Total synthesis of (+)-obtusenyne

Full text of DOI:10.1021/jo990212i

2616
J . Org. Chem. 1999, 64, 2616-2617
Sch em e 1
Tota l Syn th esis of (+)-Obtu sen yn e
Kenshu Fujiwara, Daisuke Awakura, Misa Tsunashima,
Akira Nakamura, Teruki Honma, and Akio Murai*,†
Division of Chemistry, Graduate School of Science,
Hokkaido University, Sapporo 060-0810, J apan
Received February 4, 1999
The Laurencia species of red algae produce a wide variety
of cyclic ethereal C15 acetogenins.1 Among them, (+)-ob-
tusenyne (1),2 isolated from Laurencia obtusa by the Imre2a
and Fenical2b groups independently, has the particular
features of an unusual nine-membered cyclic ethereal skel-
eton, bromo (C12) and chloro (C7) substituents on the ring
both with S configurations, and a Z-enyne terminus. Since
these features present synthetic challenge,3,4,5 we have
programmed the enantioselective total synthesis of 1. The
straightforward strategy for the synthesis of 1 was evolved
from the retrosynthetic analysis shown in Scheme 1. The
nine-membered lactone 6 was to be converted to dienyl ether
5 via ethylation of the corresponding vinyl triflate with an
organocopper reagent. On the basis of our previous observa-
tion in a simple monocyclic system,4b epoxidation of 5 with
Sch em e 2a
† Corresponding author. Phone: +81-11-706-2714. Fax: +81-11-706-
2714. E-mail: amurai@sci.hokudai.ac.jp.
(1) (a) Moore, R. E. In Marine Natural Products; Scheuer, P. J ., Ed.;
Academic Press: New York, 1978; Vol. 1, pp 43-121. (b) Erickson, K. L. In
Marine Natural Products; Scheuer, P. J ., Ed.; Academic Press: New York,
1983; Vol. 5, pp 131-257.
(2) (a) King, T. J .; Imre, S.; O¨ ztunc, A.; Thomson, R. H. Tetrahedron Lett.
1979, 1453-1454. (b) Howard, B. M.; Schulte, G. R.; Fenical, W.; Solheim,
B.; Clardy, J . Tetrahedron 1980, 36, 1747-1751. Natural isomers of 1 have
also been reported: (c) Gopichand, Y.; Schmitz, F. J .; Shelly, J .; Rahman,
A.; van der Helm, D. J . Org. Chem. 1981, 46, 5192-5197. (d) Norte, M.;
Gonza´lez, A. G.; Cataldo, F.; Rodr´ıguez, M. L.; Brito, I. Tetrahedron 1991,
47, 9411-9418.
(3) For recent approaches to the total synthesis of 1, see: (a) Carling, R.
W.; Curtis, N. R.; Holmes, A. B. Tetrahedron Lett. 1989, 30, 6081-6084.
(b) Curtis, N. R.; Holmes, A. B.; Looney, M. G. Tetrahedron 1991, 47, 7171-
7178. (c) Carling, R. W.; Clark, J . S.; Holmes, A. B. J . Chem. Soc., Perkin
Trans. 1 1992, 83-94. (d) Curtis, N. R.; Holmes, A. B.; Looney, M. G.
Tetrahedron Lett. 1992, 33, 671-674. (e) Curtis, N. R.; Holmes, A. B.
Tetrahedron Lett. 1992, 33, 675-678. (f) Elliott, M. C.; Moody, C. J .;
Mowlem, T. J . Synlett 1993, 909-910. (g) Brown, D. S.; Elliott, M. C.; Moody,
C. J .; Mowlem, T. J . J . Chem. Soc., Perkin Trans. 1 1995, 1137-1144. (h)
Brandes, A.; Eggert, U.; Hoffmann, H. M. R. Synlett 1994, 745-747. (i)
Brandes, A.; Hoffmann, H. M. R. Tetrahedron 1995, 51, 145-154. (j)
Pohlmann, J .; Sabater, C.; Hoffmann, H. M. R. Angew. Chem., Int. Ed. Engl.
1998, 37, 633-635.
a
Reagents and conditions: (a) BuLi (1.1 equiv), THF, -78 °C, 50
min, then BF3‚OEt2 (1 equiv), 30 min, then 8 (0.5 equiv), -78 °C, 1 h,
20 °C, 1 h, 85% from 8; (b) H2, Lindlar’s cat., EtOH-quinoline (250:1),
20 °C, 2 h, 98%; (c) K2CO3 (1.3 equiv), MeOH, 30 °C, 10 h, 100%; (d)
TBAF (2 equiv), THF, 20 °C, 2 h, 99%; (e) Et2NH‚HCl (5 equiv), Ti(O-
i-Pr)4 (1.5 equiv), 20 °C, 48 h, 82% (14:11 ) 4:1); (f) TBSCl (1.1 equiv),
imidazole (2.1 equiv), DMF, 0 °C, 3 h, 91%; (g) 2 M HCl, THF, 20 °C,
13 h; (h) 1,3-propanedithiol (2 equiv), BF3‚OEt2 (1.5 equiv), 20 °C, 45
min, 99% from 14; (i) TBDPSCl (1.2 equiv), imidazole (2 equiv), CH2Cl2,
20 °C, 75 min, 99%; (j) (CF3CO2)2IPh (2 equiv), MeCN-H2O (9:1), 0
(4) (a) Fujiwara, K.; Tsunashima, M.; Awakura, D.; Murai, A. Tetrahe-
dron Lett. 1995, 36, 8263-8266. (b) Fujiwara, K.; Tsunashima, M.; Awakura,
D.; Murai, A. Chem. Lett. 1997, 665-666.
(5) For recent syntheses of nine-membered cyclic ethers, see: (a) Over-
man, L. E.; Blumenkopf, T. A.; Castan˜eda, A.; Thompson, A. S. J . Am. Chem.
Soc. 1986, 108, 3516-3517. (b) Nicolaou, K. C.; McGarry, D. G.; Somers, P.
K.; Veale, C. A.; Furst, G. T. J . Am. Chem. Soc. 1987, 109, 2504-2506. (c)
Nicolaou, K. C.; McGarry, D. G.; Somers, P. K.; Kim, B. H.; Ogilvie, W. W.;
Yiannikouros, G.; Prasad, C. V. C.; Veale, C. A.; Hark, R. R. J . Am. Chem.
Soc. 1990, 112, 6263-6276. (d) Nicolaou, K. C.; Prasad, C. V. C.; Ogilvie,
W. W. J . Am. Chem. Soc. 1990, 112, 4988-4989. (e) Nicolaou, K. C.; Shi,
G.-Q.; Gunzner, J . L.; Ga¨rtner, P.; Yang, Z. J . Am. Chem. Soc. 1997, 119,
5467-5468. (f) Nicolaou, K. C.; Yang, Z.; Ouellette, M.; Shi, G.-Q.; Ga¨rtner,
P.; Gunzner, J . L.; Agrios, K. A.; Huber, R.; Chadha, R.; Huang, D. H. J .
Am. Chem. Soc. 1997, 119, 8105-8106. (g) Nicolaou, K. C.; Yang, Z.; Shi,
G.-Q.; Gunzner, J . L.; Agrios, K. A.; Ga¨rtner, P. Nature 1998, 392, 264-
269. (h) Isobe, M.; Yenjai, C.; Tanaka, S. Synlett 1994, 916-918. (i) Isobe,
M.; Hosokawa, S.; Kira, K. Chem. Lett. 1996, 473-474. (j) Inoue, M.; Sasaki,
M.; Tachibana, K. Tetrahedron Lett. 1997, 38, 1611-1614. (k) Inoue, M.;
Sasaki, M.; Tachibana, K. Angew. Chem., Int. Ed. Engl. 1998, 37, 965-
969. (l) Palazo´n, J . M.; Mart´ın, V. S. Tetrahedron Lett. 1995, 36, 3549-
3552. (m) Alvarez, E.; Delgado, M.; Dı´az, M. T.; Hanxing, L.; Pe´rez, R.;
Mart´ın, J . D. Tetrahedron Lett. 1996, 37, 2865-2868. (n) Delgado, M.;
Mart´ın, J . D. Tetrahedron Lett. 1997, 38, 6299-6300. (o) Oishi, T.; Shoji,
M.; Maeda, K.; Kumahara, N.; Hirama, M. Synlett 1996, 1165-1167. (p)
Oishi, T.; Nagumo, Y.; Hirama, M. Chem. Commun. 1998, 1041-1042. (q)
Clark, J . S.; Kettle, J . G. Tetrahedron Lett. 1997, 38, 127-130. (r) Crimmins,
M. T.; Choy, A. L. J . Org. Chem. 1997, 62, 7548-7549. (s) Amann, C. M.;
Fisher, P. V.; Puhg, M. L.; West, F. G. J . Org. Chem. 1998, 63, 2806-2807.
(t) Stefinovic, M.; Snieckus, V. J . Org. Chem. 1998, 63, 2808-2809. (u)
Mujica, M. T.; Afonso, M. M.; Galindo, A.; Palenzuela, J . A. J . Org. Chem.
1998, 63, 9728-9738. See also refs 3 and 4.
°C, 2 min; (k) NaClO2 (2 equiv), 2-methyl-2-butene (15 equiv),
NaH2PO4‚2H2O (1.5 equiv), t-BuOH-H2O (3.5:1), 0 °C, 75 min, 82%
from 16; (l) EDCI (2 equiv), DMAP‚HCl (2 equiv), DMAP (4 equiv),
CHCl3, reflux, 61 h, 83%.
dimethyldioxirane followed by reduction of the resultant 4
with DIBALH was expected to produce 3 with the desired
regio- and stereoselectivities at C12 and C13. The enyne
terminus and Br at C12 of 1 were to be installed in 3 at the
final stage of the synthesis.
Preparation of lactone 6 from epoxide 86 is shown in
Scheme 2. Acetylene 77 was coupled with 8 by Yamaguchi’s
method,8 and the resulting 9 was partially hydrogenated to
give 10 in 83% yield. Treatment of 10 with K2CO3 followed
by deprotection of the TBS group afforded hydroxy epoxide
13 (99%). The titanium tetraisopropoxide-mediated epoxide
(6) Epoxide 8 (93%ee) was synthesized by the same method as in the
previous report for the antipode of 8. See: Gao, L.-x.; Saitoh, H.; Feng, F.;
Murai, A. Chem. Lett. 1991, 1787-1790.
(7) (a) Coates, R. M.; Hutchins, C. W. J . Org. Chem. 1979, 44, 4742-
4744. (b) Trost, B. M.; Runge, T. A. J . Am. Chem. Soc. 1981, 103, 7559-
7572. (c) Ma, D.; Lu, X. Tetrahedron 1990, 46, 6319-6330.
(8) Yamaguchi, M.; Hirao, I. Tetrahedron Lett. 1983, 24, 391-394.
10.1021/jo990212i CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/26/1999

Communications
J . Org. Chem., Vol. 64, No. 8, 1999 2617
Sch em e 3a
Sch em e 4a
a
a
Reagents and conditions: (a) KHMDS (2.5 equiv), HMPA (1.5
equiv), THF, -78 °C, 1 h, then Tf2NPh (1.5 equiv), -78 °C, 1 h, then
CuI (3 equiv), Me2S (7 equiv), EtMgBr (7 equiv), -78 f -40 °C, 14 h,
71%; (b) dimethyldioxirane (0.032 M in CH2Cl2, 1.2 equiv), CHCl3-
CH2Cl2 (1:3), -60 °C, 10 min, then -20 °C, 30 min, then 2-methyl-2-
butene (1 equiv), -20 f 0 °C, 5 min, then removal of solvent at 0 °C
in vacuo (20 Torr); (c) DIBALH (0.95 M in hexane, 5 equiv), CH2Cl2,
-78 °C, 10 min, then 0 °C, 12 h, 40% of 3 from 5 (based on the
recovered 5), 15% of 19 from 5 (based on the recovered 5), 23% recovery
of 5.
Reagents and conditions: (a) CBr4 (5 equiv), Oct3P (10 equiv),
toluene, 75 °C, 14 h, 82%; (b) 47% HF-MeCN (3:7), 0 °C, 1 h, then 20
°C, 2 h, 91%; (c) Dess-Martin periodinane (2 equiv), CH2Cl2, 20 °C, 2
h, 84%; (d) CBr4 (4 equiv), PPh3 (8 equiv), CH2Cl2, 0 °C, 30 min, then
21, 0 °C, 1 h, 95%; (e) Bu3SnH (2 equiv), Pd(PPh3)4 (10 mol %), 20 °C,
5 h, 100%; (f) ethynyltrimethylsilane (8 equiv), i-Pr2NH (4.5 equiv),
Pd(PPh3)4 (10 mol %), CuI (0.4 equiv), 20 °C, 18.5 h, 66%; (g) 47% HF-1
M TBAF in THF (1:13, pH 4.5), THF, 37 °C, 2.5 h, 100%.
between the substituents at C12 and C13 in both alcohols
3
was confirmed by the large J H12-H13 (9.5-7.7 Hz) and the
ring opening of 13 with Et2NH‚HCl9 produced the desired
14 and its regioisomer 11 (14:11 ) 4:1) in 82% combined
yield. The alcohol 11 could be recycled after conversion to
10. Hydrolytic deprotection of 14, which resulted only in the
production of an oligomeric mixture, followed by treatment
with 1,3-propanedithiol and BF3‚OEt2 yielded dithiane 15
quantitatively. After protection of the primary alcohol of 15
with TBDPS (99%), the dithiane moiety of 16 was removed
to give the respective aldehyde, which was transformed into
secoic acid 17 in 82% yield from 16 with NaClO2.10 Lacton-
ization of 17 by modified Keck’s method4b,11 afforded 6 (83%)
smoothly.
The lactone 6 was easily converted to dienyl ether 5 in
71% yield via a two-step sequence:4 (i) deprotonation of 6
with KHMDS and the subsequent reaction with PhNTf2, (ii)
immediate treatment of the resultant mixture with EtMgBr
in the presence of CuI and Me2S (Scheme 3). The resulting
5 was stable enough to be handled without special care. The
oxygen functionality at C12 and the hydrogen at C13 were
introduced in a similar way to our previous report4b with a
slight modification. Epoxidation of 5 with acetone-free
dimethyldioxirane12 in CHCl3-CH2Cl2 (1:3) occurred selec-
tively on the double bond of the enol ether at -20 °C.13
Excess dimethyldioxirane was quenched by the addition of
absence of NOE (H12/H13).4 The existence of NOE (H6/H13)
in 19 and its absence in 3 confirmed the cis and trans
relationships between the substituents at C6 and C13,
respectively.3c,e,4
Installation of Br at C12 of 3 was accomplished with CBr4
and trioctylphosphine with complete inversion to give 2
(82%)15 (Scheme 4). The cis relationship of the substituents
3
at C12 and C13 was confirmed by the small J H12-H13 (2.8
Hz) and the existence of NOE (H12/H13).4 The TBDPS group
of 2 was detached using HF-acetonitrile16 (91%), and the
resulting alcohol was oxidized with Dess-Martin periodi-
nane17 to afford aldehyde 21 (84%). The aldehyde was
converted to Z-bromoalkene 23 via dibromoalkene 22 by
Uenishi’s method18 in 95% yield from 21. Sonogashira
coupling19 of 23 with ethynyltrimethylsilane followed by
removal of the TMS group with TBAF under pH-controlled
conditions (pH 4.5)20 produced (+)-obtusenyne (1) {[R]19
D
+151 (c 0.13, CHCl3); lit.2a [R]D21.6 +155 (CHCl3); lit.2b [R]20
D
+111.4 (c 2.8, CHCl3)} in 66% yield from 23.
The synthetic sample, though isolated only as a colorless
oil, had characteristics in accordance with the data supplied
by Professor Imre for the natural product in all respects (1H
and 13C NMR,21 IR, [R]D, and MS).
3
2-methyl-2-butene after consumption of about /4 of 5. After
Ack n ow led gm en t. We are grateful to Prof. S. Imre,
University of Istanbul, for the kind gifts of 1H and 13C NMR
and IR spectra of natural obtusenyne. This work was
supported by Grant-in-Aid from the Ministry of Education,
Science, Sports and Culture, J apan (No. 08245103).
solvent removal, epoxides 4 and 18 and unreacted 5 were
obtained as an acid-sensitive mixture.14 The mixture was
reduced with DIBALH in CH2Cl2 to give alcohols 3 and 19
(40% and 19%, respectively, based on the consumption of 5)
and unreacted 5 (23% recovery). The trans relationship
Su p p or tin g In for m a tion Ava ila ble: Experimental proce-
dures and spectroscopic data for synthetic intermediates and the
1H NMR spectra of synthetic 1 measured at various temperatures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(9) (a) Gao, L.-x.; Murai, A. Chem. Lett. 1991, 1503-1504. For other C4
selective halogenations of 3,4-epoxy alcohols, see: (b) Alvarez, E.; Nun˜ez,
M. T.; Mart´ın, V. S. J . Org. Chem. 1990, 55, 3429-3431.
(10) Kraus. G. A.; Taschner, M. J . J . Org. Chem. 1980, 45, 1175-1176.
(11) Boden, E. P.; Keck, G. E. J . Org. Chem. 1985, 50, 2394-2395.
(12) Review: (a) Murray, R. W. Chem. Rev. 1989, 89, 1187-1201. (b)
Adam, W.; Curci, R.; Edwards, J . O. Acc. Chem. Res. 1989, 22, 205-211.
For preparation for dimethyldioxirane-acetone, see: (c) Adam, W.; Bialas,
J .; Hadjiarapoglou, L. Chem. Ber. 1991, 124, 2377. For preparation of
acetone-free dimethyldioxirane solution, see: (d) Gibert, M.; Ferrer, M.; S.-
Baeza, F.; Messeguer, A. Tetrahedron 1997, 53, 8643-8650.
J O990212I
(15) Tsushima, K.; Murai, A. Tetrahedron Lett. 1992, 33, 4345-4348.
(16) Ogawa, Y.; Nunomoto, M.; Shibasaki, M. J . Org. Chem. 1986, 51,
1625-1627.
(13) In the oxepane-fused nine-membered dienyl ethereal system, the
selective epoxidation on the normal olefinic double bond rather than the
double bond of the enol ether has been reported. See ref 5d.
(17) Dess, D. B.; Martin, J . C. J . Org. Chem. 1983, 48, 4155-4156.
(18) Uenishi, J .; Kawahama, R.; Yonemitsu, O. J . Org. Chem. 1996, 61,
5716-5717.
(14) It was possible to estimate the ratio of 4 to 18 by 1H NMR in C6D6
without decomposition of the epoxides. The stereochemistry of the epoxides
was deduced from the fact that the proportion of 3 to 19 was closely related
to that of 4 to 18 regardless of the reaction conditions. In addition, the
solvent effect in the face selectivity at the epoxidation stage was also
observed. The ratio of 4 to 18 was 1:1.6 (3:19 ) 1:1.5) in the pentane-
acetone system, 1:1 (1:1) in CH2Cl2-acetone or CH2Cl2, and 1.8:1 (2.1:1) in
CHCl3-CH2Cl2.
(19) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467-4470.
(20) Gao, L.-x.; Murai, A. Tetrahedron Lett. 1992, 33, 4349-4352.
(21) Although good agreement of synthetic 1 with natural obtusenyne
was shown under the same measurement conditions of 1H and 13C NMR,
broadening of the signals was observed in both samples. We found slow
exchange between the two conformers of 1 and estimated its activating ∆G
to be 12.9 kcal/mol from the coalescence temperature (-5 °C).