First synthesis of a C-glycoside anologue of a tumor-associated carbohydrate antigen employing samarium diiodide promoted C-glycosylation

Article abstract of DOI:10.1039/a801196f

A hydrolytically stable analogue of the Tn antigen has been efficiently synthesized for the first time employing a SmI₂-promoted coupling of the pyridyl sulfone of N-acetylgalactosamine with an aldehydoamino acid derivative.

Full text of DOI:10.1039/a801196f

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First synthesis of a C-glycoside anologue of a tumor-associated carbohydrate
antigen employing samarium diiodide promoted C-glycosylation
Dominique Urban,a Troels Skrydstrup*a,b† and Jean-Marie Beau*a
a Universite´ Paris-Sud, Laboratoire de Synthe`se de Biomole´cules, URA CNRS 462, Institut de Chimie Mole´culaire, F-91405 Orsay
Ce´dex, France
b Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark
A hydrolytically stable analogue of the Tn antigen has been
efficiently synthesized for the first time employing a SmI2-
promoted coupling of the pyridyl sulfone of N-acetylgalacto-
samine with an aldehydoamino acid derivative.
required C-glycosidic amino acid after a subsequent deoxy-
genation step (Scheme 1). Preliminary results in this direction
showed that the carbon chain of the aldehyde precursor was of
ideal length for the coupled product to undergo in situ
lactonization upon addition of the C1-glycosyl anion to the
aldehyde CNO bond, thus complicating the following deoxy-
genation.8f To prevent this event, we decided to employ the
cyclic carbamate derivates of the type 2, the synthesis of which
is outlined in Scheme 2.
l-Aspartic acid was easily converted to alcohol 4 in four steps
adopting the protocol described by McKillop for the conversion
of amino acids to their corresponding cyclic carbamates.9
Hence, esterification in acidic MeOH and treatment with
ClCO2Et led to the dimethyl ester 3. Subsequent reduction to the
diol and cyclisation upon treatment with 2 equiv. of NaH
afforded the cyclic carbamate 4 in an overall yield of 81%.
Bis(silylation) was accomplished upon treatment of 4 with
TBDMSOTf in collidine, after which the primary alcohol could
be selectively liberated with HF in MeCN.10 Oxidation
employing the Swern conditions then gave crystalline aldehyde
2 in high yield.
The key coupling step, outlined in Scheme 3, was performed
by adding a THF solution of SmI2 (2 equiv.) to the previously
described glycosyl pyridyl sulfone 58c,f and aldehyde (1.2
equiv.) at room temperature. An immediate reaction ensued, as
monitored by the instantaneous consumption of the one-
electron reducing agent, leading after work up to the isolation of
the desired C-glycosides in high yield (82%) in favor of the
a-anomers 6‡ (a:b, 3.3:1). We note again the success of this
anionic C-glycosylation reaction even though pyridyl sulfone 5
possesses an acidic NH proton. It was necessary to install a
protecting group at the carbamate nitrogen in 2 for the coupling
reaction to succeed. In its absence (e.g. replacing the TBDMS
group by H), protonation at C1 of the sugar unit was the sole
product observed.§
The Tn (GalNAca1?O-Ser/Thr) and sialyl Tn (NeuAca-
2?6GalNAca1?O-Ser/Thr) epitopes are major tumor-asso-
ciated O-linked glycopeptide motifs of cell surface glycopro-
teins which are expressed in over 70% of human epithelial
cancers such as lung, colon, stomach and breast carcinomas.1 In
normal cells, however, the Tn antigen is cryptic, where it is
further glycosylated giving rise to complex carbohydrates of the
mucin-type glycoproteins.2 This antigen has also been identi-
fied as a partial structure of the HIV envelope glycoprotein
gp120.3 We are particularly interested in preparing analogues of
these epitopes as molecular components of potential small
molecular weight synthetic vaccines,4 with high immunoge-
nicity and in vivo stability against various carcinoma. A
C-glycoside analogue in which the interglycosidic oxygen is
replaced with a methylene group5 may well conform to such
desired properties. Here we present for the first time the
synthesis of one such mimic of these tumor-associated antigens,
namely that of O-(2-acetamido-2-deoxy-a-d-galactopyrano-
syl)-l-serine (Scheme 1).
Several recent syntheses of C-glycosidic amino acids have
now appeared in the literature, although only one has been
directly applied to 2-hexosamine derivatives. In this approach,
Kessler reported the stereoselective synthesis of a C-glycoside
mimic of N4-(2-acetamido-2-deoxy-b-d-glucopyranosyl)-
l-asparagine, an important constituent of N-glycopeptides, via
the coupling of a glycosyl dianion with a modified asparatic
acid derivative as the key step.6,7
We have recently shown that reductive samariation of
glycosyl pyridyl sulfones in the presence of carbonyl substrates
leads to a viable and rapid route to C-glycosides.8 In particular,
we have demonstrated that the anomeric organosamarium of
N-acetylgalactosamine affords predominantly a-C-glycosides
upon condensation with simple aldehydes and ketones.8c,f We
hypothesized that with a suitable aldehyde precursor of an
amino acid derivative, its coupling with 1 could lead to the
Whereas treatment of 6 (major isomer) with thiocarbonyl
diimidazole¶ surprisingly led to the formation of a cyclic
oxazoline involving the C2 acetamido group, use of the
traditional Barton deoxygenation method proved effective for
the removal of the newly created C7 hydroxy group. Thus
O
NHCO2Et
OMe
OBn
O
BnO
BnO
i, ii
L-Aspartic acid
MeO
OH
HO
HO
O
3
AcHN
O
Sml2
iii, iv
1
O
O
AcHN
O
O
O
+
TBDMS
O
X
v–vii
O
HN
H
N
H
N
O
OR3
N
H
HO
R1R2N
H
2
4
O
Scheme 2 Reagents and conditions: i, AcCl, MeOH; ii, ClCO2Et, NaHCO3,
93% (2 steps); iii, NaBH4, CaCl2, EtOH–THF (2:1); iv, NaH, THF, 87% (2
steps); v, TBDMSOTf, collidine; vi, HF, MeCN, 71% (2 steps); vii,
(COCl)2, Et3N, DMSO, 99%
Tn antigen (GalNAcα1→O-Ser) X = O
C-Glycoside analogue X = CH2
Scheme 1
Chem. Commun., 1998
955

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OBn
O
OBn
O
(α:β =3.3:1)
BnO
BnO
BnO
BnO
¶ We have recently applied this reagent successfully for the deoxygenation
of a C-disaccharide prepared via the SmI2 route [see ref. 8(b)].
i
∑ It is interesting to note that C-glycoside 8 does not occupy the normally
expected 4C1 chair conformation, as is seen from the coupling constants
(J2,3 = 4.0, J3,4 = 2.9, J4,5 = 5.6 Hz). This has also been observed for other
N-acetyl-a-C-galactosamines (ref. 12) and monosaccharides (ref. 13)
derivatives possessing benzyl protecting groups at the C3, C4 and C6
hydroxy groups. However, the corresponding deprotected or peracetylated
C-glycosides were found to possess the normal chair conformation.
AcHN
AcHN
HO
(4:1)
O
SO2Py
5
TBDMSN
O
6
ii–iv
1 G. F. Springer, Science, 1984, 224, 1198; S. Sell, Hum. Pathol., 1990,
21, 1003; S. H. Itzkowitz, M. Yuan, C. K. Montgomery, J. Kjeldsen,
H. K. Takahashi, W. L. Bigbee and Y. S. Kim, Cancer Res., 1989, 49,
197; S. Hakomori, Curr. Opin. Immunol., 1991, 3, 646.
2 K. L. Carraway and S. R. Hull, Glycobiology, 1991, 1, 131;
A. A. Gooley, A. Pisano and K. L. Williams, Trends Glycosci.
Glycotechnol., 1994, 6, 328.
OBn
O
BnO
BnO
OBn
O
BnO
BnO
AcHN
v–vii
AcHN
3 J.-E. S. Hansen, H. Clausen, C. Nielsen, L. S. Teglbjaerg, L. H. Hansen,
C. M. Nielsen, E. Dabelsteen, L. Mathiesen, S. Hakomori and
J. O. Nielsen, J. Virol., 1990, 64, 2833.
4 For previous work on the use of the Tn antigen as a potential vaccine
against epithelial cancers, see: G. D. McLean and B. M. Longenecker,
Can. J. Oncol., 1994, 4, 249; T. Toyokuni, B. Dean, S. Cai, D. Boisin,
S. Hakamori and A. K. Singhal, J. Am. Chem. Soc., 1994, 116, 395;
T. Toyokuni and A. K. Singhal, Chem. Soc. Rev., 1995, 24, 231;
B. Liebe and H. Kunz, Angew. Chem., Int. Ed. Engl., 1997, 36, 618 and
references cited therein.
5 M. H. D. Postema, C-Glycoside Synthesis, CRC Press, Boca Raton, FL,
1995; D. E. Levy and C. Tang, The Chemistry of C-Glycosides,
Pergamon, Exeter, 1995; G. Casiraghi, F. Zanardi, G. Rassu and
P. Spanu, Chem. Rev., 1995, 95, 1677; for a review on nucleophilic
C-glycosyl donors, see J.-M. Beau and T. Gallagher, Top. Curr. Chem.,
1997, 187, 1.
6 F. Burkhart, M. Hoffmann and H. Kessler, Angew. Chem., Int. Ed. Engl.,
1997, 36, 1191 and references cited therein.
7 For other syntheses of C-glycosyl amino acids not discussed in ref. 6,
see L. Lay, M. Meldal, F. Nicotra, L. Panza and G. Russo, Chem.
Commun., 1997, 1469; S. D. Debenham, J. S. Debenham, M. J. Burk and
E. J. Toone, J. Am. Chem. Soc., 1997, 119, 9897.
8 (a) D. Maze´as, T. Skrydstrup and J.-M. Beau, Angew. Chem., Int. Ed.
Engl., 1995, 34, 909; (b) O. Jarreton, T. Skrydstrup and J.-M. Beau,
Chem. Commun., 1996, 1661; (c) D. Urban, T. Skrydstrup, C. Riche,
A. Chiaroni and J.-M. Beau, Chem. Commun., 1996, 1883; (d)
O. Jarreton, T. Skrydstrup and J.-M. Beau, Tetrahedron Lett., 1997, 36,
303; (e) T. Skrydstrup, O. Jarreton, D. Maze´as, D. Urban and
J.-M. Beau, Chem. Eur. J., 1998, 4, 655; (f) D. Urban, T. Skrydstrup and
J.-M. Beau, J. Org. Chem., in the press.
O
HN
O
BocHN
O
OH
8
7
Scheme 3 Reagents and conditions: i, 2 (1.2 equiv.), SmI2 (2.2 equiv.) THF,
20 °C, 82%; ii, NaH, CS2, MeI, THF, 83%; iii, HSnBu3 (1.5 equiv.), AIBN
(0.05 equiv.), toluene, 110 °C; iv, TBAF (3.5 equiv.), THF, 95% (2 steps);
v, (Boc)2O (1.5 equiv.), Et3N (2.0 equiv.), DMAP (cat.), THF; vi, Cs2CO3
(cat.), MeOH, 83% (2 steps); vii, Jones oxidation, 70%
formation of the methyl xanthate from alcohol 6, followed by
tin hydride promoted radical deoxygenation, afforded 7 after
desilylation in 79% (three steps). A three-step procedure
involving introduction of a Boc group, selective hydrolysis of
the cyclic carbamate11 and oxidation of the primary alcohol to
its corresponding carboxylic acid 8 then completed the
synthesis of the desired C-glycosyl amino acid.∑
In conclusion, we have successfully prepared a hydrolytically
stable Tn antigen mimic via a SmI2-promoted C-glycosylation
protocol. This important amino acid building block is now ready
to be incorporated into small peptide chains in order to test them
as potentially viable synthetic peptide vaccines against various
human epithelial cancers, the results of which will be reported
in due course.
Notes and References
9 N. Lewis, A. McKillop, R. J. K. Taylor and R. J. Watson, Synth.
Commun., 1995, 25, 561.
† E-mail: ts@kemi.aau.dk
‡ Selected data: for 6 dH(250 MHz, CDCl3) (major isomer) 6.18 (d, 1 H, J
8.4, NH), 4.37 (ddd, 1 H, J 9.4, 6.5, 2.9, H5), 4.18 (dd, 1 H, J 11.2, 9.4, H6a),
4.16 (ddd, 1 H, J 8.4, 3.1, 2.2, H2), 3.88 (dd, 1 H, J 3.1, 3.1, H3), 3.76 (dd,
1 H, J 6.5, 3.1, H4), 3.74 (dd, 1 H, J 3.5, 2.2, H1), 3.69 (dd, 1 H, J 11.2, 2.9,
10 D. E. Ward and B. F. Kaller, Tetrahedron Lett., 1993, 34, 407.
11 T. Ishizuka and T. Kunieda, Tetrahedron Lett., 1987, 28, 4185.
12 G. Rubinstenn, J. Esnault, J.-M. Mallet and P. Sinay¨, Tetrahedron:
Asymmetry, 1997, 8, 1327.
H
6b).
13 T. Skrydstrup, D. Maze´as, M. Elmouchir, G. Doisneau, C. Riche,
A. Chiaroni and J.-M. Beau, Chem. Eur. J., 1997, 8, 1342.
§ The protecting group on the carbamate nitrogen also plays a pivotal role
for obtaining high coupling yields of the C-glycoside as exemplified by the
use of the Boc group, where the C-glycosylation yield was reduced by one
half. Another advantage is that it provides easily separable isomers.
Received in Glasgow, UK, 11th February 1998; 8/01196F
956
Chem. Commun., 1998