Solid-phase synthesis of new S-glycoamino acid building blocks.

Article abstract of DOI:10.1021/ol006019o

Efficient synthesis of unprotected S-glycoamino acid building blocks in the solid phase by coupling a sugar 1-thiolate with iodine activated fluoren-9-ylmethoxycarbonyl (Fmoc) protected amino acids.

Full text of DOI:10.1021/ol006019o

ORGANIC
LETTERS
2000
Vol. 2, No. 15
2265-2267
Solid-Phase Synthesis of New
S-Glycoamino Acid Building Blocks
Laurence Jobron and Gerd Hummel*
Jerini Bio Tools GmbH, Rudower Chaussee 29, 12489 Berlin, Germany
hummel@jerini.de
Received May 5, 2000
ABSTRACT
Efficient synthesis of unprotected S-glycoamino acid building blocks in the solid phase by coupling a sugar 1-thiolate with iodine activated
fluoren-9-ylmethoxycarbonyl (Fmoc) protected amino acids.
Glycoconjugates have been implicated in many biological
events important in inflammation, immune response, and
tumor metastasis.1 Of special interest are glycoproteins
containing modified glycosyl amino acids, thus exhibiting
new properties. While S-glycopeptides have been isolated
from nature,2 the driving force behind the synthesis of
S-glycosides has been the production of glycopeptido-
mimetics with enhanced stability toward chemical and
enzymatic degradation.3 Several attempts have been made
to introduce the S-linkage by chemical synthesis.4 A variety
of glycosylation methods have been applied including
Koenigs-Knorr,5 glycosyl fluorides,6 Lewis acid-catalyzed
glycosylation,7 trichloroacetimidates8 and isothiouronium
salts.9 These methods generally use protected carbohydrates
and cysteine derivatives. We present here the solid-phase
synthesis of new S-glycoamino acid building blocks. The
key feature of this method is that a nucleophilic sugar
1-thiolate without protective groups is used for coupling with
an iodine activated fluoren-9-ylmethoxycarbonyl (Fmoc)/t-
Bu protected amino acid.
Hummel and Hindsgaul10 described the use of a sugar-1-
thiolate without protective groups in the solid phase as the
nucleophile for coupling with trifluoromethanesulfonate
(triflate)-activated glycosides. Since the use of triflates with
Fmoc-protected amino acids is not compatible, we decided
to prepare amino acid iodo derivatives for the synthesis of
S-glycoamino acid building blocks.
First the free acid functions of the N-Fmoc/t-Bu ester
protected glutamic and aspartic acid derivatives were reduced
to the corresponding alcohols 2, 4, 6, and 8 (Scheme 1,
(1) Varki, A. Glycobiology 1993, 3, 97. Lee, Y. C.; Lee, R. T. Acc. Chem.
Res. 1995, 28, 322. Chambers, W. H.; Brisette-Storkus, C. S. Chem. Biol.
1995, 2, 429.
(2) Lote, C. J.; Weiss, J. B. Biochem. J. 1971, 123, 25p. Lote, C. J.;
Weiss, J. B. FEBS Lett. 1971, 16, 81. Weiss, J. B.; Lote, C. J.; Bobinski,
H. Nature New Biol. 1971, 234, 25.
Scheme 1a
(3) Michael, K.; Wittmann, V.; Ko¨nig, W.; Sandow, J.; Kessler, H. Int.
J. Pept. Protein Res. 1996, 48, 59.
(4) Taylor, C. M. Tetrahedron 1998, 54, 11317.
(5) Baran, E.; Drabarek S. Pol. J. Chem. 1978, 52, 941. Gerz, M.; Matter,
H.; Kessler, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 269.
(6) Nicolaou, K. C.; Chucholowski, A.; Dolle, R. E.; Randall, J. L. J.
Chem. Soc., Chem. Commun. 1984, 1155.
(7) Salvador, L. A.; Elofsson, M.; Kilberg J. Tetrahedron 1995, 51, 5643.
(8) Ka¨sbeck, L.; Kessler, H. Liebigs Ann./Recueil 1997, 165.
(9) Monsigny M. L. P.; Delay, D.; Vaculik, M. Carbohydr. Res. 1977,
59, 589.
(10) Hummel, G.; Hindsgaul, O. Angew. Chem., Int. Ed. 1999, 38, 1782.
10.1021/ol006019o CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/28/2000

shown for Fmoc-Asp(OtBu)) using ethyl chloroformate and
sodium borohydride in tetrahydrofuran (yields are given in
Table 1).
reaction of the alcohols 10 and 13 with triphenylphosphine-
iodine complex in the presence of imidazole in anhydrous
dichloromethane gave the iodides 11 (with inversion of
configuration) and 14 in high yield (11 88% and 14 99%)
under very mild conditions (3 h reflux for Thr 11 and 10
min for Ser 14). No elimination product was observed with
Ser and only 7% in the case of Thr. Both iodo com-
pounds were used without further purification in the follow-
ing steps.
Table 1
reduction to
alcohol
conversion into
iodide
amino acids
Fmoc-Asp(OtBu)-OH
Fmoc-Asp-OtBu
Fmoc-Glu(OtBu)-OH
Fmoc-Glu-OtBu
2 (74%)
4 (81%)
6 (86%)
8 (83%)
3 (80%)
5 (81%)
7 (95%)
9 (76%)
Thiol 15 immobilized on a trityl chloride derivatized
polystyrene resin was obtained after reduction of the disul-
fide10 (Scheme 3). The loading of the thiol on the solid
Scheme 3a
Treatment of the different alcohols 2, 4, 6, and 8 with
triphenylphosphine-iodine-imidazole11 in toluene at 120 °C
afforded the corresponding iodo derivatives 3, 5, 7, and 9 in
76-95% yields after purification by flash chromatography
on silica gel.
The same procedure applied to Fmoc-Thr-OtBu 10 and
Fmoc-Ser-OtBu 1312 gave only low yields with threonine
and no product with serine (Scheme 2). In the case of
Scheme 2a
support was determined by elemental analysis (sulfur content)
and was 1.2 mmol/g. 15 was treated for 2 h with NaOMe/
THF to enhance its nucleophilicity10 and washed with a
mixture of MeOH/THF to eliminate traces of NaOMe. The
resulting sodium thiolate was coupled with amino acid iodo
derivatives 3, 5, 7, 9, and 14 in THF at 45 °C in the presence
of [15]crown-5 as complexing agent. After 3 days, the resin
was washed and the glyco amino acids were cleaved from
the resin by treatment with 50% TFA in CH2Cl2. All final
derivatives 16-20 were obtained after reverse-phase HPLC
purification in good yields (75 to 82%).
threonine, we observed the formation of elimination product
12 and iodo compound 11 in a ratio of 1:1. No better results
could be obtained by decreasing reaction time or temperature;
therefore we tried different conditions. The use of polystyryl
diphenylphosphine-iodine (PDPI) complex was previously
described13 for the conversion of N-Fmoc â-amino alcohols
into their corresponding iodides in high yields and without
any detectable epimerization of the chiral center. Similar
Reaction of the immobilized thiolate with threonine
derivative 11 however gave only poor yields (<5%).
(11) Garegg, P. J.; Samuelsson, B. J. Chem. Soc., Chem. Commun. 1979
979.
(12) Liebe, B.; Kunz, H.. Angew. Chem., Int. Ed. Engl. 1997, 36,
618.
(13) Caputo, R.; Cassano E.; Longobardo, L.; Palumbo, G. Tetrahedron
1995, 51, 12337.
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Org. Lett., Vol. 2, No. 15, 2000

To achieve better results we studied the influence of
different conditions such as solvent (THF or DMF), tem-
perature (20 or 45 °C), and complexing agent ([15]crown-5
or Kryptofix 221). The results are summarized in Table 2.
Scheme 4a
Table 2
temperature
20 °C
solvent
THF
complexing agent
yield, %
[15]crown-5
Kryptofix 221
[15]crown-5
Kryptofix 221
[15]crown-5
Kryptofix 221
[15]crown-5
Kryptofix 221
<20
20 °C
45 °C
45 °C
DMF
THF
DMF
30
of S-glycopeptides using the Fmoc strategy. All glycosides
were obtained stereoselectively and in high yields. Using
iodine-activated amino acids gives the corresponding S-
glycosides under very mild conditions with retention of
configuration (double inversion) if the iodine is part of an
asymmetric carbon atom.
<5
50
<10
Best yields were obtained using DMF at 45 °C in the
presence of [15]crown-5 as complexing reagent (Scheme 4).
We observe that compound 21 reverts back to its threo
configuration after the substitution.
In conclusion, we present here an efficient synthesis of
new S-glycoamino acid building blocks in the solid phase
which are ideal building blocks for the solid-phase synthesis
Supporting Information Available: Experimental pro-
cedures for preparation of compounds 2-9, 11, 14, and 16-
21 and analytical data for 2-9, 11, 14, and 16-21. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL006019O
Org. Lett., Vol. 2, No. 15, 2000
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