials.

Hitherto, the clinical results of this last version of the

carbohydrate vaccine have not yet published, but considering

the good results obtained with the former versions of the vaccine

constructs,49 it can be speculated that they could be noteworthy.

The chemical reaction set applied here demonstrates, once

again, the awesome ability of the Danishefsky group handling

the total synthesis in an effort to mimic nature. In fact, such

specified structures have been produced through a rigid

synthetical control otherwise almost impossible to reproduce

only recurring to strictly biological procedures. So, total

chemical synthesis can be clearly employed to reproduce or

enhance the performance of molecules which had hitherto been

considered exclusively as ‘‘biologics’’.

2. Enzymatic strategies to prepare monosaccharidederivatives for chemoenzymatic glycosylated drugsynthesis

A huge number of methods for the regioselective protection of

monosaccharides has been reported in the literature, although

the preparation of carbohydrate based building blocks through a

regioselective deprotection of their fully protected precursors

results has not been explored so far. In fact, this approach,

especially in the case of the preparation of carbohydrate

acceptors, may be rather unfeasible due to the difficulty in

efficiently deprotecting a unique position—leaving the remaining

ones protected—when employing the same hydroxyl chemical

protecting group.

Nevertheless, deprotection at primary or secondary positions,

by the majority of reported chemical methods, does not

completely fulfill the requirement of a very regioselective

methodology. In fact, because of the difficulty to discriminate

between groups with a very similar reactivity, the chemical

deprotection is the limiting step especially when using mono-

sacharides with all the hydroxyls blocked by a unique protecting

group. By this way, a biocatalytic approach has represented a

Scheme 7

Scheme 8

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convenient alternative. Enzymes, in many cases, exhibit good

catalytic activity, specificity and regioselectivity, transforming

only a substrate in an unique product from different possible

enantio- or regioisomers.

In this way, a nice part of work is based on the development of

a combinatorial biocatalytic methodology to prepare—by one-

pot synthesis starting from cheap fully acetylated molecules—a

small library of different monohydroxy monosaccharides.50–53

The strategy is based on the complex catalytic mechanism of

lipases which implies dramatic conformational changes of the

enzyme molecule between a closed and an open form.54 Thus the

application of different immobilization protocols for a parti-

cular lipase—involving different areas of the lipase, rigidity,

micro-environments, etc.—has given access to different bioca-

talysts for the same enzyme, altering its activity and regioselec-

tivity during the hydrolysis of peracetylated carbohydrates in

aqueous media.

A case which perfectly exemplified the power of this strategy

is represented by the regioselective biocatalytic hydrolysis of

b-monosaccharides.50

For example, by this strategy, three different biocatalysts from

Thermomyces lanuginose lipase (TLL) showed quite different

catalytic properties in the hydrolysis of peracetylated b-galactose

57 (Scheme 10).51

One biocatalyst (octyl-TLL) was very specific and regioselec-

tive toward hydrolysis in C–6 with a .99% yield. However, a

second immobilized biocatalyst from the same lipase (CNBr-

TLL) was highly specific and regioselective towards the C–1

position. Moreover, a third catalyst (PEI-TLL) was poorly active

and not selective at all against one of these positions. Therefore,

from this strategy it is possible to obtain very highly selective

catalysts (Scheme 10).

This strategy was extended with success to many other lipases

demonstrating its potential as a general methodology to efficiently

prepare regioselective monodeprotected monosaccharides.50–53

Thus a combinatorial approach—via combination of different

pure lipases together with different immobilization protocols—

was developed to prepare in one step the regioselective C–6

deprotected tetraacetylated a and b monosaccharides in very

high overall yields (Scheme 11).52

Scheme 9

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The per-O-acetylated monosaccharides (60–65) were trans-

formed in the corresponding 6–OH products (66–71) with high

overall yields (from 60% to .95%) by selecting the best

biocatalyst for each monosaccharide (Scheme 11). In general,

the octyl-CRL preparation was the most interesting biocatalyst

for regioselective C–6 deprotection of monosaccharides.

A very mild controlled acyl migration applied to the different

6–OH tetraacetylated glycopyranoses (66–71)—by changing

from pH 5 (no migration conditions) up to pH 8.5 after

removing the immobilized biocatalyst from the reaction—

permitted to obtain the subsequent 3–OH and 4–OH products

(83–96) in different yields.52 In the glucopyranosidic structure, a

high amount of the 4–OH products was preferentially obtained

(70–80%) whereas in the galactopyranosidic structure the

migration produced an equimolar mix of 3–OH and 4–OH

products.

This combinatorial biocatalytic engineering approach was also

applied to different glycopyranosides with different substituents

in the anomeric position53 and especially to a more reactive

carbohydrate derivative such as glycals (Scheme 12).

Thus, by the right selection of the biocatalyst, it was possible to

hydrolyze regioselectively the C–6 and C–3 positions of perace-

tylated glucal (72) and galactal (73) in excellent yields (.95%).

Therefore, the combination of the biocatalytic strategy

together with the medium engineering has allowed to prepare

efficiently—in ‘‘one-pot’’ step and very mild conditions—a small

combinatorial library of different monohydroxy acetylated

monosaccharides in high overall yields (Scheme 13).

Scheme 10

Scheme 11

Scheme 12

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These regioselective deprotected monosaccharides, with a

great potential in glycochemistry, were used as a scaffold in

the synthesis of several biologically active molecules, such as an

lactosamine derivative antitumoral drug55 and tailor-made di-

and trisaccharides, direct precursors of Tn epitopes, Mucin

glucopeptides or sialyl lewis analogues.56

The synthesis of the antitumoral drug b-O-naphthylmethyl-

N-peracetylated lactosamine (104) was recently carried out by a

very efficient chemo-enzymatic approach (Scheme 14).55 The 4–

OH compound (87), synthesized by the chemo-enzymatic

deprotection of peracetylated monosaccharide, was used as the

acceptor for the glycosylation reaction with tetra-O-acetyl-a-D-

galactopyranosyl trichloroacetimidate (102) as the glycosyl

donor, producing the disaccharide (103) in 55% yield after

purification. The oxazoline formation and the reaction with

2-napthylmethanol yielded the target product (104) in 20%

overall yield (Scheme 14). This synthetic example clearly

demonstrates the straightforward potential derived from the

application of the chemo-enzymatic approach in glycochemistry.

In fact, comparing the entire chemo-enzymatic pathway

described above with a ‘‘canonical’’ (obtained through the sole

application of orthogonal protecting group strategy) chemical

synthesis,56 it is possible to observe an overall yield improvement

(20% vs. ,5%) together with a noteworthy reduction of the

synthetic steps (4 steps vs. 11 steps) (Scheme 14).55

Tn epitope disaccharides and linear trisaccharides were

efficiently synthesized in good yields using some monosacchar-

ides as scaffolds. The novelty of this approach relies on the use of

an acetyl group as a unique alcohol protecting group (mono-

protective approach).57

The monodeprotected acetylated building-blocks (87) and

(68)—obtained by the regioselective chemo-enzymatic

approach—were coupled to (102) in the presence of boron

trifluoride diethyl etherate (BF3*OEt2) as the activating agent

yielding the peracetylated disaccharides (103) and (108)

(Scheme 15).

Scheme 13

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For the preparation of the linear trisaccharide, the peracetylated

lactosamine (103) was converted in the glycosyl donor (109) after

the transformation in its oxazoline derivative. Finally, the

trisaccharide 110 was synthesized by the regioselective glycosylation

using monohydroxy compound 92 as the acceptor (Scheme 15).57

In summary, the great advantage relies on the consideration

that starting from a very cheap raw material, sugar, only

performing a chemical peracetylation and applying the chemo-

enzymatic strategy, it is possible to easily obtain a vast library of

high value products useful in glycochemistry.

Scheme 14

Scheme 15

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Recently, a new chemoenzymatic method for the regioselective

deprotection of monosaccharide substrates using engineered

Bacillus megaterium cytochrome P450 demethylases (P450BM3)

has been developed.58 By a fine tuned combination of protein

and substrate engineering, Lewis et al.58 obtained a wide range of

monodeprotected monosaccharides with different configurations

(a or b- gluco, galacto, or mannoside). With this straightforward

regioselective and efficient chemoenzymatic procedure it was

possible to achieve a set of key valuable intermediates easily

convertible in substituted mono- and polysaccharides useful in

chemical, biological and medicinal studies. (Scheme 16)

In fact, the monodemethylated compounds obtained from

good to high overall yield (44–98%) have been converted to a

variety of biologically relevant products using established

chemical transformations (fluorination, to modify the pharma-

cokinetic properties of molecules or for PET imaging;59

deoxygenation, to obtain deoxy sugars as important therapeutic

agents;60 glycosidation, to obtain more complex oligosacchar-

ides) (Scheme 17).

3. Conclusion

This review has shown novel recent strategies to achieve the

synthesis of different monosaccharide derivatives. Using the

applicability of known orthogonal protection methods in a

different combined way or selective deprotection strategies it has

been possible to prepare such a high amount of different

activated monosaccharides in excellent yields.

The building-blocks prepared with the techniques elucidated

above permitted the preparation of a lot of glycoderivatives with

important biological effects. For example, among all the

examples reported here, the preparation of a glycopeptidic based

anticancer vaccine construct,48 the virus binding oligosacchar-

ides involved in the H5N1 avian influenza,27 some glycomimetics

with glucosidase inhibitory effects44,55 or some oligosaccharidic

cancer epitopes useful in immunolo-stimulation57 have been

described.

To sum up, in this work we have reviewed some of the most

recent advances that present a great potential in future

Scheme 16

Scheme 17

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glycochemistry and offer new straightforward tools for the

glycobiology and medicinal chemistry advancement.

Abbreviations

Et3SIH triethylsilane

TBAF Tetrabutylammonium fluoride

DDQ 2,3-dichloro-5,6-dicyanobenzoquinone

TMS tetramethylsilane

TfO2 trifluoromethanesulfonyl anhydride

TBDPS tert-butyldiphenylsilyl

Fmoc Fluorenylmethyloxycarbonyl

Boc di-tert-butyl dicarbonate

Troc 2,2,2-trichloroethoxycarbonyl

KLH keyhole limpet hemocyanin

MOM methoxymethyl

Bn benzyl

BSP 1-benzenesulfinyl piperidine

Bz benzoyl

CSA (2) camphor-10-sulphonic acid

TBPy 2,4,6-tri-tert-butylpyrimidine

CRL Candida Rugosa lipase

PFL Pseudomonas fluorescens lipase

PCL Pseudomonas Cepacia lipase

RML Rhizomucor Miehei lipase

Octyl octyl Sepharose

PEI poliethylenimine Sepharose

CNBr cyanogens bromide activated Sepharose

Acknowledgements

This work has been sponsored by CSIC (Intramural Project

200980I133). The authors are grateful to CSIC for the JAE-DOC

contract of Dr Marco Filice. The help and comments from

Dr Angel Berenguer (Instituto de Materiales, Universidad de

Alicante) are gratefully recognized.

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