Solid-phase synthesis of cyclic glycopeptides related to mannopeptimycin derivatives

Article abstract of DOI:10.3987/COM-03-S(P)20

The mannopeptimycins comprise a novel series of glycopeptide antibiotics that display activity against susceptible and resistant forms of gram-positive bacteria. Low isolated yields of these products from fermentation make the synthesis of easily accessible analogs attractive. A simplified hexapeptide is synthesized using a combination of solid-phase and solution-phase techniques.

Full text of DOI:10.3987/COM-03-S(P)20

HETEROCYCLES, Vol. 62, 2004, pp. 131 - 135
Received, 15th July, 2003, Accepted, 25th August, 2003, Published online, 14th October, 2003
SOLID-PHASE SYNTHESIS OF CYCLIC GLYCOPEPTIDES RELATED
TO MANNOPEPTIMYCIN DERIVATIVES
Ting-Zhong Wang,a* Karen L. Wheless,b Alan G. Sutherland,b and Russell G.
Dushinb
aChemical Development, bMedicinal Chemistry, Chemical and Screening Sciences,
Wyeth Research, Pearl River, NY 10965, USA. E-mail: Wangt@wyeth.com
Abstract – The mannopeptimycins comprise a novel series of glycopeptide
antibiotics that display activity against susceptible and resistant forms of
gram-positive bacteria. Low isolated yields of these products from fermentation
make the synthesis of easily accessible analogs attractive. A simplified
hexapeptide is synthesized using a combination of solid-phase and solution-phase
techniques.
Naturally occurring and semisynthetic mannopeptimycins comprise a novel class of glycopeptide
antibiotics that display activities against both susceptible and resistant forms of gram-positive bacteria.1~3
The key structural feature is a cyclic hexapeptide core that contains alternating D- and L-amino acids and
includes an epimeric pair of a previously unknown amino acid, β-hydroxyenduricididine. One of these
O
O
NH2
NH
HO
O
O
OH
OH
HN
O
R1
N
H
O
H
O
OH
OH
O H
H2N
O
HO
OH
HN
NH HN
H
NH
1
2
3
4
R1 = α-D-mannose, R2 = Me, R3 = Ph
R1 = α-D-mannose, R2 = H, R3 = Ph
R1 = α-D-mannose, R2 = H, R3 = cyclohexyl
R1 = H, R2 = H, R3 = cyclohexyl
HN
H
O
HO
NH HN
O
R3
2
O
R
Figure 1
_____________________________________________________________________________
Dedicated with best wishes to Professor Leo A. Paquette on the occasion of his 70th birthday

novel residues contains a mannopyranose moiety in the α-configuration while a D-tyrosine residue contains
an α-linked mannose disaccharide. In the natural series, isovalerate substitution occurs on the terminal
mannose of this disaccharide, and this substitution conveys substantial increases in antibacterial potency
over the non-derivatized parent molecule.2 Similarly, those semisynthetic variants that display potent
antibacterial activities are derivatized on the same terminal mannose.3 Notably, replacements of the
β-methylphenylalanine residue with phenylalanine or cyclohexylalanine are tolerated, and derivatives
lacking the N-linked mannose or the hydroxyl substituents on the β-hydroxyenduricididine side chains also
retain good levels of antibacterial activities. Among the more potent and most easily accessed
semisynthetic derivatives are adamantyl ketals (1-4) (Figure 1).3 The apparent tolerability of structural
variation on these three residues provided impetus to prepare simplified derivatives in which the
β-hydroxyenduricididine residues are replaced with arginines. Herein we report results on the total
synthesis of a representative compound.
Retrosynthetically, we envisioned that target compound (5) could be derived from an appropriately
protected linear glycopeptide (6) which could be assembled using solid-phase techniques (Figure 2). We
anticipated that cyclization to provide the hexapeptide core would be most readily accomplished with
amide bond formation between the L-serine and unhindered glycine residues. Five of the six requisite
amino acids are commercially available, while the D-tyrosine containing a mannose disaccharide would be
prepared synthetically.
OAc
O
OAc
OAc
OAc
O
NH
NH
NH2
BzO
HO
NH
NH
O
O
OH
NH(Pmc)
HN
O H
HN
O H
O
O
(Pmc)HN
NH
H2N
NH
O
O
O
O
OH
O
OBz OBz
O
HO
OH
NH HN
NH HN
O
H
O
H
NH
HN
H
NH
HN
H
O
O
t-BuO
HO
OH HN
NH HN
O
O
O
H2N
O
6
5
Figure 2
Synthesis of the disaccharide started with the known pentenyl-α-D-mannopyranoside (7)4 which was
selectively protected with benzoyl chloride to give pentenyl-2,3,6-tri-O-benzoyl-α-D-mannopyranoside (8)
retaining a hydroxy substituent on the 4-position open for glycosylation5 (Scheme 1). Dimannoside (10)
was obtained in modest yield by silver(I) triflate-promoted coupling of
8
with
1-α-chlorotetraacetylmannose (9).6 Prior to glycosylation, the commercially available N-Fmoc-tyrosine

(11) was reacted with pentafluorophenyl trifluoroacetate (PFPTFA) to give pentafluorophenyl ester (12)
which is suitably protected for glycosylation and activated for amide bond formation during peptide
synthesis.7 Coupling of the dimannoside (10) with 12 was mediated by N-iodosuccinimide/trimethylsilyl
triflate and gave the glycosylated tyrosine derivative (13) required for peptide synthesis.4 The
stereochemistry of both α-anomeric centers was assigned based on literature precedents4,6 and was
confirmed by 1H and 13C NMR spectroscopy.8
OCOPh
O
OH
O
O
PhCOCl
pyridine
O
HO
OCOPh
HO
OH
OCOPh
OH
7
8
OCOPh
O
O
OAc
OAc
O
Cl
O
O
OCOPh
AgOTf
8
+
OCOPh
CH2Cl2
AcO
OAc
AcO
OAc
OAc
NH(Fmoc)
OAc
10
9
O
F
O
NH(Fmoc)
OCOPh
F
F
O
O
10
O
F
NH(Fmoc)
OAc
O
F
F
NIS/TMSOTf
PFP-TFA
O
O
OCOPh
OCOPh
O
F
CH2Cl2
HO
HO
F
CH2Cl2
AcO
OAc
F
OAc
HO
F
11
12
13
Scheme 1
For solid-phase synthesis, we employed a 2-chlorotrityl chloride resin with HBTU/HOBt coupling
protocols and Fmoc protection. The synthesis commenced with attachment of O-tert-butyl-N-Fmoc-serine
to the resin. The loading of the resin was determined to be in the range of ~0.5 mmol/g by UV quantification
of the Fmoc-piperidine adduct.9 The linear peptide chain was then assembled on the resin
OAc
OAc
NH
BzO
NH
O
O
OAc
OAc
NH(Pmc)
HN
O H
O
(Pmc)HN
NH
20% piperidine in DMF
Sequential coupling with:
1. Fmoc-D-arginine(Pmc)
2. Fmoc-L-arginine(Pmc)
3. 13, DIPEA
O
t-BuO
O
O
OBz OBz
NH HN
NHFmoc
O
H
NH
O
HN
H
Resin-attached serine
4. Fmoc-L-phenylalanine
5.Fmoc-glycine
Cleavage with AcOH/TFE/CH2Cl2
O
t-BuO
OH HN
O
O
H2N
6
Scheme 2

in a sequential fashion using standard procedures (Scheme 2). Peptide coupling reactions were monitored
for completion by negative Kaiser ninhydrin test. Once peptide elongation was accomplished, the protected
linear hexapeptide (6) was released from the resin by cleavage with a mixture of acetic acid,
trifluoroethanol and dichloromethane (1:1:8). The cyclization of 6 was effected by using BOP/DIPEA as
the dehydrating reagent in a mixed solvent system of DMF/CH2Cl2 (1:1). The cyclic peptide was thereby
obtained cleanly with almost complete suppression of dimerization when the concentration of the substrate
was held at <10-2 M. Deprotection was accomplished sequentially by treatment with TFA/MeOH to remove
the tert-butyl and the 2,2,5,7,8-pentamethylchroman-6-sulfonyl (Pmc) groups, and then with
K2CO3/MeOH for hydrolysis of the acetates and benzoates. The fully deprotected glycopeptide was
converted to the final target (5) by transketalization with admantanone dimethyl ketal (Scheme 3). The
selectivity at 4,6-position of the terminal mannose is in accordance to those observed for natural
mannopeptimycins with the formation of only a small amount of the corresponding 2,3-ketal isomer. To our
surprise, the synthetic glycopeptide (5) was found to have very poor activity against a diverse panel of
bacteria. The disparity of activities between this fully synthetic analog and the semisynthetic derivatives
(1-4) demonstrates that the cyclic guanidines on the β-hydroxyenduricidine residues play a very important
role in conveying antibacterial activities of the mannopeptimycin derivatives.
OAc
OAc
NH
BzO
NH
O
O
OAc
NH(Pmc)
HN
O H
O
(Pmc)HN
NH
O
OAc
1. BOP, DIPEA, CH2Cl2/DMF, -15oC to rt
2. TFA/MeOH (12:1)
O
OBz OBz
NH HN
5
3. K2CO3, MeOH/H2O
O
H
4. adamantanone dimethyl ketal, DMSO, HCl
NH
HN
H
O
t-BuO
OH HN
O
O
H2N
6
Scheme 3
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the support of Magid Abou-Gharbia, Tarek Mansour, and Janis
Upeslacis. Xidong Feng, Kaiko Tabei, Murthy Damarla, Rachel Garlish, Mark Tischler and Joseph
Ashcroft are acknowledged for providing separations and analytical support.
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