Total synthesis of enigmazole a from cinachyrella enigmatica. Bidirectional bond constructions with an ambident 2,4-disubstituted oxazole synthon

Article abstract of DOI:10.1021/ja1016975

The first total synthesis of the cytotoxic marine macrolide enigmazole A has been completed in 22 steps (longest linear sequence). The sensitive, densely functionalized 2,4-disubstituted oxazole fragment was constructed using an efficient Negishi-type cou

Full text of DOI:10.1021/ja1016975

Published on Web 06/30/2010
Total Synthesis of Enigmazole A from Cinachyrella
enigmatica. Bidirectional Bond Constructions with an
Ambident 2,4-Disubstituted Oxazole Synthon
Colin K. Skepper,† Tim Quach,† and Tadeusz F. Molinski*,†,§
Department of Chemistry and Biochemistry and Skaggs School of Pharmacy and
Pharmaceutical Sciences, UniVersity of California, San Diego, 9500 Gilman DriVe MC0358,
La Jolla, California 92093
Received February 27, 2010; E-mail: tmolinski@ucsd.edu
Abstract: The first total synthesis of the cytotoxic marine macrolide enigmazole A has been completed in
22 steps (longest linear sequence). The sensitive, densely functionalized 2,4-disubstituted oxazole fragment
was constructed using an efficient Negishi-type coupling of an oxazol-2-ylzinc reagent formed directly from
the parent ethyl 2-iodooxazole-4-carboxylate by zinc insertion. Other key steps include a hetero-Diels-Alder
cycloaddition to form the central embedded pyran ring, a Wittig reaction to unite Eastern and Western
hemispheres, and a ring size-selective Keck macrolactonization.
Introduction
Enigmazole A (1, Figure 1) and related congeners 2 and 3
are members of a family of cytotoxic macrolides isolated
from the sponge Cinachyrella enigmatica that includes
compounds that selectively target aberrant c-Kit signaling.1-3
This appears to be a very rare phenotypic effect, seen before
in only 32 natural product extracts of a panel of 134 631
extracts that were screened in a differential response assay
with wild-type and c-Kit mutant cells. c-Kit is a type-III
transmembrane protein-tyrosine kinase4 that is important for
control of gametogenesis, hematopoiesis, mast cell develop-
ment and function, and melanogenesis5 but also implicated
in several gain-of-function mutations in humans that confer
constitutive, ligand-independent kinase activity. The latter
has implications for cancer patients with a specific c-Kit
genotype that undergo treatment with Gleevec (Novartis
Pharmaceuticals Corp.; common name: imatinib mesylate)
for gastrointestinal stromal cell tumors (GISTs).6 Enigmazole
A (1) exhibits potent in vitro cytotoxicity toward IC-2 mast
cells (IC50 0.37 µg/mL).1
Figure 1. Structure of the cytotoxic macrolide enigmazole A (1).
appended at C17 and a phosphate ester at C5, a rare feature found
in only a few polyketide natural products, including calyculin A.7
Calyculin A (4) is a potent phosphatase inhibitor that has been the
subject of several total syntheses.8 We report here the first total
synthesis of enigmazole A (1) which deploys bidirectional C-C
bond construction around a preformed 2,4-disubstituted oxazole
unit. This strategy exploits both electrophilic and (latent) nucleo-
philic reactivity of a versatile oxazole synthon, ethyl 2-aminoox-
azole-4-carboxylate (11), which is readily available in one step from
urea and ethyl bromopyruvate.9
The structure of 1 consists of an 18-membered macrolide ring
decorated with a densely functionalized 2,4-disubstituted oxazole
† Department of Chemistry and Biochemistry.
§ Skaggs School of Pharmacy and Pharmaceutical Sciences.
(1) See preceding article in this issue: Oku, N.; Takada, K.; Fuller, R. W.;
Wilson, J. A.; Peach, M. L.; Pannell, L. K.; McMahon, J. B.;
Gustafson, K. R. J. Am. Chem. Soc. 2010, 130, DOI: 10.1021/
ja1016766.
(6) (a) Boyar, M. S.; Taub, R. N. Cancer InVest. 2007, 25, 328–335. (b)
Heinrich, M. C.; Corless, C. L.; Blanke, C. D.; Demetri, G. D.;
Joensuu, H.; Roberts, P. J.; Eisenberg, B. L.; von Mehren, M.; Fletcher,
C. D. M.; Sandau, K.; McDougell, K.; Ou, W.-B.; Chen, C.-J.; Fletcher,
J. A. J. Clin. Oncol. 2006, 24, 4764–4774. (c) Antonescu, C. R.;
Besmer, P.; Guo, T.; Arkun, K.; Hom, G.; Koryotowski, B.; Leversha,
M. A.; Jeffrey, P. D.; Desantis, D.; Singer, S.; Brennan, M. F.; Maki,
R. G.; DeMatteo, R. P. Clin. Cancer Res. 2005, 11, 4182–4190. (d)
Debiec-Rychter, M.; Cools, J.; Dumez, H.; Sciot, R.; Stul, M.;
Mentens, N.; Vrankx, H.; Wasag, B.; Prenen, H.; Roesel, J.; Hage-
meijer, A.; Van Oosterom, A.; Marynen, P. Gastroenterology 2005,
128, 270–279, and references therein.
(2) Henrich, C. J.; Goncharova, E. I.; Wilson, J. A.; Gardella, R. S.;
Johnson, T. R.; McMahon, J. B.; Takada, K.; Bokesch, H. R.;
Gustafson, K. R. Chem. Biol. Drug. Des. 2007, 69, 321–330.
(3) Takada, K. Presented at the 47th Annual Meeting of the American
Society of Pharmacognosy, Arlington, VA, 2006.
(4) (a) Roskoski, R., Jr. Biochem. Biophys. Res. Commun. 2005, 338,
1307–1315. (b) Ro¨nnstrand, L. Cell. Mol. Life Sci. 2004, 61, 2535–
2548.
(5) (a) Lennartsson, J.; Jelacic, T.; Linnekin, D.; Shivakrupa, R. Stem Cells
2005, 23, 16–43. (b) Taylor, M. L.; Metcalfe, D. D. Hematol. Oncol.
Clin. North Am. 2000, 14, 517–535.
9
10286 J. AM. CHEM. SOC. 2010, 132, 10286–10292
10.1021/ja1016975  2010 American Chemical Society

Total Synthesis of Enigmazole A
A R T I C L E S
Scheme 1. Retrosynthetic Analysis of Enigmazole A (1)
Scheme 2. Formation of Oxazol-2-ylzinc Reagent 9 and Negishi
Coupling with Vinyl Iodide 10a
a Reagents and conditions: (a) MnO2, CH2Cl2, rt, 1 h, 82%; (b) (+)-
MIB, Me2Zn, hexanes, 0 °C f rt, overnight, 60%; (c) (i) NaH, THF,
imidazole (cat.), 0 °C f rt, 2 h, then (ii) MeI, 1.5 h, 80%; (d) Zn, LiCl,
THF, rt, 10 min; (e) Pd(PPh3)4, rt, 1 h, 86%.
by transmetalation of 2-lithiooxazole with ZnCl2; however, the
strongly basic conditions required for C2 deprotonation are
mutually exclusive with the preparation of oxazole zincates
bearing reactive substituents such as carboxylic acid esters.13
The 2-aminooxazole carboxylate ester 11 embodies ambident
reactivity that can be revealed in stages: the electrophilicity of
the carboxylate at C4 and latent nucleophilicity at C2 that can
be unmasked through diazonium salt formation, conversion to
the iodide 12, and direct metalation with Zn0 to give the oxazol-
2-yl zincate 9. While the corresponding ethyl 2-bromooxazole-
4-carboxylate has been used in Pd-promoted Stille coupling of
oxazoles,14 our preliminary surveys showed the latter were poor
substrates for Zn insertion. Iodooxazole 12 (Scheme 2), a new
oxazole synthon, was conveniently prepared on a multigram
scale by condensation of urea and ethyl bromopyruvate to
provide 11, followed by diazotization and iodide displacement
(see Supporting Information). Iodide 12 was then smoothly
transformed into the oxazol-2-yl zincate 9 under Knochel
conditions (Zn, LiCl, THF, 10 min).15 Solutions of 9 in THF
were appreciably stable; a stock solution of 9 (0.5 M) stored in
the dark under N2 at 4 °C lost less than 23% of its titer over 1
month.16
Synthetic Plan
Antithetic analysis (Scheme 1) shows major disconnections
at the macrolide ester bond and C12-C13 bond leading to the
“Eastern” fragment 7 that could be united with the remainder
of the molecule by Wittig olefination. The central embedded
pyran ring was envisioned to arise in turn from a diastereose-
lective hetero-Diels-Alder (HDA) cycloaddition between al-
dehyde 5 and diene 6. We were cognizant that the major
challenges would lie in construction of the Eastern fragment
bearing the oxazole. Commonly, methods for construction of
2,4-disubstituted oxazoles are reliant upon de noVo assembly
of the oxazole ring from serine-derived fragments by
cyclodehydration-oxidation.10 We sought to introduce the
oxazole ring of 1 at an early stage by “grafting” of C2 and C4
substituents to a preformed oxazole synthon, represented by
oxazol-2-ylzinc reagent 9, in anticipation of C20-C21 bond
formation through Negishi coupling with 10.11
The use of metalated oxazoles, in particular 2-oxazole
zincates, is underdeveloped in natural product synthesis.12
Simple oxazol-2-ylzinc reagents have been prepared previously
(7) (a) Kato, Y.; Fusetani, N.; Matsunaga, S.; Hashimoto, K.; Fujita, S.;
Furuya, T. J. Am. Chem. Soc. 1986, 108, 2780–2781. (b) Kato, Y.;
Fusetani, N.; Matsunaga, S.; Hashimoto, K.; Koseki, K. J. Org. Chem.
1988, 53, 3930–3932. (c) Matsunaga, S.; Fujiki, H.; Sakata, D.;
Fusetani, N. Tetrahedron 1991, 47, 2999–3006. (d) Matsunaga, S.;
Fusetani, N. Tetrahedron Lett. 1991, 32, 5605–5606.
Synthesis of the Eastern Hemisphere
Synthesis of the key intermediate 7 began with preparation
of vinyl iodide 10. Oxidation of allylic alcohol 1317 gave the
geometrically unstable aldehyde 14, which was immediately
(8) (a) Evans, D. A.; Gage, J. R.; Leighton, J. L. J. Am. Chem. Soc. 1992,
114, 9434–9453. (b) Tanimoto, N.; Gerritz, S. W.; Sawabe, A.; Noda,
T.; Filla, S. A.; Masamune, S. Angew. Chem. 1994, 106, 674–677.
(c) Smith, A. B.; Friestad, G. K.; Duan, J. J.-W.; Barbosa, J.; Hull,
K. G.; Iwashima, M.; Qiu, Y.; Spoors, P. G.; Bertounesque, E.;
Salvatore, B. A. J. Org. Chem. 1998, 63, 7596–7597. (d) Smith, A. B.;
Friestad, G. K.; Barbosa, J.; Bertounesque, E.; Hull, K. G.; Iwashima,
M.; Qiu, Y.; Salvatore, B. A.; Spoors, P. G.; Duan, J. J.-W. J. Am.
Chem. Soc. 1999, 121, 10468–10477. (e) Smith, A. B.; Friestad, G. K.;
Barbosa, J.; Bertounesque, E.; Duan, J. J.-W.; Hull, K. G.; Iwashima,
M.; Qiu, Y.; Spoors, P. G.; Salvatore, B. A. J. Am. Chem. Soc. 1999,
121, 10478–10486.
(12) For examples of metalated oxazoles in natural products synthesis, see:
(a) Mulder, R. J.; Shafer, C. M.; Molinski, T. F. J. Org. Chem. 1999,
64, 4995–4998. (b) Bobeck, D. R.; Warner, D. L.; Vedejs, E. J. Org.
Chem. 2007, 72, 8506–8518. (c) Anderson, B. A.; Becke, L. M.;
Booher, R. N.; Flaugh, M. E.; Harn, N. K.; Kress, T. J.; Varie, D. L.;
Wepsiec, J. P. J. Org. Chem. 1997, 62, 8634–8639.
(13) (a) Crowe, E.; Hossner, F.; Hughes, M. J. Tetrahedron 1995, 51, 8889–
8900. (b) Harn, N. K.; Gramer, C. J.; Anderson, B. A. Tetrahedron
Lett. 1995, 36, 9453–9456. (c) Anderson, B. A.; Harn, N. K. Synthesis
1996, 583–585. (d) Reeder, M. R.; Gleaves, H. E.; Hoover, S. A.;
Imbordino, R. J.; Pangborn, J. J. Org. Process Res. DeV. 2003, 7,
696–699.
(9) Crank, G.; Foulis, M. J. J. Med. Chem. 1971, 14, 1075–1077.
(10) Selected examples of natural product syntheses involving 2,4-
substituted oxazole construction: (a) Wipf, P.; Graham, T. H. J. Am.
Chem. Soc. 2004, 126, 15346–15347. (b) Wipf, P.; Lim, S. J. Am.
Chem. Soc. 1995, 117, 558–559. (c) Bull, J. A.; Balskus, E. P.; Horan,
R. A. J.; Langner, M.; Ley, S. V. Chem.sEur. J. 2007, 13, 5515–
5538. (d) Linder, J.; Moody, C. J. Chem. Commun. 2007, 1508–1509.
(e) Panek, J. S.; Liu, P. J. Am. Chem. Soc. 2000, 122, 11090–11097,
and prior studies. (f) Smith, A. B., III; Minbiole, K. P.; Verhoest,
P. R.; Schelhaas, M. J. Am. Chem. Soc. 2001, 123, 10942–10943.
(11) Negishi, E.; Van Horn, D. E.; King, A. O.; Okukado, N. Synthesis
1979, 501–502.
(14) Hodgetts, K. J.; Kershaw, M. T. Org. Lett. 2002, 4, 2905–2907.
(15) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P. Angew.
Chem., Int. Ed. 2006, 45, 6040–6044.
(16) Determined by periodic quenching of aliquots with an excess of I2
solution in THF and back-titration with standard aqueous 0.1 M
Na2S2O3; see Supporting Information.
(17) (a) Duboudin, J. G.; Jousseaume, B.; Bonakdar, A. J. Organomet.
Chem. 1979, 168, 227–232. (b) Wiemer, D. F.; Han, Q. J. Am. Chem.
Soc. 1992, 114, 7692–7697. (c) Larock, R. C.; Doty, M. J.; Han, X.
J. Org. Chem. 1999, 64, 8770–8779.
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A R T I C L E S
Skepper et al.
Scheme 3. Synthesis of Eastern Hemisphere Phosphonium Salt 7a
a Reagents and conditions: (a) DIBAL, CH2Cl2, toluene, -90 °C, 2 h, 89%; (b) N,N′-((1S,2S)-1,2-diphenylethane-1,2-diyl)bis(toluenesulfonamide) (18),
BBr3, CH2Cl2, 0 °C f rt, 1 h, then 17, 0 °C f rt, 15.5 h, then 8, -78 °C, 1.5 h, 88%, dr 24:1; (c) (i) OsO4, K3Fe(CN)6, K2CO3, NaHCO3, DABCO,
t-BuOH/H2O, rt, 2.5 h, then (ii) NaIO4, THF/H2O, 0 °C f rt, 30 min, 60%; (d) Et2BOMe, NaBH4, THF, MeOH, -78 °C, 4 h; (e) 2,2-dimethoxypropane,
CSA, rt, 1 h, 89% (two steps); (f) DIBAL, CH2Cl2, toluene, -78 f -10 °C, 2 h, 80%; (g) PPh3, imidazole, I2, THF, 0 °C, 1 h, 89%; (h) PPh3, Li2CO3,
MeCN, toluene, microwave, 130 °C, 30 min, 75%.
treated with Me2Zn in the presence of (+)-MIB18 to give
secondary alcohol 15 in good yield and high enantiomeric excess
(60%, 93% ee from Mosher’s ester analysis;19 see Supporting
Information), with retention of olefin geometry. Methylation of
the newly formed alcohol under standard conditions provided
10. Treatment of 9 with vinyl iodide 10 in the presence of Pd(0)
resulted in smooth Negishi cross-coupling to give 16 in 86%
overall yield (based on 10 as the limiting reagent). To the best
of our knowledge, this represents the first preparation and
utilization of an oxazol-2-ylzinc reagent by direct Zn0 insertion;
in our hands, multigram quantities of 16 could be prepared in
three steps from the known aminoxazole 11.
terminal iodide 22 to phosphonium salt 7 proceeded without
incident (Scheme 3).25,26
Synthesis of the Western Hemisphere
Synthesis of aldehyde 5 commenced with dehydration of 23
(prepared by resolution of the racemic acid as the phenethy-
lamine salt)27 to give the cyclic anhydride 24 (quantitative),
which was subjected to methanolysis in the presence of pyridine
to give 25 without epimerization (87%) (Scheme 4).28 Reduction
of 25 using conditions described by Lautens and co-workers
gave alcohol 2628,29 (82%), which was oxidized to the corre-
sponding aldehyde under Swern conditions (85%).30
Several methods for allylation of 27 were surveyed. Substrate-
directed allylations with a range of standard Lewis acid catalysts
gave poor ratios of 29a:29b, indicating that 27 lacks significant
intrinsic diastereofacial bias. Hence we turned our attention to
reagent-controlled alternatives (Table 1). The recently reported
asymmetric Barbier-type allylation developed by Singaram and
co-workers31 conveniently gave 29a:29b in good yield (Table
1, entries 1 and 2; 81% and 83%, respectively) with operational
simplicity but only modest diastereoselectivity (dr 3.9:1).
Further, it was found the diastereomers were difficult to separate.
Chiral Brown allylborane addition32 gave the desired product
Treatment of 16 with DIBAL at low temperature (-90 °C)
gave the corresponding aldehyde 8 in good yield (Scheme 3).
Williams and co-workers have previously demonstrated20 the
utility of Corey’s C2-symmetric catalyst21 18 in allylations of
oxazole-4-carboxaldehydes en route to phorboxazole A. In our
hands, reaction of 8 with 17 in the presence of 18 gave 19 with
1
excellent diastereoselectivity (dr 24:1), as determined by H
NMR integration of the corresponding Mosher’s ester.19
Oxidative cleavage of the vinylidene double bond (OsO4,
K3[Fe(CN)6], then NaIO4) gave ketone 20, which was subse-
quently reduced under Narasaka’s conditions22 to give the syn-
1,3-diol (dr > 95:5) and immediately protected as the acetonide
21.23,24 Reductive removal of the benzoate protecting group and
sequential elaboration of the resulting alcohol through the
(24) The relative configuration of the major diol was confirmed using
Rychnovsky’s method: Rychnovsky, S. D.; Rogers, B.; Yang, G. J.
Org. Chem. 1993, 58, 3511–3515.
(18) (a) Nugent, W. A. J. Chem Soc., Chem. Commun. 1997, 1369–1370.
(b) Kim, H. Y.; Lurain, A. E.; Gacia-Garcia, P.; Carroll, P. J.; Walsh,
P. J. J. Am. Chem. Soc. 2005, 127, 13138–13139.
(25) Smith, A. B., III; Zhu, W.; Shirakami, S.; Sfouggatakis, C.; Doughty,
V. A.; Bennett, C. S.; Sakamoto, Y. Org. Lett. 2003, 5, 761–764.
(26) Best yields of phosphonium salt 7 were achieved under microwave
conditions (130 °C, 30 min) in the presence of Li2CO3 to suppress
acetonide loss and H2O elimination catalyzed by trace HI.
(27) Gruenfeld, N.; Stanton, J. L.; Yuan, A. M.; Ebetino, F. H.; Browne,
L. J.; Gude, C.; Huebner, C. F. J. Med. Chem. 1983, 26, 1277–1282.
(28) Hoffman, R. W.; Ditrich, K.; Koster, G.; Sturmer, R. Chem. Ber. 1989,
122, 1783–1789.
(19) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem.
Soc. 1991, 113, 4092–4096.
(20) (a) Williams, D. R.; Brooks, D. A.; Meyer, K. G.; Clark, M. P.
Tetrahedron Lett. 1998, 39, 7251–7254. (b) Williams, D. R.; Kiryanov,
A. A.; Emde, U.; Clark, M. P.; Berliner, M. A.; Reeves, J. T. Proc.
Natl. Acad. Sci. U.S.A. 2004, 101, 12058–12063.
(21) (a) Corey, E. J.; Imwinkelried, R.; Pikul, S.; Xiang, Y. B. J. Am. Chem.
Soc. 1989, 111, 5493–5494. (b) Pikul, S.; Corey, E. J. Org. Synth.
1993, 71, 22–26.
(29) Lautens, M.; Colucci, J. T.; Hiebert, S.; Smith, N. D.; Bouchain, G.
Org. Lett. 2002, 4, 1879–1882.
(30) Leopold, E. J. Org. Synth. 1986, 64, 164–171.
(22) Narasaka, K.; Pai, F.-C. Tetrahedron 1984, 40, 2233–2238.
(23) Chen, K.-M.; Hardtmann, G. E.; Prasad, K.; Repic, O.; Shapiro, M. J.
Tetrahedron Lett. 1987, 28, 155–158.
(31) Hirayama, L. C.; Gamsey, S.; Knueppel, D.; Steiner, D.; DeLaTorre,
K.; Singaram, B. Tetrahedron Lett. 2005, 46, 2315–2318.
(32) Racherla, U. S.; Brown, H. C. J. Org. Chem. 1991, 56, 401–404.
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Total Synthesis of Enigmazole A
A R T I C L E S
Scheme 4. Synthesis of Western Hemisphere Aldehyde 5a
a Reagents and conditions: (a) AcCl, 50 °C, 30 min, quantitative; (b) MeOH, pyridine, CH2Cl2, 3 h, 87%; (c) BH3 ·DMS, B(OMe)3, THF, 0 °C f rt, 3 h,
82%; (d) Cl(CO)2Cl, DMSO, CH2Cl2, -78 °C, 30 min, then 26, -78 °C, 30 min, then Et3N, -78 f 0 °C, 2 h, 85%; (e) 28, 4 Å MS, toluene, -78 °C, 2 h,
85%; (f) TBSCl, imidazole, DMF, 17 h, 90%; (g) O3, CH2Cl2, -78 °C, then PPh3, CH2Cl2, -78 °C f rt, 1 h, then rt, 30 min, 99%.
Table 1. Optimization of Allylation of Aldehyde 27a
yield) to give a separable mixture of three of the four possible
diastereomers (34a-c; dr 1.0:4.2:0.25); the stereochemistry of
the diastereomers was assigned by NOE analysis and Mosher’s
ester analysis.35
Hydrogenolysis of 34b, followed by Swern oxidation of
product 35, gave aldehyde 36 that was immediately subjected
to Wittig reaction with the ylide derived from 7. Selective
hydrogenation of the C12-C13 olefin of Wittig product 37 in
the presence of the more substituted C21-C22 double bond
(Wilkinson’s catalyst, H2) proceeded slowly but with excellent
regioselectivity to give 38.36,37
allyl donor
additive
solvent
temp, °C
t, h
yield, % (29a:29b)
1
2
3
4
32
32
31
28
33
33
N/A
THF
THF
Et2O
-78
-95
-95
-78
1.5
1.5
0.5
2
81 (3.9:1)
83 (3.9:1)
29 (g10:1)
85 (9:1)
Saponification of methyl ester 38 and acid-promoted liberation
of the C15, C17-diol 39 set the stage for macrolide formation.
Unfortunately, application of standard macrolactonization condi-
tions to 39 (Yamaguchi38 or Yonemitsu’s variant39 or Shiina’s
4 Å MS toluene
a See Supporting Information for experimental details.
(35) The relative configuration of 34b were assigned from observation of
NOEs between the axial 1H NMR signals of H7 and H11. The absolute
configuration of 34b was assigned from a parallel analysis of the
analogue S9b and its diastereomers, obtained under the same condi-
tions: liberation of the corresponding ketone S10 (p-TSA, H2O-
CH2Cl2) followed by reduction (LiAlH(t-OBu)3, THF) gave equatorial
alcohol S11, Mosher’s analysis of which confirmed the (7R,9S,11R)
configuration; see Supporting Information. The relative configuration
of the anti-isomers 34a and 34c were assigned by similar NOE and J
analyses (arbitrary absolute configurations); however, the fourth
expected syn diastereomer could not be detected (HPLC).
with high diastereoselectivity (g10:1); however, isolation from
byproducts proved troublesome (entry 3). Roush allylation
conditions33 gave optimal yields and dr for high throughput of
material, providing 29a in good yield (85%) and dr (9:1).
Protection of the newly formed alcohol as its TBS ether (90%)
followed by ozonolysis (99%) gave the key aldehyde 5 in 46%
overall yield from 23.
Fragment Coupling and Macrolactonization
Unification of “Eastern” and “Western” hemispheres of
enigmazole A began with HDA cycloaddition between aldehyde
5 and diene 6 (Scheme 5). All attempts to induce a reagent-
controlled HDA reaction (Jacobsen’s chiral Cr(salen) catalyst34)
failed to give product. It was found that treating a mixture of 5
and 6 with catalytic BF3 ·OEt2 led to a highly diastereoselective
substrate-controlled HDA reaction. Optimization of the reaction
(see Supporting Information) included in situ, one-pot conver-
sion to dimethylketal 34b (CH(OCH3)3, MeOH, and CSA, 81%
(36) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem.
Soc. (A) 1966, 1711–1732.
(37) Superior reaction rates and yields of 37 were achieved when the
hydrogenation was conducted in THF/t-BuOH (1:1) according to the
Zhu modification: Jourdant, A.; Gonzlez-Zamora, E.; Zhu, J. J. Org.
Chem. 2002, 67, 3163–3164.
(33) (a) Roush, W. R.; Walts, A. E.; Hoong, L. K. J. Am. Chem. Soc. 1985,
107, 8186–8190. (b) Roush, W. R.; Palkowitz, A. D.; Ando, K. J. Am.
Chem. Soc. 1990, 112, 6348–6359.
(34) Dossetter, A. G.; Jamison, T. F.; Jacobsen, E. N. Angew. Chem., Int.
Ed. 1999, 38, 2398–2400.
(38) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989.
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A R T I C L E S
Skepper et al.
Scheme 5. Unification of Eastern and Western Hemispheres and Attempted Macrolactonizationa
a Reagents and conditions: (a) BF3 ·OEt2 (0.2 equiv), CH2Cl2, -78 °C, 40 min, then CH(OCH3)3, MeOH, CSA, -78 °C f rt, 1 h, 81%; (b) H2, Pd/C,
EtOAc, 3 h, 94%; (c) (COCl)2, DMSO, CH2Cl2, - 78 °C, 10 min, then 35, -78 °C, 30 min, then Et3N, -78 °C, 30 min, then 0 °C, 30 min, 99%; (d) (i)
LiHMDS, 7, THF, -78 °C, 30 min, then 0 °C, 30 min, then (ii) 36, -78 °C, 75 min, then 0 °C, 45 min, 71%; (e) (PPh3)3RhCl, H2, THF/t-BuOH (1:1), 50
°C, 6 h, 83%; (f) (i) LiOH, MeOH/H2O, 80 °C, 1 h, then (ii) CSA, MeOH, rt, 1 h, quant.
method40) gave only complex, intractable mixtures. Ring closure
was achieved only by using Mukaiyama or Keck conditions,
but to our disappointment the predominant product was the
undesired 16-membered macrolide 40b, formed by attack of
the C15 hydroxyl upon the activated carboxylate ester, rather
than the 18-membered ring lactone 40a.41,42 All attempts to
optimize the Keck macrolactonization of 39 were unsuccessful
(Table 2). Under standard prescribed conditions (reflux,
(CH2)2Cl2), the smaller-ring macrolide 40b was obtained
exclusively (entry 1), while at lower temperature no product
was formed (entry 2). Variation of the solvent gave little
improvement. Replacement of (CH2)2Cl2 with toluene gave only
slightly better yield (40a:b 1:3, 30% yield, entry 4), while use
of CHCl3 led predominantly to the desired macrolide 40a;
however, difficulties in purification led to an unsatisfactory yield
(entry 5). Switching to DCC/PPTS/pyridine (entry 7)43 gave
40b exclusively in reasonable yield, and similar results were
obtained using EDC ·HCl/DMAP (entry 8).44
unwanted smaller ring size.45 Paterson observed the opposite
problem in the syntheses of aplyronine and scytophicinsformation
of an undesired kinetic macrolactonization productswhich could
be corrected by inducing 1,3-acyl migration in the unwanted
product [Ti(Oi-Pr)4] to set the desired macrolide ring size.46 In
our case, treatment of the C9-keto derivative of 40b with Ti(Oi-
Pr)4 returned only the 16-membered macrolide.
We reasoned that if the trajectory of attack of the C15-OH
group to the activated carboxylate ester intermediate was more
favorable than that of the C17-OH, and the product more stable,
then alteration of the bond geometry in the vicinity of C15 and
C17 may impose an energetically more favorable reaction
trajectory and/or product stability by altering the conformation
in the precursor. Consequently, the order of hydrogenation-
macrolactonization was reversed. Deprotection of 37 (Scheme
6) gave the corresponding dihydroxy acid with the Z-C12-C13
olefin intact, and we were gratified to find that Keck macro-
(44) Hanessian, S.; Ma, J.; Wang, W. J. Am. Chem. Soc. 2001, 123, 10200–
10206.
These results suggested strongly that ring closure of 39 was
both kinetically and thermodynamically biased toward the
(45) Evidence that 40b is the kinetic and thermodynamic product of
macrolactonization arises from our observation that the dihydro-41
(Vide infra and ref 46) undergoes an anomalous reaction upon
attempted Wittig olefination. The Wittig reagent was sufficiently basic
to cleave the acetate ester, presumably liberating an alkoxide, which
undergoes spontaneous ring contraction (1,3-acyl migration) to the
smaller macrolide ring. Since the alkoxide obtained from base-
promoted deacetylation of dihydro-41 and the alkoxide of 40bsan
intermediate of 1,3-acyl migration that would equilibrate 40a,bsare
identical, it is likely that 40b is both the kinetic and thermodynamic
product of macrolactonization.
(39) (a) Hikota, M.; Tone, H.; Horita, K.; Yonemitsu, O. J. Org. Chem.
1990, 55, 7–9. (b) Tone, H.; Nishi, T.; Oikawa, Y.; Hikota, M.;
Yonemitsu, O. Tetrahedron Lett. 1987, 28, 4569–4572. (c) Hikota,
M.; Sakurai, Y.; Horita, K.; Yonemitsu, O. Tetrahedron Lett. 1990,
31, 6367–6370.
(40) Shiina, I.; Kubota, M.; Oshiumi, H.; Hashizume, M. J. Org. Chem.
2004, 69, 1822–1830.
(41) Boden, E.; Keck, G. E. J. Org. Chem. 1985, 50, 2394–2395.
(42) Mukaiyama, T.; Usui, M.; Saigo, K. Chem. Lett. 1976, 49.
(43) Kageyama, M.; Tamura, T.; Nantz, M. H.; Roberts, J. C.; Somfai, P.;
Whritenour, D. C.; Masamune, S. J. Am. Chem. Soc. 1990, 112, 7407–
7408.
(46) (a) Paterson, I.; Watson, C.; Yeung, K.-S.; Ward, R. A.; Wallace, P. A.
Tetrahedron 1998, 54, 11955–11970. (b) Paterson, I.; Woodrow,
M. D.; Cowden, C. J. Tetrahedron Lett. 1998, 39, 6041–6044.
9
10290 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Total Synthesis of Enigmazole A
A R T I C L E S
Table 2. Attempted Keck Macrolactonization of Dihydroxy Acid 39
solvent
conditionsa
temp, °C
addition time, h
yield, % (40a:40b)
1
2
3
4
5
6
7
8
9
Cl(CH2)2Cl
Cl(CH2)2Cl
THF
toluene
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
A
A
A
A
A
A
B
C
A
83
60
66
110
61
61
61
61
40
16
16
16
16
16
2
16
16
16
47 (0:100)b
0c
0c
31 (1:3)
28 (1:3.7)d
0c
50 (0:100)
30 (∼1:10)
0c
a Conditions A: slow addition (syringe pump) of 39 to a refluxing solution of DCC (25 equiv), DMAP (22 equiv), and DMAP ·HCl (24 equiv) in the
indicated solvent (c ) 0.3 mM). Conditions B: slow addition of 39 to a refluxing solution of DCC (20 equiv), pyridine (100 equiv), and PPTS (20
equiv) in the indicated solvent (c ) 0.3 mM). Conditions C: slow addition of 39 to a refluxing solution of EDC ·HCl (5 equiv) and DMAP (5 equiv) in
the indicated solvent (c ) 0.3 mM). b Reaction conducted on C9 keto derivative. c Neither 40a or 40b was observed in crude NMR. d Extensive
side-product formation, requiring HPLC purification.
Scheme 6. Successful Keck Macrolactonization and Completion of
protected the C15 hydroxyl as the acetate ester 41 in preparation
for final elaboration to the desired natural product.
the Synthesisa
Completion of the Synthesis
Compound 41 was selectively hydrogenated as before using
Wilkinson’s catalyst followed by acetal hydrolysis to provide
ketone 42 (Scheme 6). Standard Wittig olefination of 42
(Ph3PCH2, THF, 0 °C) gave poor conversions;47 however, the
nonbasic Lombardo’s reagent48 (Zn, TiCl4, CH2Br2) led to
smooth olefination of ketone 42 to 43 (78%).
Compound 43 proved unexpectedly resistant to desilylation
(no reaction with TBAF). The best conditions (HF/MeCN/H2O,
2 days) gave the desired alcohol in 72% average yield.49 The
phosphate was introduced to the C5 secondary hydroxyl group
of 44 as a protected phosphoramidite (i-Pr2NP(OFm)2) to give
the fully protected natural product 45 in 61% yield.50 Treatment
of 45 with K2CO3 in MeOH/H2O smoothly cleaved the C15
acetate and both 9-fluorenylmethyl groups of the phosphate ester,
providing enigmazole A (1) as the K+ salt in near-quantitative
yield. Ion exchange of the product51 gave the Na+ salt of 1,
a Reagents and conditions: (a) LiOH, MeOH/H2O, 80 °C, 1 h; (b) CSA,
identical in all respects to the natural product (1H, 13C, and 31
P
MeOH, rt, 1 h; (c) (i) DCC, DMAP, DMAP ·HCl, CHCl3, reflux, 15.5 h,
then (ii) AcOH, MeOH, concentration, 35% (three steps, after HPLC); (d)
(PPh3)3RhCl, H2, THF/t-BuOH (1:1), 50 °C, 5 h, 70%; (e) CSA, acetone,
rt, 4.5 h, 82%; (f) (i) Zn, Br2CH2, TiCl4, THF, -78 f 0 °C, 2 d, then (ii)
42, CH2Cl2, 0 °C f rt, 30 min, 78%; (g) 48% HF/MeCN/H2O (5:86:9), rt,
2 d, 72% mean yield (based on recovered 43) + 13-27% recovered 43;
(h) (i) i-Pr2NP(OFm)2, 1H-tetrazole, MeCN/CH2Cl2, rt, 40 min, then (ii)
30% H2O2, 0 °C, 10 min, 61%; (i) K2CO3, MeOH, H2O, rt, 25 h, 98%; (j)
RP HPLC, Phenomenex Luna 5 µm C18, 28 f 53% MeCN in 100 mM
NaClO4, 0.75 mL/min. Fm ) 9-fluorenylmethyl.
(47) The Wittig reagent was sufficiently basic to induce cleavage of the
C15 acetate and subsequent 1,3-acyl migration of the 18- to 16-
membered macrolide (39%), leaving only 46% of the desired 43.
Lowering the temperature to -40 °C suppressed this side reaction but
did not improve the yield of 43.
(48) Lombardo, L. Org. Synth. 1987, 65, 81–85.
(49) Based on recovered starting material (59% average isolated yield, 13-
27% recovered starting material).
(50) (a) Watanabe, Y.; Nakamura, T.; Mitsumoto, H. Tetrahedron Lett.
1997, 38, 7407–7410. (b) Bialy, L.; Waldmann, H. Chem.sEur. J.
2004, 10, 2759–2780. (c) Lawhorn, B. G.; Boga, S. B.; Wolkenberg,
S. E.; Colby, D. A.; Gauss, C.-M.; Swingle, M. R.; Amable, L.;
Honkanen, R. E.; Boger, D. L. J. Am. Chem. Soc. 2006, 128, 16720–
16732.
lactonization of this precursor now furnished the desired 18-
membered macrolide 41 cleanly and exclusively (35% yield over
three steps after HPLC). Quenching the reaction mixture, containing
overequivalents of DCC, with excess AcOH also conveniently
(51) HPLC (Phenomenex Luna 5 µm C18, 28-53% CH3CN in 100 mM
NaClO4, 0.75 mL/min), followed by desalting over a C18 cartridge.
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10291

A R T I C L E S
Skepper et al.
oxazol-2-ylzinc reagent 9, prepared from an ambident 2,4-
disubstituted oxazole synthon 11 by conversion to iodooxazole
12 and direct Zn0 insertion under Knochel conditions.15 Other
key features of the synthesis include substrate-directed hetero-
Diels-Alder cycloaddition to form the embedded 2,6-syn pyran
ring, a Wittig coupling to unite key fragments 7 and 36, and a
strategic, selective, conformationally directed Keck macrolac-
tonization of an advanced 1,3-dihydroxy carboxylic acid
intermediate to the desired 18-membered macrolide.
Acknowledgment. We are grateful to K. Gustafson for helpful
discussions and a generous gift of natural 1. We thank K. Elbel for
assistance in preparations and stability studies of 9, and A. Mrse
for help with NMR experiments. HRMS measurements were
provided by Y. Su, UCSD Small-Molecule MS Facility, R. New,
UC Riverside mass spectrometric facilities, and G. Suizdak, Scripps
Research Institute. Credit for the sponge image on the cover goes
to Coral Reef Research Foundation/Patrick L. Colin, and we are
grateful to J. Molinski for assistance with rendering. Funding for
purchase of the 500 MHz NMR spectrometers was provided by a
grant from NSF (CRIF, CHE0741968). C.K.S was supported by
the Teddy Traylor Fellowship, Department of Chemistry and
Biochemistry, UC San Diego.
Figure 2. CD spectra (MeOH, 25 °C) of natural and synthetic enigmazole
A (1).
NMR, UV, HPLC retention time). Finally, CD spectra of natural
and synthetic 1 were superimposable (Figure 2).
Supporting Information Available: Experimental procedures,
optimization of the synthesis of 34b (Scheme S2), full charac-
terization and 1H and 13C NMR spectra for all new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Conclusions
In summary, we have completed the first total synthesis of
enigmazole A (1) in 22 steps and 0.41% overall yield from the
known aldehyde 14. Our approach to the key oxazole-containing
side chain employs an efficient Negishi coupling with the
JA1016975
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10292 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010