Enantioselective total synthesis of cylindramide

Article abstract of DOI:10.1002/anie.200461823

Key steps in the convergent enantio-selective synthesis of the cytotoxic natural products (see structural formula) are tandem Michael addition/electrophilic trapping reactions, Sonogashira coupling, Julia-Kocienski olefination, and macro-cyclization. Form

Full text of DOI:10.1002/anie.200461823

Communications
Natural Product Synthesis
Enantioselective Total Synthesis of
Cylindramide**
Nicolai Cramer, Sabine Laschat,* Angelika Baro,
Harald Schwalbe, and Christian Richter
Dedicated to Professor Franz Effenberger
on the occasion of his 75th birthday
Marine organisms produce a tremendous variety of bioactive
secondary metabolites,[1] which are often used as lead
structures in the development of novel pharmaceuticals. In
1993 Fusetani et al. isolated cylindramide (1) from the marine
sponge Halichondria cylindrata, and it was found to exhibit
pronounced cytotoxicity against B16 melanoma cells.[2] Cylin-
dramide belongs to the class of complex tetramic acid lactams,
which also includes structurally related compounds such as
discodermide from the Caribbean marine sponge Discoder-
mia dissoluta,[3] alteramide A from a bacterium Alteromo-
nas sp. associated with the sponge Halichondria okadai,[4]
aburatubolactam A isolated from a cultured broth of a
Streptomyces culture stemming from a mollusk,[5] maltophilin
produced by Stenotrophomonas maltophilia,[6] and geodin A
magnesium salt, which was recently isolated from the South-
ern Australian marine sponge Geodia.[7] Besides the tetramic
acid unit, a substituted bicyclo[3.3.0]octane skeleton is the
characteristic feature of these macrocyclic compounds. How-
ever, to our knowledge, syntheses of this type of macrocycles
have not yet been reported, with exception of one model
study[8] and two total syntheses of the related ikarugamycin, in
which the bicyclooctane moiety is replaced by a decahydroin-
dacene framework.[9,10]
Scheme 1. Retrosynthesis of cylindramide (1).
Michael addition and an electrophilic trapping reaction. It
should be possible to couple hydroxyornithine 2 and phenyl-
tetrazolyl (PT) sulfone 3 to pentalene 4 by Sonogashira
coupling and Julia–Kocienski olefination, respectively. After
macrocyclization, the formation of the tetramic acid unit
should conclude the synthesis.
The generation of building block 2 (Scheme 2) started
with a Wittig olefination of the Boc-protected b-aminoalde-
hyde 6[11] to give derivative 7, which was converted into
alcohol 8 by Sharpless asymmetric dihydroxylation and
Our synthetic strategy was based on the retrosynthetic
degradation of cylindramide (1) into the three fragments 2–4
(Scheme 1). The pentalene derivative 4 should be accessible
from pentalenone 5 by a tandem process consisting of a
subsequent regioselective nosylation.[12]
A sequence of
nucleophilic substitution with tetramethylguanidinium azide
(TMGN3),[13] protection of the OH function, and removal of
the Boc protecting group afforded a-azidoester 9. Following
the method of Shioiri et al.,[14] we finally coupled derivative 9
with b-iodoacrylic acid 10 to give fragment 2 in 83% yield.
Sulfone 3 was prepared from precursor 2,2,5-trimethyl-
4H-1,3-dioxin-4-one (11)[15] (Scheme 3). Alkylation, ozonol-
ysis, and reductive workup gave alcohol 13. The phenyl-
tetrazolthiol was introduced by a Mitsunobu reaction, and the
resulting intermediate 14 oxidized to PT-sulfone 3 with H2O2
in the presence of a Mo catalyst.[16]
Enantiopure bicyclo[3.3.0]octanonacetal 15, which is
available in five steps by transannular Pd-catalyzed coupling
of cycloocta-1,5-diene followed by enzymatic resolution,[17]
served as the starting material for the pentalene unit. We
employed the IBX method of Nicolaou et al.[18] to oxidize
ketone 15 affording enone 16 in 63% yield (Scheme 4).
The 1,4-addition of Me2CuLi to enone 16 and trapping
with Comins reagent (17)[19] resulted, however, in a mixture of
vinyl triflate 19 (47%) and ketone 18 (39%). Fortunately, the
latter could be converted into the desired triflate 19 in 72%
[*] Dipl.-Chem. N. Cramer, Prof. S. Laschat, Dr. A. Baro
Institut fꢀr Organische Chemie
Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax: (+49)711-685-4285
E-mail: sabine.laschat@oc.uni-stuttgart.de
Prof. H. Schwalbe, Dr. C. Richter
Institut fꢀr Organische Chemie und Chemische Biologie
Zentrum fꢀr Biomolekulare Magnetische Resonanz
Johann Wolfgang Goethe-Universitꢁt Frankfurt
Marie-Curie-Strasse 11, 60439 Frankfurt am Main (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft,
the Ministerium fꢀr Wissenschaft, Forschung und Kunst des Landes
Baden-Wꢀrttemberg, and the Fonds der Chemischen Industrie
(fellowship for N.C.). We thank Dipl.-Chem. N. Steinke and M.
Buchweitz for their assistance, Prof. N. Fusetani and Dr. S.
Matsunaga for reference spectra, and the Amersham Buchler
company for chemicals.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
820
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DOI: 10.1002/anie.200461823
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Angewandte
Chemie
Scheme 2. Reagents and conditions: a) 1. AD-mix-a, tBuOH/H2O,
MeSO2NH2, 08C, 1 d, 95%, 96% ee, 2. NsCl (1 equiv), NEt3, CH2Cl2,
08C, 12 h, 65%; b) 1. tetramethylguanidinium azide (TMGN3)
(6 equiv), DMF, 508C, 15 h, 96%, 85:15 d.r., 2. TBSCl, DMAP, DMF,
97%, 3. TFA, CH2Cl2, 08C, 1 h, quant. Boc=tert-butoxycarbonyl,
DEPC=diethyl cyanophosphonate, DMAP=4-dimethylaminopyridine,
DMF=dimethylformamide, Ns=4-nitrobenzenesulfonyl, TFA=tri-
fluoroacetic acid, TBS=tert-butyldimethylsilyl.
Scheme 4. Reagents and conditions: a) LDA, THF, TMSCl, ꢀ788C!
08C, 4-methoxypyridin-N-oxide (MPO), IBX, DMSO, CH2Cl2, RT,
45 min, 63%; b) Me2CuLi, THF, 17, ꢀ788C!08C; c) 1. PPTS, acetone,
H2O, reflux, quant., 2. LDA, THF, TMSCl, ꢀ788C!08C, then MPO,
ꢁ
IBX, DMSO, CH2Cl2, RT, 30 min, 87%; d) TMS-C CCH3, tBuLi,
TMEDA, THF, ꢀ408C, 1 h, CuI, TMSCl, THF, ꢀ788C, 5, 2 h, then
BF3·OEt2, HC(OMe)3, CH2Cl2, ꢀ208C, 1 h, 53%; e) 1. NaBH4, MeOH,
08C, 2. (Im)2CS (5 equiv), DMAP (5 equiv), DCE, reflux, 16 h,
3. Bu3SnH, AIBN, toluene, 1108C, 45 min, 57% over 3 steps, 4. EtOH,
H2O, AgNO3, 08C, 3 h, KCN, 30 min, 85%; f) 2, [Pd(PPh3)4], CuI, NEt3,
THF, 91%; g) 1. PPTS, acetone, H2O, reflux, 2. 3 (3 equiv), NaHMDS,
DME, ꢀ558C!RT, 52%; h) 1. PPh3, THF, H2O, RT, 24 h, 2. toluene,
2.5ꢂ10ꢀ4 m, reflux, 10 h, 82%, 3. H2, Pd/BaSO4, quinoline, EtOH, 66%
(referred to recovered starting material), 4. HF/MeCN, RT, 3 h, 91%.
AIBN=azobisisobutyronitrile, DCE=dichloroethane, DME=dime-
thoxyethane, dppf=1,1’-bis(diphenylphosphanyl)ferrocene, PPTS=pyr-
idinium p-toluenesulfonate, NaHMDS=sodium hexamethyldisilazide,
TMEDA=N,N,N’,N’-tetramethylethylenediamine, TMS=trimethylsilyl.
reaction of 5 with TMS-protected propynyl cuprate[21] and
subsequent addition of orthoformate and BF3·OEt2, the
pentalene derivative 21 was isolated in 53% yield as a
single diastereoisomer, as supported by NMR and GC data.
Compound 21 was reduced with NaBH4 to give a 1:1 mixture
of diastereomeric alcohols. Despite the steric hindrance of the
convex bicyclo[3.3.0]octane moiety, Barton deoxygenation
followed by desilylation to 22 (48% yield) was successful.
After all the required coupling components were avail-
able, we followed our strategy to create the macrocycle.
Scheme 3. Reagents and conditions: a) 1. LDA, DMPU, THF, 11, 08C,
1 h, 12, ꢀ408C!RT, 16 h, 50%; 2. O3, MeOH/CH2Cl2/pyridine (3:3:1),
ꢀ788C, then NaBH4, 64%. DEAD=diethylazodicarboxylate,
DMPU=N,N’-dimethyl-N,N’-propylene urea.
yield by a second deprotonation with LDA and reaction with
17. Pentalenacetal 20, prepared from 19 in 90% yield by Pd-
catalyzed reduction using Et3SiH,[20] was deprotected and
transformed into the pentalenone 5 in 87% yield. After
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Communications
Sonogashira coupling[22] of pentalene 22 with iodoacrylate 2
[6] M. Jakobi, G. Winkelmann, D. Kaiser, C. Kempter, G. Jung, G.
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[12] P. R. Fleming, K. B. Sharpless, J. Org. Chem. 1991, 56, 2869 –
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[13] The replacement of ONs with the N3 group proceeded with
partial epimerization. The mixture of diastereomers (85:15) of
both TBS-protected azides, however, could be separated by
chromatography.
[14] S. Takuma, K. Hamada, T. Shioiri, Chem. Pharm. Bull. 1982, 30,
3147 – 3153.
provided the enyne 23 in 91% yield. The dimethyl acetal
group was hydrolyzed to give the corresponding intermediate
aldehyde, which was olefinated with sulfone 3 under Julia–
Kocienski conditions[16] giving the E-alkene 24 (52%).[23]
Cyclization to the macrolactam could be realized in 82%
yield by Staudinger reduction of the azide function, subse-
quent dilution in toluene (2.5 ꢀ 10ꢀ4 m), and heating the dilute
solution at reflux. The Lindlar reduction of the enyne moiety
to yield the Z,E-diene turned out to be difficult. Only
incomplete conversion resulted in 66% yield for the desired
diene, whereas overreduction was observed at longer reaction
times. Removal of the silyl protecting group afforded the
macrocyclic b-hydroxy ester 25 in 91% yield. Brief heating of
25 with NaOMe in MeOH[24] finally gave the target macro-
lactam 1 with partial epimerization (3:1). The major diaster-
eomer (2S,3S)-1 could be isolated in pure form by reversed-
phase chromatography, and the spectroscopic data of syn-
thetic cylindramide (1, major diastereomer) are in accordance
with those of the natural product.[25]
[15] H. Graalfs, R. Frꢁhlich, C. Wolff, J. Mattay, Eur. J. Org. Chem.
1999, 1057 – 1073.
[16] P. R. Blakemore, J. Chem. Soc. Perkin Trans. 1 2002, 2563 – 2585.
[17] a) Y.-W. Zhong, X.-S. Lei, G.-Q. Lin, Tetrahedron: Asymmetry
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[18] K. C. Nicolaou, D. L. F. Gray, T. Montagnon, S. T. Harrison,
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In summary, the described convergent route was used for
the first total synthesis of the cytotoxic tetramic acid lactam 1
in 29 steps and in 1.0% overall yield, with a longest linear
sequence of 18 steps. The synthetic strategy should allow
access to the other interesting tetramic acid natural products
mentioned in the introduction.
[19] D. L. Comins, A. Dehghani, Tetrahedron Lett. 1992, 33, 6299 –
6302.
Received: August 30, 2004
Published online: December 28, 2004
[20] W. J. Scott, J. K. Stille, J. Am. Chem. Soc. 1986, 108, 3033 – 3040.
[21] E. J. Corey, H. A. Kirst, Tetrahedron Lett. 1968, 5041 – 5043.
[22] K. Sonogashira in Metal-Catalyzed Cross-Coupling Reactions
(Eds.: F. Diederich, P. J. Stang), Wiley-VCH, Weinheim, 1997,
pp. 203 – 230.
Keywords: macrolactams · tetramic acid · total syntheses
.
[23] Based on the NMR spectra and gas chromatograms, epimeriza-
tion in the conversion of 23 to 24 can be excluded.
[24] R. N. Lacey, J. Chem. Soc. 1954, 850 – 854.
[1] Review: R. J. Capon, Eur. J. Org. Chem. 2001, 633 – 645.
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34, 1065 – 1068.
[25] In the 13C NMR spectra the missing C O signals of the tetramic
=
acid unit of synthetic cylindramide (1) indicates the presence of
complex tautomer equilibria. Broadening of some 1H NMR
signals indicates that compound 1 complexes metal ions. This
interpretation is supported by FAB mass spectra, in which
[MꢀH + 2Na]+ adducts with high intensity are observed.
Tautomer equilibria and metal-ion complexation are currently
under investigation.
[3] S. P. Gunasekera, M. Gunasekera, P. McCarthy, J. Org. Chem.
1991, 56, 4830 – 4833.
[4] H. Shigemori, M.-A. Bae, K. Yazawa, T. Sasaki, J. Kobayashi, J.
Org. Chem. 1992, 57, 4317 – 4320.
[5] M.-A. Bae, K. Yamada, Y. Ijuin, T. Tsuji, K. Yazawa, Y. Tomono,
D. Uemura, Heterocycl. Commun. 1996, 2, 315 – 318.
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