Orthogonally protected sugar diamino acids as building blocks for linear and branched oligosaccharide mimetics

Article abstract of DOI:10.1002/anie.200462595

It takes two: Sugar diamino acids which differ from established sugar amino acids by an additional amino group provide access to novel branched oligosaccharide mimetics and a novel class of aminoglycoside mimetics such as 1. The latter are of great signif

Full text of DOI:10.1002/anie.200462595

Communications
Carbohydrate Mimetics
Orthogonally Protected Sugar Diamino Acids as
Building Blocks for Linear and Branched
Oligosaccharide Mimetics**
Frank Sicherl and Valentin Wittmann*
Sugar amino acids (SAAs),[1] which are carbohydrate deriv-
atives with both an amino group and a carboxyl group
connected to the carbohydrate frame, have found wide
application as building blocks for oligosaccharide[2,3] and
peptide mimetics,[4,5] as secondary-structure inducing ele-
ments, and as pharmacophore-presenting scaffolds[6] for the
generation of combinatorial libraries.[7] Used as monomers
with a rigid pyran ring, functional pharmacophoric groups
attached to the hydroxy, amino, and carboxyl groups can be
presented in a distinct spatial arrangement as was demon-
strated in seminal studies by Hirschmann et al.[8] Linear and
cyclic oligomers of SAAs have been synthesized, taking
advantage of well-established peptide chemistry, and in
certain cases they adopt defined secondary structures.[3,5]
Sugar amino acids with an additional amino group, that is,
sugar diamino acids, would be an attractive extension of this
concept, giving access to novel branched oligomeric struc-
tures.[9] However, their synthesis has not been reported until
now.
Here we introduce the protected derivatives 1 and 2 of
2,6-diamino-2,6-dideoxy-b-d-glucopyranosyl carboxylic acid,
the first examples of sugar diamino acids (SDAs) that are
amenable to peptide synthesis following standard Fmoc
strategy (Fmoc = 9-fluorenylmethoxycarbonyl) in solution
and on solid phase. The additional amino group in SDAs
can be used to form branched amide-linked oligosaccharide
mimetics. Besides that, oligomeric SDAs with unprotected
amino groups represent a new class of potential aminoglyco-
side mimetics.[10] Such structures are of great significance as
potential ligands for the new RNA targets emerging in the
post-genome era.[11]
[*] Dipl.-Chem. F. Sicherl, Prof. Dr. V. Wittmann
Fachbereich Chemie
Universitꢀt Konstanz
Fach M 709, 78457 Konstanz (Germany)
Fax: (+49)7531-88-4573
E-mail: mail@valentin-wittmann.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(SFB 579 “RNA–Ligand Interactions”). We thank the coordinator of
the SFB 579, Prof. Joachim W. Engels, for his support and Aventis
AG for providing an HPLC system.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2096 ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200462595
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Angewandte
Chemie
Whereas the protecting-group pattern of SDA 1 was
conceived for the construction of linear oligomers through the
amino group in the 6-position[12] following Fmoc strategy,
SDA 2 can be employed for the synthesis of branched
structures since the two amino groups are orthogonally
protected with the Fmoc group and as an azide, respectively.
We decided to use methoxymethyl (MOM) groups for
hydroxy protection, because they are small and easily
removable together with Boc (tert-butyloxycarbonyl) groups
under acidic conditions, and they do not deactivate adjacent
nucleophiles as it is known from electron-withdrawing acyl or
sterically demanding benzyl protecting groups.[3] Finally, since
2 has protected hydroxy groups, a large excess of the activated
amino acid can be used during peptide-bond formation
typically employed during solid-phase peptide synthesis.
SDA 1 was synthesized from the known glycosyl cyanide
3[13] (Scheme 1). After O-deacetylation, triol 4 was tosylated
regioselectively at the 6-position. Nucleophilic substitution
with sodium azide gave 5 in 86% yield over three steps. MOM
groups were introduced by treatment with dimethoxyme-
[14]
thane and P2O5 (in order to circumvent toxic MOM-Cl)
leading to 6. Initial attempts to hydrolyze the nitrile and
acetamide simultaneously under basic conditions with aque-
ous Ba(OH)2, however, failed. Under these conditions the
reaction stopped at the acetamidocarboxylate stage (8).
Other bases such as methanolic KOH and aqueous NaOH
either led to the same result or to complete decomposition.
Finally, heating 5 in 2n aqueous HCl at reflux successfully
effected amide and nitrile hydrolysis to give the free amino
acid 9. To facilitate its purification, methyl ester 10 was
formed by treatment with dimethoxypropane and HCl. The
amino group of 10 was protected by the Boc group with
concomitant cleavage of the methyl ester. Compound 11 was
obtained by MOM protection using the procedure mentioned
earlier. The MOM ester in 11 was cleaved with NaOH.
Finally, hydrogenation of the azide and subsequent Fmoc
protection of the amine gave SDA building block 1 (Table 1).
SDA 2 was synthesized commencing with methyl ester 10
(Scheme 2). The 2-amino function was masked temporarily,
Table 1: Selected physical properties of compounds 1, 2, 19, and 20.
1: White amorphous solid; Rf =0.24 (silica, MeOH/CH2Cl2 9/1);
1H NMR (600 MHz, DMSO, 300 K, TMS) (major conformation): d=7.87
(d, J=7.4 Hz, 2H, arenes), 7.68 (m, 2H, arenes), 7.38–7.40 (m, 3H,
arenes, NH-Fmoc), 7.29–7.32 (m, 2H, arenes), 6.86 (brs, 1H, NH-Boc),
4.75 (d, J=6.4 Hz, 1H, O-CH2-O), 4.62–4.65 (m, 3H, O-CH2-O), 4.11–
4.24 (m, 3H, H-9Fmoc, CH2Fmoc), 3.64 (m, 1H, H-1), 3.55 (m, 1H, H-2),
3.55 (m, 1H, H-3), 3.51 (m, 1H, H-6), 3.30 (s, 3H, O-CH3), 3.26 (s, 3H,
O-CH3), 3.24 (m, 1H, H-5), 3.23 (m, 1H, H-4), 3.00 (m, 1H, H-6’),
1.33 ppm (s, 9H, C(CH3)3); 13C NMR (150 MHz, DMSO, 300 K, TMS):
=
=
d=171.1 (COOH), 156.4 (C O), 155.0 (C O), 143.9, 140.8, 127.7,
127.2, 125.3, 120.2 (arenes), 97.9 (O-CH2-O), 97.3 (O-CH2-O), 81.3 (C-3),
79.2 (CMe3), 78.3 (C-1), 77.8 (C-5), 77.1 (C-4), 65.6 (CH2Fmoc), 56.0 (O-
CH3), 55.5 (O-CH3), 53.8 (C-2), 46.7 (CHFmoc), 42.0 (C-6), 28.3 ppm
(C(CH3)3); MS (MALDI-TOF), calcd for C31H40N2NaO11 [M+Na+]:
639.25, found: 639.5.
1
2: White amorphous solid; Rf =0.63 (silica, H2O/MeCN 1/4); H NMR
(600 MHz, DMSO, 300 K, TMS): d=7.86 (m, 2H, arenes), 7.69 (d,
J=7.4 Hz, 1H, arenes), 7.66 (d, J=7.4 Hz, 1H, arenes), 7.46 (d, 1H,
NH), 7.39 (m, 2H, arenes), 7.30 (m, 2H, arenes), 4.72 (d, J=6.5 Hz, 1H,
O-CH2-O), 4.59 (d, J=6.5 Hz, 1H, O-CH2-O), 4.57 (d, J=6.5 Hz, 1H, O-
CH2-O), 4.53 (d, J=6.5 Hz, 1H, O-CH2-O), 4.29 a. 4.20 (2 m, 2H,
CH2Fmoc), 4.15 (m, 1H, H-9Fmoc), 3.73 (d, J=10.0 Hz, 1H, H-1), 3.61 (m,
1H, H-2), 3.55 (m, 1H, H-3), 3.54 a. 3.45 (2 m, 2H, 2 H-6), 3.38 (m, 1H,
H-4), 3.33 (m, 1H, H-5), 3.27 (s, 3H, O-CH3), 3.09 ppm (s, 3H, O-CH3);
13
=
C NMR (150 MHz, DMSO, 300 K, TMS): d=170.9 (COOH), 155.8 (C
OFmoc), 144.0, 143.8, 140.8, 127.6, 127.1, 125.3, 120.1 (arenes), 97.9 (O-
CH2-O), 97.3 (O-CH2-O), 81.3 (C-3), 79.2 (C-1), 77.5 (C-5), 76.9 (C-4),
65.4 (CH2 Fmoc), 55.9 (O-CH3), 55.4 (O-CH3), 54.2 (C-2), 51.0 (C-6),
46.7 ppm (CHFmoc); MS (MALDI-TOF), calcd for C26H30N4NaO9
[M+Na+]: 565.19, found: 565.4.
19: HRMS (MALDI-FTICR), calcd for C83H110N8O27: 1673.73730
[M+Na+], found: 1673.73587, Dm=0.8 ppm.
20: HRMS (ESI-FTICR, MeCN/H2O), calcd for C31H50N8O13: 743.35696
[M+H+], found: 743.35563, Dm=1.8 ppm.
Scheme 1. Synthesis of SDA 1; rfl.=reflux.
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synthesis in solution to be able to follow the outcome of each
step. Starting with b-alanine amide 13, stepwise coupling of 1
using 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate (HBTU) and 1-hydroxy-1H-benzotria-
zole (HOBt) as coupling reagents led to linear pseudotrisac-
charide 16 in six steps. When SDA building block 1 was
applied in excess, peptide couplings proceeded smoothly with
no side products detectable by TLC and yields up to 95%
after column chromatography. The integrity of the chirality at
the a-carbon of the sugar diamino acids was verified by
3
1H NMR spectroscopy (e.g. 14: dH-1 = 3.56 ppm, JH-1,H-2
=
9.8 Hz).
The utilization of the orthogonally protected SDA 2 for
the preparation of the branched oligomer 20 is demonstrated
in Scheme 4. After coupling of 2 to b-alanine amide 13, the
amine at the 2-position was deprotected by treatment with
piperidine. Coupling of SDA 1 to this sterically hindered
amine proceeded in a yield of 79% after column chromatog-
raphy on silica gel to give 18. In this case O-(7-azabenzo-
Scheme 2. Synthesis of SDA 2.
and introduction of MOM groups gave 12, which was treated
with NaOH. Reprotection of the amino group at the 2-
position gave building block 2.
Scheme 3 illustrates the application of SDA building
block 1 in the peptide synthesis of aminoglycoside mimetic 16
following the standard Fmoc protocol. We carried out the
triazol-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophos-
phate (HATU) and 1-hydroxy-7-aza-1H-benzotriazole
(HOAt) were used as coupling reagents. Reduction of the
Scheme 3. Synthesis of linear oligomer 16; Bn=benzyl.
Scheme 4. Synthesis of branched oligomer 20.
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Kunwar, A. Mathur, R. Sharma, N. Gupta, S. Prasad, Tetrahe-
azide in presence of the hydrogenolytically labile Fmoc group
was accomplished under Staudinger conditions.[15] Subse-
quent coupling with 1 provided the branched oligomer 19 in a
yield of 73% over two steps. Complete deprotection of 19 was
carried out by a three-step procedure. Treatment with TFA/
CHCl3 (1:1) led to complete Boc removal and partial cleavage
of the MOM groups. The latter could be completed by
addition of a small amount of water. A cleaner product,
however, was obtained removing the MOM groups with conc.
HCl in methanol. Finally, Fmoc groups were cleaved by
treatment with piperidine in DMF to yield 20, which was
purified by RP-HPLC with added ion-pairing reagent penta-
fluoropropionic acid.[16] Compound 20 is the first sugar amino
acid oligomer in which branching is achieved through two
amide linkages.
In conclusion, we have presented a divergent synthesis of
the SDA building blocks 1 and 2 and their application in the
efficient assembly of oligosaccharide mimetics 16 and 20,
which are the first examples of a new class of aminoglycoside
mimetics. The protecting-group scheme of 1 and 2 is compat-
ible with conventional Fmoc solid-phase peptide synthesis
and includes the option of generating branched structures.
Future applications include the utilization of SDA building
blocks in the preparation of combinatorial libraries of amino-
glycoside mimetics. Due to the various possibilities with
which SDAs can be connected to each other, a high degree of
diversity can be achieved by employing only a small set of
different sugar diamino acids.
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Received: November 12, 2004
Published online: February 25, 2005
Keywords: amino acids · aminoglycosides · carbohydrates ·
.
C-glycosides · peptides
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