Total synthesis and structural assignment of spongidepsin through a stereodivergent ring-closing-metathesis strategy

Article abstract of DOI:10.1002/anie.200453663

Eight diastereomeric probes (1) were generated to synthesize the novel, anti-proliferative marine natural product spongidepsin (2) and to determine its absolute stereochemistry. A key step was the formation of the macrocycle by ring-closing metathesis.

Full text of DOI:10.1002/anie.200453663

Communications
Natural Products Synthesis
Total Synthesis and Structural Assignment of
Spongidepsin through a Stereodivergent Ring-
Closing-Metathesis Strategy**
Jiehao Chen and Craig J. Forsyth*
Spongidepsin (1) is a remarkable natural product isolated
recently from a Spongia sp. sponge collected off the Vanuatu
Islands, Australia, by Riccio and co-workers.[1] Its cytotoxic
and antiproliferative activities against J774.A1, WEHI-164,
and HEK-293 cancer cell lines are accompanied by an
unprecedented structure.[1] The genus Spongia is a well-
known source of diterpenoid and polyketide natural products,
such as epispongiadiol[2] and spongistatin,[3] respectively.
However, 1 reflects a distinct biogenetic origin that combines
[*] J. Chen, Prof. Dr. C. J. Forsyth
Department of Chemistry, Institute of Technology
University of Minnesota
Minneapolis, MN 55455 (USA)
Fax: (+1)612-626-7541
E-mail: forsyth@chem.umn.edu
[**] This publication was made possible by generous unrestricted grant
support from Bristol-Myers Squibb. We thank Prof. Riccio for copies
of 1H NMR spectra of naturally occurring spongidepsin and J. Xu for
provision of alcohol 7.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200453663
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Chemie
amino acid and unprecedented ketide motifs within a 13-
membered macrocycle. The ketide domain is comprised of a
9-hydroxy-2,4,7-trimethyltetradeca-14-ynoic acid, while the
amino acid was established as (S)-N-methylphenylalanine by
Marfey analysis of the acidic hydrosylate of 1.[1,4] The
dimethyl substitution at C2 and C4 of 1 was determined to
be syn by application of Murata's NMR spectroscopic-based
method, but the absolute configuration was not estab-
lished.[1,5] Neither the relative, nor the absolute stereochem-
istry of the two remaining stereogenic centers at C7 and C9
were originally assigned, partly owing to unfavorable
1H NMR spectral overlap. Hence, the actual structure of 1
could have been one of eight possible stereoisomers (2S,4S or
2R,4R + 7R/S,9R/S). The complete structural definition of 1
should extend our understanding of the complex biosynthetic
diversity of Spongia isolates, while the development of a total
synthesis should facilitate the complete biological evaluation
of 1. For this, a stereodivergent total synthesis strategy that
features macrocycle formation through ring-closing meta-
thesis (RCM) as the key step was employed. The successful
implementation of this plan culminated in the full structural
elucidation and total synthesis of spongidepsin, as summar-
ized herein.
from the known l-malate-derived epoxide 6, which bears a C9
stereogenic center.[6]
The synthesis began with the CuI-mediated opening of
epoxide 6 with a 2-bromopropene-derived Grignard reagent
to give secondary alcohol 8 (Scheme 2). The hydroxy group of
The stereochemical-determination strategy relied on the
preparation of all eight possible diastereoisomers of the
macrolide-containing portion 2 of spongidepsin incorporating
(S)-N-methylphenylalanine. These include both the 2S,4S and
2R,4R enantiomers of the syn-2,4-dimethyl moiety conjoined
with the four diastereomeric combinations of R,S isomers at
C7 and C9. Comparison of the spectral data of each of the
diastereomeric probes 2 with those of natural spongidepsin
would, ideally, indicate which isomer to advance selectively in
the total synthesis of 1. As shown in Scheme 1, the 13-
membered macrolides 2 would be prepared by RCM of dienes
3 and subsequent alkene hydrogenation. The RCM substrates
Scheme 2. Synthesis of esters 4a–d. Reagents and conditions: a) 2-
bromopropene (3 equiv), Mg, CuI (0.3 equiv), THF, À608C, 30 min,
86%; b) TESCl (1.5 equiv), imidazole (3 equiv), DMAP (0.1 equiv),
CH2Cl2, 1 h, 97%; c) BH3·THF (2.2 equiv), THF 08C, 2 h; NaOH, H2O2,
2 h, 95%; d) TPAP (0.08 equiv), NMO (1.5 equiv), molecular sieves
(4 Š; 500 mgmmolÀ1), CH2Cl2, 30 min; e) CH2Br2, Zn, TiCl4, CH2Cl2,
10 min, 75% over two steps; f) TBAF (1.5 equiv), THF, 1 h, 99%;
g) DIAD (3 equiv), Ph3P (3 equiv), THF, N-Me-N-Boc-Phe (1.5 equiv),
10 min, 91%; h) TBSOTf (1.5 equiv), 2,6-lutidine (2 equiv), CH2Cl2,
1.5 h; TBAF (1.1 equiv), THF, 1 h, 82% over two steps for 4a and 4b,
85% over two steps for 4c and 4d; i) Cl3C6H2COCl (1.2 equiv), DIPEA
(3 equiv), N-Me-N-Boc-Phe (1.1 equiv), THF; DMAP, toluene, 89%. N-
Me-N-Boc-Phe=(S)-N-methyl-N-Boc-phenylalanine, PMB=4-methoxy-
benzyl, TES=triethylsilyl, Boc=tert-butyl carbamate, Bn=benzyl,
DMAP=4-dimethylaminopyridine, TPAP=tetrapropylammonium per-
ruthenate, NMO=4-methylmorpholine N-oxide, TBAF=tetra-n-butyl-
ammonium fluoride, DIAD=diisopropyl azodicarboxylate, TBS=tert-
butyldimethylsilyl, Tf=trifluoromethanesulfonyl.
À
À
3, in turn, be derived from the C1 C5 and C6 C11 fragments
5 and 4, respectively. The two enantiomers of syn-2,4-
dimethyl carboxylic acid 5 are derivable from acetate alcohol
7 by alternative manipulations of the terminal functional
groups. Each of the four stereoisomers of 4 would originate
8 was converted into TES ether 9, and the alkene was
subjected to a hydroboration–oxidation sequence to install
the C7 stereogenic center intentionally as an approximately
equal molar ratio of primary alcohols (7R,9R)-10a and
(7S,9R)-10b. Attempts to separate 10a and 10b or various
simple derivatives thereof from one another were unsuccess-
ful. It was anticipated, however, that the C7 epimers would be
separated at the stage of the conformationally constrained 13-
membered macrolides resulting from RCM. Thus, the dia-
stereomeric mixture of 10a and 10b was converted into the
corresponding alkenes 11a and 11b through an oxida-
tion[7]–olefination[8] sequence. Liberation of the secondary
hydroxy group of 11a and 11b followed by Mitsunobu
esterification[9] with (S)-N-methyl-N-Boc-phenylalanine
Scheme 1. Retrosynthesis of macrolides 2.
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yielded C9-inverted esters (7R,9S)-13a and (7S,9S)-13b.
Stepwise cleavage of the N-Boc carbamates[10] from 13a and
13b generated secondary amines 4a and 4b. The other two
À
C7 C9 stereoisomers (7R,9R)-4c and (7S,9R)-4d were pre-
pared from 12a and 12b and (S)-N-methyl-N-Boc-phenyl-
alanine with retention of (9R)-configuration via Yamaguchi
esterification.[11]
The enantiomeric syn-2,4-dimethyl-substituted carboxylic
acids (2S,4R)-5a and (2R,4S)-5b, one of which corresponds to
À
the C1 C5 moiety of 1, were prepared from monoacetate
(2S,4R)-7 (Scheme 3). Acetate 7, in turn, was obtained by
Scheme 3. Synthesis of carboxylic acids 5a and 5b. Reagents and
conditions: a) TBDPSCl (1.5 equiv), imidazole (2.5 equiv), DMAP
(0.1 equiv), CH2Cl2, 1.5 h, 93%; b) K2CO3 (1.5 equiv), MeOH, 4 h,
ꢀ87%; c) TPAP (0.08 equiv), NMO (1.5 equiv), molecular sieves (4 Š;
500 mgmmolÀ1), CH2Cl2, 20 min; d) CH2Br2, Zn, TiCl4, CH2Cl2, 10 min,
ꢀ67% over two steps; e) TBAF (1.5 equiv), THF, 3 h, ꢀ86%; f) Jones
reagent (excess), acetone, 30 min, 65%. TBDPS=tert-butyldiphenyl-
silyl.
Scheme 4. Synthesis of macrolides 2a–b. Reagents and conditions:
a) PyAOP (1.2 equiv), DIPEA (2 equiv), DMF, 24 h, 87%; b) second-
generation Grubbs catalyst[14] (0.1 equiv), toluene, 1108C, 20 min, 80%
combined yield; c) silica gel chromatographic separation; d) H2, Pd/C
(0.1 equiv), EtOAc, 8 h, ꢀ88%. PyAOP=(7-azabenzotriazole-1-yloxy)-
tripyrrodinophosphonium hexafluorophosphate,
enzymatic resolution of the corresponding meso diol.[12] For
the synthesis of 5b, alcohol 7 was silylated to yield 14, then the
acetate terminus was converted into an alkene (16b) in a
stepwise fashion culminating in a Lombardo olefination.[8]
Desilylation of 16b followed by Jones oxidation of the
resultant alcohol 17b furnished carboxylate 5b. Its enan-
tiomer 5a was similarly obtained from monoacetate 7 through
the complementary set of terminal functionalizations indi-
cated in Scheme 3.
With each of the four amine diastereomers 4a–d and the
two enantiomeric carboxylic acids 5a and 5b available, the
synthesis of the eight diastereomeric macrolides 2 was
addressed. PyAOP-mediated amide formation[13] of the
diastereomeric mixture of amines 4a and 4b with carboxylic
acid 5b proceeded smoothly to afford the corresponding
amides 3a and 3b (Scheme 4). Exposure of 3a and 3b to the
second-generation Grubbs catalyst[14] in refluxing toluene
yielded the four possible macrocycle 5E/Z,7R/S diastereom-
ers in 80% combined yield. The two E alkenes (18a and 18b)
were obtained in a 1:1 ratio and predominated over the
Z isomers by > 10:1. As anticipated, the two C7 epimers 18a
and 18b were separated from one another easily by flash
column chromatography. The absolute stereochemical assign-
ment of C7 in compounds 18a and 18b was not made at this
stage, although each isomer could be obtained in diastereo-
merically pure form. The two diastereomeric alkenes 18a and
18b were separately subjected to palladium-catalyzed hydro-
DIPEA=diisopropylethylamine, DMF=N,N-dimethylformamide.
genation to provide the corresponding saturated macrolides
2a and 2b.
The remaining six diastereoisomeric macrolides 2c–h
were prepared in a similar fashion from the corresponding
acids and amines through amide formation, RCM, and
hydrogenation (Scheme 5). Among the eight diastereoisom-
ers of 2 prepared, the 1H and 13C NMR spectral data of
(2R,4R,9S,16S)-2a best matched those of natural spongidep-
sin.[15] To determine the configuration at C7, the RCM adduct
18a (the direct precursor to macrolide 2a) was chosen for
degradative analysis. First, the PMB ether 18a was converted
into TBDPS ether 20 as a prelude to ozonolytic cleavage of
the alkene moiety (Scheme 6). Ozonolysis of 20 followed by
reductive workup afforded diol 21. Hydrolysis of the ester
moiety of 21 with LiOH in aqueous tBuOH yielded 1,4-diol
22, which was oxidatively cyclized into five-membered
lactone 23 with TEMPO/BAIB.[16] Extensive NOE studies
and detailed 1H–1H coupling-constant analysis with reference
to analogous known cis and trans lactones,[17,18] indicated that
the methyl and (silyloxy)ethyl substituents were cis to each
other in lactone (S,S)-23. Hence, macrolide 2a was assigned
the corresponding 7R,9S stereochemistry. Given that the
configuration at C9 is established from l-malic acid via
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Scheme 5. Synthesis of macrolides 2c–h. Reagents and conditions:
a) second-generation Grubbs catalyst[14] (0.1 equiv), toluene, 1108C,
20 min; b) H2, Pd/C (0.1 equiv), EtOAc, 8 h.
Scheme 7. Total synthesis of (2R,4R,7R,9R,16S)-spongidepsin (1).
Reagents and conditions: a) Ph3P (2 equiv), imidazole (3 equiv), I2
(1.5 equiv), THF, 20 min, 82%; b) allyl tri-n-butyltin (3 equiv), AIBN
(0.5 equiv), benzene, 808C, 4 h, 85%; c) K2OsO4 (0.2 equiv), NaIO4
(6 equiv), THF-H2O (2:1), 1.5 h, 87%; d) K2CO3 (1.5 equiv), Bestmann
reagent (1.5 equiv), MeOH, 3 h, 75%. AIBN=2,2’-azobisisobutyroni-
trile, Bestmann reagent=dimethyl-1-diazo-2-oxopropylphosphonate.
(2R,4R,7R,9R,16S)-1: [a]D = À 67.3 (c = 1.00, MeOH); Spon-
gia isolate 1:[1] [a]D = À 61.8 (c = 1.4, MeOH)].
In summary, this work highlights the convergence of
synthetic design, methodology, and spectroscopic analyses to
fully define the structure and provide an alternative source of
the recently described antiproliferative natural product
spongidepsin. The complete stereochemical assignment and
the total synthesis of 1 have been achieved through a
stereochemically divergent strategy that employed macro-
lide-closure by ring-closing metathesis as a key step. Finally,
the stereochemical assignments of the unprecedented 9-
hydroxy-2,4,7-trimethyltetradeca-14-ynoic acid moiety may
be relevant to biosynthetic congeners of 1.
Scheme 6. Elucidation of the configuration of 18a at C7. Reagents and
conditions: a) DDQ (5 equiv), tBuOH, H2O, CH2Cl2, 10 min sonica-
tion, 89%; b) TBDPSCl (1.5 equiv), imidazole (2 equiv), DMAP
(0.1 equiv), CH2Cl2, 3 h, 85%; c) O3, MeOH, 10 min; NaBH4 (2 equiv),
3 h, 77%; d) LiOH (6 equiv), tBuOH/H2O (4:1), 2 h, 74%; e) TEMPO
(0.3 equiv), BAIB (3 equiv), CH2Cl2, 2 h, 65%. DDQ=2,3-dichloro-5,6-
dicyanoquinone, TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxy,
BAIB=iodobenzene diacetate.
Received: January 2, 2004 [Z53663]
Keywords: cyclization · metathesis · natural products ·
.
structure elucidation · total synthesis
epoxide 6 with inversion of configuration during the forma-
tion of 13 and that (S)-N-methylphenylalanine[1] was
employed throughout, 2a was assigned the 2R,4R,7R,9S,16S
configuration.[19]
[1] A. Grassia, I. Bruno, C. Debitus, S. Marzocco, A. Pinto, L.
Gomez-Paloma, R. Riccio, Tetrahedron 2001, 57, 6257.
[2] R. Kazlauskas, P. T. Murphy, R. J. Wells, K. Noack, W. E.
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[5] N. Matsumori, D. Kaneno, M. Murata, H. Nakamura, K.
Tachibana, J. Org. Chem. 1999, 64, 866 – 876.
[6] R. D. Cink, C. J. Forsyth, J. Org. Chem. 1995, 60, 8122.
[7] W. P. Griffith, S. V. Ley, G. P. Whitcombe, A. D. White, Chem.
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[8] L. Lombardo, Tetrahedron Lett. 1982, 23, 4293.
[9] O. Mitsunobu, Synthesis 1991, 1.
To extend the stereochemical assignment of 2a unambig-
uously to 1, the former was further functionalized to complete
a total synthesis. This involved conversion of the C11 alcohol
of 2a into the alkyne-terminated side chain of
(2R,4R,7R,9R,16S)-1. First, the primary alcohol was trans-
formed into iodide 24, which was then treated with allyl tri-n-
butyltin and catalytic AIBN to generate the allylation product
25 (Scheme 7). The resultant alkene was cleaved with K2OsO4
and NaIO4 to give the corresponding aldehyde. Finally, the
Bestmann reagent[20] was employed to convert the aldehyde
into the corresponding terminal alkyne (2R,4R,7R,9R,16S)-1,
which matched natural spongidepsin by 1H and 13C NMR
spectroscopy, HRMS, and specific rotation [synthetic
[10] M. Sakaitani, Y. Ohfune, J. Org. Chem. 1990, 55, 870.
[11] J. Inanaga, K. Hirata, H. Saeki, T. Katsuki, M. Yamaguchi, Bull.
Chem. Soc. Jpn. 1979, 52, 1989.
[12] J. C. Anderson, S. V. Ley, Tetrahedron Lett. 1994, 35, 2087.
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[13] F. Albericio, M. Cases, J. Alsina, S. A. Triolo, L. A. Carpino,
S. A. Kates, Tetrahedron Lett. 1997, 38, 4853.
[14] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18.
[15] Comparative 1H NMR data are provided in the Supporting
Information.
[16] T. M. Hansen, G. J. Florence, P. Lugo-Mas, J. Chen, J. N. Abrams,
C. J. Forsyth, Tetrahedron Lett. 2003, 44, 57.
[17] A. G. Myers, L. McKinstry, J. Org. Chem. 1996, 61, 2428.
[18] NOE and comparative 1H NMR data are provided in the
Supporting Information.
[19] The 9R/9S labels differ between 2a and 1 owing to the change in
the Cahn–Ingold–Prelog priorities of C10 in each.
[20] S. Muller, B. Liepold, G. J. Roth, H. J. Bestmann, Synlett 1996,
521.
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