Synthesis of the Cyclic Peptidic Protease Inhibitor Eurystatin A Using Acyl Cyano Phosphorane Methodology

Full text of DOI:10.1021/jo9718253

8972
J . Org. Chem. 1997, 62, 8972-8973
Syn th esis of th e Cyclic P ep tid ic P r otea se
In h ibitor Eu r ysta tin A Usin g Acyl Cya n o
P h osp h or a n e Meth od ology
Harry H. Wasserman* and Anders K. Petersen
Department of Chemistry, Yale University,
New Haven, Connecticut 06520-8107
Received October 3, 1997
Eurystatins A (1a ) and B (1b),1 isolated from Strepto-
myces eurythermus R353-21, are potent inhibitors of the
serine protease prolyl endopeptidase (PED). Their struc-
tures incorporate (S)-3-amino-2-oxobutanoic acid, leucine,
and ornithine residues in a cyclic framework. Like the
thrombin inhibitors, cyclotheonamide A and B,2 and the
PED inhibitor, poststatin,3 the eurystatins contain an
R-keto amide residue considered to be the active center
for enzyme inhibition.4 In an earlier synthesis of 1a ,5
an intermediate R-hydroxy amide was oxidized to the
R-keto grouping at a late stage of the synthesis.
Of several possible pathways to the cyclic target, a
lactam-forming ring closure between the leucine and
ornithine residues was chosen. This route provided an
opportunity to explore the scope of the acyl cyano
phosphorane chemistry in two approaches to the desired
secopeptide 14, as depicted in Schemes 1 and 2.
The first synthesis (Scheme 1) began with Cbz-pro-
tected alanine (7), which was converted to the acyl cyano
phosphorane 8 with (cyanomethylene)triphenylphosphor-
ane (3) and EDCI. Ozonolysis generated the correspond-
ing diketo nitrile 9, which was reacted in situ with leucine
tert-butyl ester (10) to form the dipeptide 11 after treat-
ment with silver nitrate to decompose any cyanohydrin
formed. Removal of the Cbz-protecting group by catalytic
hydrogenolysis yielded R-amino ketone 12,11 which was
coupled with the carboxyl group of di-N-protected orni-
thine 13 to furnish the carbonyl-extended tripeptide 14.
Considering the potential for reactivity of intermediate
12, this one-pot chain elongation of 11 worked remark-
ably well, particularly on a small scale (0.46 mmol, 61%).
The second route (Scheme 2), featuring an ylide-
stabilized R-amino ketone (17),12 called for a different
timing of the ozonolysis. The required Fmoc-protected
acyl cyano phosphorane 16 was prepared from Fmoc
alanine (15) by the same procedure as described for
compound 8. The Fmoc protecting group was eliminated
smoothly with piperidine,13 and the crude amine 17 was
coupled with the ornithine derivative 13 to afford acyl
cyano phosphorane 18. Ozonolysis yielded 19 which was
trapped with leucine tert-butyl ester (10) to form the
tripeptide 14.
We have developed a novel method for forming R-keto
amide linkages from carboxylic acids 2, as reported in a
synthesis of the linear pentapeptide, poststatin.6 The
transformation involves the condensation of 2 with the
cyano phosphorane 3 to form the acyl cyano ylide 4.
Oxidation of 4 yields the labile R,â-diketo nitrile 5, which
then undergoes aminolysis to yield an R-keto amide 6
(eq 1).7-9 A significant feature of the process is its
applicability to the formation of cyclic peptides containing
R-keto amide linkages.2,10 In this report, we demonstrate
the use of this methodology in a synthesis of eurystatin
A (1a ).
(1) (a) Toda, S.; Obi, Y.; Numata, K.-i.; Hamagishi, Y.; Tomita, K.;
Komiyama, N.; Kotake, C.; Furumai, T.; Oki, T. J . Antibiot. 1992, 45,
1573. (b) Toda, S.; Kotake, C.; Tsuno, T.; Narita, Y.; Yamasaki, T.;
Konishi, M. J . Antibiot. 1992, 45, 1580. (c) Suzuki, K.; Toda, S.;
Furumai, T.; Fukagawa, Y.; Oki, T. J . Antibiot. 1994, 47, 982.
(2) (a) Fusetani, N.; Matsunaga, S.; Matsumoto, H.; Takebayashi,
Y. J . Am. Chem. Soc. 1990, 112, 7053. (b) Hagihara, M.; Schreiber, S.
L. J . Am. Chem. Soc. 1992, 114, 6570.
The (E)-6-methyl-2-heptenoic acid side chain (24)1b was
prepared from 4-methyl-1-pentanol (20) by oxidation to
the aldehyde 21.14 Wittig olefination with [(tert-butoxy-
carbonyl)methylene]triphenylphosphorane (22) afforded
(10) (a) Fusetani, N.; Sugawara, T.; Matsunaga, S.; Hirota, H. J .
Am. Chem. Soc. 1991, 113, 7811. (b) Kobayashi, J .; Itagaki, F.;
Shigemori, H.; Ishibashi, M.; Takahashi, K.; Ogura, M.; Nagasawa,
S.; Nakamura, T.; Hirota, H.; Ohta, T.; Nozoe, S. J . Am. Chem. Soc.
1991, 113, 7812. (c) Gunasekera, S. P.; Pomponi, S. A.; McCarthy, P.
J . J . Nat. Prod. 1994, 57, 79. (d) Greco, M. N.; Powell, E. T.; Hecker,
L. R.; Andrade-Gordon, P.; Kauffman, J . A.; Lewis, J . M.; Ganesh, V.;
Tulinsky, A.; Maryanoff, B. E. BioMed. Chem. Lett. 1996, 6, 2947.
(11) Amino ketone 12 proved to be sufficiently stable to undergo
peptide coupling successfully. In addition, it was found that the
precursor 11 kept its stereochemical integrity upon treatment with
weak bases such as sodium hydrogen carbonate and pyridine. However,
the stronger base triethylamine epimerized the alanine-like residue
completely within a few minutes.
(3) (a) Aoyagi, T.; Nagai, M.; Ogawa, K.; Kojima, F.; Okada, M.;
Ikeda, T.; Hamada, M.; Takeuchi, T. J . Antibiot. 1991, 44, 949. (b)
Nagai, M.; Ogawa, K.; Muraoka, Y.; Naganawa, H.; Aoyagi, T.;
Takeuchi, T. J . Antibiot. 1991, 44, 956. (c) Tsuda, M.; Muraoka, Y.;
Nagai, M.; Aoyagi, T.; Takeuchi, T. J . Antibiot. 1996, 49, 281.
(4) Kamei, H.; Ueki, T.; Obi, Y.; Fukagawa, Y.; Oki, T. J apan. J .
Pharmacol. 1992, 60, 377.
(5) Schmidt, U.; Weinbrenner, S. J . Chem. Soc., Chem. Commun.
1994, 1003.
(6) Wasserman, H. H.; Petersen, A. K. Tetrahedron Lett. 1997, 38,
953.
(7) Wasserman, H. H.; Ho, W.-B. J . Org. Chem. 1994, 59, 4364.
(8) This method7 has considerable advantages over conventional
multistep procedures for converting a carboxylic acid to the corre-
sponding carbonyl-extended R-keto amide. An example is outlined
below. (a) Wipf, P.; Kim, H.-Y. Tetrahedron Lett. 1992, 33, 4275. (b)
Wipf, P.; Kim, H. J . Org. Chem. 1993, 58, 5592.
(12) The ylide-stabilized R-amino ketone 17 was both chemically and
stereochemically stable. As reported previously,6 the cyano group may
undergo intramolecular addition of the amine during hydrogenolysis
of the Cbz-protected amine. In the present case, the problem was
circumvented by employing Fmoc-protected alanine.
(13) Carpino, L. A.; Han, G. Y. J . Org. Chem. 1972, 37, 3404.
(14) (a) Corey, E. J .; Suggs, J . W. Tetrahedron Lett. 1975, 2647. (b)
Carballeira, N.; Thompson, J . E.; Ayanoglu, E.; Djerassi, C. J . Org.
Chem. 1986, 51, 2751.
(15) Rappe, C. Organic Syntheses; Wiley: New York, 1973; Vol. 53,
p 123.
(16) Hirschmann, R.; Yao, W.; Arison, B.; Maechler, L.; Rosegay,
A.; Sprengeler, P. A.; Smith, A. B., III. Tetrahedron Lett. 1996, 37,
5637.
(9) Ylides and Imines of Phosphorus; J ohnson, A. W., Ed.; J ohn
Wiley and Sons: New York, 1993; Chapter 5.
S0022-3263(97)01825-2 CCC: $14.00 © 1997 American Chemical Society

Communications
J . Org. Chem., Vol. 62, No. 26, 1997 8973
Sch em e 1
Sch em e 2
Sch em e 3
Sch em e 4
tert-butyl (E)-6-methyl-2-heptenoate (23) as a 95:5 mix-
ture of geometric isomers. Deprotection with TFA gave
the free acid 24. Both 23 and 24 were obtained isomeri-
cally pure by fractional distillation. Using the three-step
sequence shown in Scheme 3, without isolation of inter-
mediates, an 88% overall yield was obtained for the (E)-
1b
and (Z) alkenes15 combined.
Simultaneous removal of the protecting groups at the
δ-amino and the carboxyl termini of tripeptide 14 was
accomplished with TFA.16 Cyclization of 25 was per-
formed in DMF under conditions of high dilution using
diphenyl phosphorazidate (DPPA) and sodium hydrogen
carbonate17 (cf. Scheme 4). Following hydrogenolysis of
the cyclic tripeptide 26, the amine 27 was coupled with
the R,â-unsaturated acid 24 to yield eurystatin A (1a ),
identical to the natural material.18
ditional evidence was obtained from a 1:1 mixture of 14
and its alanine epimer prepared by equilibration with
triethylamine. Cyclization, according to the protocol
shown in Scheme 4, furnished 26 and its alanine epimer
in a ratio of 1:1. This control experiment demonstrated
unambiguously that the stereochemistry of 26 was intact.
In summary, the versatility of the acyl cyano phospho-
rane methodology has been demonstrated by diverse
syntheses of tripeptide 14. In particular, it was shown
that the N-deprotection of the alanine-like residue could
be accomplished both before and after the oxidative
cleavage of the ylide; i.e., both of the R-amino ketones
12 and 17 were viable synthetic intermediates.
A noteworthy feature of this synthesis pertains to the
preservation of the stereochemical integrity throughout,
as shown by the appearance of a single set of signals in
1
the H and 13C NMR spectra of all intermediates. Ad-
Ack n ow led gm en t. We thank Dr. Owen B. Wallace
and Stella Huang of Bristol-Myers Squibb Pharmaceu-
tical Research Institute for their help in providing the
1H-NMR spectrum of eurystatin A. This work was
supported by grants from the NIH and the NSF.
(17) (a) Shioiri, T.; Ninomiya, K.; Yamada, S.-i. J . Am. Chem. Soc.
1972, 94, 6203. (b) Brady, S. F.; Freidinger, R. M.; Paleveda, W. J .;
Colton, C. D.; Homnick, C. F.; Whitter, W. L.; Curley, P.; Nutt, R. F.;
Veber, D. F. J . Org. Chem. 1987, 52, 764.
(18) In the absence of an authentic sample of eurystatin A, we relied
on the identity of the 1H NMR spectrum of our synthetic product with
the corresponding spectrum of the natural material supplied by Bristol-
Myers Squibb Pharmaceutical Research Institute. In addition, the 13C
NMR, IR, MS, HRMS, mp, optical rotation, elemental analysis, and
TLC were in agreement with the published data.1b
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and NMR spectra of obtained compounds (40 pages).
J O9718253