Total synthesis of ustiloxin D

Article abstract of DOI:10.1021/ja017277z

The total synthesis of ustiloxin D, a highly potent inhibitor of microtubule assembly, has been achieved. Notable features are the use of nucleophilic aromatic substitution (S_NAr) for the construction of a chiral tertiary alkyl-aryl ether linka

Full text of DOI:10.1021/ja017277z

Published on Web 01/05/2002
Total Synthesis of Ustiloxin D
Bin Cao, Haengsoon Park, and Madeleine M. Joullie´*
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323
Received October 10, 2001
In 1992, Iwasaki and co-workers reported the isolation of
ustiloxin A (1) from the water extract of false smut balls on the
panicles of the rice plant caused by the fungus Ustilaginoidea
Virens.1 Later, four other congeners, ustiloxin B, C, D (2), and F,
were isolated and characterized.2 These natural products are 13-
membered cyclic depsipeptides possessing a unique chiral tertiary
alkyl-aryl ether linkage. Biological evaluation showed the ustilox-
ins to be potent antimitotic agents that strongly inhibit microtubule
assembly by interfering with tubulin polymerization.3 Due to the
important role of microtubules in cell mitosis, compounds that bind
tubulin and inhibit its assembly are potential anticancer drugs.4
Ustiloxins have been shown to inhibit the growth of a variety of
human tumor cell lines. To better understand the structure-activity
relationships of tubulin binding, synthetic efforts by several groups
have been directed toward a series of simplified ustiloxin model
systems,5 semisynthetic analogues,6 the sulfinylnorvaline side chain
of 1,7 and phomopsin A.8 Herein, we describe the first total synthesis
of ustiloxin D (2, Figure 1) in an effort to facilitate analogue
preparation and provide structural variations for investigation of
tubulin binding activity.
The main challenge in the total synthesis of 2 was the formation
of the chiral tertiary alkyl-aryl ether linkage in the highly
functionalized macrocycle. The key aspects of the synthetic strategy
were the installation of the ether linkage at an early stage in order
to circumvent future functional group incompatibility and the
stereospecific generation of the â-hydroxytyrosine moiety in a single
step. The retrosynthetic analysis of 2 is shown in Figure 1.
Disconnection of the side chain as well as an amide bond within
the macrocycle of 2 gave linear precursor 3, which could be
obtained via the regioreversed Sharpless asymmetric aminohy-
droxylation9 of a cinnamate derivative. Further disconnections led
to ether 4. A number of methods have been reported for the
synthesis of tertiary alkyl-aryl ethers, including nucleophilic
substitution,10 palladium-catalyzed cross-coupling,11 and Mitsunobu
reactions.12 These approaches either failed to give the desired ether
in ustiloxin D or were not suitable for the total synthesis. After
extensive experimentation, we chose a nucleophilic aromatic
substitution (SNAr) approach to construct the ether linkage by
reaction of nitrile-activated aryl fluoride 5 with tertiary alcohol 6.
Compound 6 could be prepared from D-serine.
Figure 1. Retrosynthetic analysis of ustiloxin D.
Treatment of 14 with ethyl acrylate under Heck conditions18
afforded cinnamate 15 in 95% yield. The Sharpless regioreversed
asymmetric aminohydroxylation protocol9 was successfully applied
to afford the desired (2S,3R)-â-hydroxy amino ester 16 in a single
step (58% yield).19 After removal of the Boc group, the amine was
coupled with N-Boc-L-valine by using the DEPBT reagent.20
Treatment of 17 with TBSOTf protected the secondary hydroxyl
group as its silyl ether, as well as transformed the tert-butyl
carbamate to the corresponding tert-butyldimethylsilyl carbamate.21
Basic hydrolysis afforded amino acid 19, which was then cyclized
via macrolactamization to afford macrocycle 20 in 78% yield with
EDCI-HOBt in a mixed solvent DMF-CH2Cl2 (1:5).22
With the macrocycle in hand, we were able to remove the MOM
group (Scheme 2). To achieve selective methylation on the nitrogen
of the tyrosine moiety, the benzyl carbamate was selectively
removed by catalytic hydrogenolysis in the presence of the benzyl
ether,23 and amine 22 was subsequently protected as the 2-nitroben-
zenesulfonamide 23. Sequential treatment of the primary hydroxyl
group in 23 with Dess-Martin reagent and then with NaClO2
afforded a carboxylic acid, which was then coupled with glycine
benzyl ester to give 24. Selective deprotonation of the sulfonamide
with MTBD24 and subsequent methylation25 before deprotection
of the silyl ether afforded alcohol 25. Removal of the 2-nitroben-
zenesulfonyl group with â-mercaptoethanol gave a secondary amine.
The final step involved hydrogenolysis with Pd black to remove
both the benzyl ester and the benzyl ether to afford ustiloxin D (2)
in 85% yield. The 1H NMR, 13C NMR, HRMS, and optical rotation
of synthetic 2 were identical to those of the natural product.
In conclusion, the first total synthesis of 2 was accomplished
from readily available D-serine and aryl fluoride starting materials.
The synthesis provides a scaffold upon which we can construct
other congeners and analogues to better understand the tubulin
binding activity of the ustiloxin family of natural products.
The synthesis began with the preparation of amino alcohol 9
(Scheme 1). Alcohol 8 was synthesized from D-serine (7) in 5
steps.13 Removal of the isopropylidene acetal gave a diol in which
the liberated primary hydroxyl group was selectively protected as
its MOM ether.14 Subsequent removal of the Boc group afforded
amino alcohol 9. Treatment of 9 with 5 and KHMDS successfully
introduced the ether linkage in 10 in 78% yield.8a,15 The amino
group was then protected as its Boc carbamate, and Raney nickel
reduction converted 10 to aldehyde 11.16 Dakin oxidation converted
11 to its corresponding formate, and subsequent hydrolysis gave
phenol 13,17 which was then protected as benzyl ether (14).
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10.1021/ja017277z CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1a
Scheme 2a
a Key: (a) B-bromocatecholborane, CH2Cl2, -78 °C, 88%; (b) H2, 5%
Pd/C, 2,2′-dipyridyl, MeOH-EtOAc, 2 h; (c) 2-nitrobenzenesulfonyl
chloride, 2,4,6-collidine, 85% for 2 steps; (d) Dess-Martin periodinane,
CH2Cl2; (e) NaClO2, NaH2PO4, 2-methyl-2-butene, t-BuOH-H2O; (f)
Gly‚OBn, DEPBT, i-Pr2NEt, THF, 66% for 3 steps; (g) methyl 4-nitroben-
zenesulfonate, MTBD, DMF, 77%; (h) n-Bu4NF-HOAc (1:1.1), THF, 0-25
°C, 4 h, 96%; (i) HSCH2CH2OH, DBU, DMF, room temperature, 74%; (j)
H2, Pd black, EtOH-H2O, 1.6 equiv of HCl, 2 h, 85%.
(4) (a) Hamel, E. Med. Res. ReV. 1996, 16, 207-231. (b) Iwasaki, S. Med.
Res. ReV. 1993, 13, 183-198.
(5) For the construction of alkyl-aryl ether via the SNAr approach, see: La¨ıb,
T.; Zhu, J. Synlett 2000, 9, 1363-1365. Williamson alkylation approach,
see: (a) Mutoh, R.; Shirai, R.; Koiso, Y.; Iwasaki, S. Heterocycles 1995,
41, 9-12. (b) Takahashi, M.; Shirai, R.; Koiso, Y.; Iwasaki, S.
Heterocycles 1998, 47, 163-166.
(6) Morisaki, N.; Mitsui, Y.; Yamashita, Y.; Koiso, Y.; Shirai, R.; Hashimoto,
Y.; Iwasaki, S. J. Antibiot. 1998, 51, 423-427.
a Key: (a) Boc2O, NaHCO3, dioxane-H2O; (b) MeNH(OMe)‚HCl,
EDCI‚HCl, THF-H2O, pH 4.5, 4 h, 71% for 2 steps; (c) (MeO)2CMe2,
catalytic p-TsOH, 97%; (d) MeLi, THF, -78 to -65 °C, 72%; (e) EtMgBr,
THF, -78 to -20 °C, ds ) 9:1, 58%; (f) p-TsOH, MeOH; (g) MOMCl,
i-Pr2NEt, CH2Cl2, 0 °C to room temperature, 71% for 2 steps; (h) TFA,
CH2Cl2, 0 °C, 78%; (i) 4-bromo-2-fluorobenzonitrile (5), 1.1 equiv of
KHMDS, THF, 0 °C to room temperature, 78%; (j) Boc2O, Et3N, CH2Cl2,
81%; (k) Raney nickel, NaH2PO2, Pyr:H2O:HOAc (2:1:1), 0-55 °C, 56%;
(l) 30% H2O2, 4 mol % bis(2-nitrophenyl)diselenide, CH2Cl2, room
temperature, 24 h; (m) KOH, MeOH-H2O, 73% for 2 steps; (n) BnBr,
K2CO3, n-Bu4NI, DMF, 96%; (o) ethyl acrylate, Pd(OAc)2, (o-tolyl)3P, Et3N,
CH3CN, reflux, 6 h, 95%; (p) NaOH, BnOCONH2, t-BuOCl, K2[OsO2(OH)4],
(DHQD)2AQN, n-PrOH:H2O (1:1), 20 °C, 1 h, 58%; (q) TFA, CH2Cl2, 0
°C; (r) N-Boc-L-valine‚OH, DEPBT, i-Pr2NEt, THF, 81% for 2 steps; (s) 6
equiv of TBSOTf, 8 equiv of 2,6-lutidine, CH2Cl2, room temperature; (t)
LiOH, t-BuOH-H2O, 83% for 2 steps; (u) 5 equiv of EDCI‚HCl, 5 equiv
of HOBt, DMF-CH2Cl2 (1:5), 0 °C to room temperature, 78%.
(7) Hutton, C. A.; White, J. M. Tetrahedron Lett. 1997, 38, 1643-1646.
(8) Phomopsin A is an antimitotic agent whose structure is closely related to
that of ustiloxins. See: (a) Woiwode, T. F.; Rose, C.; Wandless, T. J. J.
Org. Chem. 1998, 63, 9594-9596. (b) Stohlmeyer, M. M.; Tanaka, H.;
Wandless, T. J. J. Am. Chem. Soc. 1999, 121, 6100-6101. (c) Woiwode,
T. F.; Wandless, T. J. J. Org. Chem. 1999, 64, 7670-7674.
(9) (a) Tao, B.; Schlingloff, G.; Sharpless, K. B. Tetrahedron Lett. 1998, 39,
2507-2510. (b) Park, H.; Cao, B.; Joullie´, M. M. J. Org. Chem. 2001,
66, 7223-7226. We thank Professors Barry Sharpless and Valery Fokin
for information on this asymmetric aminohydroxylation procedure.
(10) (a) Coutts, I. G. C.; Southcott, M. R. J. Chem. Soc., Perkin Trans. 1 1990,
767-771. (b) Masada, H.; Oishi, Y. Chem. Lett. 1978, 57-58.
(11) (a) Mann, G.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 13109-13110.
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5413-5418.
(12) (a) Subramanian, R. S.; Balasubramanian, K. K. Synth. Commun. 1989,
19, 1255-1259. (b) Bittner, S.; Assaf, Y. Chem. Ind. (London) 1975, 6,
281.
(13) Ageno, G.; Banfi, L.; Cascio, G.; Guanti, G.; Manghisi, E.; Riva, R.;
Rocca, V. Tetrahedron 1995, 51, 8121-8134.
(14) Barton, D. H. R.; Ge´ro, S. D.; Maycock, C. D. J. Chem. Soc., Perkin
Trans. 1 1982, 7, 1541-1551.
Acknowledgment. We thank the National Institutes of Health
(CA-40081), the National Science Foundation (CHE-99-01449), and
the University of Pennsylvania (Research Foundation) for financial
support. We also thank Professor Shigeo Iwasaki and Dr. Yukiko
Koiso for kindly providing samples of natural ustiloxins A and D.
(15) It is noteworthy that the reaction conditions do not tolerate an aryl fluoride
bearing a benzylic proton para to the cyano group. When the amino group
of 9 was protected as its N-carbamate, the undesired oxazolidinone was
produced via intramolecular cyclization. With free amine 9 as the substrate,
addition of excess base caused the formation of an undesired arylamine
due to a faster reaction between in situ generated amide and aryl fluoride.
(16) Heffner, T. F.; Jiang, J.; Joullie´, M. M. J. Am. Chem. Soc. 1992, 114,
10181-10189.
Supporting Information Available: Experimental procedures and
characterization data for the preparation of all new compounds and
NMR spectra comparison of synthetic and natural 2 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(17) Syper, L. Synthesis 1989, 3, 167-172.
(18) Cho, Y. J.; Rho, K. Y.; Keum, S. R.; Kim, S. H.; Yoon, C. M. Synth.
Commun. 1999, 29, 2061-2068.
(19) Regioselectivity (5:1) and diastereoselectivity (91:9) were determined by
HPLC on a Chiralpak AD column. The desired isomer can be separated
from others by flash column chromatography.
(20) DEPBT: 3-(diethoxyphosphoryloxy)-1,2,3-benzotriazin-4(3H)-one. See:
Li, H.; Jiang, X.; Ye, Y.-H.; Fan, C.; Romoff, T.; Goodman, M. Org.
Lett. 1999, 1, 91-93.
(21) Sakaitani, M.; Ohfune, Y. J. Org. Chem. 1990, 55, 870-876.
(22) Boger, D. L.; Miyazaki, S.; Kim, S. H.; Wu, J. H.; Castle, S. L.; Loiseleur,
O.; Jin, Q. J. Am. Chem. Soc. 1999, 121, 10004-10011. EDCI: 1-[3-
(dimethylamino)propyl]-3-ethylcarbodiimide. HOBt: 1-hydroxybenzotri-
azole.
(23) Sajiki, H.; Hirota, K. Tetrahedron 1998, 54, 13981-13996.
(24) MTBD: 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido [1,2-a]-pyrimidine.
(25) Miller, S. C.; Scanlan, T. S. J. Am. Chem. Soc. 1997, 119, 2301-2302.
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