A simple convergent method for the construction of fused tri- and tetracyclic ethers

Article abstract of DOI:10.1055/s-1999-2784

Starting from monocyclic ethereal aldehyde and dithioacetal S-oxide segments, trans-fused tri- and tetracyclic ethers were synthesized in high yields in short steps involving acetal cyclization and reductive etherification reactions. This sequence could offer a simple convergent approach to the synthesis of natural trans-fused polycyclic ethers.

Full text of DOI:10.1055/s-1999-2784

LETTER
1037
A Simple Convergent Method for the Construction of Fused Tri- and
Tetracyclic Ethers
Kenshu Fujiwara,* Kimiko Saka, Daisuke Takaoka, Akio Murai*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Fax +81-11-706-2714; E-mail: amurai@sci.hokudai.ac.jp
Received 5 April 1999
thesized from aldehydes 7 and dithioacetal S-oxides 85
and 9.
Abstract: Starting from monocyclic ethereal aldehyde and dithio-
acetal S-oxide segments, trans-fused tri- and tetracyclic ethers were
synthesized in high yields in short steps involving acetal cyclization
and reductive etherification reactions. This sequence could offer a
simple convergent approach to the synthesis of natural trans-fused
polycyclic ethers.
OH
OH
ref. 6
a, b
90% 5 : 1
+
10 + 11
O
O
O
Key words: polycyclic ethers, acyl anion equivalent, dithioacetal S-
10
3 : 4 11
oxide, acetal cyclization, reductive etherification
OH
OAc
OR
d, e, d
c
+
10
49%
(98%ee)
O
O
O
X
(+)-10 34% (93%ee)
12 50% (58%ee)
c
(-)-10: R=H, X=CH2
13: R=Bn, X=O
Polycyclic ethers originated from marine bioorganisms,
represented by brevetoxins1 and ciguatoxin,2 are of signif-
icant interest to synthetic chemists due to their novel
structures and strong biological activities. For the synthe-
sis of these large polycycles, efficient convergent strate-
gies have been desired and explored actively.3 In this
paper, a simple convergent method for the synthesis of
trans-fused tri- and tetracyclic ethers having a perhydro-
pyranopyran structure is described.
h, g
85%
(96%ee)
93%
f, g
i, j
78%
81%
7a
9a
Scheme 1 Reagents and conditions: (a) (COCl)2, DMSO, CH2Cl2,
-78 °C, then Et3N, -78 Æ 0 °C; (b) LiAlH4, Et2O-THF (1:1), -78 °C;
(c) lipase Amano P, vinyl acetate, 21-24 °C, 20-58 h; (d) K2CO3, Me-
OH, 21-22 °C, 3 h; (e) lipase Amano P, vinyl acetate, 21 °C, 5.5 h; (f)
TBSCl, imidazole, DMF, 22 °C, 80 min; (g) OsO4 (cat.), NMO, 1,4-
dioxane-H2O (3:1), 19-22 °C, 3-4 h, then NaIO4, 19-22 °C, 1.5 h; (h)
NaH, t-BuOK, BnBr, TBAI, THF, 0 Æ 22 °C, 5 h; (i) TMSSEt, ZnI2,
CH2Cl2, 20 °C, 2 h; (j) m-CPBA, CH2Cl2, -78 °C, 1 h.
H
H
H
H
O
O
O
O
OR
O
O
H
O
O
2
RO
H
H
O
H
1
3
Tetrahydropyran segments 7a and 9a were prepared as
shown in Scheme 1. Dihydropyran was converted to a 3:4
mixture of 10 and 11 according to Rousseau’s method.6
To attain increased yield of the desired 10, the mixture
was subjected to Swern oxidation7 followed by reduction
with LiAlH4 to give 10 in 75% yield. Optical resolution of
10 was achieved by repeated treatment with lipase Amano
P in vinyl acetate {(+)-10: 96%ee,8 [a]D23+22.6 (c 1.14,
H
H
H
H
H
H
H
H
H
O
O
O
O
5
6
8
1
O
O
O
O
O
O
H
H
5
H
H
6
H
4
O
O
OTBS
MeS
EtS
O
n
MeS
EtS
BnO
CHO
7a: n=1
n
O
OBn
8
9a: n=1
9b: n=2
CHCl3); (-)-10: 98%ee,8 [a]D -25.7 (c 1.04, CHCl3)}.9
22
7b: n=2
Protection of (+)-10 with TBSCl followed by one-pot di-
hydroxylation-oxidative cleavage led to 7a (78%). Anoth-
er alcohol (-)-10 was protected and the resultant benzyl
ether was converted to aldehyde 13 (85%), by the same
one-pot process as the above, which was transformed to
9a (81%) by Evans’ method10 followed by oxidation with
m-CPBA. On the other hand, oxepane segments 7b and 9b
were provided from the respective hydroxy epoxides 14
Figure
We designed the synthesis of polycyclic ether 1 starting
from aldehyde segment 2 and acyl anion segment 3, be-
cause the addition of 3 to 2 could execute the requisite ox-
ygen functions for the subsequent construction of the
perhydropyranopyran structure. Here, the dithioacetal S-
oxide group was adopted as an acyl anion equivalent due
to the low basicity and high nucleophilicity of its anion as
well as its easy transformation to a carbonyl group on
acidic treatment.4 On the basis of the above strategy, tri-
cyclic 43i and tetracyclic 53c and 6 were planned to be syn-
25
{88%ee,11 [a]D -29.4 (c 0.999, CHCl3)} and 19
{93%ee,11 [a]D25+25.8 (c 1.01, CHCl3)}, easily derived
from the corresponding allyl alcohols by Sharpless asym-
metric epoxidation,12 via a ring-closing metathesis
reaction13 (Scheme 2). An allyloxy group was introduced
to 14 according to Sharpless’ method14 affording 15
Synlett 1999, No. 07, 1037–1040 ISSN 0936-5214 © Thieme Stuttgart · New York

1038
K. Fujiwara et al.
LETTER
hyde 7b (91%). Another oxepane 20 {[a]D14 -3.0 (c 0.625,
CHCl3)} antipodal to 18 was prepared from 19 through a
process similar to 18. Benzylation of 20 followed by re-
moval of the p-methoxybenzyl group afforded 21 (80%),
which was transformed to 9b (82%) by successive reac-
tions (oxidation of alcohol, dithioacetal formation, and
mono-oxidation of the dithioacetal).
O
OH
OH
O
a
b, c
OBn
OBn
O
OBn
O
16
85%
61%
OH
14 (88%ee)
15
e, f
90%
OH
OH
d
g-i
7b
9b
97%
91%
OBn
OBn
O
17
O
18
OR1
O
OH
H
OMe
TBSO
SMe
a-f
i, l, m
82%
MeS
O
a
b, c
OPMB
36%
OR2
4
8
7
8
7
O
8
O
5
1
7a
(racemic)
19 (93%ee)
20: R1=H, R2=PMB
21: R1=Bn, R2=H
j, k
80%
OBn
O
OH
O
OH OBn
H
H
22 (78%)
23
(94%, α-OH : β-OH = 1 : 3)
H
H
H
H
Scheme 2 Reagents and conditions: (a) allyl alcohol, Ti(i-PrO)4, to-
luene, reflux, 5 h; (b) 2,4,6-triisopropylbenzenesulfonyl chloride,
DMAP, pyridine, CH2Cl2, 22-25 °C, 11-14 h; (c) K2CO3, MeOH, 20-
23 °C, 4-6 h; (d) CuCN, CH2=CHMgBr, THF, -78 Æ -40 °C, 1 h, then
16, -40 °C, 1.3 h; (e) (PCy3)2Cl2Ru=CHPh (cat.), CH2Cl2, reflux, 2 h;
(f) H2, RhCl(PPh3)3, benzene, 21 °C, 7-9 h; (g) TBSCl, imidazole,
DMF, 24 °C, 3 h; (h) H2, 10% Pd/C, MeOH, 21 °C, 2 h; (i) (COCl)2,
DMSO, CH2Cl2, -78 °C, then Et3N, -78 Æ 0 °C; (j) t-BuOK, BnBr,
TBAI, THF, 0 Æ 24 °C, 25 min; (k) DDQ, CH2Cl2-H2O (10:1), 23 °C,
45 min; (l) TMSSEt, ZnI2, CH2Cl2, 23 °C, 17 h; (m) m-CPBA,
CH2Cl2, -78 °C, 1.5 h.
O
O
g
O
O
OBn
O
O
H
H
H
OH
24 (65%)
26
d, e
f
(~100%, α-OH : β-OH = 3 : 1)
+
24 (72%)
+ 25 (10%)
h
H
H
H
O
O
+
4
O
O
OBn
O
O
H
H
H
25 (24%)
27
0 °C: 92% (4 : 27 = 1 : 1)
-78 → -15 °C: 91% (4 : 27 = 4 : 1)
Scheme 3 Reagents and conditions: (a) LDA (1 eq), THF, -78 °C, 5
min, then racemic 7a (0.67 eq), -78 °C, 20 min; (b) TBAF, THF, 26
°C, 40 min; (c) TsOH◊H2O (cat.), MeOH-(MeO)3CH (10:1), 25 °C, 40
min; (d) Et3SiH (6 eq), SnCl4 (2 eq), -78 Æ 0 °C, 20 min; (e) (COCl)2,
DMSO, CH2Cl2, -78 °C, then Et3N, -78 Æ 0 °C; (f) DBU (5 eq), ben-
zene-d6, 20 °C, 35 h; (g) H2, 10% Pd/C, MeOH, 25 °C, 12.5 h; (h)
Et3SiH (10 eq), TMSOTf (2 eq), CH2Cl2, 0 °C, 10 min, or -78 Æ -15
°C, 1 h.
(85%). Selective sulfonylation of the primary hydroxyl
group of 15 followed by basic treatment gave epoxide 16
(61%), which was converted to 17 (97%) on treatment
with a vinyl copper reagent. The diene 17 was cleanly cy-
clized by Grubbs’ catalyst,13 and the resultant oxepene
produced oxepane 18 {90% yield, [a]D23+4.7 (c 1.00,
CHCl3)} on hydrogenation. After a three-step sequence
(protection of the hydroxy group with TBSCl, removal of
the benzyl group, and Swern oxidation7), 18 gave alde-
H
H
H
H
H
H
H
OMe
H
H
H
H
H
H
H
O
O
O
O
O
O
O
d
n
n
n
+
n
n
n
O
BnO
O
BnO
O
BnO
HO
HO
HO
O
33a (21%)
33b (9%)
30a (40%)
30b (44%)
32a (66%)
32b (70%)
e
e, f
H
H
RO
SEt
EtS
O
35a (87%)
35b (96%)
35a (77%)
+ epimer (15%)
H
O
c
a
n
+
OMe
9
n
7
H
H
H
O
BnO
O
H
H
H
H
H
H
H
H
O
H
HO
28a: R=TBS
29a: R=H (63% from 9a)
28b: R=TBS (71% from 9b)
29b: R=H (~100%)
O
O
O
d
e
n
a series: n=1
b series: n=2
n
n
b
n
n
n
O
BnO
O
BnO
O
BnO
H
H
HO
HO
b
35a (90%)
35b (93%)
31a (49%)
31b (22%)
34a (94%)
34b (91%)
H
H
H
H
H
H
H
H
H
H
O
O
O
O
g
h
n
n
35
n
n
O
O
O
O
OH
H
36a (81%)
5: n=1 (93%)
36b (94%)
6: n=2 (89%)
Scheme 4 Reagents and conditions: (a) LDA (1 eq), THF, -78 °C, 20 min, then 7 (0.67 eq), -78 °C, 15-20 min; (b) TBAF, THF, 25 °C, 2-3
h; (c) TsOH◊H2O (cat.), MeOH-(MeO)3CH (a series, 10:1; b series, 1:1), 20-25 °C, 2-3 h; (d) Et3SiH (6 eq), SnCl4 (2 eq), -78 Æ 0 °C; (e)
(COCl)2, DMSO, CH2Cl2, -78 °C, then Et3N, -78 Æ 0 °C; (f) DBU (5 eq), benzene-d6, 20 °C, 5 days; (g) H2, 10% Pd/C, MeOH, 20-25 °C, 3-
22 h; (h) Et3SiH (10 eq), TMSOTf (2 eq), CH2Cl2, 0 °C, 40-150 min.
Synlett 1999, No. 07, 1037–1040 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
A Simple Method for the Construction of Fused Tri- and Tetracyclic Ethers
1039
Firstly, construction of tricyclic 4 from aldehyde 7a (race-
mic) and acyclic 8 was examined as the simplest model
system for convergent synthesis of polycyclic ethers
(Scheme 3). Deprotonation of 8 with LDA (1 equiv) in
THF at -78 °C followed by addition of aldehyde 7a (race-
mic, 0.67 equiv) provided 22 in good yield (78% based on
7a) as a mixture of diastereomers together with the recov-
ered 7a (22%) and 8 (45%). After desilylation and treat-
ment with p-toluenesulfonic acid in MeOH-(MeO)3CH
(9:1) system,15 22 produced the desired bicyclic com-
pounds 23 (94%) as an inseparable mixture of epimers at
C7 (a-OH:b-OH = 1:3). The acetal 23 was reduced with
Et3SiH in the presence of SnCl416 to give an isomeric mix-
ture of bicyclic ethers which afforded the desired ketone
24 (65%) and its C8-epimer 25 (24%) after oxidation.
When 25 was treated with DBU in benzene-d6 at ambient
temperature, the reaction attained equilibrium
(24:25 = 7:1) to give 24 (72%) mainly. The benzyl group
of 24 was removed by hydrogenolysis to provide cyclic
hemiacetal 26 (~100%) as an anomeric mixture. While
TMSOTf-catalyzed reduction of 26 with Et3SiH16 at 0 °C
gave a 1:1 mixture of the desired 43i,17 and its epimer
27,3i,17 the ratio of 4 to 27 was improved by lowering the
reaction temperature (-78 Æ -15 °C, 4:27 = 4:1). Thus, 4
was synthesized from 7a in 8 steps, including an isomer-
ization step. The overall yield of 4 amounted to 44% from
7a.
Acknowledgment
This work was supported by Grant-in-Aid from the Ministry of
Education, Science, Sports and Culture, Japan (No. 10308027,
A.M.). We thank Amano Pharmaceutical Co., Ltd., for a generous
gift of lipase.
References and Notes
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(2) (a) Murata, M.; Legrand, A. M.; Ishibashi, Y.; Yasumoto, T.
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A. M.; Ishibashi, Y.; Fukui, M.; Yasumoto, T. J. Am. Chem.
Soc. 1990, 112, 4380. (c) Suzuki, T.; Sato, O.; Hirama, M.;
Yamamoto, Y.; Murata, M.; Yasumoto, T.; Harada, N.
Tetrahedron Lett. 1991, 32, 4505. (d) Satake, M.; Morohashi,
A.; Oguri, H.; Oishi, T.; Hirama, M.; Harada, N.; Yasumoto,
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Chem. Soc. 1989, 111, 4136. (b) Nicolaou, K. C.; Prasad, C.
V. C.; Hwang, C.-K.; Duggan, M. E.; Veale, C. A. J. Am.
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K.; Marron, B. E.; DeFrees, S. A.; Couladouros, E. A.; Abe,
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Angew. Chem. Int. Ed. Engl. 1996, 35, 589. (f) Nicolaou, K.
C.; Sorensen, E. J. Classics in Total Synthesis; VCH,
Weinheim, 1996, Chap. 37, pp. 731-786. (g) Nicolaou, K. C.;
Yang, Z.; Shi, G.-q.; Gunzner, J. L.; Agrios, K. A.; Gärtner, P.
Nature 1998, 392, 264. (h) Alvarez. E.; Díaz, M. T.; Hanxing,
L.; Martín, J. D. J. Am. Chem. Soc. 1995, 117, 1437. (i)
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D. J. Org. Chem. 1996, 61, 3003. (j) Inoue, M.; Sasaki, M.;
Tachibana, K. Tetrahedron Lett. 1997, 38, 1611. (k) Inoue,
M.; Sasaki, M.; Tachibana, K. Angew. Chem. Int. Ed. Engl.
1998, 37, 965. (l) Sasaki, M.; Inoue, M.; Noguchi, T.;
Takeichi, A.; Tachibana, K. Tetrahedron Lett. 1998, 39, 2783.
(m) Sasaki, M.; Fuwa, H.; Inoue, M.; Tachibana, K.
Tetrahedron Lett. 1998, 39, 9027. (n) Sasaki, M.; Noguchi, T.;
Tachibana, K. Tetrahedron Lett. 1999, 40, 1337. (o) Oishi, T.;
Nagumo, Y.; Hirama M. Synlett 1997, 980. (p) Oishi, T.;
Nagumo, Y.; Hirama M. J. Chem. Soc., Chem. Commun.
1998, 1041.
Next, tetracyclic 5 and 6, as the advanced model systems,
were synthesized (Scheme 4). Both 6-membered (7a+9a)
and 7-membered cyclic series (7b+9b) gave the respec-
tive diols 29a and 29b in good yields through the segment
connection and the desilylating steps. Acidic treatment of
each diol in MeOH-(MeO)3CH15 produced separable a-
hydroxy acetals 30 and its b-isomers 31. Under reductive
etherification conditions, both 31a and 31b showed exclu-
sive production of 34a and 34b, respectively. On the other
hand, both 30 afforded mixtures of 32 and 33, and the dis-
tribution of the isomers altered with the size of the respec-
tive outside rings (32a:33a = 3:1, 32b:33b = 8:1). Swern
oxidation7 of both 32 and 34 led to the common ketones
35, respectively. The alcohol 33a was also converted to
35a in good yield via an oxidation-isomerization se-
quence. After debenzylation followed by reductive ether-
ification, ketones 35a and 35b produced tetracyclic ether
53c, 17 and 617 in good yields (overall yields, 5:34% from
7a in 8 steps, 6:29% from 7b in 7 steps), respectively, via
cyclic hemiacetals 36a and 36b.
(4) (a) Ogura, K.; Tsuchihashi, G. Tetrahedron Lett. 1972, 2681.
(b) Herrmann, J. L.; Richman, J. E.; Wepplo, P. J.;
Schlessinger, R. H. Tetrahedron Lett. 1973, 4707.
trans-Fused tri- and tetracyclic ethers 4, 5, and 6 were thus
synthesized in high yields in short steps involving acetal
cyclization and reductive etherification reactions starting
from monocyclic ethereal aldehyde 7 and dithioacetal S-
oxide segments 8 and 9. This sequence could provide a
simple convergent approach to the synthesis of various
condensed polycyclic ethers. Application of the present
work to the synthesis of natural polycyclic ethers is cur-
rently underway in our laboratory.
(5) Sulfoxide 8 was prepared from 2-phenyl-1,3-dioxane in 3
steps [(i) LiAlH4, AlCl3; (ii) PPh3, I2, imidazole; (iii) methyl
methylthiomethyl sulfoxide, BuLi].
(6) Simart, F.; Brunel, Y.; Robin, S.; Rousseau, G. Tetrahedron
1998, 54, 13557.
(7) (a) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651. (b)
Mancuso, A. J.; Huang, S.-L.; Swern, D. J. Org. Chem. 1978,
43, 2480. (c) Mancuso, A. J.; Swern, D. Synthesis 1981, 165.
(d) Tidwell; T. T. Org. React. 1990, 39, 297.
(8) The optical purity of (+)- and (-)-10 was determined by HPLC
analysis of the corresponding 3,5-dinitrobenzoates using
chiral column (CHRALCEL OD, Daicel Chemical Ind., Ltd.).
Synlett 1999, No. 07, 1037–1040 ISSN 0936-5214 © Thieme Stuttgart · New York

1040
K. Fujiwara et al.
LETTER
Their absolute configurations were determined by modified
Mosher’s method: Ohtani, I.; Kusumi, T.; Kashman, Y.;
Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092.
(14) Chong, J. M.; Sharpless, K. B. J. Org. Chem. 1985, 50, 1560.
(15) Fujiwara, K.; Murai, A; Yotsu-Yamashita, M; Yasumoto, T. J.
Am. Chem. Soc. 1998, 120, 10770.
(9) Lipase-catalyzed optical resolution of cyclic ethers, (a)
Alvarez. E.; Díaz, M. T.; Pérez, R.; Ravelo, J. L.; Regueiro,
A.; Vera, J. A.; Zurita, D.; Martín, J. D. J. Org. Chem. 1994,
59, 2848. (b) Oishi, T.; Shoji, M.; Maeda, K.; Kumahara, N.;
Hirama, M. Synlett 1996, 1165. See also ref 3o.
(16) TMSOTf-Et3SiH system, (a) Tsunoda, T.; Suzuki, M.;
Noyori, R. Tetrahedron, Lett. 1979, 4679. (b) Sassaman, M.
B.; Kotian, K. D.; Prakash, G. K. S.; Olah, G. A. J. Org. Chem.
1987, 52, 4314. (c) Sassaman, M. B.; Prakash, G. K. S.; Olah,
G. A. Tetrahedron 1988, 44, 3771. SnCl4-Et3SiH system,
(d) Mori, A.; Ishihara, K.; Yamamoto, H. Tetrahedron Lett.
1986, 27, 987. Reduction of 6-membered cyclic acetals and
hemiacetals with BF3◊OEt2-Et3SiH system, (e) Rolf, D.; Gray,
G. R. J. Am. Chem. Soc. 1982, 104, 3539. (f) Lewis, M. D.;
Cha, J. K.; Kishi, Y. J. Am. Chem. Soc. 1982, 104, 4976. See
also ref. 3d and 3m.
(17) Spectral data of the cyclic ethers 4, 27, and 5 agreed well with
those of the literatures. Representative data for 6: colorless
needles, mp. 203-206 °C (ethyl acetate); 1H NMR (400 MHz,
CDCl3) d 1.38-1.48 (4H, m, H4a and H7ax), 1.50-1.78 (6H, m,
H2a, H3a, and H3b) 1.82-1.90 (2H, H2b), 2.07-2.13 (2H,
H4b), 2.29 (2H, brtd, J = 4.4, 11.7 Hz, H7eq), 2.93-3.00 (2H,
m, H8) 3.12 (2H, dt, J = 4.4, 9.2 Hz, H5), 3.20 (2H, ddd,
J = 4.4, 9.2, 10.8 Hz, H6) 3.71 (2H, td, J = 6.0, 11.9 Hz, H1a),
3.83 (2H, ddd, J = 6.7, 7.7, 11.9 Hz, H1b); 13C NMR (100
MHz, CDCl3) d 19.9 (C3), 28.9 (C2), 34.3 (C4), 37.2 (C7),
69.0 (C1), 76.0 (C8), 77.1 (C6), 82.1 (C5); IR (KBr) n 2924,
2889, 2856, 1445, 1307, 1349, 1298, 1285, 1253, 1156, 1146,
1129, 1102, 1073, 1038, 988, 939, 896, 655, 552, 506, 478,
430 cm-1; HR-EIMS calcd. for C16H26O4 [M]:282.1831,
found:282.1833.
(10) Evans, D. A.; Truesdale, L. K.; Grimm, K. G.; Nesbitt, S. L.
J. Am. Chem. Soc. 1977, 99, 5009.
(11) The corresponding antipodes of 14 and 19 have been reported:
antipode of 14, Kozikowski, A. P.; Stein, P. D. J. Org. Chem.
1984, 49, 2301; antipode of 19, Oka, T.; Murai, A. Chem. Lett.
1994, 1611. The optical purity of 14 and 19 was determined
by HPLC analysis using chiral column (CHRALCEL OD).
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Article Identifier:
1437-2096,E;1999,0,07,1037,1040,ftx,en;Y07899ST.pdf
Synlett 1999, No. 07, 1037–1040 ISSN 0936-5214 © Thieme Stuttgart · New York