Stereoselective synthesis of α- and β-glycosylamide derivatives from glycopyranosyl azides via isoxazoline intermediates

Article abstract of DOI:10.1021/ja028694u

Treatment of 2-acetoxy glycopyranosyl azides with Ph₃P gave isoxazolines by ring closure of the phosphorimine. Coupling of in situ generated isoxazolines with acylating reagents gave mixtures of α- or β-glycopyranosyl amides. The α/β ratio depe

Full text of DOI:10.1021/ja028694u

Published on Web 03/19/2003
Stereoselective Synthesis of r- and â-Glycosylamide Derivatives from
Glycopyranosyl Azides via Isoxazoline Intermediates
Fehmi Damkaci and Philip DeShong*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received September 25, 2002; E-mail: pd10@umail.umd.edu
Recently, interest in the synthesis of glycosyl amides has grown
because of the recognition of the importance of glycoproteins in
diverse biochemical processes including intercellular communica-
tion, cell recognition, and cell growth.1 In addition, glycopeptides
can improve the absorption of poorly bioavailable drugs and
peptides by enhancing membrane transport.2 Typical naturally
occurring glycoproteins have a â-N-linkage to an N-acetylglu-
cosamine (Figure 1).
Figure 1. â-N-linkage to an N-acetylglucosamine in naturally occurring
glycoproteins.
Recently, N-glucopyranosyl- and N-galactopyranosyl glycopro-
tein derivatives have been found in nature.3,4 Nephritogenoside
(Figure 2), isolated from the glomerular basement membrane of
rats, is a glycopeptide with activity for the induction of glomerul-
nephritis.4 This is the first example of a compound incorporating
an R-N-glycosidic bond between glucose and asparagine.4
Several syntheses of nephritogenoside have been reported.5 The
stereoselective preparation of the R-N-glycopeptide linkage has
proven to be a difficult task in these approaches. The key R-N-
glucosylamide linkage in these syntheses has been made most
frequently using Jeanloz’s methodology in which a glycosyl-R-
azide is reduced to anomeric amine and coupled with an aspartyl
containing dipeptide.5a-c While this approach gave the desired
glycodipeptide in good yield (65-80%), the stereoselectivity at the
anomeric center was poor: this method resulted in R/â mixtures
of glucosylamide product with ratios ranging from 1:5 to 5:1. The
extent of anomerization of the intermediate R-glucosylamine
depends on reducing conditions and protecting groups on the
glycosyl moiety.6
Figure 2. Structure of nephritogenoside.
Scheme 1. Synthesis of Glucopyranosyl Isoxazoline (3)
Scheme 2. Mechanism for the Formation of Isoxazoline 3
Anomerization of 1-amino glycopyranosyl derivatives is so
problematic that a number of alternative approaches to the synthesis
of R-glycopeptide linkages have been developed which avoid
intermediate formation of the free amine. For example, Fraser-
Reid5d,e has shown that condensation of n-pentenyl- or thio-
glycosides5f with aspartic acid in the presence of NBS (or NIS)
and acetonitrile affords R-N-linked glycopeptides. Alternatively,
reaction of R-glycosyl azide with a carboxylic acid in the presence
of tertiary phosphine, the Staudinger reaction,7a also affords R-N-
linked glycopeptides, although anomerization remains as a signifi-
cant problem.7b-d
The goal of this research was to develop a methodology for the
stereoselective synthesis of an R-N-glycopeptide linkage from
readily available 1-azidoglucopyranosyl derivatives 1 and 2. In this
approach, the stereochemistry of the intermediate isoxazoline 3
would control the stereochemistry at the newly formed anomeric
center.
Our lab has recently reported the stereospecific synthesis of R-
and â-glucopyranosyl (and glycosyl) azides from R- or â-glyco-
pyranosyl chlorides, respectively.8 Treatment of either â-azide 1
or R-azide 2 with Ph3P in refluxing 1,2-dichloroethane in the
presence of 4 Å molecular sieves for 15 h gave isoxazoline 3
(Scheme 1).
Formation of isoxazoline 3 from either azide can be explained
by the mechanism shown in Scheme 2 involving R/â anomerization
of the intermediate phosphorimines 4 and 5.7c,d Isoxazoline forma-
tion from 4 cannot occur due to strain in the resulting product.
Accordingly, epimerization followed by cyclization gave exclusively
R-isoxazoline 3.
Monitoring the reaction mixture by 1H NMR indicated that
isoxazoline 3 was the only glucosyl derivative observed in the NMR
spectrum following the disappearance of starting material. Because
the isoxazoline was relatively moisture sensitive, preliminary
coupling studies with activated acid derivatives were undertaken
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10.1021/ja028694u CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Coupling Reactions of Glucopyranosyl Isoxazoline 3a
Scheme 4. Coupling Reactions with Di- and Triglycosyl Azides
glycosyl azides as a starting material. Studies to extend this
methodology to the synthesis of nephritogenoside, 2-N-acetamido-
2-deoxy-â-glucosylamides, and to solid-phase synthesis are under-
way and will be reported in due course.
Acknowledgment. We thank the National Cancer Institute (CA-
82169) for generous financial support. We also thank Dr. Yiu-Fai
Lam (NMR) and Mr. Noel Whittaker (mass spectrometry) for their
assistance in obtaining analytical data. This manuscript is dedicated
to our colleague Robert Dorfman on the occasion of his 60th
birthday.
a All reactions were performed in 1,2-dichloroethane in the presence of
4 Å molecular sieves. All acylation reactions were run for 24 h. A listing
of all acylating agents and reaction conditions investigated is summarized
in the table in the Supporting Information. b Used in acylation step. Additives
were added after reagents. c For acylation step. d Isolated yield (>95%).
1
e Determined by H NMR spectroscopy or HPLC analysis.
Scheme 3. Synthesis of R-N-Aspartyl Glucosylamine 8
Supporting Information Available: Extended version of table,
experimental procedures, and spectral data of all new compounds (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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Acylation of isoxazoline 3 (formed in situ) with the reactive acid
chloride gave the R-adduct in a highly stereoselective process (entry
1). However, acid chlorides were inappropriate reagents for general
glycopeptide synthesis, and alternative derivatives were investi-
gated.9 Attempts to utilize N-hydroxysuccinimidyl (entry 2) or
pentafluorophenyl esters in the acylation protocol met with mixed
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thiopyridyl ester (entry 3), on the other hand, gave the R-adduct
with high selectivity. By adding metal salts9 to coordinate the
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temperature to give exclusiVely the R-glucopyranosyl adduct in
excellent yield (entry 4).
The generality of the thiopyridyl coupling methodology was
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derivative 8. In situ isoxazoline formation by treatment of azide 1
was followed by acylation with the thiopyridyl ester of N-Z-
protected aspartic benzyl ester in the presence of CuCl2‚2H2O to
give exclusively the R-asparagine adduct (Scheme 3).
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The thiopyridyl coupling reactions were extended using di- and
triglycosyl azides (9a and 9b) to demonstrate the generality of the
methodology (Scheme 4).
In conclusion, a stereoselective synthesis of R-N-glycopyranosyl
amides has been developed that employs the readily available
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(9) A listing of all acylating agents and reaction conditions investigated is
summarized in the table in the Supporting Information.
JA028694U
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