Synthesis of a new conformationally constrained glycoamino acid building block

Article abstract of DOI:10.1016/S0040-4039(03)01586-7

The synthesis of a suitably protected β-D-glucopyranosyl-(S)-α-methylserine derivative - a new conformationally constrained glycosylated quaternary amino acid analogue of β-D-glucopyranosyl-L-serine - is described. This compound can be used as an attractive building block for the synthesis of new, constrained glycopeptides.

Full text of DOI:10.1016/S0040-4039(03)01586-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6413–6416
Synthesis of a new conformationally constrained glycoamino
acid building block
Alberto Avenoza,* Jesu´s M. Peregrina* and Emilio San Mart´ın
Departamento de Qu´ımica, Universidad de La Rioja, Grupo de S´ıntesis Qu´ımica de La Rioja, U.A.-C.S.I.C., 26006 Logron˜o,
Spain
Received 2 June 2003; revised 24 June 2003; accepted 26 June 2003
Abstract—The synthesis of a suitably protected b-
D
-glucopyranosyl-(S)-a-methylserine derivative—a new conformationally
-glucopyranosyl- -serine—is described. This compound can be
constrained glycosylated quaternary amino acid analogue of b-
D
L
used as an attractive building block for the synthesis of new, constrained glycopeptides.
© 2003 Elsevier Ltd. All rights reserved.
The synthesis of a,a-disubstituted a-amino acids has
received enormous attention because it represents a
great advance in the design of modified peptides with
altered biological activity.1,2 For example, a-methylser-
ine can be regarded as a potential C-terminal a-helix-
stabilising building block.3
regarded as potential tools in the elucidation of the
conformational requirements for binding to receptors
and enzymes. In this sense, a number of glycoamino
acids that contain conformational constraints have
been synthesised and the majority of unnatural glycosyl-
ated amino acids reported are C-glycoside deriva-
tives,12–15 including C-glycosylated a-amino acids that
possess a quaternary center (Ca).16–19 Examples of
exclusively O-glycosylated quaternary a-amino acids
have not been reported to date.
On the other hand, a vast number of natural proteins
contain oligosaccharide side chains—glycoproteins—
and the saccharide residue is covalently linked to the
peptide backbone. In the majority of cases this linkage
is O-glycosidic through serine. It is well known that the
saccharide moiety plays an important role in biological
processes by contributing to the membrane permeabil-
ity and stabilising certain secondary conformations of
the peptide backbone.4–7 Nevertheless, while significant
research has been undertaken in the design of restricted
peptides, the field of conformationally constrained gly-
copeptides remains relatively unexplored, and most of
the restrictions studied are centered in the saccharide
moiety.8–11
Taking into account this observation and the fact that
various biologically active O-glycopeptides or O-glyco-
proteins incorporate the unusual b-
serine (b- -Glc- -Ser) substructure, we decided to
synthesise a suitably protected b- -glucopyranosyl-a-
methylserine derivative b- -Glc-(S)-a-MeSer (1) as a
conformationally restricted analogue of b- -Glc- -Ser
D-glucopyranosyl-L-
D
L
D
D
D
L
(2) (Fig. 1). The novel unnatural glycoamino acid 1 is
an attractive building block for incorporation into pep-
tides, since only a limited number of glycosylated
amino acid building blocks suitable for glycopeptide
synthesis are commercially available.
We have recently combined the aspects mentioned
above and focused our attention on the synthesis of
conformationally constrained glycosylated amino acids.
Such compounds are building blocks for the construc-
tion of constrained glycopeptides, which can be
Keywords: amino acids and derivatives; azides; glycosidation; peptide
analogues/mimetics.
* Corresponding authors. Tel.: +34-941-299655; fax: +34-941-299655;
e-mail: alberto.avenoza@dq.unirioja.es; jesusmanuel.peregrina@
dq.unirioja.es
Figure 1. Protected b-
[b- -Glc-(S)-MeSer] (1) and b-
-Glc- -Ser) (2).
D
-glucopyranosyl-(S)-a-methylserine
D
D
-glucopyranosyl- -serine (b-
L
D
L
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01586-7

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A. A6enoza et al. / Tetrahedron Letters 44 (2003) 6413–6416
OPfp 9 with the donors 2,3,4,6-tetra-O-benzoyl-a-
D-
Several examples of O-glycopeptides or O-glyco-
proteins that incorporate the b- -Glc- -Ser linkage are
present in the first epidermal growth factor-like
domains,20 certain synthetic enkephalin analogues21 and
the blood-clotting factors VII and IX (two plasma
glycoproteins that are involved in the blood coagula-
tion cascade).22–24 In these proteins the serine residue
glucopyranosyl bromide28 10 with AgOTf or 2,3,4,6-
D
L
tetra-O-acetyl-a-
D
-glucopyranosyl
trichloroacetimi-
date29 11 with BF3·Et2O failed (Scheme 1).
The glycosidation reaction with these classical
approaches has given good results in the case of ser-
ine.25 However, given the disappointing results obtained
here, we decided to change the methodology and start
from compound 15—a precursor of the a-methylserine
7. Compound 15 was obtained by a modified version of
the procedure cited above27 and required formation of
the benzyl ester instead of the methyl ester. Diol 12 was
prepared by Sharpless asymmetric dihydroxylation of 3
with AD-mix a and the product was hydrolysed with
HCl under reflux, treated with Cs2CO3 in MeOH and
esterified with BnBr to give compound 13. Diol 13 was
transformed into its 2,3-cyclic sulfite 14 with thionyl
chloride and the ring-opening reaction of this sulfite
with azide ions in dimethylformamide gave a mixture of
the azido esters 15 and 16. A regioselectiviy of 4.5:1 in
favour of compound 15 was obtained and this corre-
sponds to attack at the quaternary centre with clean
inversion of configuration30 (Scheme 2).
bears either a disaccharide a-
in factor VII or a trisaccharide a-
1,3-b- -Glc chain in factor IX. More specifically, factor
D
-Xyl-1,3-b-
D
-Glc chain
D
-Xyl-1,3-a-
D
-Xyl-
D
IX consists of a single chain of 416 amino acids and the
glycopeptide sequence containing the residues 51–56,
including the trisaccharide, has been synthesised.25
It was planned to use the suitably protected a-
methylserine 5 (N-Fmoc-MeSer-OMe) as the starting
material for the glycosidation reaction with the donor
2,3,4,6-tetra-O-acetyl-a-D 6
-glucopyranosyl bromide26
using the Koenigs–Knorr methodology with silver trifl-
ate (AgOTf). Compound 5 was obtained from methyl
a-methylserinate 4, which was synthesised using our
methodology involving the Sharpless asymmetric dihy-
droxylation of the Weinreb amide of 2-methyl-2-
propenoic acid 3, followed by several transformations
(Scheme 1).27 However, this approach proved unsuc-
cessful and further attempts were made using more
conveniently protected a-methylserine 9 (N-Fmoc-
MeSer-OPfp). Compound 9 was synthesised from a-
methylserine hydrochloride 7 by protection of the
amino group with FmocCl, followed by formation of
the Pfp-ester. However, glycosidation of N-Fmoc-Ser-
The glycosidation of 15, once separated from 16 by
column chromatography, with donor 6 using the
Koenigs–Knorr methodology with a mixture of silver
carbonate (Ag2CO3) and silver perchlorate (AgClO4)
gave 70% of the undesired orthoformiate derivative 17
with 25% of unreacted 6 recovered. Fortunately, the
glycosidation of 15 using trichloloracetimidate 11 gave
50% yield of the desired derivative 18. This yield was
later improved by using the donor 2,3,4,6-tetra-O-ace-
tyl-a-
-glucopyranosyl fluoride31,32 19 in the presence
D
of BF3·Et2O and triethylamine (TEA),33 a process that
gave 18 in 60% yield. The azido group of this com-
pound was readily transformed into the amino group
by the action of Ph3P and water in tetrahydrofuran.
This reaction gave b- -Ac4Glc-(S)-MeSer-OBn (20),
D
which was easily converted into the required building
block 1 by protection of the amino group as the Fmoc
carbamate (Scheme 2).34
In conclusion, we have synthesised the suitably pro-
tected glycosylated amino acid b- -Glc-(S)-a-MeSer
D
(1). This compound is an important building block in
the synthesis of constrained glycopeptides in which the
conformational restriction is only present in the amino
acid moiety (quaternary a-amino acid). To the best of
our knowledge this is the first time that a glycosylated
amino acid with these features has been synthesised.
The nearest previous example is the glycosylated amino
acid recently obtained by Lane and Halcomb,19
although in this case the restriction affects not only the
amino acid moiety but also the carbohydrate. We
intend to explore the incorporation of other carbohy-
drates into different hydroxylated quaternary a-amino
acids, using the methodology described here, in order to
assess the applicability of this approach in the synthesis
of conformationally restricted glycopeptides.
Scheme 1. Reagents and conditions: (i) Ref. 27: (a) AD-mix a,
MeSO2NH2, tBuOH/H2O (1:1), 0°C, 12 h, 81%; (b)
LiOH·H2O, H2O/MeOH (1:3), rt, 2 h; (c) AcCl, MeOH,
reflux, 12 h, 85%; (d) SOCl2, CCl4, reflux, 4 h, 85%; (e) NaN3,
DMF, 50°C, 2 days, 75%; (f) H2/Pd–C, MeOH, rt, 24 h, 83%;
(ii) Fmoc-OSu, TEA, MeCN, rt, 12 h, 55%; (iii) 6N HCl,
reflux, 12 h, 100%; (iv) FmocCl, K2CO3, H2O/THF (1:1), 0°C
to rt, 12 h, 70%; (v) PfpOH, DCC, AcOEt, 0°C, 2 h, 70%.

A. A6enoza et al. / Tetrahedron Letters 44 (2003) 6413–6416
6415
t
Scheme 2. Reagents and conditions: (i) Ref. 27: AD-mix a, MeSO2NH2, BuOH/H2O (1:1), 0°C, 12 h, 81%; (ii) (a) 6N HCl, reflux,
1 h, (b) Cs2CO3, MeOH, rt, 20 min. (c) BnBr, DMF, rt, 48 h, 84%; (iii) SOCl2, CCl4, reflux, 4 h, 81%; (iv) NaN3, DMF, 50°C,
,
12 h, column chromatography (hexane/AcOEt=4/1), 77% of 15 and 17% of 16; (v) 6, Ag2CO3, AgClO4, CH2Cl2, MS 4 A, −10°C
to rt, 12 h; 70% (vi) 11, BF3·EtO2, CH2Cl2, rt, 12 h; 50% (vii) 19, BF3·Et2O, TEA, CH2Cl2, rt, 12 h; 60%; (viii) Ph3P (2 equiv.),
H2O (2 equiv.), THF, reflux, 4 h, 71%; (ix) FmocCl, DIEA, CH2Cl2, rt, 12 h, 58%.
Acknowledgements
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We thank the Ministerio de Ciencia y Tecnolog´ıa (pro-
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(ANGI2001/30) and the Universidad de La Rioja (pro-
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3H), 2.00 (s, 3H), 2.02 (s, 3H), 2.04 (s, 3H), 2.06 (s, 3H),
3.50 (d, 1H, J=9.6 Hz), 3.64 (ddd, 1H, J=10.0 Hz,
J=4.6 Hz, J=2.5 Hz), 4.11 (dd, 1H, J=12.3 Hz, J=2.5
Hz), 4.17 (d, 1H, J=9.6 Hz), 4.25 (dd, 1H, J=12.3 Hz,
J=4.6 Hz), 4.57 (d, 1H, J=8.0 Hz), 4.96 (dd, 1H,
J=9.6 Hz, J=8.0 Hz), 5.07 (‘t’, 1H, J=9.6 Hz), 5.15 (s,
2H), 5.18 (‘t’, 1H, J=9.6 Hz), 7.33–7.37 (m, 5H); 13C
NMR (400 MHz, CDCl3) l (ppm): 20.5, 20.6, 22.7,
58.5, 61.9, 67.0, 68.4, 71.3, 71.8, 72.7, 76.1, 101.1, 128.0,
128.3, 128.5, 132.0, 132.1, 135.7, 167.1, 169.3, 170.1,
170.5, 175.3. Anal. calcd for C25H33NO12: C, 55.65; H,
6.17; N, 2.60. Found; C, 55.71; H, 6.22; N, 2.84.
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34. Compound 20: Mp: 65–67°C. [h]2D5=−14.3 (c 1.01,
1
MeOH); H NMR (400 MHz, CDCl3) l (ppm): 1.28 (s,