Synthesis of S-Linked Glycosyl Amino Acids in Aqueous Solution with Unprotected Carbohydrates

Article abstract of DOI:10.1021/ol006908b

(Equation Presented) The cyclic sulfamidate 5 was synthesized in 60% overall yield from L-serine benzyl ester. Compound 5 reacted cleanly with the sodium thiolate salt of a variety of unprotected 1-thio sugars in aqueous buffer to afford the corresponding

Full text of DOI:10.1021/ol006908b

ORGANIC
LETTERS
2001
Vol. 3, No. 3
405-407
Synthesis of S-Linked Glycosyl Amino
Acids in Aqueous Solution with
Unprotected Carbohydrates
Scott B. Cohen and Randall L. Halcomb*
Department of Chemistry and Biochemistry, UniVersity of Colorado,
Boulder, Colorado 80309-0215
alcomb@spot.colrado.edu
Received November 21, 2000
ABSTRACT
The cyclic sulfamidate 5 was synthesized in 60% overall yield from L-serine benzyl ester. Compound 5 reacted cleanly with the sodium thiolate
salt of a variety of unprotected 1-thio sugars in aqueous buffer to afford the corresponding S-linked amino acid glycoconjugates in good
yields after hydrolysis of the N-sulfates
O-Linked glycoproteins constitute a major class of glyco-
conjugates found in mammalian cells.1 The corresponding
S-linked glycoproteins are interesting synthesis targets as a
result of their enhanced chemical stability and enzymatic
resistance.2 A major goal of our research is to develop a
convergent approach for the synthesis of S-linked glycopro-
teins as stable analogues of O-linked wild types through the
chemoselective ligation of unprotected carbohydrates with
unprotected peptides in aqueous solution.3 As a first step
toward this goal, we chose to develop the principles for this
concept within the framework of more simple glycosyl amino
acids. This report describes the synthesis of a cyclic
sulfamidate which was derived from serine and its reaction
with a variety of unprotected 1-thio sugars.
doalanine (Scheme 1).4,5 A potential limitation in the use of
â-iodoalanyl derivatives as the electrophilic components is
Scheme 1
Recent syntheses of S-linked amino acid glycosides
utilized an anomeric thiolate nucleophile on a protected or
support-bound sugar in a reaction with a protected â-io-
the propensity of these compounds to undergo elimination
of HI, affording elimination product 1. Subsequent Michael
addition of the sulfur nucleophile to 1 results in a mixture
of diastereomers that differ in configuration at the R-carbon
(1) Taylor, C. M. Tetrahedron 1998, 54, 11317-11362.
(2) Horton, D.; Wander, J. D. Carbohydrates: Chemistry and Biochem-
istry, Vol. 4B; Pigman, W. W., Horton, D., Eds.; Academic Press: New
York, 1990; pp 799-842.
(3) Chemoselective liagtions: (a) Davis, N. J.; Flitsch, S. L. Tetrahedron
Lett. 1991, 32, 6739-6796. (b) Marcaurelle, L. A.; Rodriguez, E. C.;
Bertozzi, C. R. Tetrahedron Lett. 1998, 39, 8417-8420. (c) Rodriguez, E.
C.; Marcaurelle, L. A.; Bertozzi, C. R. J. Org. Chem. 1998, 63, 7134-
7135. (d) Marcaurelle, L. A.; Bertozzi, C. R. Chem. Eur. J. 1999, 5, 1384-
1390.
(4) S-Linked amino acid glycosides: (a) Jobron, L.; Hummel, G. Org.
Lett. 2000, 2, 2265-2268. (b) Ohnishi, Y.; Ichikawa, M.; Ichikawa, Y.
Bioorg. Med. Chem. Lett. 2000, 10, 1289-1291.
10.1021/ol006908b CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/10/2001

of the amino acid (Scheme 1). Under certain experimental
conditions, such a pathway was in fact observed.4b
hydrogenolysis of the benzyl ester of 4 to produce 5 was
quantitative. Protection of the primary amine of serine benzyl
ester, while retaining the sp3 character of the nitrogen, was
necessary to form the cyclic sulfamidate. Attempts to form
a cyclic sulfamidate directly from serine benzyl ester or
N-BOC-serine benzyl ester were unsuccessful.
We hypothesized that constraining the â leaving group
into a five-membered ring could reduce the rate of elimina-
tion due to poor orbital overlap between the developing
enolate and the leaving oxygen atom (this transition state
for elimination is the same as for a 5-endo-trig cyclization).6
Cyclic sulfamidate 5 (Scheme 2) was chosen as a suitable
Before the reactivity of 5 with sulfur nucleophiles was
examined, a control experiment was performed to evaluate
the stability of 5 toward hydrolysis in aqueous buffer and to
determine the eagerness of 5 to provide the elimination
product 6 (Scheme 3). Compound 5 was incubated in D2O
Scheme 2
Scheme 3
with sodium bicarbonate at pH 8 (23 °C), and its decomposi-
tion was followed by 1H NMR. Loss of 5 proceeded slowly
(k ) 0.034 h-1, t1/2 ) 20 h) to form a mixture of the
sulfamidate hydrolysis products 7a and 7b (Scheme 3). The
elimination product 6 was not observed, suggesting that under
these reaction conditions epimerization of the R-carbon did
not occur.
“â-alanyl” equivalent. Serine-derived cyclic sulfamidates
were previously utilized in the synthesis of unnatural amino
acids.7 In a nonpeptidic system, cyclic sulfamidates were
shown to react with a protected 1-thio sugar.8
Compound 5 was synthesized in five steps and 60% overall
yield from L-serine benzyl ester (Scheme 2). First, the amine
was protected by reductive amination with p-anisaldehyde.
Cyclization of 2 with thionyl chloride cleanly afforded the
corresponding cyclic sulfamidite, which was then oxidized
to the protected sulfamidate 3 with catalytic Ru(III) and
periodate.9 Removal of the PMB protecting group was
effected with ceric ammonium nitrate,10 and subsequent
A variety of 1-thio sugars were found to add rapidly to
the â-carbon of 5. Reaction of 5 with the sodium salt of
commercially available 1-thio-â-D-glucose (8) in aqueous
sodium bicarbonate proceeded with an initial half-life of less
than 10 min to give the N-sulfatyl glycoconjugate intermedi-
ate in 95% yield (Scheme 4). The sulfamidate was hydro-
lyzed cleanly with aqueous HCl to provide 11 in 90% yield
after purification by size exclusion chromatography (Biogel
P-2, H2O mobile phase). The same procedure using 1-thio-
N-acetyl-â-D-glucosamine (9)11 proceeded equally as well
to afford 12. Addition of 1-thio-R-D-glucose (10)12 to 5 also
afforded the corresponding N-sulfatyl adduct in 95%. After
hydrolysis of the N-sulfate and size-exclusion chromatog-
(5) Thio-sugars: (a) Lu, P.; Hindsgaul, O.; Li, H.; Palcic, M. M. Can.
J. Chem. 1997, 75, 790-800. (b) Witczk, Z. J.; Chhabra, R.; Chen, H.;
Xie, X. Carbohydr. Res. 1997, 301, 167-175. (c) Eisele, T.; Windmuller,
R.; Schmidt, R. R. Carbohydr. Res. 1998, 306, 81-91. (d) Fernandez, J.
M. G.; Mellet, C. O.; Defaye, J. J. Org. Chem. 1998, 63, 3572-3580. (e)
Auzanneau, F. I.; Bennis, K.; Fanton, E.; Prome, D.; Defaye, J.; Gelas, J.
J. Chem. Soc., Perkin Trans. 1 1998, 3629-3635. (f) Hummel, G.;
Hindsgaul, O. Angew. Chem., Int. Ed. 1999, 38, 1782-1784. (g) Johnston,
B. D.; Pinto, B. M. J. Org. Chem. 2000, 65, 4607-4617. (h) Xu, W.;
Springfield, S. A.; Koh, J. T. Carbohydr. Res. 2000, 325, 169-176.
(6) Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734-736.
(7) (a) Baldwin, J. E.; Spivey, A. C.; Schofield, C. J. Tetrahedron:
Asymmetry 1990, 1, 881-884. (b) Boulton, L. T.; Stock, H. T.; Raphy, J.;
Horwell, D. C. J. Chem. Soc., Perkin Trans. 1 1999, 1421-1429.
(8) Aguilera, B.; Fernandez-Mayoralas, A. J. Org. Chem. 1998, 63,
2719-2723.
1
raphy to afford 13, H NMR indicated the presence of a
minor component (ca. 3%), which was tentatively determined
to be the product which was diastereomeric at the amino
(10) Yamaura, M.; Suzuki, T.; Hashimoto, H.; Yoshimura, J.; Okamoto,
T.; Shin, C. Bull. Chem. Soc. Jpn. 1985, 58, 1413-1420.
(11) Obtained by deacylating the peracetylated derivative with sodium
methoxide in methanol: Horton, D.; Wolfrom, M. L. J. Org. Chem. 1962,
27, 1794-1799.
(12) Obtained by deacylating the peracetylated derivative with sodium
methoxide in methanol: Gadelle, A.; DeFaye, J.; Pedersen, C. Carbohydr.
Res. 1990, 200, 497-498.
(9) Gao, Y.; Sharpless, K. B. J. Am. Chem. Soc. 1988, 110, 7538-7539.
406
Org. Lett., Vol. 3, No. 3, 2001

Scheme 4
acid R-carbon. We were unsuccessful in attempts to isolate
this component in pure form for unambiguous confirmation
of this assignment.
Foundation (Career Award), the Camille and Henry Dreyfus
Foundation (Camille Dreyfus Teacher-Scholar Award), Pfizer
(Junior Faculty Award), and Novartis (Young Investigator
Award) for support. This work was greatly facilitated by a
500 MHz NMR spectrometer that was purchased partly with
funds from an NSF Shared Instrumentation Grant (CHE-
9523034).
In summary, this report presents an efficient method for
synthesizing S-linked glycosyl amino acids using unprotected
carbohydrates in aqueous solution. An important feature of
this method is the reduced risk of elimination and subsequent
epimerization of the R-carbon by constraining the â leaving
group within a ring as a cyclic sulfamidate. Current efforts
in our laboratory focus on the incorporation of 5 into peptides
to generate reactive peptide substrates for ligations with
unprotected 1-thio sugars.
Supporting Information Available: Full experimental
1
procedures and tabulated H and 13C NMR and IR data for
all compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by the
NIH (GM55852). R.L.H. also thanks the National Science
OL006908B
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