Stereoselective synthesis of the C1-C11 and C12-C34 fragments of mycalolide A

Article abstract of DOI:10.1021/ol101145t

(Equation Presented). A convergent synthesis of the C1-C11 and C12-C34 fragments of mycalolide A is described. Synthetic highlights include a highly E-selective cross-metathesis between a vinyl-functionalized bis-oxazole unit and a polypropionate side cha

Full text of DOI:10.1021/ol101145t

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3348-3351
Stereoselective Synthesis of the
C1-C11 and C12-C34 Fragments of
Mycalolide A
Thomas J. Hoffman, Amandine Kolleth, James H. Rigby,† Stellios Arseniyadis,*
and Janine Cossy*
Laboratoire de Chimie Organique, ESPCI ParisTech, CNRS, 10 rue Vauquelin,
75231 Paris Cedex 05, France
stellios.arseniyadis@espci.fr; janine.cossy@espci.fr
Received May 18, 2010
ABSTRACT
A convergent synthesis of the C1-C11 and C12-C34 fragments of mycalolide A is described. Synthetic highlights include a highly E-selective
cross-metathesis between a vinyl-functionalized bis-oxazole unit and a polypropionate side chain to introduce the C19-C20 double bond and
an enzymatic desymmetrization of a meso-diol in addition to five stereoselective allylations/crotylations to control the 11 stereogenic centers
present in the natural product.
In 1989, Fusetani et al. reported the isolation of mycalolide
A (Scheme 1), a secondary metabolite produced by a
sponge of the genus Mycale sp. collected in the Gokasho
Bay of the Kii Peninsula in Japan.1 This secondary
metabolite exhibited antifungal activity against a variety
of pathogenic fungi as well as a strong activity against
B-16 melanoma cells with IC50 values ranging from 0.5
to 1.0 ng/mL (Scheme 1). In addition, it was shown to
selectively inhibit the actomyosin Mg2+-ATPase suggest-
ing that it acts as an actin-depolymerization agent.2
From a structural perspective, mycalolide A constitutes
one of the first known members of a vast family of marine
macrolides containing a unique tris-oxazole unit which
includes the ulapualides,3 the jaspisamides,4 the halishi-
gamides,5 the kabiramides,6 and the halichondramides.7
Its structure, which was elucidated through a combination
of chemical degradation, extensive 1H and 13C NMR
analysis, and structural correlation experiments,8 consists
of a 25-membered macrolide attached to a highly func-
tionalized aliphatic polyketide side chain bearing eight
of the 11 stereogenic centers present in the natural product.
(2) Hori, M.; Saito, S.; Shin, Y.; Ozaki, H.; Fusetani, N.; Karaki, H.
FEBS Lett. 1993, 322, 151–154.
(3) Roesner, J. A.; Scheuer, P. J. J. Am. Chem. Soc. 1986, 108, 846–
847.
(4) Kobayashi, J.; Murata, O.; Shigemori, H. J. Nat. Prod. 1993, 56,
787–791.
(5) Kobayashi, J.; Tsuda, M.; Fuse, H.; Sasaki, T.; Mikami, Y. J. Nat.
Prod. 1997, 60, 150–154.
(6) (a) Matsunaga, S.; Fusetani, N.; Hashimoto, K.; Koseki, K.; Noma,
M. J. Am. Chem. Soc. 1986, 108, 847–849. (b) Matsunaga, S.; Fusetani, N.;
Hashimoto, K.; Koseki, K.; Noma, M.; Noguchi, H.; Sankawa, U. J. Org.
Chem. 1989, 54, 1360–1363.
† Department of Chemistry, Wayne State University, Detroit, Michigan
48202-3489.
(1) Fusetani, N.; Yasumuro, K.; Matsunuga, S.; Hashimoto, K. Tetra-
hedron Lett. 1989, 30, 2809–2812.
(7) Kernan, M. R.; Molinski, T. F.; Faulkner, D. J. J. Org. Chem. 1988,
53, 5014–5020.
(8) Matsunaga, S.; Liu, P.; Celatka, C. A.; Panek, J. S.; Fusetani, N.
J. Am. Chem. Soc. 1999, 121, 5605–5606.
10.1021/ol101145t  2010 American Chemical Society
Published on Web 07/15/2010

Scheme 1. Retrosynthesis of mycalolide A.
Scheme 2. Synthesis of the C29-C34 ꢀ-Ketophosphonate 5
terminal olefin. Interestingly, however, despite the plethora of
examples which have been reported in the literature in the field
of CM within the last couple of decades,11 there has only been
a few examples involving vinyl-functionalized azoles.10s,12 In
this context, and with the scope to validate this strategy, we
embarked on the synthesis of the two CM coupling partners.
The preparation of the C20-C34 fragment of mycalolide
A relied on a Horner-Wadsworth-Emmons (HWE) reaction
between a C29-C34 ꢀ-ketophosphonate of type IV and a
C22-C28 aldehyde of type V. The synthesis of the former
began by the preparation of meso-diol 2 which was obtained
in three steps and 58% overall yield starting from methac-
rolein (1) and following a reported procedure (Scheme 2).13
The resulting meso-diene was then subjected to a diastereo-
selective double hydroboration (9-BBN, THF, -85 °C)
which, upon oxidation, delivered the meso-diol 2 in 70%
yield (dr >95:5). Desymmetrization of the latter through a
lipase-mediated acetylation (Candida rugosa, vinyl acetate,
hexane, 4 Å MS, 36 h)14 afforded the corresponding
monoacetate 3 in 97% yield as a single stereoisomer (er >95:
5).15 Protection of the remaining primary alcohol as a tert-
butyldiphenylsilyl ether (TBDPSCl, imidazole, CH2Cl2) and
saponification of the acetate group (K2CO3, MeOH) led to
alcohol 4 which was eventually oxidized under Swern condi-
tions to provide the corresponding aldehyde. The aldehyde was
then treated with a THF solution of LiCH2P(O)(OMe)2 (pre-
pared in situ from methyl dimethyl phosphonate, n-BuLi, THF,
0 °C)16 to afford the ꢀ-hydroxyphosphonate intermediate. The
latter was ultimately oxidized to the corresponding ꢀ-ketophos-
The promising biological properties displayed by my-
calolide A in conjunction with its challenging structure and
the fact that only one total synthesis has been reported so
far9,10 prompted us to initiate studies toward its total
synthesis. We describe here the results of our endeavor.
Our strategy for the synthesis of mycalolide A relied on four
key disconnections: an unusual cross-metathesis (CM) between
a vinyl-functionalized bis-oxazole unit and the C20-C34
polypropionate fragment bearing a terminal olefin, an esterifi-
cation to link the C12-C34 fragment II and the C1-C11
fragment III, a Robinson-Gabriel-type cyclodehydration to
form the third oxazole ring and concomitantly generate the
macrolide, and a Wittig olefination using an N-methylforma-
mide phosphonium salt to install the enamide moiety and
complete the synthesis of the natural product (Scheme 1).
Our first instinct when trying to devise a straightforward
strategy to access mycalolide A was to apply a CM between a
vinyl-functionalized mono-, bis-, or tris-oxazole unit and a
(9) (a) Liu, P.; Panek, J. S. J. Am. Chem. Soc. 2000, 122, 1235–1236.
(b) Panek, J. S.; Liu, P. J. Am. Chem. Soc. 2000, 122, 11090–11097.
(10) For earlier synthetic studies on mycalolide A and related tris-
oxazoles, see: (a) Knight, D. W.; Pattenden, G.; Rippon, D. E. Synlett 1990,
36–37. (b) Kiefel, M. J.; Maddock, J.; Pattenden, G. Tetrahedron Lett. 1992,
33, 3227–3230. (c) Pattenden, G. J. Heterocycl. Chem. 1992, 29, 607–618.
(d) Yoo, S.-K. Tetrahedron Lett. 1992, 33, 2159–2162. (e) Chattopadhyay,
S. K.; Pattenden, G. Tetrahedron Lett. 1995, 36, 5271–5274. (f) Panek, J. S.;
Beresis, R. T.; Celatka, C. A. J. Org. Chem. 1996, 61, 6494–6495. (g) Panek,
J. S.; Beresis, R. T. J. Org. Chem. 1996, 61, 6496–6497. (h) Liu, P.; Celatka,
C. A.; Panek, J. S. Tetrahedron Lett. 1997, 38, 5445–5448. (i) Celatka, C. A.;
Liu, P.; Panek, J. S. Tetrahedron Lett. 1997, 38, 5449–5452. (j) Chatto-
padhyay, S. K.; Pattenden, G. Synlett 1997, 1342–1344. (k) Chattopadhyay,
S. K.; Pattenden, G. Synlett 1997, 1345–1348. (l) Liu, P.; Panek, J. S.
Tetrahedron Lett. 1998, 39, 6143–6146. (m) Liu, P.; Panek, J. S. Tetrahedron
Lett. 1998, 39, 6147–6150. (n) Kempson, J.; Pattenden, G. Synlett 1999,
533–536. (o) Chattopadhyay, S. K.; Kempson, J.; McNeil, J.; Pattenden, G.;
Reader, M.; Rippon, D. E.; White, D. J. Chem. Soc., Perkin Trans. 1 2000,
2415–2428. (p) Pattenden, G.; Chattopadhyay, S. K. J. Chem. Soc., Perkin
Trans. 2000, 1, 2429–2454. (q) Panek, J. S.; Celatka, C. A. Tetrahedron
Lett. 2002, 43, 7043–7046. (r) Suenaga, K.; Kimura, T.; Kuroda, T.; Matsui,
K.; Miya, S.; Kuribayashi, S.; Sakakura, A.; Kigoshi, H. Tetrahedron 2006,
62, 8278–8290. (s) Kimura, T.; Kuribayashi, S.; Sengoku, T.; Matsui, K.;
Ueda, S.; Hayakawa, I.; Suenaga, K.; Kigoshi, H. Chem. Lett. 2007, 36,
1490–1491. (t) Pattenden, G.; Ashweek, N. J.; Baker-Glenn, C. A. G.;
Walker, G. M.; Yee, J. G. K. Angew. Chem., Int. Ed. 2007, 46, 4359–
4363.
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Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 11360–11370.
(12) For examples of CM involving vinyl-functionalized oxazoles, see:
(a) Hoffman, T. J.; Rigby, J. H.; Arseniyadis, S.; Cossy, J. J. Org. Chem.
2008, 73, 2400–2403. For examples of CM involving vinyl-functionalized
thiazoles, see: (b) Dash, J.; Arseniyadis, S.; Cossy, J. AdV. Synth. Catal.
2007, 349, 152–156. For applications in natural product synthesis, see: (c)
Gebauer, J.; Arseniyadis, S.; Cossy, J. Org. Lett. 2007, 9, 3425–3427. (d)
Gebauer, J.; Arseniyadis, S.; Cossy, J. Eur. J. Org. Chem. 2008, 2701–
2704.
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(15) The enantiomeric excess of 13 was measured by 1H NMR of the
corresponding mandelic ester, while the assignment of the absolute configuration
was made by comparison with the [R]D reported in the literature for the known
compound ([R]20D -8.2, c 2.36, CHCl3);exp ([R]20D -8.6, c 2.37, CHCl3)lit.
Org. Lett., Vol. 12, No. 15, 2010
3349

Scheme 3
.
Synthesis of the C22-C28 Aldehyde 11
Scheme 4. Synthesis of the CM Coupling Partner 14
polypropionate fragment of mycalolide A (Scheme 4). Hence,
ꢀ-ketophosphonate 5 was treated with Ba(OH)2·8H2O25 in wet THF
(THF/H2O ) 40:1) followed by aldehyde 11 to provide the coupled
product as a single (E)-isomer in 80% yield (Scheme 4). Reduction
of the enone to the corresponding ketone using Pd(OH)2/C in
MeOH under a hydrogen atmosphere occurred with concomitant
cleavage of the C22 primary TBS ether26 thus affording alcohol
12 in 85% yield. Swern oxidation to the corresponding aldehyde
and treatment with the (R,R)-[Ti]-I complex (Et2O, -78 °C)
installed the (22S) stereogenic center and provided homoallylic
alcohol 13 as a single stereoisomer in 68% yield over two steps
(dr >95:5).27 As an attempt to methylate the C22 hydroxyl group
using NaH/MeI had caused epimerization at C31, milder conditions
were applied. To our delight, exposure of 13 to MeOTf and 2,6-
di-tert-butylpyridine in refluxing CHCl3 produced the desired
methylated product 14 as a single stereoisomer in 65% yield (93%
based on recovered starting material). The preparation of the
C20-C34 polyketide fragment of mycalolide A was thus achieved
in 14 steps and 13.2% overall yield starting from methacrolein 1.
With the C20-C34 fragment secured, we next turned our
attention toward the synthesis of the vinyl-functionalized
bisoxazole subunit which would be engaged in the key CM
(Scheme 5). Hence, treatment of acrylamide 15 and ethyl
bromopyruvate 16 with NaHCO3 (THF, 55 °C) followed by
TFAA (THF, 0 °C) and saponification of the resulting ester
using LiOH·H2O in a THF/H2O mixture afforded carboxylic
acid 17 in 72% yield over three steps. The latter was then
coupled with (()-serine methyl ester hydrochloride using
standard conditions (EDC, HOBt, NMM, CH2Cl2)28 to afford
ꢀ-hydroxy amide 18 (72% yield) which in turn was engaged
phonate 5 thus completing the synthesis of the C29-C34
subunit in nine steps and 40.3% overall yield starting from
methacrolein 1 (Scheme 2).
The construction of the C22-C28 aldehyde subunit com-
menced from commercially available (R)-Roche ester and
proceeded through an initial three-step sequence which included
the protection of the primary alcohol as a TBS ether 6, the
reduction of the ester moiety, and the oxidation of the resulting
alcohol to the corresponding aldehyde (Scheme 3). The latter
was then subjected to a diastereoselective allylation using the
(R,R)-[Ti]-I complex (THF, -78 °C)17 to afford the corre-
sponding homoallylic alcohol (64.3% yield from 6, dr >95:5,
er >95:5)18,19 which was subsequently protected as a meth-
oxymethyl ether (MOMCl, DMAP, i-PrNEt2, CH2Cl2, reflux,
94% yield). Olefin 8 was then engaged in an OsO4-catalyzed
oxidative cleavage (OsO4, NaIO4, 2,6-lutidine, dioxane/
H2O),20 and the resulting aldehyde was treated with (E/Z)-
triphenyl crotylstannane 921 under Keck’s conditions22
(BF3·OEt2, CH2Cl2, -78 °C) to effect a substrate-controlled
diastereoselective crotylstannylation (83%, dr 80:20).19,23,24 The
resulting alcohol 10 was then converted to a methoxy ether
(CH3I, NaH, DMF, 89% yield) and the terminal olefin subjected
to an OsO4-catalyzed oxidative cleavage, thus completing the
synthesis of the C22-C28 aldehyde subunit 11 in nine steps
and 45.7% overall yield starting from the (R)-Roche ester.
With the two fragments 5 and 11 in hand, the stage was set for
the key HWE olefination which would afford the C22-C34
(16) (a) Heathcock, C. H.; Hadley, C. R.; Rosen, T.; Theisen, P. D.;
Hecker, S. J. J. Med. Chem. 1987, 30, 1858–1863. (b) Yasuda, N.; Hsiao,
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Org. Chem. 2004, 69, 1959–1966.
(23) The absolute configuration of the C26 stereogenic center was
1
(17) (a) Hafner, A.; Duthaler, R. O.; Marti, R.; Rihs, J.; Rothe-Streit,
P.; Schwarzenbach, F. J. J. Am. Chem. Soc. 1992, 114, 2321–2336. (b)
Duthaler, R. O.; Hafner, A. Chem. ReV. 1992, 92, 807–832.
determined as (26S) by H NMR analysis of the corresponding (R)- and
(S)-mandelic esters.
(24) (a) Reetz, M. T. Angew. Chem., Int. Ed. 1984, 23, 556–559. (b) Reetz,
M. T.; Kessler, K.; Jung, M. Tetrahedron 1984, 40, 4327–4336. (c) Reetz,
M. T.; Harms, K.; Reif, W. Tetrahedron Lett. 1988, 29, 5881–5884. Reetz,
M. T. Acc. Chem. Res. 1993, 26, 462–468.
(18) The diastereomeric ratio was determined by crude 1H NMR analysis,
and the absolute configuration of the C24 stereogenic center was determined
1
as (24S) by H NMR analysis of the corresponding (R)- and (S)-mandelic
esters.
(25) (a) Alvarez-Ibarra, C.; Arias, S.; Ban˜o´n, G.; Ferna´ndez, M.; Rod-
riguez, V.; Sinisterra, J. Chem. Soc. Chem. Commun. 1987, 1509–1511.
(b) Paterson, I.; Yeung, K. S.; Smaill, J. B. Synlett 1993, 774–776.
(26) For a review on selective monodeprotection of bis-silyl ethers, see:
Crouch, R. D. Tetrahedron 2004, 60, 5833–5871. For the use of hydrogena-
tion conditions in silyl ether deprotection, see: (a) Toshima, K.; Yanagawa,
K.; Mukaiyama, S.; Tatsuta, K. Tetrahedron Lett. 1990, 31, 6697–6698.
(b) Lee, K.; Wiemer, D. F. J. Org. Chem. 1993, 58, 7808–7812. (c) Rotulo-
Sims, D.; Prunet, J. Org. Lett. 2004, 4, 4701–4704.
(19) Seco, J. M.; Quinoa, E.; Riguera, R. Tetrahedron: Asymmetry 2001,
12, 2915–2925
(20) Yu, W.; Mei, Y.; Kang, Y.; Hua, Z.; Jin, Z. Org. Lett. 2004, 6,
3217–3219.
.
(21) Nakatsuka, M.; Ragan, J. A.; Sammakia, T.; Smith, D. B.; Uehling,
D. E.; Schrieber, S. L. J. Am. Chem. Soc. 1990, 112, 5583–5601.
(22) (a) Yamamoto, Y. Aldrichimica Acta 1987, 20, 45–49. (b) Yamamoto,
Y. Acc. Chem. Res. 1987, 20, 243–249. (c) Keck, G. E.; Savin, K. A.;
Creesman, E. N. K.; Abbot, D. E. J. Org. Chem. 1994, 59, 7889–7896. (d)
Keck, G. E.; Savin, K. A.; Weglarz, M. A.; Cressman, E. N. K. Tetrahedron
Lett. 1996, 37, 3291–3294.
(27) The absolute configuration of the C22 stereogenic center was
1
determined as (22S) by H NMR analysis of the corresponding (R)- and
(S)-mandelic esters.
3350
Org. Lett., Vol. 12, No. 15, 2010

Scheme 5. Synthesis of the C12-C34 Fragment 20
Scheme 6. Synthesis of the C1-C5 Fragment 24
NaH, DMF, 0 °C) followed by OsO4-catalyzed oxidative cleavage
of the terminal olefin then furnished aldehyde 29 in 84% yield
over two steps. Immediate treatment with LiCH2P(O)(OMe)2
(MeP(O)(OMe)2, n-BuLi, THF, 0 °C) followed by a Dess-
Martin periodinane (DMP)-mediated oxidation of the resulting
ꢀ-hydroxyphosphonate then supplied the desired ꢀ-ketophospho-
nate in 46% yield. A Ba(OH)2-mediated HWE olefination between
the C1-C5 aldehyde and the C6-C11 ꢀ-ketophosphonate finally
afforded the desired C1-C11 fragment of mycalolide A, com-
pound 30, in 69% yield (E/Z > 95:5). This sequence was thus
carried out in 10 steps and 13.1% overall yield starting from
(L)-serine 25.
in a sequential DAST-mediated cyclodehydration (CH2Cl2,
-78 °C)/dehydrobromination (BrCCl3, DBU, CH2Cl2, 0 °C)
that led to the desired bisoxazole 19 in 70% yield over two
steps. The C20-C34 polyketide fragment 14 and bisoxazole
19 were then coupled using Grubbs second-generation
catalyst, [Ru]-II (CH2Cl2, 40 °C, 16 h), to afford the
C12-C34 fragment of mycalolide A 20 in a moderate 57%
yield and an excellent E-stereoselectivity (E/Z > 20:1).
The synthesis of the C1-C11 fragment of mycalolide
A relied on a HWE between a C1-C5 aldehyde of type
VI and a C6-C11 ꢀ-ketophosphonate of type VII. The
former was prepared starting from 3-buten-1-ol (21)
(Scheme 6). Hence, 21 was first converted into the
corresponding para-methoxybenzyl ether using trichlo-
roacetimidate 22 in the presence of a catalytic amount of
La(OTf)3 (toluene, rt, 95% yield). An OsO4-catalyzed
oxidative cleavage then afforded the corresponding alde-
hyde19 which was subsequently treated with the (R,R)-
[Ti]-II complex to provide homoallylic alcohol 23 in 85%
yield over two steps (er >95:5).29 The latter was then
protected as a TBS ether, and the terminal olefin was finally
oxidatively cleaved to unveil the desired aldehyde 24 in 95%
yield over two steps.
Scheme 7. Synthesis of the C1-C11 Fragment 30
In conclusion, we have completed the synthesis of the
C1-C11 and C12-C34 fragments of mycalolide A. The
synthesis includes a highly E-selective cross-metathesis between
a vinyl-functionalized bis-oxazole unit and a polypropionate side
chain to introduce the C19-C20 double bond, an enzymatic
desymmetrization of a meso-diol to control the three stereogenic
centers at C31, C32, and C33, and five stereoselective allylations/
crotylations to control the stereogenic centers at C3, C8, C9, C22,
C24, C26, and C27. Future efforts will be dedicated in coupling
the two fragments together, performing the Robinson-Gabriel-
type cyclodehydration to form the macrolide and introducing the
enamide moiety through a Wittig olefination to complete the
synthesis. These efforts will be reported in due course.
The synthesis of the C6-C11 ꢀ-ketophosphonate, on the other
hand, began with the preparation of the Weinreb amid 26 starting
from (L)-serine 25 according to a reported procedure30 (73% over
three steps) (Scheme 7). Weinreb amide 26 was then reduced to
the corresponding aldehyde 27 using LiAlH4 (THF, 0 °C, quantita-
tive) and immediately engaged in a diastereoselective crotyltitana-
tion using the (R,R)-[Ti]-II complex (Et2O, -78 °C) to afford the
corresponding homoallylic alcohol 28 in decent yield and excellent
selectivity (67% yield, dr >95:5). Methyl ether formation (MeI,
Acknowledgment. We would like to thank the Ministe`re
de l’Education Nationale, de l’Enseignement Supe´rieur et
de la Recherche, for financial support to T.H.
(28) Chattopadhyay, S. K.; Biswas, S.; Pal, B. K. Synthesis 2006, 8,
1289–1293. For a review of peptide coupling reagents, see: Han, S.-Y.; Kim,
Y.-A. Tetrahedron 2004, 60, 2447–2467.
(29) The enantiomeric ratio and absolute configuration were determined by
crude 1H NMR analysis of the corresponding (R)-mandelic ester and confirmed
by comparison with the physical data reported in the literature (exp: [R]20D + 1.5,
c 1.0, CHCl3); lit: [R]20D + 1.0, c 1.0, CHCl3), see: Smith, A. B., III; Minbiole,
K. B.; Verhoest, P. R.; Schelhaas, M. J. Am. Chem. Soc. 2001, 123, 10942–
10953.
Supporting Information Available: Experimental details
and characterization data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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R.; Roccaa, V. Tetrahedron 1995, 51, 8121–8134. (b) Nahm, S.; Weinreb,
S. M. Tetrahedron Lett. 1981, 22, 3815–3818.
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3351