Concise total synthesis of the prolyl endopeptidase inhibitor eurystatin A via a novel Passerini reaction-deprotection-acyl migration strategy

Article abstract of DOI:10.1016/S0040-4039(01)01287-4

The Passerini reaction between suitably protected alaninal, leucine isonitrile, and ornithine components delivered adducts 10a,b in high yield. Orthogonal N-deprotection of 10a led, via a smooth O- to N-acyl migration, to 11, which constitutes the entire skeleton of the eurystatins. Subsequent deprotection, macrocyclization, elaboration, and final oxidation steps efficiently afforded eurystatin A 1a in high overall yield.

Full text of DOI:10.1016/S0040-4039(01)01287-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6271–6274
Concise total synthesis of the prolyl endopeptidase inhibitor
eurystatin A via a novel Passerini reaction–deprotection–acyl
migration strategy†
Timothy D. Owens, Gian-Luca Araldi, Ruth F. Nutt and J. Edward Semple*
Department of Medicinal Chemistry, Corvas International, Inc., 3030 Science Park Road, San Diego, CA 92121, USA
Received 14 June 2001; accepted 16 July 2001
Abstract—The Passerini reaction between suitably protected alaninal, leucine isonitrile, and ornithine components delivered
adducts 10a,b in high yield. Orthogonal N-deprotection of 10a led, via a smooth O- to N-acyl migration, to 11, which constitutes
the entire skeleton of the eurystatins. Subsequent deprotection, macrocyclization, elaboration, and final oxidation steps efficiently
afforded eurystatin A 1a in high overall yield. © 2001 Elsevier Science Ltd. All rights reserved.
Peptidyl and peptidomimetic a-ketoamide scaffolds are
useful in small molecule drug discovery programs as
transition-state analog (TSA) protease inhibitors.1 Such
covalent inhibitors generally exhibit potent in vitro
enzyme inhibitory activity, with sub-nanomolar equi-
librium inhibitor constants (Ki) being typical of repre-
sentative members.2 Accordingly, they are finding
increasing applications as potential therapeutics for
important disease indications.3–6 Eurystatins A (1a) and
B (1b) are 13-membered macrocyclic natural products
isolated from Streptomyces eurythermus R353-21 fea-
turing leucine, ornithine and a-ketoalaninamide sub-
units.7 They are reported to be potent inhibitors of the
serine protease prolyl endopeptidase (PEP). Due to
their relative structural simplicity, they serve as attrac-
tive targets for the development of new a-hydroxy-b-
amino amide and a-ketoamide methodologies.
migration strategy towards a concise total synthesis of
eurystatin A (1a).
The synthesis of isonitrile and aldehyde intermediates is
outlined in Scheme 1.12 Leucine isonitrile derivatives
8a,b were obtained from the corresponding amino
esters via N-formylation and dehydration according to
Ugi protocols.13 N-Fmoc-alaninal 9 was generated by
either LiTEPA reduction of the requisite UNCA
precursor14 or by DIBAL reduction of the correspond-
ing ester.15 These methods produced crude aldehyde 9
in high chemical and enantiomeric purity (typically
>95–97%), which was utilized immediately in the
Passerini reactions.
Utilizing the appropriate N-a-protected amino alde-
hydes 2 (Fig. 1), we recently disclosed novel variations
of the atom-economical Passerini reaction8 for the
direct production of either a-acyloxy-b-amino amides
31,9 or a-hydroxy-b-amino amide derivatives 4.9,10 We
observed that orthogonal N-deprotection of 3 under
mild conditions generates the a-acyloxy-b-amino inter-
mediate 5, which undergoes a facile O- to N-acyl shift
and delivers the stable adduct 6 in high yield. Interme-
diates 4 and 6 serve as useful advanced precursors to
a-ketoamide derivatives 7 by oxidation, preferably at a
very late synthetic stage. The recent report by Banfi et
al.11 on a related process prompts us to disclose our
application of the Passerini reaction–deprotection–acyl
Three-component reaction of leucine isonitriles 8a,b,
N-a-Fmoc-alaninal 9 and N-protected ornithine frag-
ments proceeded under very mild, essentially neutral
conditions, and provided the corresponding Passerini
adducts 10a,b in high yield and in multigram quantities
(Scheme 2).16 RP-HPLC or chiral HPLC analysis of
adducts 10a or 10b revealed two peaks in a ratio of ca.
* Corresponding author.
†
Dedicated to the memory of Joseph E. Semple.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01287-4

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T. D. Owens et al. / Tetrahedron Letters 42 (2001) 6271–6274
Figure 1. Variations of the Passerini reaction employing a-amino aldehydes. PG=protecting group.
Scheme 1. Reagents and conditions: (a) for Me ester: 1. HCO2H, NaCO2H, 70°C to rt; 2. Ac2O, 0°C to rt, 70% distilled; (b) for
Bn ester: 1. CH3CO2CHO, Et3N, 0°C to rt, ꢀ90%; (c) Cl3CO2CCl, NMM, CH2Cl2, −40 to −15°C, 86–98%; (d)
LiAl[OC(C2H5)3]3H (LiTEPA), THF, −40 to −5°C; HCl, ꢀquant.; (e) DIBAL, toluene, −78 to −20°C, 93%.
Scheme 2. Reagents and conditions: (a) 8a or 8b, 9, CH2Cl2, 0°C to rt, 3–5 days; Me ester 10a, 75%; Bn ester 10b, 80%; (b) for
Me ester 10a: Et2NH, rt, CH2Cl2, 12 h, 92%; (c) LiOH, MeOH, H2O, rt, 95%; (d) H2, Pd/C, MeOH, 45 psi, 3 days, ꢀquant. (e)
for Bn ester 10b: 1. Et2NH, CH2Cl2, rt, 75%; 2. H2, Pd/C, MeOH, 95%; (f) DPPA, DMF, NaHCO3, 0–4°C, 5 days, (0.005–0.01
M), 75–85% or HBTU, HOBt, DMF, rt, 3 days, (0.005 M), 50%; (g) HCl, EtOAc, 0°C to rt, ꢀquant.; (h) (E)-6-methyl-2-hep-
tenoic acid, EDC, HOAt, DMF, rt, 93%; (i) Pyr·SO3, DMSO, Et3N, 5°C to rt, 75%.

T. D. Owens et al. / Tetrahedron Letters 42 (2001) 6271–6274
6273
Acknowledgements
1:1.2, confirming retention of the original chirality and
formation of one diastereomeric pair at the newly-cre-
ated a-hydroxy center.8e,f Base-catalyzed Fmoc-depro-
tection of 10a led, via a smooth O- to N-acyl
migration, to 11 in high yield.17 Hydrolysis of 11 and
subsequent hydrogenolysis afforded advanced interme-
diate 12, which constitutes the entire acyclic skeleton of
the eurystatins. Deprotection–migration of 10b fol-
lowed by global hydrogenolysis provided a more direct
route to 12. Macrocyclization of the v-amino acid 12
occurred under standard high dilution conditions and
delivered the 13-membered macrocycle 13 in respectable
yields. Finally, 13 was elaborated to the target eury-
statin A 1a by sequential N-Boc deprotection, acylation
with (E)-6-methyl-2-heptenoic acid, and oxidation to
the a-ketoamide. Physical properties of our sample
agreed with literature values.18 It is of interest to note
that solubility issues with both 13 and the N-acylated
penultimate intermediate precluded the use of typical
Swern-type oxidation conditions in dichloromethane
solvent. However, the oxidation was efficiently and
reproducibly executed via the Parikh–von Doering
protocol19 in DMSO.
Stimulating discussions with T. K. Brunck, K. E. Pryor,
J. E. Reiner and O. A. Moreno are gratefully
acknowledged.
References
1. Semple, J. E.; Owens, T. D.; Araldi, G. L. Abstracts of
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cal Society, San Francisco, CA, March 26–30, 2000,
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2. (a) a-Ketoamide serine protease inhibitors: Semple, J. E.;
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T. Bioorg. Med. Chem. Lett. 2000, 10, 2807; (d) Otto, H.
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3. NS3-4A Hepatitis C protease and inhibitors thereof: (a)
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4. HIV protease inhibitors: Sheha, M. M.; Mahfouz, N. M.;
Hassan, H. Y.; Youssef, A. F.; Mimoto, T.; Kiso, Y. Eur.
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5. Calpain inhibitors: (a) De Biasi, R. L.; Edelstein, C. L.;
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Our approach to eurystatin A 1a via the key Passerini
reaction–deprotection–acyl migration strategy is con-
vergent and highly efficient. Proceeding over nine to ten
steps in 20–26% overall yield from commercially avail-
able leucine esters, it compares quite favorably to other
current routes.18,20 Although Wasserman’s elegant acyl
cyanophosphorane oxidation approach to 1a proceeded
concisely over ten steps, the base-labile a-ketoamide
functionality is revealed early in the synthesis
sequence.18 Our experience,1,2,10 as well as literature
precedent,2c,21 indicates that a-ketoamide racemization
issues may arise during completion of such a synthesis.
Schmidt’s simple Passerini–acyl cleavage method to 1a
proceeded in 14 steps and utilized benzoic acid for
adduct formation.20 The benzoyl moiety was subse-
quently cleaved and discarded, producing a simpler
a-hydroxyamide derivative akin to 4. We contrast these
approaches with our method, whereby a suitably pro-
tected ornithine derivative is used as the carboxylic acid
component in the key Passerini reaction and is retained
throughout the succeeding deprotection–migration
stages, ultimately shortening the synthesis of this target
by several steps.
6. HCMV protease inhibitors: LaPlante, S. R.; Bonneau, P.
R.; Aubry, N.; Cameron, D. R.; Deziel, R.; Grande-
Maitre, C.; Plouffe, C.; Tong, L.; Kawai, S. H. J. Am.
Chem. Soc. 1999, 121, 2974.
7. Eurystatins A and B, isolation and structure determina-
tion: Toda, S.; Kotake, C.; Tsuno, T.; Narita, Y.;
Yomasaki, T.; Konishi, M. J. Antibiot. 1992, 45, 1580.
8. Passerini reaction: (a) Passerini, M. Gazz. Chim. Ital.
1921, 51, 126; (b) Passerini, M.; Ragni, G. Gazz. Chim.
Ital. 1931, 61, 964; (c) Authoritative review: Ugi, I. In
Isonitrile Chemistry, Ugi, I., Ed.; Academic Press: New
York, 1971; Chapter 7; (d) TiCl4 activation: Carofiglio,
T., et al. Organometallics 1993, 12, 2726; (e) Seebach, D.;
Adam, G.; Gees, T.; Schiess, M.; Weigand, W. Chem.
Ber. 1988, 121, 507; (f) Seebach, D.; Schiess, M. Helv.
Chim. Acta 1983, 66, 1618.
In conclusion, the utility of this technology for the
rapid assembly of relatively complex a-ketoamide
amide-containing natural products is highlighted by a
concise total synthesis of eurystatin A (1a). The key
Passerini reaction, deprotection and acyl migration
steps proceed in moderate to high yields and under
mild conditions. Other noteworthy features of our
methodology include high atom-economy and very late
stage oxidation to the reactive a-ketoamide moiety,
which minimizes potential stability and racemization
issues. Numerous applications to the synthesis of a-
ketoamide natural products and protease inhibitors are
envisioned and will form the basis of forthcoming
publications from our laboratories.
9. Semple, J. E.; Levy, O. E. PCT Int. Appl. WO 0035868
A2, June, 2000.
10. Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E.
Org. Lett. 2000, 2, 2769.
11. Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000,
985.
12. All new compounds were characterized by full spectro-
scopic (NMR, low/high resolution MS) analysis. Yields
refer to spectroscopically and chromatographically
homogenous (]95% by HPLC, TLC) materials.
13. (a) Skorna, G.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1977,
16, 259; (b) Urban, R.; Marquarding, D.; Seidel, P.; Ugi,
I. Chem. Ber. 1977, 110, 2012; (c) Giesemann, G.; von
Hinrichs, E.; Ugi, I. J. Chem. Res. (S) 1982, 79.

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T. D. Owens et al. / Tetrahedron Letters 42 (2001) 6271–6274
14. Fehrentz, J. A.; Pothion, C.; Califano, J. C.; Loffet, A.;
Martinez, J. Tetrahedron Lett. 1994, 35, 9031.
15. Synthesis of a-amino aldehyde derivatives: (a) Jurczak, J.;
Golebiowski, A. Chem. Rev. 1989, 89, 149; (b) Ho, P. T.;
Ngu, K. J. Org. Chem. 1993, 58, 2313; (c) Hyun, S. I.;
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was removed and the residue was purified by flash chro-
matography on silica gel, eluting with dichloromethane/
isopropanol, 97:3 to afford 2.30 g (91.6% yield) of 11 as
a colorless viscous oil. TLC (silica, CH2Cl2/IPA, 4:1): Rf
ꢀ0.5, 0.45. MS: [MH]+ 595.7, [MNa]+ 617.7. RP-HPLC
1
analysis indicated two diastereomers ca. 1/1.2. H NMR
16. Experimental for 10a: A solution of a-N-Boc-v-N-Cbz-
Orn-OH (10.63 g, 29.0 mmol), isonitrile 8a (4.50 g, 29.0
mmol) and freshly prepared alaninal 9 (9.10 g, 30.8
mmol) in 60 mL of dry dichloromethane was stirred at rt
under nitrogen for 4 days. Solvent was removed and the
residue was purified by flash chromatography on silica
gel, eluting with a dichloromethane to dichloromethane/
isopropanol: 99/1 gradient system to afford 17.65 g
(74.5% yield) of 10a as a pale yellow, viscous oil. TLC
(silica, CH2Cl2/IPA, 9:1 or EtOAc): two overlapping
spots. MS: [MH]+ 817.9, [MNa]+ 839.9. RP-HPLC and
1H NMR analysis indicated two diastereomers, ca. 1:1.2.
17. Experimental for 11: To a solution of 10a (3.44 g, 4.20
mmol) in 20 mL of dry dichloromethane at rt under
nitrogen was added Et2NH (3.07 g, 42.0 mmol, 4.35 mL).
After stirring the yellow solution overnight, the solvent
(CD3OD, 400 MHz): l 0.90–1.00 (ov. d, 6H), 1.05–1.25
(m, 3H), 1.45 (s, 9H), 1.50–1.85 (br. m, 7H), 3.1 (m, 2H),
3.65+3.75 (2s, 3H total), 4.00 (br s, 2H), 4.21–4.31 (m,
1H), 4.45–4.55 (m, 1H), 5.11 (s, 2H), 7.25–7.40 (m, 5H).
18. Wasserman, H. H.; Peterson, A. K. J. Org. Chem. 1997,
62, 8972.
19. For reviews, see: (a) Tidwell, T. T. Synthesis 1990, 857;
(b) Tidwell, T. T. Org. React. 1990, 39, 297; (c) Mancuso,
A. J.; Swern, D. Synthesis 1981, 165.
20. Schmidt, U.; Weinbrenner, S. J. Chem. Soc., Chem. Com-
mun. 1994, 1003.
21. (a) For a-ketoamide racemization issues, see: Brady, S.
F.; Nutt, R. F., et al. Bioorg. Med. Chem. 1995, 3, 1063;
(b) For a discussion of related issues with P1-aldehyde
derivatives, see: Siev, D. V.; Semple, J. E. Org. Lett. 2000,
2, 19.
.