Click azide-nitrile cycloaddition as a new ligation tool for the synthesis of tetrazole-tethered C-glycosyl α-amino acids

Article abstract of DOI:10.1021/jo801670k

(Chemical Equation Presented) Glycoproteins play a key role in a multitude of biological events in living organisms. Hence, neoglycopeptides obtained from unnatural C-glycosyl α-amino acids can be used as synthetic probes in studies aiming at clarifying the role of the carbohydrate domain in glycoprotein biological activity. A new class of C-glycosyl α-amino acids featuring a nitrogenated heterocycle ring holding the carbohydrate and glycinyl moiety was designed in our laboratory. Having previously prepared isoxazole-, 1,2,3-triazole-, and pyridine-tethered compounds, the family has now been enlarged by a group of newcomers represented by tetrazole derivatives. Two sets of compounds have been prepared, one being constituted of C-galactosyl and C-ribosyl O-tetrazolyl serines while the other contains 5-tetrazolyl cysteine derivatives. In both cases, the synthetic scheme involved a two-step route, the first one being the thermal cycloaddition of a sugar azide with p-toluensulfonyl cyanide (TsCN) to give a 1-substituted 5-sulfonyl tetrazole and the second the replacement of the tosyl group with a serine or cysteine residue. For the high efficiency and operational simplicity, the azide-TsCN cycloaddition appears to be a true click process. Finally, one of the amino acids prepared was incorporated into a tripeptide.

Full text of DOI:10.1021/jo801670k

Click Azide-Nitrile Cycloaddition as a New Ligation Tool for the
Synthesis of Tetrazole-Tethered C-Glycosyl r-Amino Acids†
Mohammad Aldhoun, Alessandro Massi,* and Alessandro Dondoni*
Dipartimento di Chimica, Laboratorio di Chimica Organica, UniVersita` di Ferrara,
Via L. Borsari 46, I-44100 Ferrara, Italy
adn@unife.it; msslsn@unife.it
ReceiVed July 28, 2008
Glycoproteins play a key role in a multitude of biological events in living organisms. Hence,
neoglycopeptides obtained from unnatural C-glycosyl R-amino acids can be used as synthetic probes in
studies aiming at clarifying the role of the carbohydrate domain in glycoprotein biological activity. A
new class of C-glycosyl R-amino acids featuring a nitrogenated heterocycle ring holding the carbohydrate
and glycinyl moiety was designed in our laboratory. Having previously prepared isoxazole-, 1,2,3-triazole-,
and pyridine-tethered compounds, the family has now been enlarged by a group of newcomers represented
by tetrazole derivatives. Two sets of compounds have been prepared, one being constituted of C-galactosyl
and C-ribosyl O-tetrazolyl serines while the other contains S-tetrazolyl cysteine derivatives. In both cases,
the synthetic scheme involved a two-step route, the first one being the thermal cycloaddition of a sugar
azide with p-toluensulfonyl cyanide (TsCN) to give a 1-substituted 5-sulfonyl tetrazole and the second
the replacement of the tosyl group with a serine or cysteine residue. For the high efficiency and operational
simplicity, the azide-TsCN cycloaddition appears to be a true click process. Finally, one of the amino
acids prepared was incorporated into a tripeptide.
Introduction
disorder of glycosylation (CDG) syndrome. Specifically, fol-
lowing the original observations of a pediatrician in Belgium
in the beginning of the 1980s, further studies have revealed that
congenital underglycosylation of proteins causes severe health
problems in children and typically results in multisystemic
presentation involving interference with normal development
of the brain and functions of the liver, stomach, and nervous
and intestinal systems.4 On the other hand, the beneficial effects
of glycosylation in neuropeptides (e.g., enkephalins) in terms
of improved blood-brain barrier penetration and analgesic
potency have been recently discussed.5 The role of antifreeze
glycoproteins6 in Teleost fish is crucial to prevent in vivo ice
growth in organisms inhabiting subzero environments. Conse-
A great deal, about 50%, of proteins in humans are glyco-
sylated.1 A variety of oligosaccharides that are mostly branched
are introduced in the protein backbone mainly through O- and
N-glycosyl amino acids, being serine, threonine, hydroxyproline,
and asparagine, the most common derivatives. The carbohydrate
domain in glycoproteins is key to vital biological processes
including cell-cell recognition and interaction and it affects
protein folding, conformation, stability, and biological activity.2
In this context, defects in the attachment of carbohydrate to
protein in humans may be the main cause of undesired
biochemical events that result in serious diseases.3 A striking
example is represented by the disease known as congenital
(3) (a) Kornfeld, S. J. Clin. InVest. 1998, 101, 1293–1295. (b) Spiro, R. G.
Glycobiology 2002, 12, 43R–56R. (c) Glycoproteins and Disease; Montreuil,
J., Vliegenthart, J. F. G., Schachter, H., Eds.; Elsevier: New York, 1996.
(4) For a review with leading references, see: Lehle, L.; Strahl, S.; Tanner,
W. Angew. Chem., Int. Ed. 2006, 45, 6802–6818.
† Dedicated to the fond memory of Albert I. Meyers.
(1) Wong, C.-H. J. Org. Chem. 2005, 70, 4219–4225.
(2) (a) Essentials of Glycobiology; Varki, A., Cummings, R. D., Esko, J. D.,
Freeze, H. H., Hart, G. W., Marth, J., Eds.; Cold Spring Harbor Laboratory
Press: New York, 1999. (b) Seitz, O. ChemBioChem 2000, 1, 214–246. (c) Hang,
H. C.; Bertozzi, C. R. Acc. Chem. Res. 2001, 34, 727–736.
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10.1021/jo801670k CCC: $40.75
Published on Web 10/11/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 9565–9575 9565

Aldhoun et al.
quently, efficient protection of these organisms against cryoin-
jury and death is achieved. In general, however, numerous
mechanisms of carbohydrate action in glycoproteins are at
present poorly understood.1 Natural and unnatural glycopeptides
with a well-defined structure and composition can serve as
probes in biochemical studies, while the latter are also potential
leads for the developments of new drugs against carbohydrate-
based metabolic disorders. Hence, a great deal of efforts are
made toward the search of efficient synthetic methods of natural
O- and N-linked glycosyl amino and glycopeptides2b,7 as well
as of unnatural C-linked analogues8 to be incorporated into
peptidic chains. Attention is also drawn to the synthesis of
S-linked glycosyl amino acids and thioglycopeptides.9 While
in early studies we have carried out the synthesis of methylene
isosteres of glycosyl serines10 and ethylene isosteres of glycosyl
asparagines,10c,11 more recently we have started a program
focused on the synthesis of a newly designed family of
C-glycosyl amino acids that feature a nitrogenated heterocycle
as a tether of the carbohydrate and glycinyl moiety (Figure 1).
In our view, the heterocycle should serve not only as a passive
yet robust linker but can also act as an additional site of
interaction with target substrates through dipolar interactions
and hydrogen bonding. Therefore, glycopeptides in which such
amino acids are embedded may give rise to highly selective
molecular recognition processes. Synthetic approaches to these
FIGURE 1. Heterocycle-tethered C-glycosyl amino acids.
glycosyl amino acids having isoxazole,12 1,2,3-triazole,12 and
pyridine rings13 as linkers have been reported and reviewed.14
In this paper, we report on the first synthesis of tetrazole-tethered
C-glycosyl amino acids that relies on an very efficient
azide-nitrile cycloaddition as a key step for the heterocyclic
ring formation. This ligation strategy is quite attractive because
tetrazole is a lipophilic and highly stable unit under a variety
of reaction conditions and being densely nitrogenated may
induce strong intermolecular interactions via an extensive set
of hydrogen bondings. The biological relevance of the tetrazole
ring is well-recognized as a metabolically stable surrogate for
a carboxylic or amide group.15
Results and Discussion
(6) (a) Czechura, P.; Tam, R. Y.; Dimitrijevic, E.; Murphy, A. V.; Ben, R. N.
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Ben, R. N. Biomacromolecules 2006, 7, 565–571.
Formation of tetrazole rings by the Huisgen 1,3-dipolar
cycloaddition between nitriles and organic azides16 is in principle
the most direct method. However, this reaction is limited in
scope because only nitriles activated by strong electron-
withdrawing groups can be effectively engaged as dipolarophilic
partners by organic azides. A few years ago, Demko and
Sharpless have shown for the first time the existence of a
reaction window by coupling under solvent-free conditions
p-toluenesulfonyl cyanide (TolSO2-CN, TsCN)17 and acyl
cyanides18 with various aromatic and aliphatic azides to give
exclusively the corresponding 1,5-disubstituted tetrazoles. As
the reactions were carried out using 1 equiv of each reagent
and gave nearly quantitative conversion into almost pure
products, the process was considered as a case of click chemistry
transformation as defined by Sharpless and co-workers.19
Moreover, earlier work of Gol’tsberg and Koldobskii had
previously demonstrated that the 5-sulfonyl substituent in
tetrazoles can be readily replaced by a wide range of nucleo-
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R-Amino Acids and C-Glycopeptides. In Current Frontiers in Asymmetric
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ACS Symposium Series; Oxford University Press: Oxford, 2008;to be published.
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Sharpless, K. B. Drug DiscoVery Today 2003, 8, 1128–1137.
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1 2001, 2380–2388. (b) Dondoni, A.; Mariotti, G.; Marra, A.; Massi, A. Synthesis
2001, 2129–2137. (c) Dondoni, A.; Marra, A.; Massi, A. J. Org. Chem. 1999,
64, 933–944. (d) Dondoni, A.; Marra, A.; Massi, A. Chem. Commun. 1998, 1741–
1742. (e) Dondoni, A.; Marra, A.; Massi, A. Tetrahedron 1998, 54, 2827–2832.
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6604.
9566 J. Org. Chem. Vol. 73, No. 24, 2008

Synthesis of Tetrazole-Tethered C-Glycosyl R-Amino Acids
SCHEME 1. Azide-Nitrile Cycloaddition-Based Approach
toward the Synthesis of C-Glycosylmethyl Tetrazole Serines
and Cysteines
SCHEME 2. Synthesis of Perbenzylated Glycosylmethyl
Azides
philes.20 The click azide-TsSCN cycloaddition together with
the tosyl substitution in the formed 1-substituted 5-sulfonyl
tetrazole established a two-step route toward the synthesis of a
variety of 1,5-disubstituted tetrazoles that were hardly accessible
by the direct Huisgen azide-nitrile cycloaddition. Recently, we
have validated the effectiveness of this click ligation route for
the synthesis of complex molecules such as glycoclusters
installed on a calix[4]arene platform through tetrazole tethers.21
With this in mind, we planned the synthesis of target
C-glycosylmethyl tetrazole serines 5 and cysteines 6 as shown
in Scheme 1. This consisted of the click cycloaddition of
benzylated and acetylated glycosylmethyl azides (azidomethyl
glycosides) 1 and 2 with TsCN to give 1-glycosylmethyl-5-
tosyl tetrazoles 3 and 4 followed by the replacement of the tosyl
group by serine and cysteine derivatives acting as O- and
S-nucleophile, respectively. Azides 1 and 2 as starting materials
were selected in order to introduce a methylene bridge between
the carbohydrate and tetrazole ring, thus creating a robust
C-glycosidic linkage that would preserve the final product from
chemical and enzymatic degradation. Another point of great
importance was the careful choice of orthogonal protective
groups PG1 and PG2 in the carbohydrate and glycinyl moiety
in order to make the formed amino acids suitable building blocks
in peptide synthesis.
nearly quantitative yields by lithium aluminum hydride to
amines 8, and these were transformed in very good yields
(62-87%) into azides 1 by diazotransfer reaction using imida-
zole-1-sulfonyl azide (ISA) hydrochloride24 (Scheme 2). Key
in our method was this new, efficient, and shelf-stable diaz-
otransfer reagent. The ISA reagent provided a safer, more
effective azide transfer than the commonly used diazo transfer
reagent trifluoromethanesulfonyl azide, whose preparation and
manipulation require special caution.25 Following Scheme 2,
we have prepared a set of four benzylated glycosylmethyl azides
1a-d with R- and ꢀ-galacto and R- and ꢀ-ribo configurations.
In order to modulate the orthogonal protecting group setting
in the final amino acid, acetylated glycosylmethyl azides 2a-d
were conveniently prepared employing aminomethyl glycosides
8 (Scheme 3). Debenzylation of these compounds by hydroge-
nation over Pd(OH)2 afforded the amines 9 that, as crude
material, were subjected to the diazotransfer reaction with ISA
hydrochloride to give the azides 10. These compounds were
acetylated to give good overall yields (62-74%) of azides
2a-d.
B. Synthesis of C-Glycosylmethyl O-Tetrazolyl Serines
5. The method and conditions for the synthesis of these
compounds are illustrated by the synthesis of the ꢀ-D-galacto-
sylmethyl derivative 5b (Scheme 4). Neat tetrabenzyl ꢀ-D-
galactosylmethyl azide 1b and TsCN (2.0 equiv) were heated
at 100 °C (oil bath). At this temperature, a homogeneous liquid
resulted so that the mixture was efficiently stirred by a magnetic
bar. As the reaction proceeded, the mixture solidified. The excess
of TsCN was simply removed by sublimation under vacuum
so that nearly pure 1-galactosylmethyl-5-tosyl tetrazole 3b was
A. Synthesis of Glycosylmethyl Azides 1 and 2. Although
the preparation of ꢀ-azidomethyl glycosides was reported in
previous papers from our laboratory starting from formyl ꢀ-C-
glycosides,22 we have developed a new and more efficient,
general method starting from glycosyl cyanides. These C-
glycosides are available as either R- or ꢀ-anomers by cyanation
of glycosyl acetates with trimethylsilyl cyanide (TMSCN).23
Benzylated galactosyl and ribosyl cyanides 7 were reduced in
(20) (a) Gol’tsberg, M. A.; Koldobskii, G. I. Russ. J. Org. Chem. 1995, 31,
1552–1553. (b) Gol’tsberg, M. A.; Koldobskii, G. I. Russ. J. Org. Chem. 1996,
32, 1194–1201. (c) Gol’tsberg, M. A.; Koldobskii, G. I. Chem. Heterocycl.
Compd. 1996, 32, 1300–1304.
(21) Dondoni, A.; Marra, A. Tetrahedron 2007, 63, 6339–6345.
(22) (a) Dondoni, A.; Massi, A.; Sabbatini, S.; Bertolasi, V. J. Org. Chem.
2002, 67, 6979–6994. (b) Dondoni, A.; Marra, A. J. Org. Chem. 2006, 71, 7546–
7557.
(24) Goddard-Borger, E. D.; Stick, R. V. Org. Lett. 2007, 9, 3797–3800.
(25) (a) Nyffeler, P. T.; Liang, C.-H.; Koeller, K. M.; Wong, C.-H. J. Am.
Chem. Soc. 2002, 124, 10773–10778. (b) Lundquist, J. T., IV.; Pelletier, J. C.
Org. Lett. 2001, 3, 781–783.
(23) (a) Togo, H.; Ishigami, S.; Fujii, M.; Ikuma, T.; Yokoyama, M. J. Chem.
Soc., Perkin Trans. 1 1994, 2931–2942. (b) Igarashi, Y.; Shiozawa, T.; Ichikawa,
Y. Bioorg. Med. Chem. Lett. 1997, 7, 613–616, and references therein.
J. Org. Chem. Vol. 73, No. 24, 2008 9567

Aldhoun et al.
SCHEME 4. Synthesis of Model Tetrabenzylated
SCHEME 3. Synthesis of Peracetylated Glycosylmethyl
Azides
ꢀ-C-Glycosylmethyl Tetrazole Serine 5b
isolated (93% yield) as the sole regioisomer. The 1,5-substitution
pattern in the tetrazole ring was assumed on the basis of previous
observations.17,18,26 A highly performing click azide-nitrile
cycloaddition appeared to take place as well with a complex
alkyl azide such as 1b.27 As the substitution of the tosyl group
by an O-nucleophile required strong basic conditions, this
prevented the use of a protected serine derivative such as N-Boc-
L-serine methyl ester. The tosyl group of 3b was replaced by
reaction with N-Boc-N,O-isopropylidene-D-serinol 1128 and
NaH. This reaction provided the masked serine derivative 12b.
It has been demonstrated that compound 11 functions as a
configurationally stable equivalent of serine by virtue of
the easy transformation of the N-Boc oxazolidine ring into the
glycinyl group.10,11 Unexpectedly, treatment of 12b with the
Jones reagent (H2SO4-H2O, CrO3) at room temperature to
achieve one-pot cleavage of the oxazolidine ring and oxidation
of the amino alcohol afforded the target amino acid in very
poor yield (15%). In addition, the use of aqueous 60-80%
AcOH10 to remove the acetonide protecting group gave poor
results (25% yield of the corresponding alcohol). Fortunately,
a recent method for cleaving protected cyclic N,O-aminals by
using catalytic BiBr3 was available.29 To our satisfaction,
treatment of 12b with BiBr3 in acetonitrile at room temperature,
then oxidation of the alcohol with the Jones reagent, and finally
esterification with diazomethane afforded the orthogonally
protected N-Boc methyl serinate 5b in 60% isolated overall
yield. This final product displayed all prerequisites for being a
suitable building block in N-Boc-based peptide synthesis.
Debenzylation of the carbohydrate moiety of 5b by palladium-
catalyzed hydrogenolysis proved to be compatible with the
presence of basic nitrogen atoms of the tetrazole ring, thus
leading to the amino ester 5b′.
The cycloaddition-substitution sequence was successfully
applied to the other three benzylated glycosylmethyl azides
prepared, i.e., the R-D-galactosyl derivative 1a and the R-D- and
ꢀ-D-ribosyl derivatives 1c and 1d. As quoted in Table 1,
uniformly high yields of intermediates 3 and 12 and amino esters
5 were observed in all cases, thus confirming the efficiency of
the method.
C. Synthesis of C-Glycosylmethyl S-Tetrazolyl Cystei-
nes 6. Having in mind the final role of the amino acids under
preparation, i.e., their use in cotranslational modification of
glycopeptides, we decided to use peracetylated glycosylmethyl
azides 2 as starting material. We envisaged the acetyl group
removal by saponification of either the final amino acid or the
peptide in which it was introduced.30 In this way, we avoided
the risk that the removal of the carbohydrate benzyl groups by
Pd-catalyzed hydrogenation could fail due to the presence of
the cysteine sulfur atom. Hence, a typical synthesis of a model
(26) Butler, R. N. In ComprehensiVe Heterocyclic Compounds; Katritzky,
A. R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon: Oxford, U.K., 1966;
Vol. 4.
(27) The proficuous choice of thermal conditions to perform the cycloaddition
of 1b with TsCN became apparent when the reaction was carried out under
Cu(I) catalysis as described by Bosch and Vilarrasa: Bosch, L.; Vilarrasa, J.
Angew. Chem., Int. Ed. 2007, 46, 3926–3930. In our hands, treatment of 1b
with TsCN (2.0 equiv) in CH2Cl2 (room temperature) in the presence of Cu2(OTf)
2 · C6H6 (0.1 equiv) as the catalyst produced a complex reaction mixture from
which the cycloadduct 12b was isolated in very low yield (18%).
(28) Dondoni, A.; Perrone, D. Org. Synth. 1999, 77, 64–70.
(29) Cong, X.; Hu, F.; Liu, K.-G.; Liao, Q.-J.; Yao, Z.-J. J. Org. Chem.
2005, 70, 4514–4516. It is worth noting that errors in some references cited in
this paper as author given names instead of family names have been reported.
9568 J. Org. Chem. Vol. 73, No. 24, 2008

Synthesis of Tetrazole-Tethered C-Glycosyl R-Amino Acids
TABLE 1. Glycosylmethyl Tetrazole Serines Prepareda
a See the Experimental Section for details. b Isolated yield by chromatography.
compound is succinctly illustrated here (Scheme 5). The reaction
of the tetraacetyl azidomethyl ꢀ-D-galactoside 2b with excess
TsCN (2.0 equiv) was run neat at 100 °C, and the isolated
5-tosyl tetrazole 4b (93% yield) was treated with N-fluorenyl-
methoxycarbonyl (N-Fmoc) cysteine 1331 (1.1 equiv) under mild
basic conditions (K2CO3) at room temperature. The cysteine
13 was added in two portions to 4b in order to reduce its
transformation into N-Fmoc cystine. The final C-galactosylm-
ethyl S-tetrazolyl N-Fmoc cysteine 6b was isolated in high yield
(90%) without any apparent loss of configurational integrity of
the glycinyl group. This approach appeared to be more
straightforward than that followed in the synthesis of the serine
derivatives 5 (see previous section) in which a masked amino
acid nucleophile had to be employed because of the strong basic
conditions required for the substitution of the tosyl group.
The same reaction sequence was applied to the other
peracetylated glycosylmethyl azides shown in Scheme 3, i.e.,
compounds 2a, 2c, and 2d. Both the cycloadducts 4a, 4c, and
4d and the tetrazole-tethered C-glycosyl cysteines 6a, 6c, and
6d were isolated in good to excellent yields (Table 2). It is worth
noting that the set of orthogonal protective groups in amino
acids 6a-d makes these compounds all suitable for the N-Fmoc-
based automated peptide synthesis.30
SCHEME 5. Synthesis of Model Tetracetylated
ꢀ-C-Glycosylmethyl Tetrazole Cysteine 6b
D. Synthesis of Tetrazole-Tethered Glycopeptides. In order
to demonstrate the potential of the prepared C-glycosyl amino
(30) As a selected example of this protection strategy in glycopeptide
synthesis, see: Liu, S.; Ben, R. N. Org. Lett. 2005, 7, 2385–2388.
(31) Lumbierres, M.; Palomo, J.; Kragol, G. S. R.; Mu¨ller, O.; Waldmann,
H. Chem.sEur. J. 2005, 11, 7405–7415.
acids as orthogonally protected building blocks for the cotrans-
lational modification of glycopeptides, the O-serine derivative
J. Org. Chem. Vol. 73, No. 24, 2008 9569

Aldhoun et al.
TABLE 2. Glycosylmethyl Tetrazole Cysteines Prepareda
a See the Experimental Section for details. b Isolated yield by chromatography.
14 was selected as a prototype in this crucial validating test
(Scheme 6). A crude sample of this amino acid as obtained from
the cleavage of the oxazolidine precursor 12b was employed
in the coupling with phenylalanine ethyl ester hydrochloride
(H-Phe-OEt·HCl). The reaction was carried out at ambient
temperature under activation of the condensation agent (ben-
zotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophos-
phate (PyBOP) in the presence of the Hu¨nig’s base (diisopro-
pylethyl amine, DIPEA). From this reaction, the dipeptide 15
was isolated by chromatography in very good yield (80%). The
removal of the N-Boc protective group from 15 was readily
carried out upon treatment with diluted trifluoroacetic acid
(TFA). The amine that formed was used in the condensation
with N-Boc alanine (Boc-Ala-OH) under the same coupling
conditions described above. The tetrazole-tethered glycopeptide
16 was isolated after chromatography in 72% yield. This
tripeptide can be inserted in a more complex peptide via N-Boc-
based peptide synthesis.
SCHEME 6. Synthesis of Tripetide 16
Conclusions
In summary, we have demonstrated the efficiency of the
Sharpless-Demko azide-sulfonyl cyanide cycloaddition/sul-
fonyl substitution route for the synthesis of tetrazole-tethered
C-glycosyl R-amino acids. A new ligation tool has been
established for unnatural C-glycosyl R-amino acid synthesis,
the building blocks required for the cotranslational modification
of glycopeptides. The key step in this strategy was the thermal
azide-cyanide cycloaddition, a reaction that is highlighted as
a click process when the organic cyanide is activated by a strong
9570 J. Org. Chem. Vol. 73, No. 24, 2008

Synthesis of Tetrazole-Tethered C-Glycosyl R-Amino Acids
H, J5,6a ) 7.3 Hz, J6a,6b) 10.7 Hz, H-6a), 3.82-3.70 (m, 3 H, H-2,
H-3, H-6b), 3.60 (dd, 1 H, J1,1′a ) 8.5 Hz, J1′a,1′b ) 12.9 Hz, H-1′a),
3.27 (dd, 1 H, J1,1′b ) 4.8 Hz, H-1′b). 13C NMR: δ ) 138.3 (2C),
137.8 (2C), 128.5-127.4 (20C), 75.9, 75.5, 73.7, 73.3, 73.1, 73.2,
73.0, 72.9, 70.2, 66.6, 49.2. MALDI-TOF MS: 602.9 (M+ + Na).
Anal. Calcd for C35H37N3O5 (579.27): C, 75.52; N, 7.25; H, 6.43.
Found: C, 75.50; N, 7.27; H, 6.41.
electron-withdrawing substituent such as the sulfonyl group. This
and previous work reported from our laboratory21 demonstrated
the fidelity of this thermally induced metal-free process even
when applied to complex systems. This reaction combined with
the sulfonyl group substitution step appears to have its own value
as a new ligation tool. The tetrazole ring that is formed plays
the role of a robust molecular keystone holding the biologically
active residues. So far the glycosyl amino acids prepared were
serine and cysteine derivatives. It can be envisaged that
substitution of the sulfonyl group with N-, Se-, and C-
nucleophile amino acids or their synthetic equivalents will
expand the scope of this new synthetic method.
(2,3,5-Tri-O-benzyl-r-D-ribofuranosyl)methyl Azide (1c). Col-
umn chromatography with 9:1 cyclohexane-AcOEt afforded 1c
1
(285 mg, 62%) as a white foam. [R]D ) 40.3 (c 2.8, CHCl3). H
NMR (400 MHz): δ ) 7.40-7.20 (m, 15 H, Ph), 4.77 and 4.55 (2
d, 2 H, J ) 11.5 Hz, PhCH2), 4.62 and 4.54 (2 d, 2 H, J ) 11.8
Hz, PhCH2), 4.57 and 4.49 (2 d, 2 H, J ) 12.0 Hz, PhCH2), 4.25
(ddd, 1 H, J3,4 ) 5.5 Hz, J4,5a ) 3.6 Hz, J4,5b ) 3.4 Hz, H-4), 4.21
(ddd, 1 H, J1,2 ) 4.8 Hz, J1,1′a ) 8.0 Hz, J1,1′b ) 5.0 Hz, H-1), 4.12
(dd, 1H, J2,3 ) 4.8 Hz, H-2), 4.07 (dd, 1 H, H-3), 3.65 (dd, 1 H,
J1′a,1′b ) 12.8 Hz, H-1′a), 3.59 (dd, 1 H, J5a,5b ) 10.7 Hz, H-5a),
3.51 (dd, 1 H, H-5b), 3.46 (dd, 1 H, H-1′b). 13C NMR: δ ) 138.0,
137.8, 137.7, 128.4-127.6 (15C), 80.4, 79.0, 78.7, 77.5, 73.4, 73.1,
72.6, 69.9, 50.9. MALDI-TOF MS: 460.4 (M+ + H). Anal. Calcd
for C27H29N3O4 (459.22): C, 70.57; N, 9.14; H, 6.36. Found: C,
70.53; N, 9.11; H, 6.38.
Experimental Section
Cyanides 7a-d,23 amines 8b,22a 8d,22a and 9a,32a 9b,32 azides
1b,22a 1d,22a and 2b,22b tetrazoles 3b21and 3d,21 serinol 11,28 and
cysteine 1331 are known compounds, and their spectroscopic data
were identical to those reported.
General Procedure for the Synthesis of Benzylated Amines
8. To a stirred suspension of LiAlH4 (304 mg, 8.00 mmol) in
anhydrous THF (10 mL) was slowly added a solution of cyanide 7
(1.10 g, 2.00 mmol) in anhydrous THF (4 mL). The resulting
mixture was stirred under reflux until disappearance of starting
cyanide (TLC analysis, typically 1 h), cooled to room temperature,
and then diluted with 28% aqueous NH4OH (2 mL). The resulting
mixture was stirred at room temperature until the formation of a
white precipitate was observed (typically 30 min). The mixture was
then filtered over a pad of Celite and washed thoroughly with
AcOEt. The combined filtrates were concentrated to give the
corresponding crude amine 8 in almost quantitative yield. Each
amine 8 was used in the next step without any purification.
(2,3,4,6-Tetra-O-benzyl-r-D-galactopyranosyl)methylamine (8a).
1H NMR: δ ) 7.50-7.20 (m, 20 H, Ph), 4.75 and 4.61 (2 d, 2 H,
J ) 11.8 Hz, PhCH2), 4.73 and 4.51 (2 d, 2 H, J ) 12.0 Hz,
PhCH2), 4.66 and 4.64 (2 d, 2 H, J ) 11.5 Hz, PhCH2), 4.56 and
4.52 (2 d, 2 H, J ) 12.2 Hz, PhCH2), 4.10-4.04 (m, 1 H, H-5),
4.02 (dd, 1 H, J4,5 ) 2.8 Hz, J3,4 ) 3.8 Hz, H-4), 3.95-3.81 (m,
2 H, H-2, H-3), 3.81-3.71 (m, 2 H, H-1, H-6a), 3.66 (dd, 1 H,
J5,6b ) 4.2 Hz, J6a,6b ) 10.5 Hz, H-6b), 2.97 (dd, 1 H, J1,1′a ) 8.9
Hz, J1′a,1′b ) 13.5 Hz, H-1′a), 2.78 (dd, 1 H, J1,1′b ) 4.2 Hz, H-1′b).
(2,3,5-Tri-O-benzyl-r-D-ribofuranosyl)methylamine (8c). 1H
NMR: δ ) 7.40-7.25 (m, 15 H, Ph), 4.78 and 4.59 (2 d, 2 H, J )
11.8 Hz, PhCH2), 4.62 and 4.52 (2 d, 2 H, J ) 12.0 Hz, PhCH2),
4.57 and 4.54 (2 d, 2 H, J ) 12.2 Hz, PhCH2), 4.20 (ddd, 1 H, J3,4
) 7.0 Hz, J4,5a ) 3.3 Hz, J4,5b ) 4.10 Hz, H-4), 4.08 (dd, 1 H, J1,2
) 4.8 Hz, J2,3 ) 4.8 Hz, H-2), 4.06-3.98 (m, 2 H, H-1, H-3), 3.60
(dd, 1 H, J5a,5b ) 10.6 Hz, H-5a), 3.49 (dd, 1 H, H-5b), 3.01 (dd,
1 H, J1,1′a ) 6.7 Hz, J1′a,1′b ) 13.2 Hz, H-1′a), 2.92 (dd, 1 H, J1,1′b
) 5.1 Hz, H-1′b).
General Procedure for the Synthesis of Benzylated Azides
1. A mixture of crude amine 8 (553 mg, ∼1.00 mmol), imidazole-
1-sulfonyl azide hydrochloride (251 mg, 1.20 mmol), K2CO3 (235
mg, 1.70 mmol), CuSO4 ·5H2O (2.5 mg, 0.01 mmol), and MeOH
(5 mL) was stirred at room temperature for 4 h. The mixture was
then concentrated, diluted with H2O (15 mL), acidified with concd
HCl, and extracted with AcOEt (3 × 25 mL). The combined organic
layers were dried (Na2SO4), concentrated, and eluted from a column
of silica gel with the suitable elution system to give the corre-
sponding azide 1.
(2,3,4,6-Tetra-O-benzyl-r-D-galactopyranosyl)methyl azide
(1a). Column chromatography with 9:1 cyclohexane-AcOEt af-
forded 1a (452 mg, 79%) as white foam. [R]D ) 16.2 (c 1.3,
CHCl3). 1H NMR: δ ) 7.42-7.20 (m, 20 H, Ph), 4.72 and 4.59 (2
d, 2 H, J ) 12.0 Hz, PhCH2), 4.70 and 4.51 (2 d, 2 H, J ) 11.8
Hz, PhCH2), 4.62 and 4.60 (2 d, 2 H, J ) 11.5 Hz, PhCH2), 4.60
and 4.53 (2 d, 2 H, J ) 12.2 Hz, PhCH2), 4.22-4.08 (m, 2 H, H-1,
H-5), 4.05 (dd, 1 H, J4.5 ) 2.7 Hz, J3,4 ) 4.2 Hz, H-4), 3.92(dd, 1
General Procedure for the Synthesis of Unprotected Ami-
nes 9. A vigorously stirred mixture of amine 8 (553 mg, ∼1.00
mmol), 20% palladium hydroxide on carbon (227 mg), and AcOH
(8 mL) was degassed under vacuum and saturated with hydrogen
(by a H2-filled balloon) three times. The suspension was stirred at
room temperature for 12 h under a positive pressure of hydrogen
(8 bar), filtered through a plug of cotton, and concentrated to give
crude amine 9 in almost quantitative yield. Each amine 9 was used
in the next step without any purification.
1
(r-D-Ribofuranosyl)methylamine (9c). H NMR (D2O): δ )
4.19 (dd, 1 H, J1,2 ) 4.0 Hz, J2,3 ) 4.5 Hz, H-2), 4.15 (ddd, 1 H,
J1,1′a ) 6.0 Hz, J1,1′b ) 4.5 Hz, H-1), 4.03 (dd, 1 H, J3,4 ) 7.8 Hz,
H-3), 3.81 (ddd, 1 H, J4,5a ) 2.8 Hz, J4,5b ) 5.4 Hz, H-4), 3.66
(dd, 1 H, J5a,5b ) 12.5 Hz, H-5a), 3.49 (dd, 1 H, H-5b), 3.15 (dd,
1 H, J1′a,1′b ) 13.0 Hz, H-1′a), 3.11 (dd, 1 H, H-1′b). 13C NMR
(D2O): δ ) 81.9, 75.9, 72.5, 71.8, 61.5, 39.6.
(ꢀ-D-Ribofuranosyl)methylamine (9d). 1H NMR: δ ) 3.93 (dd,
1 H, J1,2 ) 5.1 Hz, J2,3 ) 5.0 Hz, H-2), 3.90 (ddd, 1 H, J1,1′a ) 3.2
Hz, J1,1′b ) 13.3 Hz, H-1), 3.84 (dd, 1 H, J3,4 ) 5.2 Hz, H-3), 3.82
(ddd, 1 H, J4.5a ) 3.2 Hz, J4,5b ) 4.9 Hz, H-4), 3.17 (dd, 1 H,
J1′a,1′b ) 13.3 Hz, H-1′a), 2.91 (dd, 1 H, H-1′b). 13C NMR (D2O):
δ ) 84.4, 78.5, 72.7, 71.2, 61.5, 41.7.
General Procedure for the Synthesis of Acetylated Azides 2. A
mixture of crude amine 9 (193 mg, ∼1.00 mmol), imidazole-1-
sulfonyl azide hydrochloride (251 mg, 1.20 mmol), K2CO3 (235
mg, 1.70 mmol), CuSO4 ·5H2O (2.5 mg, 0.01 mmol), and MeOH
(5 mL) was stirred at room temperature for 4 h. The mixture was
then concentrated and coevaporated with toluene (2 × 10 mL).
Acetic anhydride (3 mL) and pyridine (3 mL) were added to the
residue, and the resulting mixture was stirred for an additional 5 h,
diluted with H2O (10 mL), and extracted with AcOEt (3 × 25 mL).
The combined organic layers were dried (Na2SO4), concentrated,
and eluted from a column of silica gel with the suitable elution
system to give the corresponding azide 2.
(2,3,4,6-Tetra-O-acetyl-r-D-galactopyranosyl)methyl azide (2a).
Column chromatography with 2:1 cyclohexane-AcOEt afforded
2a (248 mg, 64%) as a yellow syrup. [R]D ) 53.0 (c 1.3, CHCl3).
1H NMR (400 MHz): δ ) 5.44 (dd, 1 H, J3.4 ) 3.3 Hz, J4,5 ) 3.5
Hz, H-4), 5.27 (dd, 1 H, J1,2 ) 4.6 Hz, J2,3 ) 8.5 Hz, H-2), 5.21
(dd, 1 H, H-3), 4.39 (dd, 1 H, J5.6a ) 8.0 Hz, J6a,6b ) 11.7 Hz,
H-6a), 4.37 (ddd, 1 H, J1,1′a ) 8.8 Hz, J1,1′b ) 3.7 Hz, H-1), 4.24
(ddd, 1 H, J5,6b ) 4.7 Hz, H-5), 4.10 (dd, 1 H, H-6b), 3.61 (dd, 1
H, J1′a,1′b ) 13.5 Hz, H-1′a), 3.28 (dd, 1 H, H-1′b), 2.12, 2.10, 2.07,
and 2.05 (4s, 12 H, CH3). 13C NMR: δ ) 170.7, 169.8, 169.6, 169.5,
70.8, 69.9, 67.7, 67.6, 66.9, 60.7, 48.3, 20.7 (3C), 20.6. MALDI-
TOF MS: 426.4 (M+ + K). Anal. Calcd for C15H21N3O9 (387.13):
C, 46.51; N, 10.85; H, 5.46. Found: C, 46.50; N, 10.88; H, 5.45.
J. Org. Chem. Vol. 73, No. 24, 2008 9571

Aldhoun et al.
(2,3,5-Tri-O-acetyl-r-D-ribofuranosyl)methyl Azide (2c). Column
chromatography with 2.5:1 cyclohexane-AcOEt afforded 2c (208
C, 65.61; N, 8.74; H, 5.66; S, 5.00. Found: C, 65.63; N, 8.71; H,
5.67; S, 5.02.
1
mg, 66%) as a white foam. [R]D ) 55.8 (c 1.4, CHCl3). H NMR
1-(2,3,4,6-Tetra-O-acetyl-r-D-galactopyranosylmethyl)-5-(p-tolu-
ensulfonyl)-1H-tetrazole (4a). Column chromatography with 2:1
cyclohexane-AcOEt afforded 4a (267 mg, 94%) as a white
amorphous solid. [R]D ) 59.7 (c 1.0, CHCl3). 1H NMR (400 MHz):
δ ) 8.01-7.97 (m, 2 H, Ar), 7.49-7.42 (m, 2 H, Ar), 5.49 (dd, 1
H, J3,4 ) 3.3 Hz, J4,5 ) 3.2 Hz, H-4), 5.43 (dd, 1 H, J1,2 ) 4.8 Hz,
(400 MHz): δ ) 5.54 (dd, 1 H, J1,2 ) 4.7 Hz, J2,3 ) 5.0 Hz, H-2),
5.32-5.26 (m, 1 H, H-3), 4.38 (ddd, 1 H, J1,1′a ) 7.2 Hz, J1,1′b
)
5.5 Hz, H-1), 4.33-4.24 (m, 2 H, H-4, H-5a), 4.16-4.08 (m, 1 H,
H-5b), 3.51 (dd, 1 H, J1′a,1′b ) 13.0 Hz, H-1′a), 3.40 (dd, 1 H,
H-1′b), 2.16, 2.12, and 2.08 (3s, 9 H, CH3). 13C NMR: δ ) 170.6,
169.6, 169.5, 77.8, 77.6, 71.8, 71.5, 63.4, 50.2, 20.8, 20.6, 20.5.
MALDI-TOF MS: 338.1 (M+ + Na). Anal. Calcd for C12H17N3O7
(315.11): C, 45.71; N, 13.33; H, 5.43. Found: C, 45.74; N, 13.35;
H, 5.40.
J2,3 ) 8.5 Hz, H-2), 5.28 (dd, 1 H, H-3), 5.06 (dd, 1 H, J1,1′a
)
10.8 Hz, J1′a,1′b ) 14.5 Hz, H-1′a), 4.95 (dd, 1 H, J1,1′b ) 3.0 Hz,
H-1′b), 4.75 (ddd, 1 H, H-1), 4.35 (ddd, 1 H, J5,6a ) 8.5 Hz, J5,6b
) 4.5 Hz, H-5), 4.21 (dd, 1 H, J6a,6b ) 11.8 Hz, H-6a), 4.04 (dd,
1 H, H-6b), 2.49 (s, 3 H, CH3), 2.21, 2.11, 2.08 and 1.93 (4s, 12
H, CH3). 13C NMR: δ )170.6, 169.8, 169.7, 169.6, 154.9, 147.6,
134.1, 130.4 (2C), 129.3 (2C), 70.4, 69.7, 67.3, 67.0, 66.8, 60.4,
(2,3,5-Tri-O-acetyl-ꢀ-D-ribofuranosyl)methyl azide (2d). Column
chromatography with 2.5:1 cyclohexane-AcOEt afforded 2d (214
mg, 68%) as a white foam. [R]D ) -76.3 (c 1.7, CHCl3). 1H NMR
(400 MHz): δ ) 5.18-5.13 (m, 2 H, H-2, H-3), 4.34 (dd, 1 H,
J4.5a ) 2.9 Hz, J5b,5a ) 11.9 Hz, H-5a), 4.21 (ddd, 1 H, J3,4 ) 5.5
Hz, J4,5b ) 5.0 Hz, H-4), 4.15 (dd, 1 H, H-5b), 4.13 (ddd, 1 H, J1,2
) 5.5 Hz, J1,1′a ) 3.1 Hz, J1,1′b ) 4.2 Hz, H-1), 3.60 (dd, 1 H,
J1′a,1′b ) 13.3 Hz, H-1′a), 3.36 (dd, 1 H, H-1′b), 2.11, 2.09, and
2.07 (3s, 9 H, CH3). 13C NMR: δ ) 170.6, 169.7, 169.6, 80.6,
79.4, 71.6, 71.4, 63.2, 51.6, 20.7, 20.5 (2C). MALDI-TOF MS:
338.4 (M+ + Na). Anal. Calcd for C12H17N3O7 (315.11): C, 45.71;
N, 5.43; H, 35.5. Found: C, 45.73; N, 13.34; H, 5.44.
General Procedure for the Synthesis of 1-Glycosylmethyl-5-
tosyl tetrazoles 3 and 4. A mixture of either azide 1 or 2 (0.50
mmol) and commercially available p-toluensulfonyl cyanide (181
mg, 1.00 mmol) was stirred at 100 °C in the absence of solvent
under a nitrogen atmosphere and then cooled to room temperature.
The excess of p-toluensulfonyl cyanide was then removed by
sublimation under vacuum to give nearly pure tetrazole 3 or 4.
Analytical quality samples of 3 and 4 were obtained by eluting the
corresponding crude derivative from a column of silica gel with
the suitable elution system.
46.4, 21.9, 20.7 (2C), 20.6 (2C). MALDI-TOF MS: 607.5 (M+
+
K). Anal. Calcd for C23H28N4O11S (568.15): C, 48.59; N, 9.85; H,
4.96; S, 5.64. Found: C, 48.57; N, 9.83; H, 4.92; S, 5.66.
1-(2,3,4,6-Tetra-O-acetyl-ꢀ-D-galactopyranosylmethyl)-5-(p-tolu-
ensulfonyl)-1H-tetrazole (4b). Column chromatography with 2:1
cyclohexane-AcOEt afforded 4b (264 mg, 93%) as a white
1
amorphous solid. [R]D ) -7.8 (c 1.4, CHCl3). H NMR: δ )
8.01-7.95 (m, 2 H, Ar), 7.47-7.42 (m, 2 H, Ar), 5.42 (dd, 1 H,
J3,4 ) 3.3 Hz, J4,5 ) 0.5 Hz, H-4), 5.28 (dd, 1 H, J1,2 ) 9.8 Hz, J2,3
) 9.9 Hz, H-2), 5.13 (dd, 1 H, H-3), 5.08 (dd, 1 H, J1,1′a ) 9.2 Hz,
J1′a,1′b ) 14.5 Hz, H-1′a), 4.87 (dd, 1 H, J1,1′b ) 2.5 Hz, H-1′b),
4.05-3.90 (m, 3 H, H-1, 2H-6), 3.79 (ddd, 1 H, J5,6a ) 6.7 Hz,
J5,6b ) 6.8 Hz, H-5), 2.49 (s, 3 H, CH3), 2.18, 2.15, 2.01, and 1.94
(4s, 12 H, CH3). 13C NMR: δ ) 170.6, 170.5, 170.4, 170.2, 155.4,
147.6, 134.3, 130.4 (2C), 129.3 (2C), 76.4, 74.2, 71.5, 67.4, 67.3,
61.1, 50.6, 21.9, 20.7, 20.6, 20.5, 20.4. MALDI-TOF MS: 591.4
(M+ + Na). Anal. Calcd for C23H28N4O11S (568.15): C, 48.59; N,
9.85; H, 4.96; S, 5.64. Found: C, 48.57; N, 9.83; H, 4.92; S, 5.66.
1-(2,3,5-Tri-O-acetyl-r-D-ribofuranosylmethyl)-5-(p-toluensulfo-
nyl)-1H-tetrazole (4c). Column chromatography with 1.5:1 cyclo-
hexane-AcOEt afforded 4c (243 mg, 98%) as a white amorphous
solid. [R]D ) 48.8 (c 1.2, CHCl3). 1H NMR (400 MHz): δ )
8.01-7.97 (m, 2 H, Ar), 7.47-7.42 (m, 2 H, Ar), 5.56 (dd, 1 H,
J1,2 ) 5.5 Hz, J2,3 ) 5.2 Hz, H-2), 5.34 (dd, 1 H, J3,4 ) 5.0 Hz,
H-3), 5.23 (dd, 1 H, J1,1′a ) 9.8 Hz, J1′a,1′b ) 14.5 Hz, H-1′a),
4.88-4.80 (m, 2 H, H-1, H-1′b), 4.46 (ddd, 1 H, J4,5a ) 3.2 Hz,
J4,5b ) 4.8 Hz, H-4), 4.22 (dd, 1 H, J5a,5b ) 12.2 Hz, H-5a), 4.14
(dd, 1 H, H-5b), 2.50 (s, 3 H, CH3), 2.19, 2.14, and 2.09 (3s, 9 H,
CH3). 13C NMR: δ ) 170.4, 169.7, 169.3, 155.5, 147.5, 134.3,
130.3 (2C), 129.2 (2C), 79.1, 77.4, 71.9, 71.0, 63.2, 49.3, 21.9,
20.7, 20.6, 20.4. MALDI-TOF MS: 497.4 (M+ + H). Anal. Calcd
for C20H24N4O9S (496.13): C, 48.38; N, 11.28; H, 4.87; S, 6.46.
Found: C, 48.35; N, 11.26; H, 4.85; S, 6.47.
1-(2,3,4,6-Tetra-O-benzyl-r-D-galactopyranosylmethyl)-5-(p-tolu-
ensulfonyl)-1H-tetrazole (3a). Column chromatography with 4.5:1
cyclohexane-AcOEt afforded 3a (319 mg, 84%) as a white
amorphous solid. [R]D ) 47.1 (c 1.1, CHCl3). 1H NMR (400 MHz):
δ ) 8.00-7.90 (m, 2 H, Ar), 7.50-7.20 (m, 20 H, Ar), 7.18-7.10
(m, 2 H, Ar), 5.10 (dd, 1 H, J1,1′a ) 10.3 Hz, J1′a,1′b ) 14.2 Hz,
H-1′a), 4.89 (dd, 1 H, J1,1′b ) 3.6 Hz, H-1′b), 4.75 and 4.57 (2 d,
2 H, J ) 11.8 Hz, PhCH2), 4.74 and 4.67 (2 d, 2 H, J ) 11.5 Hz,
PhCH2), 4.71 and 4.66 (2 d, 2 H, J ) 11.8 Hz, PhCH2), 4.65 (ddd,
1 H, J1,2 ) 4.5 Hz, H-1), 4.30 and 4.23 (2 d, 2 H, J ) 12.0 Hz,
PhCH2), 4.18-4.08 (m, 2 H, H-4, H-5), 4.03 (dd, 1 H, J2,3 ) 7.5
Hz, H-2), 3.79 (dd, 1 H, J3.4 ) 2.6 Hz, H-3), 3.63 (dd, 1 H, J5,6a
)
6.0 Hz, J6a,6b ) 10.4 Hz, H-6a), 3.60 (dd, 1 H, J5,6b ) 7.0 Hz,
H-6b), 2.43 (s, 3 H, CH3). 13C NMR: δ ) 155.1, 147.2, 138.3,
138.1, 137.6, 137.5, 134.4, 130.3 (2C), 129.2 (2C), 127.9-127.0
(20C), 75.2, 75.1, 73.7, 73.6, 73.4, 73.2, 72.9, 72.8, 70.9, 66.9,
47.7, 21.9. MALDI-TOF MS: 783.4 (M+ + Na). Anal. Calcd for
C43H44N4O7S (760.29): C, 67.88; N, 7.36; H, 5.83; S, 4.21. Found:
C, 67.89; N, 7.32; H, 5.81; S, 4.23.
1-(2,3,5-Tri-O-acetyl-ꢀ-D-ribofuranosylmethyl)-5-(p-toluensulfo-
nyl)-1H-tetrazole (4d). Column chromatography with 1.5:1 cyclo-
hexane-AcOEt afforded 4d (223 mg, 90%) as a white amorphous
1
solid. [R]D ) -23.6 (c 1.3, CHCl3). H NMR (400 MHz): δ )
8.02-7.97 (m, 2 H, Ar), 7.45-7.40 (m, 2 H, Ar), 5.19-5.12 (m,
3 H, H-2, H-3, H-1′a), 5.03 (dd, 1 H, J1,1′b ) 5.8 Hz, J1′a,1′b ) 14.1
Hz, H-1′b), 4.54-4.48 (m, 1 H, H-1), 4.20 (ddd, 1 H, J3,4 ) 4.0
1-(2,3,5-Tri-O-benzyl-r-D-ribofuranosylmethyl)-5-(p-toluensul-
fonyl)-1H-tetrazole (3c). Column chromatography with 4:1 cyclo-
hexane-AcOEt afforded 3c (256 mg, 80%) as a white amorphous
solid. [R]D ) 43.6 (c 2.0, CHCl3). 1H NMR (400 MHz): δ )
8.00-7.90 (m, 2 H, Ar), 7.50-7.30 (m, 15 H, Ar), 7.30-7.10 (m,
2 H, Ar), 5.37 (dd, 1 H, J1,1′a ) 9.9 Hz, J1′a,1′b ) 14.2 Hz, H-1′a),
4.82 (dd, 1 H, J1,1′b ) 2.8 Hz, H-1′b), 4.74 and 4.59 (2 d, 2 H, J
) 11.8 Hz, PhCH2), 4.70 and 4.63 (2 d, 2 H, J ) 12.0 Hz, PhCH2),
4.64 (ddd, 1 H, J1,2 ) 6.5 Hz, H-1), 4.53 (ddd, 1 H, J3,4 ) 2.8 Hz,
J4,5a ) 3.6 Hz, J4,5b ) 4.0 Hz, H-4), 4.51 and 4.43 (2 d, 2 H, J )
12.2 Hz, PhCH2), 4.29 (dd, 1 H, J2,3 ) 5.5 Hz, H-2), 4.06 (dd, 1
H, H-3), 3.47 (dd, 1 H, J5a,5b ) 10.5 Hz, H-5a), 3.44 (dd, 1 H,
H-5b), 2.45 (s, 3 H, CH3). 13C NMR: δ ) 155.4, 147.1, 137.9,
137.7, 137.4, 134.2, 130.2 (2C), 129.1 (2C), 128.5-127.6 (15C),
81.6, 78.2, 77.9, 77.8, 73.5, 72.9, 72.4, 70.2, 50.7, 21.8. MALDI-
TOF MS: 641.3 (M+ + H). Anal. Calcd for C35H36N4O6S (640.24):
Hz, J4,5a ) 3.2 Hz, J4,5b ) 3.0 Hz, H-4), 4.11 (dd, 1 H, J5a,5b
)
12.3 Hz, H-5a), 4.06 (dd, 1 H, H-5b), 2.48 (s, 3 H, CH3), 2.10,
2.08, and 2.03 (3s, 9 H, CH3). 13C NMR: δ ) 170.5, 169.7 (2C),
155.7, 147.5, 134.5, 130.3 (2C), 129.3 (2C), 80.7, 78.1, 71.9, 71.7,
63.0, 50.2, 21.8, 20.7, 20.5, 20.3. MALDI-TOF MS: 519.7 (M+
+
Na). Anal. Calcd for C20H24N4O9S (496.13): C, 48.38; N, 11.28;
H, 4.87; S, 6.46. Found: C, 48.35; N, 11.26; H, 4.85; S, 6.47.
General Procedure for the Synthesis of Masked Glycosyl
Amino Acids 12 (Table 1). To a stirred solution of D-serinol 11
(92 mg, 0.40 mmol) in anhydrous DMF (4 mL) was added NaH
(32 mg, 0.80 mmol of a 60% dispersion in oil). The mixture was
stirred at room temperature for 30 min and then diluted with a
solution of sugar tetrazole 3 (304 mg, 0.40 mmol) in anhydrous
DMF (2 mL). The resulting solution was stirred at room temperature
9572 J. Org. Chem. Vol. 73, No. 24, 2008

Synthesis of Tetrazole-Tethered C-Glycosyl R-Amino Acids
for 16 h, diluted with saturated aqueous NH4Cl (10 mL), and
extracted with AcOEt (3 × 50 mL). The combined organic phases
were dried (Na2SO4), concentrated, and eluted from a column of
silica gel with the suitable elution system to give the corresponding
adduct 12.
(4R)-2,2-Dimethyl-4-[1-(2′,3′,4′,6′-tetra-O-benzyl-r-D-galactopy-
ranosylmethyl)-1H-tetrazol-5-yloxymethyl)]oxazolidine-3-carboxy-
lic Acid tert-Butyl Ester (12a). Column chromatography with 3:1
cyclohexane-AcOEt afforded 12a (268 mg, 80%) as a white foam.
[R]D ) 37.4 (c 1.3, CHCl3). 1H NMR (DMSO-d6, 120 °C) selected
data: δ ) 7.40-7.20 (m, 20 H, Ph), 4.72 and 4.62 (2 d, 2 H, J )
11.5 Hz, PhCH2), 4.70 and 4.54 (2 d, 2 H, J ) 11.8 Hz, PhCH2),
4.25-4.18 (m, 1 H), 4.16-4.08 (m, 1 H), 3.64 (d, 2 H, J ) 6.0
Hz), 1.45 and 1.43 (2s, 6 H), 1.42 (s, 9 H). MALDI-TOF MS:
858.8 (M+ + Na). Anal. Calcd for C47H57N5O9 (835.42): C, 67.53;
N, 8.38; H, 6.87. Found: C, 67.55; N, 8.34; H, 6.81.
(4R)-2,2-Dimethyl-4-[1-(2′,3′,4′,6′-tetra-O-benzyl-ꢀ-D-galactopy-
ranosylmethyl)-1H-tetrazol-5-yloxymethyl)]oxazolidine-3-carboxy-
lic Acid tert-Butyl Ester (12b). Column chromatography with 3:1
cyclohexane-AcOEt afforded 12b (284 mg, 85%) as a yellow
syrup. [R]D ) 1.5 (c 2.3, CHCl3). 1H NMR (DMSO-d6, 120 °C): δ
) 7.40-7.10 (m, 20 H, Ph), 4.92 and 4.56 (2 d, 2 H, J ) 11.5 Hz,
PhCH2), 4.83 and 4.67 (2 d, 2 H, J ) 11.2 Hz, PhCH2), 4.81 and
4.72 (2 d, 2 H, J ) 12.0 Hz, PhCH2), 4.54 and 4.42 (2 d, 2 H, J )
10.5 Hz, PhCH2), 4.46-4.34 (m, 3 H), 4.28-4.14 (m, 2 H), 4.06
(dd, 1 H, J ) 1.0 Hz, J ) 2.5 Hz), 3.92 (d, 2 H, J ) 4.5 Hz),
3.82-3.72 (m, 3 H), 3.67 (ddd, 1 H, J ) 1.0 Hz, J ) 6.0 Hz, J )
7.5 Hz), 3.56 (dd, 1 H, J ) 6.0 Hz, J ) 10.5 Hz), 3.46 (dd, 1 H,
J ) 6.5 Hz, J ) 10.5 Hz), 1.49 and 1.43 (2s, 6 H), 1.42 (s, 9 H).
MALDI-TOF MS: 836.7 (M+ + H). Anal. Calcd for C47H57N5O9
(835.42): C, 67.53; N, 8.38; H, 6.87. Found: C, 67.54; N, 8.33; H,
6.82.
prepared 1 M Jones reagent (0.60 mL, 0.60 mmol) was added in
one portion. The mixture was allowed to warm to room temperature
in 30 min, stirred at room temperature for an additional 1.5 h, and
then diluted with i-PrOH (0.5 mL). The suspension was neutralized
with saturated aqueous NaHCO3, diluted with AcOEt (80 mL), and
then washed with brine (2 × 5 mL). The organic phase was dried
(Na2SO4) and concentrated to afford the corresponding crude
carboxylic acid derivative.
To a stirred, cooled (0 °C) solution of the above crude carboxylic
acid in CH2Cl2 (2.0 mL) was added an ethereal solution of CH2N2
until a pale yellow color persisted. The solution was stirred for an
additional 15 min at 0 °C and then dried under vacuum. The residue
was eluted from a column of silica gel with the suitable elution
system to give the corresponding glycosylmethyl tetrazole serine
5.
(2S)-2-tert-Butoxycarbonylamino-3-[1-(2′,3′,4′,6′-tetra-O-benzyl-
r-D-galactopyranosylmethyl)-1H-tetrazol-5-yloxy]propionic Acid
Methyl Ester (5a). Column chromatography with 2.5:1 cyclo-
hexane-AcOEt afforded 5a (102 mg, 62%) as a white foam. [R]D
1
) 16.1 (c 1.1, CHCl3). H NMR (400 MHz): δ ) 7.44-7.14 (m,
20 H, Ph), 6.07 (d, 1 H, J2,NH ) 7.5 Hz, NH), 4.80-4.62 (m, 5 H,
2H-3, 3 CHPh), 4.62-4.46 (m, 4 H, H-2, 3 CHPh), 4.45-4.36 (m,
1 H, H-1′), 4.35 (s, 2 H, CH2Ph), 4.25 (d, 2 H, J ) 8.0 Hz, 2H-
1′′), 4.16-4.09 (m, 1 H, H-5′), 4.02-3.94 (m, 2 H, H-2′, H-4′),
3.76 (dd, 1 H, J2′,3′ ) 7.6 Hz, J3′,4′ ) 2.7 Hz, H-3′), 3.70-3.60 (m,
1 H, H-6′a), 3.68 (s, 3 H, CH3), 3.54 (dd, 1 H, J5′,6′b ) 4.5 Hz,
J6′a,6′b ) 11.5 Hz, H-6′b), 1.43 (s, 9 H, t-Bu). 13C NMR: δ ) 169.5,
161.1, 155.3, 138.0 (2C), 137.7, 137.6, 128.6-127.4 (20C), 80.2,
75.3, 73.8, 73.5, 73.4, 73.3, 73.0, 72.9, 72.3, 70.5, 68.1, 53.1, 52.7,
43.6, 29.7, 28.3 (3C). MALDI-TOF MS: 846.7 (M+ + Na). Anal.
Calcd for C45H53N5O10 (823.38): C, 65.60; N, 8.50; H, 6.48. Found:
C, 65.62; N, 8.53; H, 6.41.
(4R)-2,2-Dimethyl-4-[1-(2′,3′,5′-tri-O-benzyl-r-D-ribofuranosyl-
methyl)-1H-tetrazol-5-yloxymethyl)]oxazolidine-3-carboxylic Acid
tert-Butyl Ester (12c). Column chromatography with 3:1 cyclo-
hexane-AcOEt afforded 12c (237 mg, 83%) as a white foam. [R]D
) 49.2 (c 1.4, CHCl3). 1H NMR (DMSO-d6, 120 °C): δ )
7.40-7.20 (m, 15 H, Ph), 4.75 and 4.62 (2 d, 2 H, J ) 11.5 Hz,
PhCH2), 4.68 and 4.62 (2 d, 2 H, J ) 11.8 Hz, PhCH2), 4.57 (dd,
1 H, J ) 4.5 Hz, J ) 10.5 Hz), 4.52-4.30 (m, 5 H), 4.48 (s, 2 H,
PhCH2), 4.22-4.15 (m, 2 H), 4.12 (t, 1 H, J ) 4.5 Hz), 4.00-3.88
(m, 2 H), 3.51 (d, 2 H, J ) 5.5 Hz), 1.45 and 1.43 (2s, 6 H), 1.42
(s, 9 H). MALDI-TOF MS: 836.7 (M+ + K). Anal. Calcd for
C39H49N5O8 (715.36): C, 65.44; N, 9.78; H, 6.90. Found: C, 65.44;
N, 9.72; H, 6.93.
(4R)-2,2-Dimethyl-4-[1-(2′,3′,5′-tri-O-benzyl-ꢀ-D-ribofuranosyl-
methyl)-1H-tetrazol-5-yloxymethyl)]oxazolidine-3-carboxylic Acid
tert-Butyl Ester (12d). Column chromatography with 3:1 cyclo-
hexane-AcOEt afforded 12d (223 mg, 78%) as a white foam. [R]D
) -3.4 (c 1.3, CHCl3). 1H NMR (DMSO-d6, 120 °C): δ )
7.40-7.20 (m, 15 H, Ph), 4.62 and 4.53 (2 d, 2 H, J ) 11.5 Hz,
PhCH2), 4.58 (dd, 1 H, J ) 3.2 Hz, J ) 10.0 Hz), 4.56 (s, 2 H,
PhCH2), 4.48 (s, 2 H, PhCH2), 4.44 (dd, 1 H, J ) 7.0 Hz, J ) 10.0
Hz), 4.34-4.18 (m, 4 H), 4.14-4.08 (m, 1 H), 4.02-3.94 (m, 4
H), 3.47 (dd, 1 H, J ) 4.5 Hz, J ) 10.5 Hz), 3.43 (dd, 1 H, J )
5.0 Hz, J ) 10.5 Hz), 1.48 and 1.44 (2s, 6 H), 1.42 (s, 9 H).
MALDI-TOF MS: 738.4 (M+ + Na). Anal. Calcd for C39H49N5O8
(715.36): C, 65.44; N, 9.78; H, 6.90. Found: C, 65.41; N, 9.73; H,
6.92.
(2S)-2-tert-Butoxycarbonylamino-3-[1-(2′,3′,4′,6′-tetra-O-benzyl-
ꢀ-D-galactopyranosylmethyl)-1H-tetrazol-5-yloxy]propionic Acid
Methyl Ester (5b). Column chromatography with 3:1 cyclo-
hexane-AcOEt afforded 5b (99 mg, 60%) as a white foam. [R]D
) 4.5 (c 1.0, CHCl3). 1H NMR (400 MHz): δ ) 7.45-7.10 (m, 20
H, Ph), 5.85 (d, 1 H, J2,NH ) 8.5 Hz, NH), 5.02 and 4.74 (2 d, 2 H,
J ) 11.2 Hz, PhCH2), 4.90 and 4.52 (2 d, 2 H, J ) 11.5 Hz,
PhCH2), 4.80-4.69 (m, 2 H, 2H-3), 4.76 and 4.66 (2 d, 2 H, J )
11.8 Hz, PhCH2), 4.66-4.59 (m, 1 H, H-2), 4.42 and 4.36 (2 d, 2
H, J ) 11.8 Hz, PhCH2), 4.32 (dd, 1 H, J1′,1′′a ) 2.8 Hz, J1′′a,1′′b
)
14.1 Hz, H-1”a), 4.09 (dd, 1 H, J1′,1′′b ) 8.0 Hz, H-1′′b), 3.96 (dd,
1 H, J3′,4′ ) 2.6 Hz, J4′,5′ ) 0.5 Hz, H-4′), 3.80 (dd, 1 H, J1′,2′ ) 9.0
Hz, J2′,3′ ) 9.5 Hz, H-2′), 3.67 (dd, 1 H, H-3′), 3.65 (ddd, 1 H,
H-1′), 3.60 (s, 3 H, CH3), 3.52-3.45 (m, 3 H, H-5′, 2H-6′), 1.45
(s, 9 H, t-Bu). 13C NMR: δ ) 169.4, 161.5, 155.1, 138.5, 138.0,
137.9, 137.8, 129.0-127.0 (20C), 84.6, 80.5, 77.4, 76.6, 75.7, 75.1,
74.6, 73.4, 72.1, 68.5, 53.2, 52.8, 47.0, 26.7, 28.3 (3C). MALDI-
TOF MS: 824.2 (M+ + H). Anal. Calcd for C45H53N5O10 (823.38):
C, 65.60; N, 8.50; H, 6.48. Found: C, 65.62; N, 8.53; H, 6.41.
(2S)-2-tert-Butoxycarbonylamino-3-[1-(2′,3′,5′-tri-O-benzyl-r-D-
ribofuranosylmethyl)-1H-tetrazol-5-yloxy]propionic Acid Methyl
Ester (5c). Column chromatography with 2:1 cyclohexane-AcOEt
afforded 5c (94 mg, 67%) as a white foam. [R]D ) 44.9 (c 1.2,
CHCl3). 1H NMR (400 MHz): δ ) 7.40-7.20 (m, 15 H, Ph), 5.63
(d, 1 H, J2,NH ) 8.5 Hz, NH), 4.83 (d, 2 H, J ) 3.0 Hz, 2H-3),
4.71 and 4.52 (2 d, 2 H, J ) 11.5 Hz, PhCH2), 4.74-4.67 (m, 1 H,
H-2), 4.67 and 4.61 (2 d, 2 H, J ) 12.0 Hz, PhCH2), 4.50-4.34
(m, 6 H, 2H-1”, H-1′, H-4′, PhCH2), 4.22 (dd, 1 H, J1′,2′ ) 5.5 Hz,
J2′,3′ ) 5.3 Hz, H-2′), 4.02 (dd, 1 H, J3′,4′ ) 3.5 Hz, H-3′), 3.66 (s,
3 H, CH3), 3.52 (dd, 1 H, J4′,5′a ) 4.5 Hz, J5′a,5′b ) 10.5 Hz, H-5′a),
3.47 (dd, 1 H, J4′,5′b ) 4.0 Hz, H-5′b), 1.41 (s, 9 H, t-Bu). 13C
NMR: δ ) 169.5, 161.5, 155.1, 137.8, 137.6, 137.5, 128.5-127.5
(15C), 81.4, 80.5, 77.9, 77.6, 73.5, 72.9, 72.8, 72.3, 70.3, 53.2,
52.8, 46.4, 28.2 (4C). MALDI-TOF MS: 742.8 (M+ + K). Anal.
Calcd for C37H45N5O9 (703.32): C, 63.14; N, 9.95; H, 6.44. Found:
C, 63.17; N, 9.91; H, 6.45.
General Procedure for the Synthesis of Glycosylmethyl Tetra-
zole Serines 5 (Table 1). To a stirred solution of adduct 12 (167
mg, 0.20 mmol) in MeCN (2 mL) was added BiBr3 (9 mg, 0.02
mmol) in one portion. The resulting mixture was stirred at room
temperature for 6 h, diluted with saturated aqueous NaHCO3
solution (5 mL), and extracted with AcOEt (3 × 25 mL). The
combined organic phases were dried (Na2SO4), and concentrated
to give the corresponding crude N-Boc amino alcohol.
To a cooled (0 °C), stirred solution of the above crude N-Boc
amino alcohol (159 mg, ∼0.20 mmol) in acetone (6 mL) freshly
J. Org. Chem. Vol. 73, No. 24, 2008 9573

Aldhoun et al.
(2S)-2-tert-Butoxycarbonylamino-3-[1-(2′,3′,5′-tri-O-benzyl-ꢀ-D-
ribofuranosylmethyl)-1H-tetrazol-5-yloxy]propionic Acid Methyl
Ester (5d). Column chromatography with 2:1 cyclohexane-AcOEt
afforded 5d (86 mg, 61%) as a white foam. [R]D ) -38.5 (c 1.2,
CHCl3). 1H NMR (400 MHz): δ ) 7.50-7.10 (m, 15 H, Ph), 5.83
(d, 1 H, J2,NH ) 8.5 Hz, NH), 4.89 (dd, 1 H, J2,3a ) 2.8 Hz, J3a,3b
) 10.3 Hz, H-3a), 4.62 (ddd, 1 H, J2,3b ) 3.4 Hz, H-2), 4.95 and
4.55 (2 d, 2 H, J ) 11.5 Hz, PhCH2), 4.54 (dd, 1 H, H-3b), 4.45
(s, 2 H, PhCH2), 4.38 and 4.20 (2 d, 2 H, J ) 12.0 Hz, PhCH2),
4.33-4.27 (m, 3 H, H-1′, 2H-1”), 4.16 (ddd, 1 H, J3′,4′ ) 2.2 Hz,
J4′,5′a ) 3.8 Hz, J4′,5′b ) 3.3 Hz, H-4′), 3.84 (dd, 1 H, J1′,2′ ) 8.0
Hz, J2′,3′ ) 5.0 Hz, H-2′), 3.75 (s, 3 H, CH3), 3.63 (dd, 1 H, H-3′),
3.17 (dd, 1 H, J5′a,5′b ) 10.2 Hz, H-5′a), 3.13 (dd, 1 H, H-5′b),
1.48 (s, 9 H, t-Bu). 13C NMR: δ ) 169.6, 161.6, 155.3, 137.5 (3C),
128.5-127.6 (15C), 82.3, 80.4, 78.1, 78.0, 77.2, 73.3, 72.8, 71.8,
solid. [R]D ) 15.8 (c 1.2, CHCl3). 1H NMR: δ ) 7.80-7.73 (m, 2
H, Ar), 7.63-7.55 (m, 2 H, Ar), 7.45-7.35 (m, 2 H, Ar), 7.35-7.27
(m, 2 H, ar), 6.39 (d, 1 H, J2,NH ) 6.5 Hz, NH), 5.40 (dd, 1 H, J3′,4′
) 3.0 Hz, J4′,5′ ) 0.5 Hz, H-4′), 5.15 (dd, 1 H, J1′,2′ ) 9.5 Hz, J2′,3′
) 9.8 Hz, H-2′), 5.05 (dd, 1 H, H-3′), 4.81-4.70 (m, 1 H, H-2),
4.50-4.26 (m, 4 H, 2H-1′′, FmocCH2), 4.22 (t, 1 H, J ) 7.0 Hz,
FmocCH), 4.04 (dd, 1 H, J5′,6′a ) 6.5 Hz, J6′a,6′b ) 11.0 Hz, H-6′a),
3.98-3.80 (m, 3 H, H-3a, H-1′, H-6′b), 3.79-3.61 (m, 2 H, H-3b,
H-5′), 2.10, 2.00, and 1.90 (3s, 12 H, CH3). 13C NMR: δ ) 171.4,
171.0, 170.1, 170.0, 169.9, 156.2, 155.4, 143.6 (2c), 141.3 (2C),
127.8 (2C), 127.1 (2C), 125.1 (2C), 120.0 (2C), 76.2, 74.2, 71.4,
67.4, 67.2, 67.1, 61.3, 53.8, 48.7, 47.0, 35.7, 20.7, 20.6, 20.5 (2C).
MALDI-TOF MS: 801.6 (M+ + K). Anal. Calcd for C34H37N5O13S
(755.21): C, 54.03; N, 9.27; H, 4.93; S, 4.24. Found: C, 54.03; N,
9.25; H, 4.91; S, 4.24.
69.3, 53.0, 52.9, 45.6, 28.3 (4C). MALDI-TOF MS: 726.4 (M+
+
(2R)-2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-[1-(2′,3′,5′-tri-
O-acetyl-r-D-ribofuranosylmethyl)-1H-tetrazol-5-ylsulfanyl)propi-
onic Acid (6c). Column chromatography with 19:1 AcOEt-AcOH
afforded 6c (155 mg, 91%) as a white amorphous solid. [R]D ) 36.8
Na). Anal. Calcd for C37H45N5O9 (703.32): C, 63.14; N, 9.95; H,
6.44. Found: C, 63.13; N, 9.94; H, 6.43.
(2S)-2-tert-Butoxycarbonylamino-3-[1-(ꢀ-D-galactopyranosylm-
ethyl)-1H-tetrazol-5-yloxy]propionic Acid Methyl Ester (5b′). A
vigorously stirred mixture of 5b (165 mg, 0.20 mmol), 20%
palladium hydroxide on carbon (80 mg), and MeOH (4 mL) was
degassed under vacuum and saturated with hydrogen (by an H2-
filled balloon) three times. The suspension was stirred at room
temperature for 4 h under a slightly positive pressure of hydrogen
(balloon) and then filtered off through a plug of cotton and washed
thoroughly with MeOH (5 mL) and water (2 mL). The combined
filtrates were concentrated to afford crude 5b′ (88 mg, 95%) at
least 90% pure as established by 1H NMR analysis. 1H NMR
(CD3OD): δ ) 4.83-4.75 (m, 2 H, 2H-3), 4.72-4.65 (m, 1 H,
H-2), 4.51 (dd, 1 H, J1′,1′′a ) 3.5 Hz, J1′′a,1′′b ) 14.0 Hz, H-1′′a),
4.31 (dd, 1 H, J1′,1′′b ) 7.5 Hz, H-1”b), 3.85 (dd, 1 H, J3′,4′ ) 3.5
Hz, J4′,5′ ) 0.5 Hz, H-4′), 3.70-3.40 (m, 6 H, H-1′, H-2′, H-3′,
H-5′, 2H-6′), 3.77 (s, 3 H, CH3), 1.45 (s, 9 H, t-Bu). 13C NMR
(CD3OD): δ ) 169.6, 161.1, 155.8, 78.8, 77.5, 74.7, 71.9, 69.2,
68.9, 61.1, 53.1, 51.9, 47.1, 27.4 (4C). MALDI-TOF MS: 464.8
(M+ + H), 486.1 (M+ + Na).
General Procedure for the Synthesis of Glycosylmethyl Tetra-
zole Cysteines 6 (Table 2). To a stirred mixture of glycosylmethyl
tetrazole 4 (142 mg, 0.25 mmol), K2CO3 (103 mg, 0.75 mmol),
and anhydrous MeCN (2.5 mL) was added cysteine 13 (45 mg,
0.13 mmol) in one portion. The resulting mixture was stirred at
room temperature for 2 h, and then a second portion of cysteine
13 (45 mg, 0.13 mmol) was added. The mixture was stirred for an
additional 3 h and then concentrated. The resulting residue was
eluted from a column of silica gel with the suitable elution system
to give the corresponding glycosylmethyl tetrazole cysteine 6.
(2R)-2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-[1-(2′,3′,4′,6′-
tetra-O-acetyl-r-D-galactopyranosylmethyl)-1H-tetrazol-5-ylsulfa-
nyl)propionic Acid (6a). Column chromatography with 32.3:1
AcOEt-AcOH afforded 6a (174 mg, 92%) as a white amorphous
solid. [R]D ) 47.7 (c 0.6, CHCl3). 1H NMR (400 MHz): δ )
7.79-7.73 (m, 2 H, Ar), 7.64-7.55 (m, 2 H, Ar), 7.44-7.36 (m,
2 H, Ar), 7.35-7.28 (m, 2 H, Ar), 6.44 (d, 1 H, J2,NH ) 6.4 Hz,
NH), 5.45 (dd, 1 H, J ) 3.0 Hz, J ) 3.5 Hz, H-4′), 5.34-5.21 (m,
2 H, H-2′, H-3′), 4.82-4.68 (m, 1 H, H-2), 4.66-4.57 (m, 1 H,
H-1′), 4.57-4.43 (m, 2 H, 2H-1′′), 4.43-4.27 (m, 4 H, FmocCH2,
2H-6′), 4.27-4.19 (m, 1 H, FmocCH), 3.97-3.77 (m, 3 H, 2H-3,
H-5′), 2.15, 2.09, and 1.97 (3s, 12 H, CH3). 13C NMR: δ ) 171.3,
169.7 (2C), 169.6 (2C), 156.3, 154.1, 143.6 (2C), 141.3 (2C), 127.8
(2C), 127.1 (2C), 125.2 (2C), 120.0 (2C), 70.5, 68.6, 67.8, 67.5,
67.3, 66.3, 59.9, 47.0, 45.8, 36.0, 20.7 (2C), 20.6 (2C). MALDI-
TOF MS: 778.4 (M+ + Na). Anal. Calcd for C34H37N5O13S
(755.21): C, 54.03; N, 9.27; H, 4.93; S, 4.24. Found: C, 54.01; N,
9.26; H, 4.94; S, 4.23.
1
(c 1.1, CHCl3). H NMR: δ ) 7.74-7.60 (m, 2 H, Ar), 7.56-7.40
(m, 2 H, Ar), 7.37-7.20 (m, 2 H, Ar), 7.29-7.10 (m, 2 H, Ar), 6.42
(d, 1 H, J2,NH ) 7.5 Hz, NH), 5.49 (dd, 1 H, J1′,2′ ) 5.0 Hz, J2′,3′
)
5.5 Hz, H-2′), 5.27 (dd, 1 H, J3′,4′ ) 5.5 Hz, H-3′), 4.74 (ddd, 1 H,
J2,3a ) 3.5 Hz, J2,3b ) 6.5 Hz, H-2), 4.59 (ddd, 1 H, J1′,1”a ) 6.0 Hz,
J1′,1”b ) 6.0 Hz, H-1′), 4.46 (d, 2 H, 2H-1′′), 4.42-4.28 (3 H, H-4′,
FmocCH2), 4.22-4.12 (m, 2 H, H-5′a, FmocCH), 4.07 (dd, 1 H, J4′,5′a
) 4.5 Hz, J5′a,5′b ) 8.5 Hz, H-5′b), 3.93 (dd, 1 H, J3a,3b ) 14.5 Hz,
H-3a), 3.76 (dd, 1 H, H-3b), 2.11, 2.05, and 2.01 (3s, 9 H, CH3). 13
C
NMR: δ ) 171.8, 170.8, 169.6, 169.5, 156.2, 154.6, 143.6, 143.5,
141.2, 141.1, 127.7 (2C), 127.1 (2C), 125.1 (2C), 120.0 (2C), 78.5,
77.2, 71.9, 71.1, 67.3, 63.2, 54.0, 47.3, 46.9, 35.8, 20.7, 20.5 (2C).
MALDI-TOF MS: 684.6 (M+ + H). Anal. Calcd for C31H33N5O11S
(683.19): C, 54.46; N, 10.24; H, 4.87; S, 4.69. Found: C, 54.48; N,
10.23; H, 4.81; S, 4.64.
(2R)-2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-[1-(2′,3′,5′-
tri-O-acetyl-ꢀ-D-ribofuranosylmethyl)-1H-tetrazol-5-ylsulfanyl)pro-
pionic Acid (6d). Column chromatography with 24:1 AcOEt-AcOH
afforded 6d (154 mg, 90%) as a white foam. [R]D ) -17.4 (c 1.2,
CHCl3). 1H NMR (400 MHz): δ ) 7.77-7.68 (m, 2 H, Ar),
7.65-7.55 (m, 2 H, Ar), 7.41-7.35 (m, 2 H, Ar), 7.30-7.25 (m,
2 H, Ar), 6.55 (d, 1 H, J2,NH ) 7.5 Hz, NH), 5.05 (dd, 1 H, J2′,3′
)
5.5 Hz, J3′,4′ ) 4.0 Hz, H-3′), 4.96 (dd, 1 H, J1′,2′ ) 7.5 Hz, H-2′),
4.73 (ddd, 1 H, J2,3a ) 3.5 Hz, J2,3b ) 7.0 Hz, H-2), 4.55 (dd, 1 H,
J1′,1′′a ) 4.0 Hz, J1′′a,1′′b ) 14.5 Hz, H-1”a), 4.50 (dd, 1 H, J1′,1”b
)
4.5 Hz, H-1′′b), 4.40-4.30 (m, 3 H, H-1′, FmocCH2), 4.25-4.10
(m, 3 H, H-4′, H-5′a, FmocCH), 4.05 (dd, 1 H, J4′,5′b ) 3.5 Hz,
J5′a,5′b ) 12.0 Hz, H-5′b), 3.83 (dd, 1 H, J3a,3b ) 14.5 Hz, H-3),
3.73 (dd, 1 H, H-3b), 1.99, 2.06, and 2.07 (3s, 9 H, CH3). 13C NMR:
δ ) 171.9, 170.9, 169.8 (2C), 156.3, 155.4, 143.6 (2C), 141.2 (2C),
127.7 (2C), 127.1 (2C), 125.2 (2C), 119.9 (2C), 80.6, 78.2, 71.4,
67.5, 63.2, 53.9, 48.1, 47.0, 35.6, 20.7, 20.5, 20.4. MALDI-TOF
MS: 706.4 (M+ + Na). Anal. Calcd for C31H33N5O11S (683.19):
C, 54.46; N, 10.24; H, 4.87; S, 4.69. Found: C, 54.44; N, 10.21;
H, 4.87; S, 4.63.
Dipetide (15). To a cooled (0 °C), stirred solution of crude acid
14 (81 mg, ∼0.10 mmol), L-phenylalanine ethyl ester hydrochloride
(34 mg, 0.15 mmol), and (benzotriazol-1-yloxy)tripyrrolidinophos-
phonium hexafluorophosphate (62 mg, 0.12 mmol) in anhydrous
CH2Cl2 (2.0 mL) was added N,N-diisopropylethylamine (52 µL,
0.30 mmol). The solution was warmed to room temperature, stirred
for an additional 2 h, and then concentrated. The residue was
suspended with AcOEt (80 mL) and washed with H2O (2 × 10
mL). The organic phase was dried (Na2SO4), concentrated, and
eluted from a column of silica gel with 1:1 cyclohexane-AcOEt
to give dipeptide 15 (79 mg, 80%) as a white foam. [R]D ) 5.5 (c
1
(2R)-2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-[1-(2′,3′,4′,6′-
tetra-O-acetyl-ꢀ-D-galactopyranosylmethyl)-1H-tetrazol-5-ylsulfa-
nyl)propionic Acid (6b). Column chromatography with 24:1
AcOEt-AcOH afforded 6b (170 mg, 90%) as a white amorphous
1.0, CHCl3). H NMR (400 MHz) selected data: δ ) 7.40-7.00
(m, 25 H, Ar), 6.90 (bs, 1 H, NH), 5.60 (bs, 1 H, NH), 5.01 and
4.70 (2 d, 2 H, J ) 11.5 Hz, PhCH2), 4.88 and 4.72 (2 d, 2 H, J )
12.0 Hz, PhCH2), 4.42 and 4.34 (2 d, 2 H, J ) 11.8 Hz, PhCH2),
9574 J. Org. Chem. Vol. 73, No. 24, 2008

Synthesis of Tetrazole-Tethered C-Glycosyl R-Amino Acids
4.08 (q, 2 H, J ) 7.0 Hz, CH2CH3), 3.86 (dd, 1 H, J3,4 ) 3.5 Hz,
J4,5 ) 0.5 Hz, H-4sug), 3.81 (dd, 1 H, J1,2 ) 9.0 Hz, J2,3 ) 9.5 Hz,
H-2sug), 3.68 (ddd, 1 H, J1,1′a ) 1.5 Hz, J1,1′b ) 2.0 Hz, H-1sug),
3.62 (dd, 1 H, H-3sug), 3.52-3.46 (m, 3 H, H-5sug, 2H-6sug), 3.06
(dd, 1 H, J ) 6.0 Hz, J ) 14.5 Hz), 2.98 (dd, 1 H, J ) 7.0 Hz, J
) 14.5 Hz), 1.40 (s, 9 H, t-Bu), 1.18 (t, 3 H, J ) 7.0 Hz, CH2CH3).
MALDI-TOF MS: 985.8 (M+ + H). Anal. Calcd for C55H64N6O11
(984.46): C, 67.06; N, 8.53; H, 6.55. Found: C, 67.03; N, 8.52; H,
6.58.
Tripeptide (16). To a cooled (0 °C), stirred solution of dipeptide
15 (79 mg, 0.08 mmol) in CH2Cl2 (2.0 mL) was slowly added a
solution of TFA-CH2Cl2 (0.50 mL-1.50 mL). Stirring was
continued at 0 °C for an additional 30 min, and then the solution
was warmed to room temperature. After 30 min at room temper-
ature, the solution was neutralized at 0 °C with saturated aqueous
Na2CO3 and extracted with CH2Cl2 (2 × 50 mL). The combined
organic phases were dried (Na2SO4) and concentrated to give the
corresponding crude free amine (70 mg), which was used for the
following reaction without any purification.
To a cooled (0 °C), stirred solution of the above crude amine
(70 mg, ∼0.08 mmol), tert-butoxycarbonyl-L-alanine (22 mg, 0.12
mmol), and (benzotriazol-1-yloxy)tripyrrolidinophosphonium hexaflu-
orophosphate (73 mg, 0.14 mmol) in anhydrous CH2Cl2 (2.0 mL)
was added N,N-diisopropylethylamine (60 µL, 0.35 mmol). The
solution was warmed to room temperature, stirred for an additional
2 h, and then concentrated. The residue was suspended with AcOEt
(80 mL) and washed with saturated aqueous NaHCO3 (10 mL) and
brine (10 mL). The organic phase was dried (Na2SO4), concentrated,
andelutedfromacolumnofsilicagelwith1.5:1cyclohexane-AcOEt
to give tripeptide 16 (60 mg, 72% from 15). [R]D ) -14.8 (c 0.7,
1
CHCl3). H NMR (400 MHz) selected data: δ ) 7.40-7.00 (m,
27 H, Ar, 2 NH), 5.08 (bs, 1 H, NH), 5.00 and 4.70 (2 d, 2 H, J )
11.5 Hz, PhCH2), 4.80 and 4.56 (2 d, 2 H, J ) 11.0 Hz, PhCH2),
4.09 (q, 2 H, J ) 7.0 Hz, CH2CH3), 3.98 (dd, 1 H, J3,4 ) 3.5 Hz,
J4,5 ) 0.5 Hz, H-4sug), 3.80 (dd, 1 H, J1,2 ) 9.0 Hz, J2,3 ) 9.5 Hz,
H-2sug), 3.73-3.64 (m, 2 H, H-1sug, H-3sug), 3.52-3.40 (m, 3 H,
H-5s, 2H-6s), 3.10 (dd, 1 H, J ) 6.0 Hz, J ) 14.5 Hz), 3.00 (dd,
1 H, J ) 7.0 Hz, J ) 14.5 Hz), 1.40 (s, 9 H, t-Bu), 1.28 (d, 3 H,
J ) 7.0 Hz, CH3), 1.17 (t, 3 H, J ) 7.0 Hz, CH2CH3). MALDI-
TOF MS: 1056.9 (M+ + H). Anal. Calcd for C58H69N7O12
(1055.50): C, 65.95; N, 9.28; H, 6.58. Found: C, 65.93; N, 9.23;
H, 6.56.
Acknowledgment. We greatly acknowledge the University
of Ferrara for financial support.
Supporting Information Available: General experimental
methods and NMR spectra for new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO801670K
(32) (a) BeMiller, J. N.; Yadav, M. P.; Kalabokis, V. N.; Myers, R. W.
Carbohydr. Res. 1990, 200, 111–126. (b) Bhat, A. S.; Gervay-Hague, J. Org.
Lett. 2001, 3, 2081–2084.
J. Org. Chem. Vol. 73, No. 24, 2008 9575