Total synthesis of syringolin A

Article abstract of DOI:10.1021/ol101252y

(Equation Presented). A convergent, efficient synthesis of syringolin A has been accomplished in 13 steps from commercially available materials, Garners aldehyde and l-valine. The unnatural 3,4-dehydrolysine fragment was prepared using successive Johnson-Claisen/Curtius rearrangement reactions. The macrolactamization and late-stage introduction of the side chain will provide convenient access to analogues of this promising proteasome inhibitor.

Full text of DOI:10.1021/ol101252y

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3453-3455
Total Synthesis of Syringolin A
Chunhui Dai and Corey R. J. Stephenson*
Department of Chemistry, Boston UniVersity, Boston, Massachusetts 02215
crjsteph@bu.edu
Received May 31, 2010
ABSTRACT
A convergent, efficient synthesis of syringolin A has been accomplished in 13 steps from commercially available materials, Garner’s aldehyde
and L-valine. The unnatural 3,4-dehydrolysine fragment was prepared using successive Johnson-Claisen/Curtius rearrangement reactions.
The macrolactamization and late-stage introduction of the side chain will provide convenient access to analogues of this promising proteasome
inhibitor.
Syringolin A was isolated from strains of the plant pathogen
Pseudomonas synringae pV Syringae (Pss) in 1998.1 It
possesses a unique 12-membered dipeptide ring structure
containing two (E)-configured double bonds and a urea side
chain. It was initially reported to induce death of hypersensi-
tive cells colonized by powdery mildew in wheat and other
cereals2 and more recently to inhibit cell proliferation and
induce apoptosis in neuroblastoma and ovarian cancer cells.3
Studies revealed that syringolin A inhibited both p53 wild-
type NB cell lines SK-N-SH and the p53 mutant NB cell
line LAN-1 with IC50 values between 20 and 25 µM.
Furthermore, it was shown to selectively inhibit all three
catalytic activities of eukaryotic proteasomes by covalent
modification of a threonine residue in the active site.4 The
proteasome is essential for protein degradation5,6 and has
recently been validated in the clinic as a biological target
for the treatment of multiple myeloma with the launch of
bortezomib by Millennium Pharmaceuticals (now Takeda
Oncology).7 Unfortunately, expansion of the clinical utility
of bortezomib beyond multiple myeloma has yet to be
achieved, and its use has been limited by poor pharmacologi-
cal properties, a requirement for intravenous administration,
neurotoxicity, and selectivity for only one of the three
enzymatic functions of the 20S proteasome.8 Therefore, the
development of new proteasome inhibitors that display
improved activity and improved properties has been under
intense investigation. Given the promising bioactivities and
unique structure of syringolin A, it has drawn considerable
attention to investigate its synthesis along with evaluation
of the biological activity of its analogues.9,10
To synthesize syringolin A and its analogues for biological
evaluation, we sought a convergent and flexible approach
utilizing commercially available amino acids to introduce
all of the stereocenters (Figure 1). Meanwhile, we also
(1) (a) Wa¨spi, U.; Blanc, D.; Winkler, T.; Ruedi, P.; Dudler, R. Mol.
Plant-Microbe Interact. 1998, 11, 727. (b) Wa¨spi, U.; Hassa, P.; Staempfli,
A. A.; Molleyres, L. P.; Winker, T.; Dudler, R. Microbiol. Res. 1999, 154,
89.
(7) (a) Adams, J. Cancer Cell 2004, 5, 417. (b) Kyle, R. A.; Rajkumar,
S. V. N. Engl. J. Med. 2004, 351, 1860. (c) Rajkumar, S. V.; Richardson,
P. G.; Hideshima, T.; Anderson, K. C. J. Clin. Oncol. 2005, 23, 630. (d)
Borissenko, L.; Groll, M. Chem. ReV. 2007, 107, 687.
(2) Wa¨spi, U.; Schweizer, P.; Dudler, R. Plant Cell 2001, 13, 153.
(3) Coleman, C. S.; Rocetes, J. P.; Park, D. J.; Wallick, C. J.; Warn-
Cramer, B. J.; Michel, K.; Dudler, R.; Bachmann, A. S. Cell Proliferation
2006, 39, 599.
(8) Chauhan, D.; Hideshima, T.; Anderson, K. C. Annu. ReV. Pharmacol.
Toxicol. 2005, 45, 465.
(4) Groll, M.; Schellenberg, B.; Bachmann, A. S.; Archer, C. R.; Huber,
R.; Powell, T. K.; Lindow, S.; Kaiser, M.; Dudler, R. Nature 2008, 452,
755.
(9) (a) Clerc, J.; Groll, M.; Illich, D. J.; Bachmann, A. S.; Huber, R.;
Schellenberg, B.; Dudler, R.; Kaiser, M. Proc. Natl. Acad. Sci. U.S.A. 2009,
106, 6507. (b) Clerc, J.; Schellenberg, B.; Groll, M.; Bachmann, A. S.;
Huber, R.; Dudler, R.; Kaiser, M. Eur. J. Org. Chem. 2010, published online
(5) (a) Ciechanover, A. Angew. Chem., Int. Ed. 2005, 44, 5944. (b)
Goldberg, A. L. Biochem. Soc. Trans. 2007, 35, 12
(6) For a discussion of the proteasome as a target for cancer chemo-
therapy, see: Almond, J. B.; Cohen, G. M. Leukemia 2002, 16, 433
.
May 19, 2010; DOI: 10.1002/ejoc.201000317
(10) Pirrung, M. C.; Biswas, G.; Ibarra-Rivera, T. R. Org. Lett. 2010,
12, 2402
.
.
.
10.1021/ol101252y  2010 American Chemical Society
Published on Web 07/02/2010

Scheme 1. Synthesis of Fragment 3
Figure 1. Retrosynthetic analysis of syringolin A.
tion to avoid the formation of the corresponding symmetrical
urea. Intermediate 11 was obtained with 81% yield and then
selectively deprotected using p-TsOH to afford alcohol 12.
Subsequent oxidation to corresponding acid using two-step
protocols proved to be problematic because of the instability
of the aldehyde intermediate.17,18 Gratifyingly, after extensive
studies on the oxidation conditions, we found that the Jones
reagent efficiently converted alcohol 12 to the desired amino
acid 4 in 1 h at 0 °C in 78% yield (86% brsm) without any
observable epimerization of the stereogenic center (Scheme
2).19
envisioned introduction of the side chain at a late stage,
which would expedite the synthesis of other analogues. The
key 12-membered macrocycle core 2 would be formed
through an intramolecular peptide coupling at the less
hindered position. Horner-Wadsworth-Emmons olefination
would provide the R,ꢀ-unsaturated ester 3. The (E)-olefin
of 4 could be furnished by Johnson-Claisen rearrangement,
and its precursor could be generated from easily accessible
Garner’s aldehyde 5.11 Herein, we report the successful
execution of this strategy to the enantioselective synthesis
of syringolin A.
The synthesis of the R,ꢀ-unsaturated ester 3 began with
Boc protection of L-valine, followed by coupling with N,O-
dimethylhydroxylamine to afford Weinreb amide 6.12 Reduc-
tion of the amide to the corresponding aldehyde with LiAlH4,
followed by immediate subjection of the crude material to
Horner-Wadsworth-Emmons olefination provided the (E)-
olefin 7 in 91% yield and >99% ee.13,14 Removal of the
N-Boc protecting group provided amine 3 in quantitative
yield (Scheme 1).
Scheme 2. Synthesis of Fragment 4
To access the 3,4-dehydrolysine fragment,15 Garner’s
aldehyde 5, prepared from D-serine according to the literature
procedure,11 was treated with vinyl magnesium bromide in
THF for 30 min at -78 °C and then for 2 h at room
temperature to afford vinyl alcohol 8 in 86% yield. Johnson-
Claisen rearrangement employing excess CH3C(OMe)3 in
xylene at reflux provided the desired R,ꢀ-unsaturated ester
9 in high yield and excellent stereoselectivity (E/Z > 95:
5).16 Hydrolysis led to the carboxylic acid 10, which was
subjected to Curtius rearrangement using DPPA, Et3N, and
tert-butanol in the presence of activated 4 Å molecular sieves.
The use of sieves was found to be crucial in this transforma-
(11) Garner, P.; Park, J. M. J. Org. Chem. 1987, 52, 2361.
(12) Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22, 3815.
(13) The stereochemical integrity of the R-stereogenic center and % ee
of 7 were determined by HPLC analysis.
(14) For seminal studies on the preparation and utility of vinylogous
amino acids, see: (a) Hagihara, M.; Anthony, N. J.; Stout, T. J.; Clardy, J.;
Schreiber, S. L. J. Am. Chem. Soc. 1992, 114, 6568. (b) Hagihara, M.;
With both coupling fragments 3 and 4 in hand, the first
peptide bond formation was accomplished using EDCI,
(17) Decomposition and epimerization were always observed when
handling the aldehyde. In contrast, the carboxylic acid was found to be
Schreiber, S. L. J. Am. Chem. Soc. 1992, 114, 6570
.
(15) During the preparation of this manuscript, Kaiser and coworkers
described a similar approach to fragment 4; see ref 9b.
(16) E/Z selectivity of compound 9 was determined by analysis of the
crude 1H NMR.
quite stable and did not undergo noticeable epimerization .
(18) For an example of the lability of the products of oxidation of
vinylglycinol derivatives, see: Campbell, A. D.; Taylor, R. J. K.; Raynham,
T. M. Chem. Commum. 1999, 24, 245
.
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Org. Lett., Vol. 12, No. 15, 2010

i
HOBt, and Pr2NEt in dichloromethane over 12 h to afford
13 in 70% yield. Removal of the C- and N-terminal
protecting groups generated the macrolactamization precursor
14, which was then treated with peptide coupling reagents
BOP and HOAt in DMF for 48 h, providing the macrocycle
220 in 15% yield (Scheme 3).21 Although the yield of this
Scheme 4. Completion of the Synthesis
Scheme 3. Synthesis of Macrolactam 2
A were identical in all respects with those previously reported
for the natural product.24
In summary, utilizing commercially available amino acids,
we have developed a convergent synthesis of the proteasome
inhibitor syringolin A in 13 steps from Garner’s aldehyde.
In addition, it demonstrates that the macrolactamization of
the 12-membered cycle core can be accomplished via peptide
coupling with reasonable yield, which will allow for facile
modification of both the macrocycle and side chain. Studies
directed toward the preparation of analogues of syringolin
A and their evaluation as proteasome and protease inhibitors
are currently underway and will be reported in due course.
macrolactamization is low, this approach provides the most
straightforward approach to syringolin A and its analogues.
Similar macrolactamization approaches to syringolin A have
previously been reported to be unsuccessful.9 However, the
macrolactamization containing a lysine residue in place of
3,4-dehydrolysine has been reported to provide the macro-
lactam of syringolin B, in 30% yield.9
The final stage of the synthesis required introduction of
the urea side chain 17, which was obtained by treatment of
benzyl (S)-(-)-2-isocyanato-3-methylbutyrate22 with tert-
butyl valine in the presence of iPr2NEt,23 followed by benzyl
deprotection via hydrogenolysis with 10% Pd/C in methanol.
Removal of Cbz group in macrocycle core 2 with HBr in
acetic acid afforded the corresponding free amine in quan-
titative yield, which was then coupled with side chain 17
Acknowledgment. Boston University and the Department
of Chemistry are gratefully acknowledged for financial
support. NMR (CHE-0619339) and MS (CHE-0443618)
facilities at Boston University are supported by the NSF.
We thank Professor John Porco (Boston University) for
helpful discussions and suggestions.
i
using BOP, HOAt, and Pr2NEt to afford 18 in 85% yield.
Supporting Information Available: Experimental pro-
1
cedures and H and 13C NMR spectra for all compounds.
The tert-butyl protecting group was cleaved under mild acidic
condition using formic acid, affording syringolin A in 94%
isolated yield (Scheme 4). The data for synthetic syringolin
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL101252Y
(19) The enantiomeric purity of compound 4 was determined by its
coupling with (S)-(-)-R-methylbenzylamine (EDCI, HOBt) and analysis
of the crude 1H NMR as compared with the coupling of 4 with (()-R-
methylbenzylamine. See Supporting Information for further details.
(20) This intermediate was also prepared in the recently reported
synthesis by Pirrung and coworkers using a Horner-Wadsworth-Emmons
macrocyclization. See ref 10 for details.
(21) Analysis of the crude UPLC-MS data indicated that dimeric and
trimeric products are also formed during the formation of the strained 12-
membered macrocycle.
(22) The isocyanate was prepared using triphosgene; see: Sunder-
Plassmann, N.; Sarli, V.; Gartner, M.; Utz, M.; Seiler, J.; Huemmer, S.;
Mayer, T. U.; Surrey, T.; Giannis, A. Biorg. Med. Chem. 2005, 13, 6094.
(23) Majer, P.; Randad, R. S. J. Org. Chem. 1994, 59, 1937.
(24) Data for synthetic syringolin A (1). Rf (85:15 CH2Cl2/MeOH, 2% v/v
AcOH): 0.28; 1H NMR (400 MHz, DMSO-d6): δ 12.5 (br s, 1 H), 7.98-8.06
(m, 2 H), 7.47 (t, J ) 7.2 Hz, 1 H), 6.68 (dd, J ) 15.5, 5.5 Hz, 1 H), 6.29 (d,
J ) 8.6 Hz, 1 H), 6.25 (d, J ) 9.0 Hz, 1 H), 6.09 (d, J ) 15.2 Hz, 1 H), 5.59
(dt, J ) 15.8, 7.6 Hz, 1 H), 5.41 (dd, J ) 15.6, 7.8 Hz, 1 H), 4.85 (m, 1 H),
4.02-4.12 (m, 2 H), 3.94 (dd, J ) 8.8, 4.9 Hz, 1 H), 3.08-3.25 (m, 2 H),
2.23-2.32 (m, 1 H), 1.86-2.02 (m, 3 H), 1.68-1.78 (m, 1 H), 0.94 (d, J )
6.7 Hz, 3 H), 0.90 (d, J ) 6.7 Hz, 3 H), 0.82-0.87 (m, 9 H), 0.77 (d, J ) 6.9
Hz, 3 H); 13C NMR (100 MHz, DMSO-d6): δ 173.5, 171.6, 168.9, 166.3, 157.6,
143.3, 133.0, 126.1, 121.5, 58.2, 57.6, 55.5, 53.6, 42.5, 35.0, 31.5, 30.9, 30.6,
19.8, 19.4, 19.3, 19.2, 17.8, 17.6; HRMS (ESI) m/z calcd for C24H39N5O6 [M
+ H]+ 494.2979, found 494.3003.
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