Ring-opening of aziridine-2-carboxamides with carbohydrate C1-O-nucleophiles. Stereoselective preparation of α- and β-O-glycosyl serine conjugates

Article abstract of DOI:10.1021/ja804589j

The stereoselective formation of the α-GalNAc-Ser linkage via the ring opening of aziridine-2-carboxamides with pyranose C1-O-nucleophiles is described. The process is tolerant to the native C2-NHAc group, can be modulated to provide either the α- or β-glycoside through judicious choice of solvent and metal counterion, and is amenable to other classes of O-glycosyl-Ser constructs such as the β-GlcNAc-Ser and α-Man-Ser linkages. This coupling reaction also led to the development of the o-allylbenzyl (ABn) moiety as a new C-terminus carboxyl protective group, which allows for the use of novel methods for N- and C-terminus extension of amino acids following carbohydrate conjugation. Copyright

Full text of DOI:10.1021/ja804589j

Published on Web 10/25/2008
Ring-Opening of Aziridine-2-Carboxamides with Carbohydrate C1-O-Nucleophiles.
Stereoselective Preparation of r- and ꢀ-O-Glycosyl Serine Conjugates
Daniel A. Ryan and David Y. Gin*
Memorial Sloan-Kettering Cancer Center, 1275 York AVenue, New York, New York 10065
Received June 16, 2008; E-mail: gind@mskcc.org
Chart 1
O-Glycosylation of Ser/Thr residues within glycoproteins are
common post-translational modifications, often serving as key
determinants of biological function. R-D-GalNAc (1, Chart 1, found
in mucins, blood group determinants, tumor antigens, antifreeze
glycoproteins),1-3 ꢀ-D-GlcNAc (2, present in eukaryotic regulatory
proteins),4 and R-D-Man (3, identified in yeasts, molds, and
mammals)5 comprise some of the more common natural carbohy-
drate anchor residues. Of these, the R-D-GalNAc-Ser/Thr connection
(1) is the most difficult to construct by chemical glycosylation, as
it incorporates a pyranosyl C2-acetamido group in a 1,2-cis
configuration that directly opposes traditional neighboring group
participatory effects. Current approaches employ either electrophilic
glycosyl donors with nonparticipatory C2-aza groups, such as azide6
and oxazolidinone7 for R-glycosylation of Ser/Thr nucleophiles,
or O-conjugate addition to C2-nitro-glycals.8 These methods require
subsequent C2-N-functional group interchange to access an ap-
propriate glycosyl amino acid for use in solution or solid phase
peptide synthesis. Remarkably, the complementary coupling ap-
proach employing pyranose C1-O-hemiacetal nucleophiles with Ser-
derived electrophiles has not been reported. We describe herein
the stereoselective formation of the R-GalNAc-Ser, ꢀ-GlcNAc-Ser,
and R-Man-Ser linkages via the ring opening of aziridine-2-
carboxamides with pyranose C1-O-nucleophiles.
Table 1
Reports of regioselective ring-opening of aziridine-2-carboxylate
derivatives with oxygen nucleophiles are uncommon.9 Although
10,11
BF3 ·OEt2
and CuOAc/DBU12 have been shown to be effective
aziridinolysis promoters with alkyl and aryl alcohol nucleophiles, the
analogous process involving hemiacetal nucleophiles, characteristic of
carbohydrates, remains elusive. Initial attempts (Table 1, entries 1 and
2) at regioselective aziridinolysis of N-benzyl-1-p-nosyl-L-aziridine-
2-carboxamide (L-6) with 2-acetamido-2-deoxy-3,4,6-tri-O-benzyl-D-
glucopyranose (4) by applying these existing methods proved inef-
fective, requiring the development of an alternate coupling procedure.
It was found that the use of alkali metal salts of carbohydrate
hemiacetals provided rapid regio- and stereoselective aziridine
opening. For example (entries 3 and 4), treatment of a mixture of
the protected GlcNAc hemiacetal 4 and aziridine L-6, with either
KH or NaH in DMF, led to good yields of the glycoconjugate 7,
favoring the contrasteric R-anomer. Conversely, when THF solvent
was employed (entries 5 and 6), high ꢀ-anomeric selectivities in 7
were achieved with similar yields.13 Application of these coupling
conditions to 2-acetamido-2-deoxy-3,4,6-tri-O-benzyl-D-galacto-
pyranose (5, entries 7 and 8) with aziridine L-6 proceeded with
comparable efficiencies, obtaining either high R-selectivity (KH/
DMF) to directly afford the Tn-tumor antigen R-GalNAc-Ser
linkage (8r, entry 7), or high ꢀ-selectivity (NaH/THF) to furnish
its anomeric counterpart (8ꢀ, entry 8). Importantly, the analogous
couplings (KH/DMF or NaH/THF) of protected GalNAc hemiacetal
5 with the D-aziridine carboxamide D-6 (entries 9 and 10), led to
the R- and ꢀ-GalNAc-D-Ser diastereomers 9r and 9ꢀ, with high
anomeric selectivity. These latter examples not only indicate the
suitability of aziridine electrophiles of the D-configuration in this
reaction, but also implicitly verify the absence of amino acid
epimerization during the process. Taken together, these initial
experiments signal the versatility of anomeric stereocontrol in C1-
O-alkylation with judicious selection of solvent and metal
counterion,14-16 effects which have received limited attention for
the C2-NHAc class of carbohydrates17-20 and heretofore have
remained unexplored for the synthesis of natural glycopeptide
conjugates.
However, this coupling reaction of 4/5 with 6 was found to be
best applied to secondary amides of aziridine-2-carboxamides.21
Adhering to this constraint, a new secondary amide protective group
for carboxylic acids was developed to expand the versatility of this
coupling reaction. Consideration of an o-allylbenzyl derivative
(ABn) as a C-terminus amide protective group proved fruitful in
this regard, both in terms of its compatibility with the aziridine
opening process, as well as its ability to undergo multifaceted
functionalization.
In establishing the former criterion (Chart 2), the ABn-derivatized
p-Ns-aziridine-2-carboxamide (11, pNs-Azy-NHABn) was coupled
with various carbohydrate hemiacetals 10, using either optimized
conditions “A” (KH, 18-cr-6, DMF) or “B” (NaH, PBu3,22 1,4-
9
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10.1021/ja804589j CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Chart 2a
Scheme 1a
a Reagents and conditions: (a) Boc-Lys(CBz)-SPh, PhSH, K2CO3,
DMSO, CH3CN, 23f90 °C, 80% (19), 76% (20); (b) OsO4, (nBu4N)2IO4,
2,6-lutidine, acetone, H2O, 23 °C; TFA, CH3CN, 23 °C, 70% (from 15),
82% (from 17); (c) (py-H)2CeCl6, THF, H2O, 23 °C, 83% (21), 79% (22);
(d) H-Glu(Bn)-OBn, Et3N, (py-H)2CeCl6, CH2Cl2, 23 °C, 72% (23), 74%
(24).
to new building blocks for glycopeptide synthesis, with particular
attention to the challenging R-GalNAc-Ser motif.28
Acknowledgment. This research was supported by the NIH-
NIGMS (GM58833). A portion of this work was performed in the
Dept. of Chemistry, University of Illinois, Urbana, IL.
Supporting Information Available: Experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
a Reagents and conditions: (A) KH, 18-cr-6, DMF, 0 °C; (B) NaH, PBu3,
1,4-dioxane, 23 °C.
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The feasibility of aziridine 2-carboxamide ring-opening with
pyranose C1-O-nucleophiles has been established. The process
provides good levels of anomeric selectivity and is tolerant to the
native C2-NHAc group. This, in addition to novel methods for N-
and C-terminus extension of amino acids, should provide access
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