New building blocks for peptide and depsipeptide modification N-glycosylated L-malic and L-citramalic acid derivatives

Article abstract of DOI:10.1016/S0040-4039(02)02285-2

Neoglycoconjugates have been synthesized from L-malic acid/L-citramalic acid and β-D-Ac₄Glc-NH₂/β-D-Bzl₄Glc-NH ₂ using hexafluoroacetone as protecting and activating agent.

Full text of DOI:10.1016/S0040-4039(02)02285-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9711–9714
New building blocks for peptide and depsipeptide modification
N-glycosylated -malic and
-citramalic acid derivatives†,‡
L
L
Christoph Bo¨ttcher and Klaus Burger*
Department of Organic Chemistry, University of Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
Received 19 August 2002; accepted 4 October 2002
Abstract—Neoglycoconjugates have been synthesized from
L
-malic acid/L-citramalic acid and b-
D
-Ac4Glc-NH2/b-D-Bzl4Glc-NH2
using hexafluoroacetone as protecting and activating agent. © 2002 Elsevier Science Ltd. All rights reserved.
Peptide drugs are a fast growing class of therapeutics.
However, they show a series of severe disadvantages,
like proteolytic instability and low lipophilicity. Fur-
thermore, the lack of specific transport systems to direct
peptide drugs into cells or across the blood brain
barrier is the reason why cell membranes generally
resist passage of most peptides. Therefore, peptide
drugs are rapidly degraded and excreted.3,4
conjugates isolated from biological sources are often
microheterogeneous and cannot be applied for mecha-
nistic studies. Therefore, the development of new
methodology for the assembly of glycopeptide mimetics
by chemoselective ligation is of current interest. As part
of an ongoing program we synthesized a series of
glycosylated a-hydroxy acids20 via the new ‘hex-
afluoroacetone route’.1,2
The high conformational flexibility of peptides causes
another problem. Bioactive peptides often bind to dif-
ferent receptor sites, causing undesired side effects.5,6
Different strategies have been developed to overcome
these drawbacks. The most versatile approach is the
rational design of peptidomimetics6–8 by backbone-
modification,6,9–11 incorporation of a-C-alkyl and a-N-
alkyl amino acids6,12–14 and incorporation of
glycosylated peptide fragments15,16 into key positions of
the peptide chain.
a-Hydroxy acids, including multifunctional species like
L
-malic (1a) and -citramalic acid (1b), react with hex-
L
afluoroacetone in DMSO at room temperature to give
regiospecifically five-membered lactones (2a,b) in excel-
lent yields.21 In only one step, protection of both the
a-hydroxy and the adjacent carboxy group can be
achieved. Concomitantly, the a-carboxy group is selec-
tively activated toward nucleophiles. The b-carboxy
groups of malic and citramalic acid remain unaffected
and can be derivatized in a consecutive step.22 Com-
pounds 2 can be easily prepared in a 50–100 g scale. On
exclusion of moisture 2,2-bis-(trifluoromethyl)-1,3-diox-
olan-4-ones (2) can be stored in a fridge, at −30°C, for
months without decomposition.
Since glycopeptides are ubiquitous in nature, playing a
key role in various biological recognition processes17,18
and glycoclusters exhibiting enhanced binding proper-
ties,19 the synthesis of glycopeptide mimetics has
received significant attention. Deciphering these roles in
controlled studies requires convenient synthetic access
to a large number of different binding motifs. Glyco-
Upon treatment with thionyl chloride or DAST, com-
pounds 2 can be transformed into acid chlorides 323
and acid fluorides 4,24 respectively. Compounds of type
3 and 4 represent double activated malic and citramalic
acid derivatives with two electrophilic centers of differ-
ent reactivity. Consequently, a new efficient, prepara-
tively simple method for regioselective derivatization of
a-functionalized a,b-dicarboxylic acids is now available.
Recently, N-protected amino acid chlorides and amino
acid fluorides have received significant attention as
acylating agents in N-glycopeptide synthesis.25 There-
fore, we decided to study scope and limitation of com-
Keywords: malic acid; citramalic acid; hexafluoroacetone; glycosyl
amines; N-glycosylation; neoglycoconjugates.
* Corresponding author. Fax: (49) 341 9736599; e-mail: burger@
organik.chemie.uni-leipzig.de
† Hexafluoroacetone as protecting and activating agents in glycopep-
tide and glycopeptoid chemistry. Part 3. For Parts 1 and 2, see Refs.
1 and 2.
‡
Dedicated to Professor Dr. P. Welzel on the occasion of his 65th
birthday.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02285-2

9712
C. Bo¨ttcher, K. Burger / Tetrahedron Letters 43 (2002) 9711–9714
Scheme 1. Reagents and conditions: (i) hexafluoroacetone, DMSO, rt; (ii) SOCl2, reflux; (iii) DAST, CH2Cl2, 0°C; (iv) 3a or 3b,
pyridine, CH2Cl2, 0°C.
pounds 3 and 4 as acyl transfer reagents. As model acyl
acceptors we choose glycosyl amines b-
D-Ac4Glc-NH2
(5a)26 and b- -Bzl4Glc-NH2 (5b).27
D
We found that acid chlorides 3a,b and glycosyl amines
5a,b react readily at 0°C in DCM in the presence of 1
equiv. of pyridine or 2,6-lutidine to give the acylated
products 6a–d within minutes in high yields (73–91%)
(Scheme 1).
1H NMR analysis (300 MHz) of the crude products
reveals that the b-anomer (3J12=9.0–9.3 Hz) is formed
exclusively in the case of 6a–c. While for 6d, after
isolation of the b-anomer by crystallization, the a-
Scheme 2. Reagents and conditions: (i) 5 equiv. pyridine,
CHCl3, reflux, 16 h; (ii) 1 equiv. base, EtOAc, rt, 1–4 days.
1
anomer could be detected by H NMR (3J12=5.8 Hz)
and TLC in the concentrated mother liquor. It was
isolated and fully characterized. Purification of com-
pounds 6 by crystallization is the method of choice. On
work-up by column chromatography the yields are
notoriously lower. When stronger bases like NEM and
DIEA are used in the acylation process, a competing
reverse Michael addition is the reason for decreasing
yields (40–60%).
diastereomeric mixture of 7a and maleimido sugar 8a28
as elimination product (Table 1).
Compound 6b reacts analogously, but since there is no
a-proton (R=CH3) present epimerization cannot be
observed.
Finally, we demonstrated that glycosylated a-hydroxy
acid derivatives 6a–d are excellent acyl transfer
reagents. With amino acid and dipeptide esters or
amides, they react to give N-glycoconjugates 9a–d
which can be applied as new building blocks for peptide
and depsipeptide modification (Scheme 3, Table 2).
The acid fluorides 4a,b are noticeable less reactive than
the corresponding acid chlorides 3a,b. Nevertheless,
with 4a,b clean and regioselective acylation of 5a,b can
be achieved at 0°C in the presence of pyridine. With
stronger bases like NEM and DIEA, the yields are
lower.
As by-products of the aminolytic cleavage, compounds
7a–d are formed with variable yields. In all cases stud-
ied so far, chromatographic separation of compounds 9
and 7 was achieved without problems. As expected, the
sterically more demanding compounds 6b and 6d
exhibit lower reactivity toward N-nucleophiles. Conse-
quently, an increasing tendency to undergo the
intramolecular cyclization process is observed. Further-
more, the amount of by-products is increasing with the
sterical demand of the amino acid side-chain (H-Gly-
OtBu<H-Ala-OtBu<H-Val-OtBu).
On treatment with bases compounds 6a–d undergo an
intramolecular cyclocondensation. When the cyclization
process is performed in the presence of pyridine in
boiling chloroform 6a–d are transformed to give
stereoselectively 3-hydroxysuccinimido sugars 7a–d in
acceptable yields, which were purified by column chro-
matography (Scheme 2).
Treatment of a solution of 6a in ethyl acetate with
bases like NEM, DIEA or DBU gives rise to a

C. Bo¨ttcher, K. Burger / Tetrahedron Letters 43 (2002) 9711–9714
9713
Table 1.
Base
Product ratioa 7a:8a
Yield (%)b
Ratioc 3-(S):3-(R)-7a
Product ratio 7b:8b
Yield (%)b
DBU
DIEA
NEM
5:1
3:1
25:1
57
63
73
5:3
7:3
7:3
4:1
15:1
8b n.d.
65
68
72
a Determined by integration of the 1H NMR spectra of the crude products.
b Isolated yields after chromatography [7+8].
c Determined by integration of the 1H NMR spectra of an isolated diastereomeric mixture of 7a.
Table 2.
No.
Xaa
Product ratioa 9:7
Yield (%)b
No.
Xaa
Product ratio 9:7
Yield (%)b
9a
9a
9a
9a
9a
9b
9b
9b
9b
9b
Gly
Ala
Val
Phe
Pro
Gly
Ala
Val
Phe
Pro
\95:5
8:1
7:1
7:1
\95:5
5:1
76
72
82
80
72
78
72
82
82
76
9c
9c
9c
Phe
Phe-Ala
Pro
\95:5
\95:5
\95:5
71
74
67
9d
9d
9d
Phe
Phe-Ala
Pro
5:1
5:1
1.4:1
69
88
80
2:1
1.3:1
1.2:1
B5:95
a Determined by integration of the 1H NMR spectra of the crude products.
b Isolated yields after chromatography [9+7].
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3. Hruby, V. J.; Al-Obeidi, F.; Kazmierski Biochem. J. 1990,
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4. DeGrado, W. F. Adv. Protein Chem. 1988, 39, 51–124.
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OtBu, 1.2 equiv. NEM, EtOAc/DMF=1:1.
7. Olson, G. L.; Bolin, D. R.; Pat Bonner, M.; Bo¨s, M.;
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malic, thiomalic acid and a-, b-, g-amino acid deriva-
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