Total synthesis of cyclic tetrapeptide FR235222, a potent immunosuppressant that inhibits mammalian histone deacetylases

Article abstract of DOI:10.1021/ol050991r

(Chemical Equation Presented) The total synthesis of FR235222, a potent immunosuppressant with in vivo activities, has been achieved. The key steps include assembling its (2S,9R)-2-amino-9-hydroxy-8-oxodecanoic acid residue via an olefin cross-metathesis of a methyl (R)-lactate-derived homoallyl ketone with protected allyl amino acid and constructing its trans-(2R,4S)-4-methylproline unit from protected (R)-pyroglutamic acid in seven steps.

Full text of DOI:10.1021/ol050991r

ORGANIC
LETTERS
2005
Vol. 7, No. 13
2775-2777
Total Synthesis of Cyclic Tetrapeptide
FR235222, a Potent Immunosuppressant
that Inhibits Mammalian Histone
Deacetylases
Weiqing Xie,† Bin Zou,† Duanqing Pei,‡ and Dawei Ma*,†
State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, China, and Department of Pharmacology, UniVersity of Minnesota,
Minneapolis, Minnesota 55455
madw@pub.sioc.ac.cn
Received May 2, 2005
ABSTRACT
The total synthesis of FR235222, a potent immunosuppressant with in vivo activities, has been achieved. The key steps include assembling
its (2S,9R)-2-amino-9-hydroxy-8-oxodecanoic acid residue via an olefin cross-metathesis of a methyl (R)-lactate-derived homoallyl ketone with
protected allyl amino acid and constructing its trans-(2R,4S)-4-methylproline unit from protected (R)-pyroglutamic acid in seven steps.
FR235222 (1, Figure 1) is a cyclic tetrapeptide that was
isolated from the fermentation broth of a fungus, Acremonium
sp. No. 27082.1 This compound displayed a potent in vitro
inhibitory effect on both lymphocyte proliferation and
lymphokine production (IC50 ) 0.35∼4.7 ng/mL).2 In animal
models, it showed marked immunosuppressive effects on
mouse ex vivo splenic T-lymphocyte proliferation, mouse
delayed type hypersensitivity (DTH) response, rat adjuvant-
induced arthritis (AA), and rat heterotopic cardiac transplant-
ation.3 These facts, together with the low cytotoxicity of
FR235222, make this cyclopeptide promising as a novel
immunosuppressive drug. Like other cyclic tetrapeptides such
as trapoxin B (2),4 chlamydocin (3),5 and diheteropeptin (4),6
FR235222 is also a potent inhibitor for mammalian histone
deacetylases (HDAC, IC50 ) 9.7 ng/mL).2 However,
FR235222 possesses unique structural features that are
distinct from the other cyclic tetrapeptides, which include
isovaline, hydroxyketone, and methylproline units. The
former two are rare, and the latter one is unprecedented in
the other cyclic tetrapeptides. It was recognized that some
of these units might play an essential role for its in vivo
immunosuppressive effects because those related cyclic
tetrapeptides bearing an epoxyketone element did not perform
similar activities.3 The presence of these elements in
FR235222 is correlated to the agent’s oral bioavailability
† Chinese Academy of Sciences.
‡ University of Minnesota.
(4) Itazaki, H.; Nagashima, K.; Sugita, K.; Yoshida, H.; Kawamura, Y.;
Yasuda, Y.; Matsumoto, K.; Ishii, K.; Uotani, N.; Nakai, H.; Terui, A.;
Yoshimatsu, S.; Ikenishi, Y.; Nakagawa, Y. J. Antibiot. 1990, 43, 1524.
(5) Closse, A.; Huguenin, R. HelV. Chim. Acta. 1974, 57, 533.
(6) (a) Masuoka, Y.; Shin-Ya, K.; Furihata, K.; Hayakawa, Y.; Seto, H.
J. Antibiot. 1997, 50, 1058. (b) Masuoka, Y.; Shin-Ya, K.; Kim, Y.-B.;
Yoshida, M.; Nagai, K.; Suzuki, K.-I.; Hayakawa, Y.; Seto, H. J. Antibiot.
2000, 53, 788.
(1) Mori, H.; Urano, Y.; Kinoshita, T.; Yoshimura, S.; Hino, M. J.
Antibiot. 2003, 56, 181.
(2) Mori, H.; Urano, Y.; Abe, F.; Furukawa, S.; Tsurumi, Y.; Sakamoto,
K.; Hashimoto, M.; Takase, S.; Hino, M.; Fujii, T. J. Antibiot. 2003, 56,
72.
(3) Mori, H.; Abe, F.; Furukawa, S.; Furukawa, S.; Sakai.; Hino, M.;
Fujii, T. J. Antibiot. 2003, 56, 80.
10.1021/ol050991r CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/25/2005

acid with N,O-dimethyl hydroxylamine afforded Weinreb
amide 5. Grignard reaction of 5 with 3-butenylmagnesium
bromide delivered homoallyl ketone 6 in 89% yield. Next,
we planned to use an intermolecular olefin cross-metathesis
of 6 with protected allyl amino acid 7 to elaborate the desired
olefins 8.9a To our delight, reaction of 6 and 7 under the
action of second-generation Grubbs RCM catalyst A10
afforded 8 as a mixture of (E)- and (Z)-isomers in 55% yield.
Finally, reduction of the olefin moiety of 8, accompanied
by removal of the benzyl group, upon Pd/C-catalyzed
hydrogenation, provided acid 9 in 94% yield.
We next moved our attention to the elaboration of (2R,4S)-
4-methylproline. Literature surveys11-13 indicated that al-
though considerable efforts have been directed toward the
synthesis of 4-substituted prolines and their derivatives, none
of them is satisfactory for assembling this trans-4-methyl-
proline. For example, a recently reported protocol by
Koskinen and co-workers suffers from long operation periods
(10 steps from Garner aldehyde) and production of a
diastereomeric mixture at a late stage.11 Goodman’s method12
would require unnatural 4-hydroxyproline as a starting
material and difficult to prepare Ir(COD)PyPCy3PF6 as a
hydrogenation catalyst. Drawbacks such as these prompted
us to develop a more practical route.
Figure 1. Structures of FR235222 and other related cyclic
tetrapeptides.
and/or selectivity for immunorelated HDAC isozymes.
Further SAR studies are required to clarify these issues.
Toward this goal, we disclose here the first total synthesis
of FR235222.7
As depicted in Scheme 2, protected (R)-pyroglutamic acid
On the basis of the structure of FR235222 and previous
synthetic investigations toward cyclic tetrapeptides,8 we
realized that its (2R,4S)-4-methylproline site should be an
ideal juncture for macrocyclization. Thus, our initial synthetic
efforts were devoted to the assembly of FR235222’s two
unusual amino acid units, (2S,9R)-2-amino-9-hydroxy-8-
oxodecanoic acid (Aoh) and (2R,4S)-4-methylproline. The
production of the protected Aoh 9 is outlined in Scheme 1.
Scheme 2
Scheme 1
10 was condensed with formaldehyde under the action of
TiCl4/i-Pr2NEt-delivered aldol adducts in 58% yield,14 which,
upon treatment with TBSCl, produced a separable mixture
of silyl ethers 11 and 12. Stepwise reduction15 of the 2-oxo
unit in pyrrolidinone 11 with DIBAL-H and then Et3SiH/
BF3‚OEt2 gave pyrrolidine 13 in 95% yield. Cleavage of the
(7) For other synthetic efforts toward the cyclopeptides from our group,
see: (a) Yu, S.; Pan, X.; Lin, X.; Ma, D. Angew. Chem., Int. Ed. 2005, 44,
135. (b) Zhu, J.; Ma, D. Angew. Chem., Int. Ed. 2003, 42, 5348. (c) Zou,
B.; Wei, J.; Cai, G.; Ma, D. Org. Lett. 2003, 5, 3503. (d) Ma, D.; Wu, Q.
Tetrahedron Lett. 2001, 42, 5279. (e) Ma, D.; Wu, Q. Tetrahedron Lett.
2000, 41, 9089.
Treatment of methyl (R)-lactate with TBDPSCl, followed
by hydrolysis of the ester group, and coupling of the resultant
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Org. Lett., Vol. 7, No. 13, 2005

mediated by EDC resulted in tripeptide 18. Fmoc cleavage
with diethylamine of 18 in acetonitrile produced the free
amine of 18, which was then coupled with the acid 9 to afford
linear tetrapeptide 19 in 78% yield. Liberation of the ester
moiety in 19 with Pd/C-catalyzed hydrogenolysis followed
by coupling with pentafluorophenol delivered an activated
ester. This ester was treated with TFA to effect cleavage of
the Boc group and then subjected to macrocyclization in
methylene chloride mediated by aqueous NaHCO3 to provide
cyclic tetrapeptide 20 in 56% yield. It is noteworthy that
attempts at DPPA-mediated macrolactamization gave 20 in
less than 20% yield. Finally, removal of the silyl protecting
group in 20 with TBAF furnished FR235222 1.18 Importantly,
analytical data obtained for synthetic 1 were found to be
indistinguishable from literature precedence.1
Scheme 3
In conclusion, we have described here the first total
synthesis of FR235222 featuring a novel and convenient
protocol for elaboration of protected trans-4-methylproline
and a facile synthesis of amino acid 9 via olefin metathesis
with Grubbs RCM catalyst. Further investigations on the
structure-activity relationship of this immunosuppressant
made possible by the advances described here are in progress.
Acknowledgment. The authors are grateful to the Chinese
Academy of Sciences (Grant KGCX2-SW-209), National
Natural Science Foundation of China (Grant 20321202), and
Science and Technology Commission of Shanghai Munici-
pality (Grant 04DZ14901) for their financial support.
Supporting Information Available: Experimental pro-
cedures and characterizations for compounds 5-9, 11-14,
17-20, and 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
silyl ether of 13 with TBAF followed by mesylation of
resultant hydroxy group and reductive deoxygenation with
Zn/NaI16 afforded N-Boc-O-Bzl-trans-4-methylproline 14.
The overall yield for seven steps from 10 was about 25%.
Substitution of formaldehyde with other aldehydes at the first
step is envisioned to allow rapid assembly of other trans-
4-alkyl-prolines. Thus, the simplicity and generality of the
presented methodology should be comparable to existing
methods.11-13
OL050991R
(9) (a) Gibson, S. E.; Gibson, V. C.; Keen, S. Chem. Commun. 1997,
1107. For a review on recent developments in olefin cross-metathesis, see:
(b) Connon, S. J.; Blechert, S. Angew. Chem., Int. Ed. 2003, 42, 1900.
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(15) Ezquerra, J.; Pedregal, C.; Escribano, A.; Carren˜o, M. C.; Garcia
Ruano, J. L. Tetrahedron Lett. 1994, 35, 2053.
With synthons 9 and 14 in hand, we set out to construct
FR235222 as shown in Scheme 3. Coupling of Fmoc-
isovaline 15, prepared by Mutter’s procedure,17 with (S)-
phenylalanine benzyl ester 16, afforded dipeptide 17 in 90%
yield. Hydrogenolysis of the benzyl group in 17 and
subsequent connection of the liberated amine from 14
(16) Fujimoto, Y.; Tatsuno, T. Tetrahedron Lett. 1976, 37, 3325.
(17) Nebel, K.; Mutter, M. Tetrahedron 1988, 44, 4793.
(18) Selected data for 1: [R]23 ) -129.1 (c 0.5, CHCl3), [lit1: [R]23
D
D
) -129.1 (c 0.5, CHCl3)]; 1H NMR (500 MHz, CDCl3) δ 7.52 (d, J )
10.2 Hz, 1H), 7.29-7.20 (m, 5H), 7.17 (d, J ) 10.6 Hz, 1H), 5.83 (s, 1H),
5.16 (ddd, J ) 6.3, 9.7, 9.9 Hz, 1H), 4.24-4.17 (m, 2H), 4.05 (dd, J )
7.8, 9.7 Hz, 1H), 3.54 (d, J ) 4.5 Hz, 1H), 3.24 (dd, J ) 9.9, 13.6 Hz,
1H), 2.95 (dd, J ) 6.1, 13.5 Hz, 1H), 2.73 (t, J ) 7.8 Hz, 1H), 2.63 (m,
1H), 2.54-2.28 (m, 4H), 2.16 (m, 1H), 1.80 (m, 1H), 1.63 (m, 3H), 1.38
(d, J ) 7.1 Hz, 3H), 1.33 (m, 5H), 1.28 (s, 3H), 0.88 (d, J ) 6.7 Hz, 3H),
0.84 (t, J ) 7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 214.4, 175.6, 171.1,
171.8, 137.1, 129.1, 128.6, 126.7, 72.6, 63.1, 58.0, 54.5, 53.9, 53.4, 37.3,
35.8, 33.1, 32.9, 28.8, 27.9, 25.3, 23.3, 22.4, 19.9, 18.2, 16.5, 8.4; ESIMS
m/z 557 (M + H)+, 579 (M + Na)+; HRMS for C30H44N4O6Na (M +
Na)+ calcd 579.3158, found 579.3156.
(8) (a) Pastuszak, J.; Gardner, J. H.; Singh, J.; Rich, D. H. J. Org. Chem.
1982, 27, 1877. (b) Kawai, M.; Gardner, J. H.; Rich, D. H. Tetrahedron
Lett. 1986, 27, 1877. (c) Schmidt, U.; Beutler, U.; Lieberknecht, A. Angew.
Chem., Int. Ed. Engl. 1989, 28, 333. (d) Baldwin, J. E.; Adlington, R. M.;
Godfrey, C. R. A.; Patel, V. K. Tetrahedron 1993, 49, 7837. (e) Taunton,
J.; Collins, J. L.; Schreiber, S. L. J. Am. Chem. Soc. 1996, 118, 10412. (f)
Durand, P.; Peralba, P.; Derain, V.; Komesli, S.; Renaut, P. Tetrahedron
Lett. 2001, 42, 2121. For a theoretical study for cyclization of cyclotet-
rapeptides, see: (g) Cavelier-Frontin, F.; Pepe, G.; Verducci, J.; Siri, D.;
Jacquier, R. J. Am. Chem. Soc. 1992, 114, 8885.
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