Total synthesis of the cyclic heptapeptide Argyrin B: A new potent inhibitor of T-cell independent antibody formation

Article abstract of DOI:10.1021/ol017184m

(equation presented) Argyrin B (1) The total synthesis of Argyrin B (1) is presented using a synthetic plan that is convergent and flexible and conserves the stereogenic centers. The unusual amino acid 4-methoxy tryptophan (6) was obtained via an enzymati

Full text of DOI:10.1021/ol017184m

ORGANIC
LETTERS
2002
Vol. 4, No. 5
711-714
Total Synthesis of the Cyclic
Heptapeptide Argyrin B: A New Potent
Inhibitor of T-Cell Independent Antibody
Formation
,†
Steven V. Ley,* Alain Priour,† and Christoph Heusser‡
Department of Chemistry, UniVersity of Cambridge, Lensfield Road,
Cambridge CB2 1EW, U.K., and NoVartis Pharma AG, Transplantation Research,
CH-4002 Basel, Switzerland
sVl1000@cam.ac.uk
Received December 6, 2001
ABSTRACT
The total synthesis of Argyrin B (1) is presented using a synthetic plan that is convergent and flexible and conserves the stereogenic centers.
The unusual amino acid 4-methoxy tryptophan (6) was obtained via an enzymatic resolution. Cyclization followed by oxidative elimination of
the phenylseleno cysteine to the sensitive dehydroalanine afforded synthetic 1.
In a continuing search for new drugs that selectively inhibit
antibody formation and subsequently may prove to be effec-
tive in xenotransplantation by modulating the progression
of antibody-mediated rejection events, the cyclic heptapeptide
Argyrin B (1)1,2 has been recently identified at Novartis as
a potential candidate. Argyrin B and its congeners were ori-
ginally discovered at GBF Braunschweig during the screen-
ing of myxobacteria for the production of new antibiotics.3
T-cell independent antibody formation. Moreover the two
way murine mixed lymphocyte reaction (MLR), a cellular
model for alloantigenic-mediated T-cell activation and
proliferation,6 was also inhibited by Argyrin B. Cytotoxicity
was low, since 1 did not affect the proliferation of human
Jurkat T cells. Argyrin B was further shown to be a potent
inhibitor of in vitro IgG production by CD40L-stimulated
murine and human B-cells.
Using established assays for both murine4 and human
Owing to this interesting biological profile and the unusual
features of this natural product we have undertaken a
B-cells,5 Argyrin B was found to be a potent inhibitor of
† University of Cambridge.
(4) Skelly, R. R.; Munkenbeck, P.; Morisson, D. C. Infect. Immun. 1979,
23, 287. Pike, B. L.; Nossal, G. J. V. J. Immunol. 1984, 132, 1687. Morgan,
E. L.; Weigle, W. O. 1983, 130, 1066.
(5) Brinkmann, V.; Kristofic, C. J. Immunol. 1995, 154, 3078. Levy, F.;
Kristofic, C.; Heusser, C.; Brinkmann, V. Int. Arch. Allergy Immunol. 1997,
112, 49.
‡ Novartis Pharma AG.
(1) Full details of the biological data, the isolation, and the X-ray crystal
structure characterization will be reported separately.
(2) Ho¨fle, G. GBF Scientific Annual Report; Walsdorff, H.-J., Ed.;
GBF: Braunschweig, 1996; p 112.
(3) Revised structure and absolute configuration can be found in: Ho¨fle,
G.; Vollbrecht, L. GBF Scientific Annual Report; Walsdorff, H.-J., Ed.;
GBF: Braunschweig, 2000/2001; p 99.
(6) Sanglier, J J.; Quesniaux, V.; Fehr, T.; Hofmann, H.; Mahnke, M.;
Memmert, K.; Schuler, W.; Zenke, G., Gschwind, L.; Maurer, C.; Schilling,
W. J. Antibiot. 1999, 52, 466.
10.1021/ol017184m CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/05/2002

Scheme 2. Synthesis of Fragment 3a
Figure 1. Structure and retrosynthetic analysis of Argyrin B.
a Reagents and conditions: (a) penicillin G acylase immobilized,
MeOH/H2O, rt; (b) CbzCl, NaHCO3, THF/H2O, rt (44%, 2 steps);
(c) Gly-OMe, EDC, HOBt, CH2Cl2, rt (94%); (d) H2, Pd/C, MeOH/
aquous HCl, rt; (e) Cbz-L-Trp, EDC, HOBt, DIPEA, CH2Cl2, rt
(81%, 2 steps). DIPEA ) N,N-diisopropylethylamine; EDC )
1-ethyl-3-(3′-dimethyl aminopropyl)carbodiimide hydrochloride.
synthesis of 1, the details of which are reported below. In
particular we were interested in the absolute stereochemistry,
which was not known at the start of the work, and especially
we wished to determine methods for the introduction of both
the unusual 4-methoxy tryptophan and dehydroalanine units.
While it is possible to conceive of a large variety of
alternative coupling arrangements for the various amino acid
fragments, we have chosen a plan outlined in Figure 1
because this best serves to protect the stereogenic centers at
crucial stages of the synthesis. The route also should facilitate
later deprotection, coupling, and the difficult cyclizing event
as required.
This approach therefore requires the synthesis of ap-
propriately protected fragments, namely, the thiazole 2, the
tripeptide unit 3 containing the 4-methoxy tryptophan, and
the selenophenyl substituted tripeptide 4, which in turn
derives from D-aminobutyric acid and sarcosine.
Scheme 1. Synthesis of Fragment 2a
For the preparation of 2, this follows a straightforward
route7 from N-Boc-D-alanine requiring transformation to the
thioamide 5 via the intermediate amide and thiolation with
Belleau’s reagent8 (Scheme 1). The compound 5 was reacted
with bromo ethyl pyruvate to afford an intermediate that on
treatment with trifluoroacetic anhydride and 2,6-lutidine gave
2.9
a Reagents and conditions: (a) DCC, HOBt, NH3, CH2Cl2, 0 °C
(97%); (b) Belleau’s reagent, THF, 0 °C (79%); (c) (i) BrCH2-
COCO2Et, KHCO3, DME, (ii) TFAA, 2,6-lutidine, DME, -15 °C
(41%). DCC ) N,N′-dicyclohexylcarbodiimide; HOBt ) N-hydroxy-
benzotriazole; Belleau’s reagent ) 2,4-bis(4-phenoxyphenyl)-
1,3,2,4-dithiaphosphetane-2,4-disulfide; DME ) 1,2-dimethoxy-
ethane; TFAA ) trifluoroacetic anhydride.
Next the tripeptide 3 was assembled; however, this first
required an efficient synthesis of the key 4-methoxy tryp-
(7) Kigoshi, H.; Yamada, S. Tetrahedron 1999, 55, 12301.
(8) Lajoie, G.; Le´pine, F.; Maziak, L.; Belleau, B. Tetrahedron Lett. 1983,
24, 3815.
(9) Aguilar, E.; Meyers, A. I. Tetrahedron Lett. 1994, 35, 2473.
712
Org. Lett., Vol. 4, No. 5, 2002

the enzyme resolution route using immobilized Penicillin G
acylase.10 This was achieved by highly selective hydrolysis
of the racemic amide 7,11 which afforded 8 and the non-
hydrolyzed R-isomer 9. Protection of 8 with the Cbz group
gave 6 in an overall 44% yield.
Scheme 3. Synthesis of Fragment 4a
This compound was then coupled using standard peptide
coupling techniques with methyl glycine and after removal
of the Cbz group reacted with NR-Cbz-L-tryptophan to afford
the tripeptide in excellent yield (Scheme 2).
Last, the tripeptide containing the selenophenyl group 4
was constructed again following the established protocols
from N-Boc-L-serine (Scheme 3). This route proceeded via
the â-lactone 11, which after ring opening afforded the
selenide 12.12 This was coupled with ethyl sarcosine using
PyBroP13 to give 13, which after deprotection to remove the
butyloxycarbonyl group was coupled with N-Boc-D-amino-
butyric acid to give 4.
a Reagents and conditions: (a) DEAD, PPh3, THF; -78 °C to
rt; (b) PhSeSePh, NaBH(OMe)3, EtOH then 11, rt (34%, 2 steps);
(c) HCl-Sar-OEt, PyBroP, DIPEA, CH2Cl2, rt (84%); (d) TFA/
CH2Cl2, rt; (e) Boc-D-Abu, EDC, HOBt, DIPEA, CH2Cl2, rt (71%,
2 steps). DEAD ) diethyl Azodicarboxylate; Sar ) sarcosine;
PyBroP ) bromo tripyrrolidino phosphonium hexafluorophosphate;
TFA ) trifluoro acetic acid; Abu ) aminobutyric acid.
With all the fragments in hand we could now progress
these to the natural product (Scheme 4). Removal of the Cbz
protection from 3 and coupling with the free acid from 2
gave the product 14 in 96% yield. Similarly, hydrolysis of
14 to the acid and coupling with the Boc deprotected
fragment from 4 gave the fully assembled heptapeptide 15.
This was then efficiently deprotected at the two termini and
finally cyclized to 16 in an overall yield of 60%. Lastly,
syn-elimination of the selenide was achieved using periodate
tophan amino acid derivative 6 (Scheme 2). Although we
investigated a number of alternative methods to this fragment,
none proved satisfactory on a gram scale until we studied
Scheme 4. Synthesis of Argyrin Ba
a Reagents and conditions: (a) LiOH, THF/MeOH/H2O, rt; (b) H2, Pd/C, MeOH, rt; (c) EDC, HOBt, CH2Cl2, rt (60%, 3 steps); (d)
LiOH, THF/MeOH/H2O, rt; (e) TFA/CH2Cl2, rt; (f) EDC, HOBt, DIPEA, CH2Cl2 (90%, 2 steps); (g) LiOH, THF/MeOH/H2O, rt; (h)
TFA/Anisole/CH2Cl2, rt; (i) TBTU, HOBt, DIPEA, CH2Cl2, rt (60%, 2 steps); (j) NaIO4, CH3CN/H2O, rt; (k) NaHCO3, CH3CN/H2O, rt
(52%, 2 steps). TBTU ) 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate.
Org. Lett., Vol. 4, No. 5, 2002
713

and bicarbonate to give Argyrin B 1 identical in all respects
to an authentic sample of the natural product.14
18 steps (20.5% yield from the enantiopure NR-Cbz-L-4-
methoxy-tryptophan 6, 13 steps).
In summary, we have achieved the first synthesis of a new
cyclic heptapeptide Argyrin B from 4-methoxy indole in
5.6% overall yield with the longest linear sequence being
Acknowledgment. This work was financially supported
by the Novartis Research Fellowship (S.V.L.), the BP En-
dowment (S.V.L.), and the Cambridge European Trust (A.P.).
Supporting Information Available: Characterization
data for Argyrin B (1)15 and synthetic intermediates 2, 3, 4,
6, and 16. This material is available free of charge via the
Internet at http://pubs.acs.org.
(10) Meseguer, B.; Alonso-D´ıaz, D.; Griebenow, N.; Herget, T.; Wald-
mann H. Chem. Eur. J. 2000, 6, 3943.
(11) The racemic N-phenylacetyl-4-methoxy tryptophan was obtained
from 4-methoxy indole (4 steps, 63% yield).
(12) Okeley, N. M.; Zhu, Y.; van der Donk, W. A. Org. Lett. 2000, 2,
3603. Sakai, M.; Hashimoto, K.; Shirahama, H. Heterocycles 1997, 44,
319.
(13) Coste, J.; Fre´rot, E.; Jouin, P.; Castro, B. Tetrahedron Lett. 1991,
32, 1967.
OL017184M
(15) We note the data are remarkably similar to those of antibiotic
A21459 A (Selva, E.; Gastaldo, L., Saddler, G. S., Toppo, G, Ferrari, P.;
Carniti, G.; Goldstein, B. P. J. Antibiot. 1996, 49, 150.
(14) We thank Prof. Dr. G. Ho¨fle from the GBF, Braunschweig for an
authentic sample of Argyrin B (1).
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Org. Lett., Vol. 4, No. 5, 2002