Studies toward the synthesis of furanocembrane bipinnatin J: Synthesis of a 2,3,5-trisubstituted furfuryl ether intermediate

Article abstract of DOI:10.3987/COM-04-10317

Synthesis of 2,3,5-trisubstituted furfuryl ether (29), a potentialintermediate in a synthetic approach to bipinnatin J, was achieved by a combination of etherification and Stille cross-coupling reaction. Unexpectedly, an intramolecular coupling reaction o

Full text of DOI:10.3987/COM-04-10317

HETEROCYCLES, Vol. 65, No. 3, 2005
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HETEROCYCLES, Vol. 65, No. 3, 2005, pp. 531 - 540
Received, 22nd December, 2004, Accepted, 21st January, 2005, Published online, 21st January, 2005
STUDIES TOWARD THE SYNTHESIS OF FURANOCEM-
BRANE BIPINNATIN J: SYNTHESIS OF A 2,3,5-TRI-
SUBSTITUTED FURFURYL ETHER INTERMEDIATE
Masayoshi Tsubuki,* Kazunori Takahashi, Ken Sakata, and Toshio Honda*
Faculty of Pharmaceutical Sciences, Hoshi University, 2-4-41 Ebara, Shinagawa,
Tokyo 142-8501, Japan
e-mail:tsubuki@hoshi.ac.jp
Abstract – Synthesis of 2,3,5-trisubstituted furfuryl ether (29), a potential
intermediate in a synthetic approach to bipinnatin J, was achieved by a
combination of etherification and Stille cross-coupling reaction. Unexpectedly, an
intramolecular coupling reaction of 29 proceeded through SN2’ substitution to
give 15-membered furfuryl ether (30). Formation of γ-butenolide (34) was also
accomplished by a ruthenium-catalyzed carbonylation of allenic alcohol (33).
INTRODUCTION
Marine furanocyclic natural products, such as lophotoxin,1 bipinnatins,2 kallolides,3 and pinnatins,4 were
isolated from gorgonians and soft corals in tropical and temperate seas (Figure 1). Lophotoxin and
bipinnatins are members of the lophotoxin family of marine neurotoxins, which block nicotinic
acetylcholinergic neurotransmission in autonomic ganglia causing paralysis and asphyxiation.5 Kallolide
A is an antiinflammatory agent with activity comparable to that of indomethacin.3a Pinnatins are potent
antitumor agents.4a Although several total syntheses of furanocembranes (acerosolide and rubifolide) and
pseudopteranes (gorgiacerone, kallolides A and B) have been achieved by Paquette et al.6 and Marshall et
al.,7 no total synthesis of lophotoxin, bipinnatins, and pinnatins has been achieved yet owing to their
structural complexity and lability of furan derivatives.8
We have recently reported the stereospecific construction of anti- and syn-isopropenyl alcohol moieties at
the C(2) and C(3) positions of 2,5-bridged furanocycles, such as kalollide A and pinnatin A, employing
the [2,3] Wittig rearrangement of cyclic furfuryl ethers.9 In this regard, we have intended to synthesize
furanocembrane bipinnatin J (1)2b because of its potent biological activity and also structural similarity to

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HETEROCYCLES, Vol. 65, No. 3, 2005
CHO
2
3
H
OH
OH
O
O
OH
O
O
O
O
O
O
O
OAc
O
O
O
O
O
lophotoxin
bipinnatin J 1
pinnatin A
kallolide A
Figure 1.
lophotoxin. Bipinnatin J (1) bears three chiral centers with anti-homoallylic hydroxy and γ-butenolide
moieties, whose stereoselective introduction seems to be a central issue of the synthesis. The
retrosynthetic plan is illustrated in Scheme 1. A fundamental premise from the outset was that the
γ–butenolide moiety could be introduced in a stereoselective manner late in the synthesis.7 Thus,
14-membered furanocycle (2) would be postulated as an intermediate. Introduction of the γ-butenolide
into furanocycle (2) was envisioned to be realized by conversion of propargylic alcohol into allene10
followed by ruthenium-catalyzed cyclic carbonylation11 of the corresponding allenic alcohol.
anti-Homoallylic alcohol (2) could be synthesized by Wittig rearrangement of cyclic furfuryl ether (3)
based on our previous result.9 Key 17-membered cyclic ether (3) could be synthesized by either an
intramolecular Nozaki-Hiyama-Kishi reaction12 of ω–iodoalkynyl aldehyde (4) or an intramolecular
coupling reaction13 of ω-alkynylated allylic halide (5). We describe here a synthesis of 2,3,5-trisubstituted
O
O
a
OH
O
CHO
I
R1O
O
O
4
1
b. coupling
R1O
R1O
O
b
R2O
R2O
O
a. Nozaki-Hiyama-
Kishi reaction
3
2
X
R1O
R2O
5
Scheme 1.

HETEROCYCLES, Vol. 65, No. 3, 2005
533
furfuryl ether (29), a potent intermediate in a synthetic approach to bipinnatin J (1), by a Stille coupling
between bromofurfuryl ether (25) and vinylstannane (10) followed by introduction of acetylene moiety.
We also demonstrated the stereoselective construction of γ–butenolide moiety using a model compound.
RESULTS AND DISCUSSION
We first prepared chiral segments forthe left parts of intermediates (4) and (5) from a known chiral ketone
(6)14 as shown in Scheme 2. Highly Z-selective Wittig iodomethylenation of aldehyde was described by
Stork et al. and Bestmann et al.15 but there has been no report on that of unsymmetrical ketone to date.
Although reaction of 6 with iodomethyltriphenylphosphorane underthe reported condition15a gave only a
trace of vinyl iodide as a E/Z mixture probably due to elimination of β–alkoxy moiety in 6, the addition of
LiBr suppressed side reaction to afford vinyl iodide in 94% yield but in low selectivity (E/Z=48/52).16
The observed LiBr effect might be explained by increasing the reactivity of ketone carbonyl group with
coordination of lithium cation.17 Acid treatment of 7 gave diol (8), whose selective protection of primary
hydroxy group with TBSCl followed by reaction of secondary hydroxy group with MOMCl furnished 9.
Vinyl iodide (9) was further converted into vinylstannane (10) by trapping of the lithiated intermediate
with tributyltin chloride. Transformation of 9 to α–alkoxy aldehyde (11) was carried out by deprotection
of silyl moiety and oxidation of the resulting alcohol with sulfurtrioxide-pyridine complex in DMSO
(Parikh-Doering oxidation).
R2O
MOMO
TBSO
O
O
O
R1O
i
ii
O
O
v
10
6
7
I
I
SnBu3
8 R1=R2=H
vi, vii
iii, iv
9 R1=TBS, R2=MOM
MOMO
OHC
11
I
Scheme 2. Reagents and conditions: (i) ICH2PPh3I (3 equiv.), LiHMDS (3 equiv.), LiBr (6 equiv.), THF-HMPA (10 : 1),
-30°C, 1.5 h, 94%, E/Z=48/52; (ii) 0.5 M HCl (2.7 equiv.), THF, reflux, 12 h, 97%; (iii) TBSCl (1.6 equiv.), Et3N (2.3 equiv.),
DMAP (0.1 equiv.), CH2Cl2, rt, 12 h, 92%; (iv) MOMCl (10 equiv.), i-Pr2NEt (28 equiv.), Bu4NI (0.4 equiv.), rt, 21 h, 95%; (v)
t-BuLi (2.1 equiv.), THF, -78°C, then Bu3SnCl (1.1 equiv.), from –30°C to rt, 1.25 h, 75%; (vi) TBAF (1.2 equiv.), THF, rt, 36
h, 98%; (vii) SO3-pyridine complex (4 equiv.), Et3N (10 equiv.), DMSO (20 equiv.), CH2Cl2, 0°C, 4 h, 76%.
Enyne (15), the right part for intermediate (4), was prepared as shown in Scheme 3. Stille cross-coupling
reaction of vinylstannane (12), the silyl ether of a known alcohol,18 with 3-trimethylsilylpropargyl

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HETEROCYCLES, Vol. 65, No. 3, 2005
bromide gave skipped enyne (13) in 67% yield. The use of Ph3As in place of Ph3P and DMA ratherthan
THF provided improvement in chemical yield.19 Treatment of 13 with HF in MeCN resulted in selective
deprotection to afford allylic alcohol (14). Exchanging terminal trimethylsilyl moiety in 13 into bromide
was carried out by reaction of NBS and AgNO3 in acetone to furnish bromoalkyne (15).20
TMS
CH2Br
i
iii
OTBDPS
OR
+
Br
TMS
OTBDPS
Bu3Sn
15
13 R=TBDPS
14 R=H
ii
12
Scheme 3. Reagents and conditions: (i) 12 (1 equiv.), 3-trimethylsilylpropargyl bromide (1 equiv.), Pd2(dba)3 (5 mol%),
Ph3As (10 mol%), DMA, 110°C, 0.5 h, 67%; (ii) HF (2 equiv.), MeCN, rt, 4 h, 83%; (iii) NBS (1.4 equiv.), AgNO3 (0.5 equiv.),
acetone, rt, 3 h, 89%.
With left and right parts of the intermediates in hand, we examined the synthesis of ω–iodoalkynyl
aldehyde (4) as follows. (Schemes 4 and 5) Reduction of 5-bromo-3-methylfuroate (16)21 with DIBAL
gave relatively unstable furfuryl alcohol (17), which was treated with Cl3CCN and DBU22 to afford
imidate (18) in 82% yield (2 steps). Etherification of allylic alcohol (14) with 18 was carried out in the
presence of PPTS to provide furfuryl ether (19) in 76% yield.23 Unfortunately, attempt to prepare
vinylfuran (20) using Stille reaction of vinylstannane (10) with 19 failed under various conditions.
10
O
O
Pd(0)
ii
iii
O
Br
O
CCl3
O
R
Br
Br
O
O
NH
16 R=CO2Me
17 R=CH2OH
18
i
MOMO
TBSO
TMS
TMS
19
20
Scheme 4. Reagents and conditions: (i) DIBAL (2.2 equiv.), CH2Cl2, -78°C, 1 h; (ii) Cl3CCN (1.3 equiv.), DBU (0.2 equiv.),
CH2Cl2, 0°C, 20 min, 82% (2 steps); (iii) 14 (1 equiv.), 18 (2 equiv.), PPTS (1 equiv.), CH2Cl2, 0°C, 1 h, 76% from 14
We also tried changing the order of these reactions to obtain 20. Stille reaction of 10 with bromofuroate
(16) underwent smoothly to produce vinylfuroate (21), whose reduction with DIBAL gave furfuryl

HETEROCYCLES, Vol. 65, No. 3, 2005
535
14
R
O
O
i
20
16
O
NH
CCl3
PPTS
MOMO
MOMO
TBSO
TBSO
21 R=CO2Me
22 R=CH2OH
23
ii
Scheme 5. Reagents and conditions: (i) 10 (1 equiv.), 16 (1.1 equiv.), Pd(Ph3P)4 (5 mol%), i-Pr2NEt (1.5 equiv.), DMF,
110°C, 3.25 h, 72% from 10; (ii) DIBAL (2.2 equiv.), CH2Cl2, -78°C, 40 min, 91%.
alcohol (22). Trichloroacetimidation of 22 did not give 23 owing to its instability. Although this
discrepancy in the reactivity toward Stille reaction would not be rationalized, the 1,4-enyne part might
disturb the oxidative addition of palladium species by its palladium coordination.
We next examined the synthesis of 17-membered cyclic furfuryl ether (3) through ω-alkynylated allylic
halide (5) as shown in Scheme 6. Etherification of 17 with allylic chloride (24), the ethoxyethyl ether of a
known alcohol,24 under usual Williamson reaction condition gave furfuryl ether (25) in moderate yield.
Stille reaction of 10 with 25 furnished vinylfurfuryl ether (26), whose treatment with TBAF followed by
Parikh-Doering oxidation of the resulting alcohol provided α–alkoxy aldehyde (27). Addition of
ethynylmagnesium bromide to 27 gave an inseparable diastereomeric mixture of propargylic alcohol,
which was protected as silyl etherto afford 28. Compound (28) was converted to 29, an important
precursor for 3, by removal of ethoxyethyl moiety followed by chlorination of the resulting allylic
alcohol.25 The stage was now set forthe examination of the ring closure by coupling reaction of
ω-alkynylated allylic chloride.13d Treatment of 29 with CuI (10 equiv.), NaI (15 equiv.), and Cs2CO3 (10
equiv.) in DMF (2.5 mM concentration) at 80°C brought about SN2’ substitution to provide 15-membered
cyclic furfuryl ether (30) in 45% yield. Since the coupling reaction of copper(I) alkynides with allylic
halides has produced preferentially 1,4-enynes, SN2 products,13e, 26 we expected SN2 product (31) as a
major product ratherthan SN2’ product (30) in this reaction. Moreover, semi-empirical MO calculations
using the MNDO Hamiltonian have shown 16.4 kcal/mol as the energy difference between the two
optimized conformations of 15- and 17-membered model compounds, thus indicating that 17-membered
furfuryl ether (31) should be thermodynamically more stable than 15-membered ether (30) (Figure 2).
This observed siteselectivity might be explained by assuming that the participation of the neighboring
alkoxy group in furfuryl ether resulted in SN2’ substitution to form three membered oxonium intermediate,
which could be attacked at the quaternary carbon by acetylide.

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HETEROCYCLES, Vol. 65, No. 3, 2005
O
ii
O
v, vi
i
Cl
Br
O
O
R
OEE
24
25
OEE
OEE
MOMO
iii, iv
26 R=CH2OTBS
27 R=CHO
O
O
Ο
O
O
O
ix
R
MOMO
MOMO
MOMO
TBSO
TBSO
30
TBSO
31
28 R=OEE
29 R=Cl
vii, viii
Scheme 6. Reagents and conditions: (i) 17 (1.1 equiv.), NaH (ca 50% purity, 1 equiv.), DMF, 0°C, 2 h, 59%; (ii) 10 (1.2
equiv.), Pd2(dba)3 (5 mol%), Ph3As (21 mol%), i-Pr2NEt (1.2 equiv.), DMA, 80°C, 2 h, 78%; (iii) TBAF (1.5 equiv.), CH2Cl2,
rt, 3 h, 89%; (iv) SO3-pyridine complex (4 equiv.), i-Pr2NEt (10 equiv.), DMSO (20 equiv.), CH2Cl2, 0°C, 2 h; (v)
ethynylmagnesium bromide (7 equiv.), THF, form -78°C to -20°C, 1 h; (vi) TBSOTf (6 equiv.), 2,6-lutidine (12 equiv.),
CH2Cl2, -50°C, 1 h, 55% (3 steps); (vii) 0.5 M HCl (1.5 equiv.), THF, rt, 3 h, 77%; (viii) MsCl (6.5 equiv.), 2,6-lutidine (8.7
equiv.), LiCl (6.2 equiv.), DMF, -5°C, 6 h, 60%; (ix) CuI (10 equiv.), NaI (15 equiv.), Cs2CO3 (10 equiv.), DMF, 80°C, 45%.
Figure 2. Conformations of furfuryl ether. Left:15-membered compound. Right:17-membered compound.
In view of the synthesis of naturally occurring furanocyclic diterpenes, we then examined the construction
of γ-butenolide from propargylic alcohol, such as 2, using a model compound. (Scheme 7) Reaction of
EtMgBr with alkynyl bromide (15) followed by the addition of the resulting acetylide to aldehyde (11)
gave an inseparable mixture of propargylic alcohol (32) quantitatively. Compound (32) was converted to
allenic alcohol (33) by the procedure reported by Myers et al.10 and subsequent removal of

HETEROCYCLES, Vol. 65, No. 3, 2005
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methoxymethyl moiety with CBr4 in i-PrOH.27 Ru-catalyzed cyclocarbonylation11 of 33 was successfully
carried out to provide a desired γ–butenolide (34) in 66% yield.28
OTBDPS
I
TBDPSO
TBDPSO
I
I
ii, iii
i
iv
15
•
MOMO
HO
O
HO
32
O
33
34
Scheme 7. Reagents and conditions: (i) 15 (2 equiv.), EtMgBr (2.1 equiv.), THF, 0°C, 15 min, then 11 (1 equiv.), from -78°C
to 0°C, 1.5 h, 99%; (ii) Ph3P (5 equiv.), DEAD (5 equiv.), o-nitrobenzenesulfonyl hydrazide (5 equiv.), THF, -15°C, 7 h, then
from -15°C to rt, 2 h, 51%; (iii) CBr4 (0.2 equiv.), i-PrOH, reflux, 3 h, 67%; (iv) Ru3(CO)12 (5 mol%), Et3N (3 equiv.), CO (10
atm), 100°C., 119 h, 66%.
In summary, we have achieved the synthesis of a 2,3,5-trisubstituted furfuryl ether, a potent intermediate
in a synthetic approach to bipinnatin J, employing a combination of etherification and Stille coupling
reaction. An intramolecular coupling reaction of ω-alkynylated allylic chloride gave unanticipated SN2’
product, 15-membered furfuryl ether. We have also demonstrated the stereoselective formation of
γ-butenolide from propargylic alcohol. Further synthetic studies toward bipinnatin J are currently under
investigation.
ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
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1
25. Data for an inseparable mixture (1 : 1) of 29: IR ν max 1040, 1080, and 1100 cm-1; H NMR
(CDCl3; 270 MHz) δ 0.14 (6H, s, Si(CH3)2), 0.90 and 0.91 (each 4.5H, each s, C(CH3)3), 1.72 (3H, s,
2”-CCH3), 1.94 (3H, s, 2’-CCH3), 2.00 (3H, s, 3-CCH3), 2.45 (1H, d, J=2.1 Hz, 7’-CH), 2.53 and
2.63 (each 0.5H, each dd, J=3.5, 13.5 and 2.3, 13.8 Hz, 3’-CHH), 2.84 and 2.88 (each 0.5H, each dd,
J=2.8, 13.5 and 3.8, 13.8 Hz, 3’-CHH), 3.31 and 3.32 (each 1.5H, each s, CH3O), 3.76-4.04 (1H, m,
4’-CH), 3.88 (2H, s, 1“-CH2), 4.10 (2H, d, J=7.9 Hz, 4”-CH2), 4.35 (2H, s, 2-CCH2), 4.44 and 4.54
(each 0.5H, each dd, J=2.1, 3.5 and 2.1, 5.1 Hz, 5’-CH), 4.61 and 4.63 (each 0.5H, each d, each
J=7.1 Hz, OCHHO), 4.71 and 4.79 (each 0.5H, each d, each J=6.9 Hz, OCHHO), 5.70 (1H, t, J=7.9
13
Hz, 3”-CH), 6.12 (1H, s, CH), 6.29 and 6.30 (each 0.5H, s, CH); C NMR (CDCl3; 67.8 MHz)
δ -5.0, -4.8, -4.5 (2), 9.9, 10.0, 13.7 (2), 18.1, 18.2, 25.2, 25.5, 25.7 (3), 25.8 (3), 34.3, 35.0, 40.1
(2), 55.7 (2), 61.7, 61.8, 66.0, 66.1, 73.9, 74.1, 74.2 (2), 78.5, 78.7, 82.9 (2), 96.4, 97.1, 110.7,
111.2, 117.0, 117.2, 120.6 (2), 122.4 (2), 135.6, 136.2, 138.7 (2), 145.1, 145.3, 151.8, 151.9; MS
(EI): 510 (M+), 512 (M++2); HRMS (EI): calcd for C27H43O5ClSi: 510.2568. Found: 510.2573.

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HETEROCYCLES, Vol. 65, No. 3, 2005
26. (a) R. Rossi, A. Carpita, and M. G. Quirici, Gazz. Chim. Ital., 1981, 111, 173. (b) J. Rokach, J.
Adams, and R. Perry, Tetrahedron Lett., 1983, 24, 5185. (c) E.-I. Negishi, Y. Zhang, F. E.
Cederbaum, and M. B. Webb, J. Org. Chem., 1986, 51, 4080. (d) R. Bloch, G. Gasparini, and C.
Girard, Chem. Lett., 1988, 1927.
27. A. S.-Y. Lee, Y.-J. Hu, and S.-F. Chu, Tetrahedron, 2001, 57, 2121.
23
1
28. Data for 34: [α]D +10.6 ° (c 1.07, CHCl3) ;IR ν max 1110 and 1760 cm-1 ; H-NMR (CDCl3 ; 270
MHz) δ 0.95-1.15 (9H, br s, C(CH3)3), 1.60 (3H, s, 4’-CCH3), 1.97 (3H, s, 2”-CCH3), 2.35 (2H, s,
2-CH2), 2.49 (1H, dd, J=7.6 and 13.5 Hz, 1’-CHH), 2.63 (1H, dd, J=5.8 and 13.5 Hz, 1’-CHH), 4.05
(2H, s, 5’-CH2), 5.01 (1H, ddd, J=0.7, 5.8 and 7.6 Hz, 5-CH), 5.44 (1H, s, 3’-CH), 6.09 (1H, s,
13
3”-CH), 7.05 (1H, d, J=0.7 Hz, 4-CH), 7.25-7.50 (6H, m, Ar), 7.55-7.75 (4H, m, Ar); C-NMR
(CDCl3 : 125.65 MHz) δ 13.6, 19.3, 24.8, 25.1, 25.2, 26.8 (3), 42.4, 68.6, 78.4, 79.4, 122.3, 127.6 (4),
129.6 (2), 133.7 (2), 134.1, 135.5 (4), 142.4, 147.6, 173.3 ; MS (CI): 601 (M+1); HRMS (CI): calcd
for C30H37O3ISi+H : 601.1635. Found ; 601.1609.