Synthesis of dibarrelane, a dibicyclo[2.2.2]octane hydrocarbon

Article abstract of DOI:10.1021/jo5003455

The synthesis of a novel hydrocarbon, dibarrelane, has been accomplished in 11 steps via an intramolecular REDDA reaction of a masked o-benzoquinone, followed by Clemmensen reduction and Barton decarboxylation. The twisted structure of the tetracyclic dibarrelane skeleton was also clarified via X-ray crystallography. Finally, it was proposed that dibarrelane has C₂ symmetry rather than C_2v symmetry.

Full text of DOI:10.1021/jo5003455

Note
pubs.acs.org/joc
Synthesis of Dibarrelane, a Dibicyclo[2.2.2]octane Hydrocarbon
Takahiro Suzuki,*,†,‡ Hiroshi Okuyama,† Atsuhiro Takano,† Shinya Suzuki,‡ Isao Shimizu,‡
and Susumu Kobayashi*,†
†Faculty of Pharmaceutical Sciences, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan
‡Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
S
* Supporting Information
ABSTRACT: The synthesis of a novel hydrocarbon,
dibarrelane, has been accomplished in 11 steps via an
intramolecular REDDA reaction of a masked o-benzoquinone,
followed by Clemmensen reduction and Barton decarbox-
ylation. The twisted structure of the tetracyclic dibarrelane
skeleton was also clarified via X-ray crystallography. Finally, it
was proposed that dibarrelane has C2 symmetry rather than C2v symmetry.
ighly symmetrical cage hydrocarbons such as prismane,
cubane, dodecahedrane, and fullerene have attracted
possesses one quaternary carbon which is shared by six six-
membered rings. All cyclohexanes may adopt a boatlike
conformation. Dibarrelane 2 is a potent component of
polycyclic hydrocarbons (Figure 2). Fusion of bicyclo[2.2.2]-
H
chemists with their simple and beautiful chemical structures
(Figure 1a).1 While the synthesis of symmetrical cage
Figure 2. Example of cage hydrocarbons based on dibarrelane.
octane to dibarrelane at the C1−C10 and C1−C11 bonds gives
a hexacyclic compound, tribarrelane 3. The further extension of
bicyclo[2.2.2]octanes in the same fashion will give the D5h-
symmetric cyclopentabarrelane 4.7 On the other hand, fusion of
bicyclo[2.2.2]octane to 2 at the C1−C2 and C1−C10 bonds
gives an isomer of 3, isotribarrelane 5, which possesses axial
chirality. The intriguing structural features of 2 provided a
strong motivation for the synthesis of dibarrelane hydro-
carbons. We report herein the synthesis, structure, and
properties of 2.
The synthetic strategy to attain dibarrelane is shown in
Scheme 1. It was envisaged that the construction of the
dibarrelane skeleton 8 could be achieved by an intramolecular
reverse-electron-demand Diels−Alder (REDDA) reaction of a
masked o-benzoquinone (MOB) 7.8 MOB 7 would be prepared
from tetralone 6 by introduction of methoxycarbonyl and allyl
groups. Due to the dense array of functional groups on 9, there
are a variety of options for the order of defunctionalization.
Dibarrelane-1-carboxylic acid 9 was chosen as the precursor of
2,9 and Barton decarboxylation10 of 7 completes the synthesis
of dibarrelane 2.
Figure 1. (a) Example of highly symmetrical cage hydrocarbons. (b)
Structures of atropurupuran (1) and its core skeleton, dibarrelane (2).
hydrocarbons continues to be a challenging subject in organic
chemistry, synthetic efforts have resulted in novel method-
ologies in organic synthesis and have also provided valuable
knowledge in physical organic chemistry. Most of the
symmetrical skeletons have been artificially designed through
imagination and computational study; however, nature occa-
sionally produces an unprecedented cage compound possessing
a highly symmetrical skeleton.2
We have been investigating the total synthesis of a unique
pentacyclic diterpene, atropurpuran3,4 (1) (Figure 1b).
Atropurpuran possesses a cage-like skeleton with C2v symmetry,
tetracyclo[5.3.3.04,9.04,12]tridecane (2), which includes two
bicyclo[2.2.2]octane units fused by two C−C single bonds.5
We have named this simple C13H20 hydrocarbon “dibarrelane”
after “barrelene”, bicyclo[2.2.2]octa-2,5,7-triene.6 Dibarrelane
Received: February 13, 2014
Published: February 24, 2014
© 2014 American Chemical Society
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Note
Scheme 1. Synthetic Strategy toward Dibarrelane
Scheme 3. Synthesis of Dibarrelane
Synthesis of the tetracyclic compound 8 commenced with
the methoxycarbonylation of tetralone 6, which was prepared
from o-eugenol on a multigram scale4a (Scheme 2). The
reduction of a secondary alcohol took place under these
reaction conditions.13 Finally, Barton decarboxylation of 9
occurred via acyloxypyridine-2-thione intermediate 16 to afford
the volatile hydrocarbon 2 in 94% yield. Sublimation of 2 at
45−50 °C (bath temperature) gave a dendritic crystal (mp 90−
93 °C in a sealed capillary). Dibarrelane 2 has a terpene-like
odor, and the 13C NMR in CDCl3 has six sharp signals at 38.2,
32.3, 31.2, 27.4, 27.1, and 26.7 ppm, indicative of the
symmetrical structure of dibarrelane.
Scheme 2. Construction of Tetracyclic Skeleton
Although the crystals were not suitable for X-ray analysis, the
dibarrelane skeleton was unambiguously confirmed via X-ray
crystallographic analysis of carboxylic acid 914 (Figure 3).
requisite vinyl group (dienophile for the REDDA reaction) was
introduced by allylation of the active methine carbon of 10 to
give keto ester 11. After removal of a Bn group with BCl3,
reduction of ketone under Luche conditions afforded alcohol
12 as a single isomer.11 The resulting secondary alcohol 12 was
transformed into 13 by silyl etherification and selective
deprotection of phenol. Oxidative dearomatization of phenol
13 with (diacetoxy)iodobenzene (PIDA) in the presence of
MeOH provided MOB 7. The intramolecular REDDA reaction
of MOB 7 in toluene (180 °C, in a sealed tube) successfully
proceeded to give tetracyclic compound 8 in high yield (94%).
Defunctionalization of the dibarrelane skeleton was then
accomplished (Scheme 3). First, hydrogenation of the anti-
Bredt double bond of 8 was carried out to obtain the α-keto
acetal 14 in 86% yield. Treatment of 14 with an 80% aqueous
TFA solution in CH2Cl2 caused hydrolysis of the dimethyl
acetal concurrent with deprotection of the secondary alcohol to
give the α-diketone 15. Surprisingly, 15 was directly converted
into dibarrelane-1-carboxylic acid 9 under Clemmensen
reduction conditions.12 It is noteworthy that the unexpected
Figure 3. Ortep drawings of carboxylic acid 9: (a) dimeric structure;
(b) monomeric structure. Thermal ellipsoids are drawn at the 50%
probability level.
Carboxylic acid 9 forms a dimer via intermolecular hydrogen
bonding in its crystalline form, and it was surprising to find that
the dimer is a set of enantiomers (Figure 3a), because
carboxylic acid 9 was thought to be achiral. The ORTEP
structure and representative details of the monomer (right side
of Figure 3a) are shown in Figure 3b and Table 1. Similar to the
case for adamantane, all bond lengths are 1.54 0.02 Å and all
C−C−C bond angles are 109.5
1.5° except for the
bridgehead carbons (C4, C9, and C12). The bond angles of
bridgehead methines C8−C9−C10 and C11−C12−C13 are
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Note
In summary, the synthesis of a novel hydrocarbon,
dibarrelane, has been accomplished in 11 steps from tetralone
6 via an intramolecular REDDA reaction of a masked o-
benzoquinone, followed by Clemmensen reduction and Barton
decarboxylation. The twisted structure of the tetracyclic
dibarrelane skeleton was also clarified via X-ray crystallography.
Finally, it was proposed that dibarrelane has C2 symmetry
rather than C2v symmetry. Further structural analyses of the
dibarrelane skeleton along with a synthetic study of cage
hydrocarbons based on dibarrelane are currently underway.
Table 1. Selected Geometric Parameters for Carboxylic Acid
9 and Dibarrelane 2
9
a
b
b
exptl
theor
2 theor
Distances (Å)
1.548(2)
1.542(2)
1.545(3)
1.532(3)
1.550(2)
1.553(2)
Angles (deg)
111.5(1)
106.6(1)
113.4(1)
113.4(1)
9.3(2)
C1−C11
C4−C12
C6−C7
C7−C13
C11−C12
C12−C13
1.552
1.555
1.540
1.542
1.561
1.559
1.540
1.557
1.540
1.542
1.563
1.558
EXPERIMENTAL SECTION
■
General Methods. Flash column chromatography was performed
with silica gel (50−200 μm). Solvents for chromatography are listed as
volume/volume ratios. Analytical thin-layer chromatography was
performed using commercial silica gel plates. Infrared spectra were
recorded in reciprocal centimeters (cm−1). High-resolution mass
spectra (HRMS) were obtained by a hybrid quadrupole time-of-flight
(TOF) mass spectrometer for electrospray ionization (ESI) and a
double-focusing magnetic sector (DFS) mass spectrometer for fast
atom bombardment (FAB) and electroimpact ionization (EI). 1H
NMR spectra were acquired at 400 and 600 MHz. Chemical shifts are
reported in delta (δ) units in parts per million (ppm) relative to the
singlet at 7.26 ppm for chloroform-d and the quintet at 2.05 ppm for
acetone-d6. 13C NMR spectra were acquired at 100 and 125 MHz.
Chemical shifts are reported (ppm) relative to the central line of the
triplet at 77.0 ppm for chloroform-d and the septet at 29.84 ppm for
acetone-d6.
C3−C4−C5
C9−C4−C12
C8−C9−C10
C11−C12−C13
C1−C2−C3−C4
C4−C5−C6−C7
C7−C8−C9−C4
C4−C9−C10−C1
C1−C11−C12−C4
C4−C12−C13−C7
111.8
106.3
113.9
113.5
12.7
11.8
9.7
111.7
106.3
113.9
113.9
11.1
11.1
9.1
10.7(2)
7.7(2)
12.6(2)
15.7
9.8
14.8
9.1
7.9(2)
12.9(2)
15.2
14.8
a
b
X-ray structure reported in this work. DFT calculations were
performed with the Gaussian 09 program for structure minimization
with the B3LYP/6-31G* method.
Methyl 5-(Benzyloxy)-6-methoxy-1-oxo-1,2,3,4-tetrahydro-
naphthalene-2-carboxylate (10). To a solution of NaH (55%,
0.926 g, 21.24 mmol) and dimethyl carbonate (2.76 mL, 31.86 mmol)
in toluene (20 mL) was added tetralone 64a (1.00 g, 3.54 mmol) at
room temperature. The reaction mixture was stirred at 80 °C for 10 h.
The reaction mixture was cooled to 0 °C, and H2O followed by 1 M
aqueous HCl was carefully added to the reaction mixture. The aqueous
layer was extracted with CH2Cl2,, and the combined organic layers
were washed with brine, dried over Na2SO4, and concentrated under
reduced pressure. The residue was purified by silica gel chromatog-
raphy with (hexane/EtOAc 8/1 to 5/1) to afford keto ester 10 (1.06 g,
88% yield) as a mixture of tautomers (keto form:enol form 2:1): Rf =
113.4°, and those of quaternary carbon C3−C4−C5 and C9−
C4−C12 are 111.5 and 106.6°, respectively. These results are
consistent with those of computational calculation.15
Other characteristic structural features of carboxylic acid 9
are follows: the C1−C2−C3−C4 and C4−C5−C6−C7 torsion
angles are ca. 10°, resulting in a twist in the two bicyclo[2.2.2]-
octanes (the torsion angle between C2−C3 and C5−C6 is
17.85°). The distances between the 1,5-diaxial hydrogens on
the bottom crownlike eight-membered ring are not the same;
e.g. the H8ax−H11ax distance is 2.720 Å, and that between
H10ax and H13ax is 3.864 Å. These results demonstrate that
the dibarrelane skeleton apparently does not have σ symmetry
in the crystalline state. The twisted dibarrelane skeleton might
be the result of a diminished H−H diaxial interaction (between
H8ax−H10ax and H11ax−H13ax). The H8ax−H10ax and
H11ax−H13ax distances are 2.162 and 2.167 Å, respectively,
still smaller than the van der Waals radii. As a result, the
dibarrelane skeleton becomes chiral, and the crystals of
carboxylic acid 9 pack so as to grow a dimer composed of a
set of enantiomers.
Although an X-ray crystallographic analysis of dibarrelane 2
itself was unsuccessful, the structural analysis of carboxylic acid
9 strongly suggests a distorted structure for dibarrelane. In
other words, dibarrelane might have C2 symmetry rather than
the originally anticipated C2v symmetry. A precise structural
analysis of the dibarrelane skeleton, particularly an experimental
proof of the chiral nature (both in crystalline form and in
solution), would be quite interesting. The results of DFT
calculations for geometry optimization of dibarrelane 2
indicated that the C2-symmetric conformations are the most
stable (Table 1). However, there is almost no difference in
energy between C2-symmetric and C2v-symmetric conforma-
tions (ca. 1.1 kJ/mol).16 Therefore, it is suggested that the
energy barrier for conformational interconversion is quite
low.17
1
0.43 (hexane/EtOAc 3/1); H NMR (400 MHz, CDCl3) δ 12.43 (s,
1/3H), 7.88 (d, J = 8.8 Hz, 2/3H), 7.56 (d, J = 8.5 Hz, 1/3H), 7.44−
7.28 (m, 5H), 6.92 (d, J = 8.8 Hz, 2/3H), 6.84 (d, J = 8.5 Hz, 1/3H),
5.55 (s, 4/3H), 4.97 (s, 2/3H), 3.94 (s, 2H), 3.91 (s, 1H), 3.78 (s,
1H), 3.75 (s, 2H), 3.50 (dd, J = 10.3, 4.6 Hz, 2/3H), 3.00 (ddd, J =
17.5, 5.9, 4.6 Hz, 2/3H), 2.73−2.64 (m, 4/3H), 2.39 (dd, J = 8.9, 7.0
Hz, 2/3H), 2.32 (dddd, J = 13.5, 9.9, 9.9, 4.7 Hz, 2/3H), 2.20 (dddd, J
= 13.5, 5.9, 4.9, 4.9 Hz, 2/3H); 13C NMR (100 MHz, CDCl3) δ 192.1,
173.0, 170.8, 165.2, 157.2, 155.0, 143.8, 138.2, 137.4, 137.2, 133.9,
128.41, 128.36, 128.30, 128.28, 128.1, 128.0, 125.4, 125.2, 123.3,
121.1, 110.3, 109.5, 95.1, 74.6, 74.4, 55.8, 55.7, 53.9, 52.2, 51.4, 25.8,
21.8, 21.1, 20.0; IR (ATR) 2950, 1738, 1676, 1590, 1438, 1270, 1205,
1075 cm−1; HRMS (ESI-TOF) calcd for C20H20NaO5 363.1202, found
363.1212.
Methyl 2-Allyl-5-(benzyloxy)-6-methoxy-1-oxo-1,2,3,4-tetra-
hydronaphthalene-2-carboxylate (11). To a solution of keto ester
10 (2.13 g, 6.27 mmol) in THF (60 mL) was added NaH (55%, 0.274
g, 6.27 mmol) portionwise at 0 °C. After the mixture was stirred for 10
min at 0 °C, allyl iodide (2.29 mL, 25.1 mmol) was added. The
reaction mixture was stirred at 0 °C for 3 h. The reaction was
quenched with saturated aqueous NH4Cl solution, and the aqueous
layer was extracted with CH2Cl2. The combined organic layers were
washed with brine, dried over Na2SO4, and concentrated under
reduced pressure. The residue was purified by silica gel chromatog-
raphy with (hexane/EtOAc 6/1) to afford keto ester 11 (2.35 g, 98%
yield) as a colorless oil: Rf = 0.41 (hexane/EtOAc 4/1); 1H NMR (400
MHz, CDCl3) δ 7.89 (d, J = 8.8 Hz, 1H), 7.44−7.30 (m, 5H), 6.93 (d,
J = 8.8 Hz, 1H), 5.79 (dddd, J = 16.9, 10.6, 7.2, 7.2 Hz, 1H), 5.14−
5.06 (m, 2H), 5.00 (s, 2H), 3.96 (s, 3H), 3.65 (s, 3H), 2.88 (ddd, J =
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Note
17.9, 5.2, 5.2 Hz, 1H), 2.73 (ddd, J = 17.9, 10.0, 4.9 Hz, 1H), 2.69 (dd,
J = 14.1, 7.2 Hz, 1H), 2.60 (dd, J = 14.1, 7.2 Hz, 1H), 2.41 (ddd, J =
13.9, 5.2, 4.9 Hz, 1H), 1.98 (ddd, J = 13.9, 10.0, 5.2 Hz, 1H); 13C
NMR (100 MHz, CDCl3) δ 194.0, 172.1, 157.0, 143.7, 138.0, 137.3,
133.5, 128.41, 128.36, 128.2, 125.7, 125.5, 118.8, 110.4, 74.3, 56.9,
55.8, 52.3, 38.5, 29.9, 20.2; IR (ATR) 3019, 2950, 1731, 1676, 1589,
1489, 1440, 1280, 1214, 1076 cm−1; HRMS (ESI-TOF) calcd for
C23H24NaO5 403.1515, found 403.1521.
5.10−5.03 (m, 2H), 4.90 (brs, 1H), 3.85 (s, 3H), 3.50 (s, 3H), 2.79
(ddd, J = 18.2, 7.6, 3.7 Hz, 1H), 2.72 (ddd, J = 18.2, 9.5, 7.0 Hz, 1H),
2.60 (dd, J = 13.7, 7.0 Hz, 1H), 2.35 (dd, J = 13.7, 7.8 Hz, 1H), 2.18−
2.02 (m, 2H), 0.85 (t, J = 7.9 Hz, 9H), 0.50 (q, J = 7.9 Hz, 6H); 13C
NMR (100 MHz, CDCl3) δ 174.7, 145.4, 142.7, 134.0, 131.4, 122.7,
120.5, 117.6, 107.6, 71.8, 55.9, 51.9, 51.3, 40.4, 23.9, 20.2, 6.9, 5.4; IR
(ATR) 3435, 2955, 2876, 1716, 1495, 1277, 1053, 1007 cm−1; HRMS
(ESI-TOF) calcd for C22H34NaO5Si 429.2067, found 429.2070.
(1R*,2S*)-Methyl 2-Allyl-6,6-dimethoxy-5-oxo-1-
((triethylsilyl)oxy)-1,2,3,4,5,6-hexahydronaphthalene-2-car-
boxylate (7). To a solution of phenol 13 (5.24 g, 12.8 mmol) in
MeOH/CH2Cl2 (2/3 v/v, 100 mL) was added PIDA (4.53 g, 14.1
mmol) portionwise at 0 °C. The reaction mixture was stirred for 30
min at 0 °C, and saturated NaHCO3 solution was added. The resulting
mixture was extracted two times with CH2Cl2, and the organic layers
were dried over Na2SO4, filtered, and concentrated under reduced
pressure. The residue was purified by silica gel chromatography with
hexane/EtOAc (15/1) to afford MOB 7 (5.26 g, 94% yield) as a
(1R*,2S*)-Methyl 2-Allyl-1,5-dihydroxy-6-methoxy-1,2,3,4-
tetrahydronaphthalene-2-carboxylate (12). To a solution of Bn
ether 11 (1.27 g, 4.37 mmol) in CH2Cl2 (30 mL) was added dropwise
a 1.0 M solution of BCl3 in heptane (11.37 mL, 11.37 mmol) at −78
°C. After it was stirred for 3 h at −78 °C, the reaction mixture was
quenched with MeOH (0.5 mL) and diluted with water. The aqueous
layer was extracted with CH2Cl2. The combined organic layer was
washed with brine, dried over Na2SO4, filtered, and concentrated in
vacuo. The residue was purified by flash column chromatography
(hexane/EtOAc 6/1 to 2/1) to afford the phenol (0.984 g, 97%) as a
white solid: Rf = 0.24 (hexane/EtOAc 4/1); mp 105 °C (from EtOAc/
hexane); 1H NMR (400 MHz, CDCl3) δ 7.69 (d, J = 8.8 Hz, 1H), 6.85
(d, J = 8.8 Hz, 1H), 5.84 (dddd, J = 16.9, 10.1, 7.4, 7.0 Hz, 1H), 5.75
(s, 1H), 5.18−5.08 (m, 2H), 3.95 (s, 3H), 3.67 (s, 3H), 2.99 (ddd, J =
17.9, 5.3, 5.3 Hz, 1H), 2.88 (ddd, J = 17.9, 9.7, 4.8 Hz, 1H), 2.74 (dd, J
= 14.0, 7.0 Hz, 1H), 2.66 (dd, J = 14.0, 7.4 Hz, 1H), 2.54 (ddd, J =
13.8, 5.3, 4.9 Hz, 1H), 2.10 (ddd, J = 13.8, 9.7, 5.3 Hz, 1H); 13C NMR
(100 MHz, CDCl3) δ 194.2, 172.1, 150.0, 141.8, 133.5, 129.2, 125.8,
121.0, 118.7, 108.6, 57.1, 56.1, 52.3, 38.5, 29.8, 19.4; IR (ATR) 3461,
3219, 2948, 1728, 1493, 1435, 1276, 1218, 996 cm−1; HRMS (ESI-
TOF) calcd for C16H18NaO5 313.1046, found 313.1043. To a solution
of the phenol (3.30 g, 11.3 mmol) and CeCl3·7H2O (10.9 g, 29.4
mmol) in MeOH (100 mL) was added NaBH4 (1.14 g, 29.4 mmol) at
−10 °C. The reaction mixture was stirred at the same temperature for
3 h. The reaction mixture was quenched with saturated aqueous
NH4Cl solution, and MeOH was removed by a rotary evaporator. The
residue was diluted with EtOAc and H2O, and the mixture was
separated. The aqueous layer was extracted with CH2Cl2, and the
combined organic layers were washed with brine, dried with Na2SO4,
and concentrated in vacuo. The residue was purified by flash column
chromatography (hexane/EtOAc 8/1 to 2/1) to afford 12 (2.63 g,
79%) as a white solid: Rf = 0.19 (hexane/EtOAc = 4/1); mp 136−138
°C (from EtOAc/hexane); 1H NMR (400 MHz, CDCl3) δ 7.00 (d, J =
8.3 Hz, 1H), 6.78 (d, J = 8.3 Hz, 1H), 5.86 (dddd, J = 16.9, 10.2, 7.4,
7.0 Hz, 1H), 5.68 (s, 1H), 5.14−5.05 (m, 2H), 4.93 (brs, 1H), 3.87 (s,
3H), 3.65 (s, 3H), 2.79 (ddd, J = 18.2, 6.3, 6.3 Hz, 1H), 2.68 (ddd, J =
18.2, 7.2, 7.2 Hz, 1H), 2.63 (dd, J = 13.8, 7.0 Hz, 1H), 2.37−2.29 (m,
2H), 2.12−2.00 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 175.6,
145.3, 142.5, 133.9, 130.8, 121.9, 119.7, 118.0, 108.8, 70.5, 56.0, 51.7,
50.2, 37.4, 25.1, 20.0; IR (ATR) 3460, 3229, 2947, 1727, 1493, 1434,
1276, 1218, 996 cm−1; HRMS (ESI-TOF) calcd for C16H20NaO5
315.1202, found 315.1205.
(1R*,2S*)-Methyl 2-Allyl-5-hydroxy-6-methoxy-1-
((triethylsilyl)oxy)-1,2,3,4-tetrahydronaphthalene-2-carboxy-
late (13). To a solution of alcohol 12 (5.10 g, 17.4 mmol) in DMF
(50 mL) was added imidazole (5.93 g, 87.2 mmol), TESCl (13.1 mL,
87.2 mmol), and DMAP (0.53 g, 4.35 mmol) at 0 °C, and the mixture
was stirred at room temperature for 16 h. The reaction was quenched
with a saturated NaHCO3 solution, and the organic layer was
separated. The aqueous layer was extracted with ether, and the
combined organic layers were washed with brine, dried over Na2SO4,
filtered, and concentrated under reduced pressure. To the crude di-
TES ether were added THF (80 mL) and 1 M NaOH solution (100
mL) at room temperature, the mixture was stirred for 4 h, and the
mixture was neutralized with 1 M HCl solution. The resulting mixture
was extracted with EtOAc, and the organic layer was washed with
brine, dried over Na2SO4, filtered, and concentrated under reduced
pressure. The residue was purified by silica gel chromatography with
hexane/EtOAc (10/1) to afford 13 (5.24 g, 74%) as a white solid: Rf =
0.51 (hexane/EtOAc = 4/1); mp 64−65 °C (from EtOAc/hexane);
1H NMR (400 MHz, CDCl3) δ 6.75 (d, J = 8.3 Hz, 1H), 6.67 (d, J =
1
yellow oil: Rf = 0.35 (hexane/EtOAc = 6/1); H NMR (400 MHz,
CDCl3) δ 6.29 (m, 2H), 5.76 (dddd, J = 16.9, 10.7, 8.0, 6.8 Hz, 1H),
5.10−5.03 (m, 2H), 4.53 (s, 1H), 3.61 (s, 3H), 3.36 (s, 3H), 3.33 (s,
3H), 2.53 (dd, J = 13.5, 6.8 Hz, 1H), 2.41 (ddd, J = 19.9, 6.8, 3.1 Hz,
1H), 2.33−2.21 (m, 2H), 1.99 (ddd, J = 13.5, 6.3, 3.1 Hz, 1H), 1.86
(ddd, J = 13.5, 9.6, 6.8 Hz, 1H), 0.96 (t, J = 7.9 Hz, 9H), 0.66 (q, J =
7.9 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 195.8, 174.3, 145.6,
133.2, 132.3, 130.4, 129.1, 118.2, 91.2, 70.8, 51.6, 51.3, 50.2, 50.1, 38.9,
24.1, 19.6, 6.9, 5.6; IR (ATR) 2953, 2877, 1731, 1672, 1435, 1217,
1061, 1004, 728 cm−1; HRMS (ESI-TOF) calcd for C23H36NaO6Si
459.2173, found 459.2174.
(2S*,4aR*,7S*)-Methyl 6,6-Dimethoxy-5-oxo-1-
((triethylsilyl)oxy)-2,3,4,5,6,7-hexahydro-1H-2,4a-ethano-4,7-
methanonaphthalene-2-carboxylate (8). A solution of MOB 7
(5.26 g, 12.0 mmol) in dry toluene (100 mL) was heated to 180 °C for
16 h in a sealed tube. The reaction mixture was transferred into a
round-bottom flask, and the solvent was removed in vacuo. The
residue was purified by silica gel column chromatography with hexane/
EtOAc (50/1 to 10/1) to afford cycloadduct 8 (4.95 g, 94% yield) as a
1
colorless oil: Rf = 0.38 (hexane/EtOAc = 6/1); H NMR (400 MHz,
CDCl3) δ 6.31 (dd, J = 6.7, 1.8 Hz, 1H), 4.78 (d, J = 1.8 Hz, 1H), 3.65
(s, 3H), 3.35 (s, 3H), 3.27 (s, 3H), 2.34−2.23 (m, 2H), 2.18 (ddd, J =
9.3, 6.2, 6.2 Hz, 1H), 2.03−1.90 (m, 2H), 1.83 (dd, J = 13.7, 9.0 Hz,
1H), 1.64−1.54 (m, 2H), 1.36, (ddd, J = 14.0, 5.7, 3.1 Hz, 1H), 1.32
(dd, J = 13.7, 4.6 Hz, 1H), 0.96 (t, J = 7.8 Hz, 9H), 0.63 (q, J = 7.8 Hz,
6H); 13C NMR (100 MHz, CDCl3) δ 205.1, 173.9, 143.6, 125.5, 95.3,
76.2, 54.2, 52.7, 51.6, 50.3, 49.4, 42.2, 39.6, 34.8, 34.5, 27.9, 18.5, 6.8,
4.8; IR (ATR) 2953, 2876, 1731, 1241, 1153, 1115, 1090, 1055, 734
cm−1; HRMS (ESI-TOF) calcd for C23H36NaO6Si 459.2173, found
459.2170.
(2S*,4aR*,7S*)-Methyl 6,6-Dimethoxy-5-oxo-1-
((triethylsilyl)oxy)octahydro-1H-2,4a-ethano-4,7-methano-
naphthalene-2-carboxylate (14). To a solution of unsaturated
ester 8 (0.745 g, 1.71 mmol) in EtOH (10 mL) was added 10% Pd/
90% C (0.200 g, 10 mol % Pd) under an argon atmosphere. The argon
was purged with H2, and the mixture was stirred under an H2
atmosphere for 2.5 h. The reaction mixture was filtered over Celite
and concentrated in vacuo. The residue was purified by silica gel
column chromatography with hexane/EtOAc (10/1) to give ester 14
(0.644 g, 86%) as a colorless oil: Rf = 0.51 (hexane/EtOAc = 4/1); 1H
NMR (400 MHz, CDCl3) δ 4.19 (dd, J = 9.0, 1.4 Hz, 1H), 3.65 (s,
3H), 3.30 (s, 3H), 3.29 (s, 3H), 2.35 (m, 1H), 2.24−2.00 (m, 4H),
1.95−1.82 (m, 3H), 1.77−1.67 (m, 2H), 1.58−1.46 (m, 3H), 0.95 (t, J
= 7.8 Hz, 9H), 0.58 (q, J = 7.8 Hz, 6H); 13C NMR (100 MHz, CDCl3)
δ 209.3, 175.7, 97.0, 70.8, 51.5, 50.1, 49.3, 47.3, 45.7, 36.1, 33.8, 32.0,
30.7, 30.2, 27.4, 21.4, 20.5, 6.8, 4.7; IR (ATR) 2950, 2875, 1729, 1459,
1246, 1152, 1106, 1055 cm−1; HRMS (ESI-TOF) calcd for
C23H38NaO6Si 461.2329, found 461.2326.
(2S*,4aR*,7S*)-Methyl 1-Hydroxy-5,6-dioxooctahydro-1H-
2,4a-ethano-4,7-methanonaphthalene-2-carboxylate (15). To
a solution of TES ether 14 (0.911 g, 2.08 mmol) in CH2Cl2 (4 mL)
was added a solution of TFA (80% v/v in H2O, 6.0 mL) at room
8.3 Hz, 1H), 5.82 (dddd, J = 16.9, 10.6, 7.8, 7.0 Hz, 1H), 5.62 (s, 1H),
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Note
temperature, and the mixture was vigorously stirred for 2 h. The
reaction mixture was concentrated in vacuo. The resulting residue was
purified by silica gel column chromatography (hexane/EtOAc = 2/1)
to afford 15 (559 mg, 97% yield) as a pale yellow solid: Rf = 0.15
Notes
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
(hexane/EtOAc = 1/1); mp 140−142 °C (from CH2Cl2); H NMR
■
(400 MHz, CDCl3) δ 4.23 (d, J = 9.8 Hz, 1H), 3.73 (s, 3H), 3.27 (br,
1H), 2.89 (ddd, J = 14.7, 3.9, 3.9 Hz, 1H), 2.83 (ddd, J = 5.8, 3.3, 2.8
Hz, 1H), 2.36 (dddd, J = 13.9, 9.8, 4.4, 2.5 Hz,), 2.29−2.01 (m, 4H),
1.97 (ddd, J = 12.7, 3.0, 3.0 Hz, 1H), 1.92−1.76 (m, 4H), 1.72−1.64
(m, 1H); 13C NMR (100 MHz, CDCl3) δ 199.4, 198.3, 177.0, 67.7,
52.3, 47.5, 45.9, 44.1, 33.4, 32.3, 31.9, 28.6, 26.0, 21.5, 20.6; IR (ATR)
3473, 2953, 2875, 1785, 1717, 1432, 1243, 1049, 1007 cm−1; HRMS
(FAB-DFS) calcd for C15H19O5 279.1232, found 279.1234.
This research was supported in part by a Grant-in-Aid for
Young Scientists (B, KAKENHI no. 22790022) from the Japan
Society for the Promotion of Science. We thank the Naito
Foundation, the Uehara Memorial Foundation, and the
Research Foundation for Pharmaceutical Sciences for financial
support. We also thank Prof. K. Miyamura and Mr. K. Ueji
(Department of Chemistry, Tokyo University of Science) for
assistance with X-ray analysis, Mr. N. Sugimura (Materials
Characterization Central Lab, Waseda University) for assistance
with computational studies, and Prof. H. Natsugari and Prof. H.
Takahashi (Faculty of Pharmaceutical Sciences, Teikyo
University) for helpful discussions.
Dibarrelane-1-carboxylic Acid (9). To a solution of HgCl2 (330
mg, 1.22 mmol) in H2O (24 mL) was added Zn powder (12.0 g, 184
mmol), and the mixture was vigorously stirred for 1 h. The
supernatant liquid was removed by decantation. To the residue were
added concentrated HCl (60 mL), H2O (40 mL), and a solution of
diketone 15 (559 mg, 2.02 mmol) in toluene (14 mL), and the two-
phase mixture was vigorously stirred and heated to reflux. Zn powder
(9.0 g) was added portionwise until products converged on a single
spot by TLC analysis. After it was cooled to room temperature, the
mixture was diluted with Et2O/H2O and separated. The aqueous layer
was extracted with CH2Cl2. The combined organic layers were washed
with brine, dried over Na2SO4, filtered, and concentrated in vacuo.
The residue was purified by silica gel chromatography with hexane/
EtOAc (5/1) to afford 9 (269 mg, 61% yield) as a colorless solid: Rf =
0.38 (hexane/EtOAc 2/1); mp 144−145 °C (from CH2Cl2/hexane);
1H NMR (400 MHz, CDCl3) δ 2.10 (dddd, J = 13.3, 10.8, 1.4, 1.4 Hz,
2H), 1.89 (ddddd, J = 13.3, 10.8, 2.9, 1.4, 1.4 Hz, 2H), 1.73−1.58 (m,
5H), 1.53−1.44 (m, 4H), 1.33−1.22 (m, 6H); 13C NMR (100 MHz,
CDCl3) δ 185.0, 41.1, 39.5, 37.5, 31.8, 30.7, 30.3, 28.7, 28.0, 26.6, 26.4;
IR (ATR) 2925, 2855, 2654, 1686, 1458, 1412, 1282, 952 cm−1;
HRMS (ESI-TOF) calcd for C14H20NaO2 243.1355, found 243.1350.
Dibarrelane (2). To a solution of carboxylic acid 9 (78.3 mg, 0.355
mmol) in benzene (4.0 mL) were added DMF (0.08 mL) and
(COCl)2 (0.095 mL, 1.11 mmol). The reaction mixture was stirred for
2 h at room temperature and then concentrated in vacuo. The crude
acid chloride was obtained and used for the next step without
purification. N-Hydroxypyridine-2-thione sodium salt 17 (64.0 mg,
0.429 mmol), DMAP (9.0 mg, 0.0737 mmol), and tert-butyl
mercaptan (0.40 mL, 3.55 mmol) in benzene were refluxed for 10
min, and then the acid chloride in benzene (2.7 mL) was added
dropwise to the refluxing solution while irradiating with a 250 W
tungsten lamp. After 20 min, the resulting mixture was cooled to room
temperature and concentrated in vacuo. The residue was purified by
silica gel chromatography with pentane to afford 2 (62.7 mg, 94%
yield) as a colorless solid; sublimation at 45−50 °C (bath
temperature) gave a dendritic crystal (mp 90−93 °C in a sealed
capillary): 1H NMR (400 MHz, CDCl3) δ 1.91−1.83 (m, 4H), 1.65−
1.60 (m, 2H), 1.58−1.49 (m, 2H), 1.48−1.42 (m, 4H), 1.27−1.21 (m,
4H), 1.20−1.15 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 38.2 (CH2,
4C), 32.3 (CH, 2C), 31.2 (CH2, 2C), 27.4 (C, 1C), 27.1 (CH, 2C),
26.7 (CH2, 2C); IR (ATR) 2915, 2853, 1455, cm−1; HRMS (EI-DFS)
calcd for C13H20 176.1565, found 176.1564.
REFERENCES
■
(1) (a) Hopf, H. Classics in Hydrocarbon Chemistry; Wiley-VCH:
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Hydrocarbons; Wiley: New York, 1990.
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(c) Henry, G. E.; Jacobs, H.; Sean Carrington, C. M.; McLean, S.;
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adamantane and higher diamondoids have also been isolated from
petroleum; see: (d) Dahl, J. E.; Liu, S. G.; Carlson, M. K. Science 2003,
299, 96−99 and references therein.
(3) Tang, P.; Chen, Q.-H.; Wang, F.-P. Tetrahedron Lett. 2009, 50,
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(4) (a) Suzuki, T.; Sasaki, A.; Egashira, N.; Kobayashi, S. Angew.
Chem., Int. Ed. 2011, 50, 9177−9179. (b) Hayashi, R.; Ma, Z.-X.;
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Dissertation; Princeton University, Princeton, NJ, 2013. (d) Chen, H.;
Xhang, D.; Xue, F.; Qin, Y. Tetrahedron 2013, 69, 3141−3148.
(5) For other cage hydrocarbons consisting of two bicyclo[2.2.2]
octane units fused by one or three C−C single bonds, see: (a) Park,
H.; King, P. F.; Paquette, L. A. J. Am. Chem. Soc. 1979, 101, 4773−
4774. (b) Hirao, K.-i.; Iwakuma, T.; Taniguchi, M.; Abe, E.;
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(6) Zimmerman, H. E.; Paufler, R. M. J. Am. Chem. Soc. 1960, 82,
1514−1515.
(7) Considering the structural analogy between 4 and corannulene,
spherical higher barrelanes are possible. For example, the C100H60
hydrocarbon 18 is a C80-fullerene-like molecule possessing a
dodecahedrane core (blue line in 18) and 4 (red in 18) on the surface.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and a CIF file giving H and 13C NMR spectra
1
of all new compounds, results of DFT calculations of
compounds 2 and 9, and crystallographic data for 9. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(8) For reviews on MOBs, see: (a) Liao, C.-C.; Peddinti, R. K. Acc.
Chem. Res. 2002, 35, 856−866. (b) Magdziak, D.; Meek, S. J.; Pettus,
T. R. R. Chem. Rev. 2004, 104, 1383−1429. (c) Liao, C.-C. Pure Appl.
Chem. 2005, 77, 1221−1234. (d) Roche, S. P.; Porco, J. A., Jr. Angew.
Chem., Int. Ed. 2011, 50, 4068−4093.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for T.S.: suzuki-t@aoni.waseda.jp.
*E-mail for S.K.: kobayash@rs.noda.tus.ac.jp.
■
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The Journal of Organic Chemistry
Note
(9) Due to low volatility and moderate polarity, the handling of a
carboxylic acid is much easier than that of other functional groups
toward the last step.
(10) (a) Barton, D. H. R.; Crich, D.; Motherwell, W. B. Tetrahedron
1985, 41, 3901−3924. (b) Barton, D. H. R.; Lacher, B.; Zard, S. Z.
Tetrahedron 1987, 43, 4321−4328.
(11) The stereochemistry of the secondary alcohol of 10 was
determined by NOESY experiments after transformation to 5. The
stereoselective reduction can be explained by considering it to occur
via a chelation-controlled hydride attack.
(12) Martin, E. L. Org. React. 1942, 1, 155−209.
(13) It was reasoned that an SN1 reaction of the secondary alcohol
gave the corresponding chloride, which was smoothly reduced by
excess zinc.
(14) Crystal data for 6: Mr = 220.30, monoclinic, space group P21/c,
a = 6.7640(11) Å, b = 10.5710(17) Å, c = 15.916(3) Å, α = 90.00°, β =
93.658(2)°, γ = 90.00°, V = 1135.7(3) Å3, Z = 4, μ(Mo Kα) = 0.084
mm−1, T = 173 K, 6005 (2560) collected (independent) reflections,
146 parameters refined, R1(all data) = 0.0671, R1(I > 2σ(I)) =0.0639,
wR2(all data) = 0.1735, wR2(I > 2σ(I)) = 0.1699, GOF = 1.045,
largest difference peak and hole 0.379 and −0.304 e Å−3. CCDC
944735 contains supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
(15) Frisch, M. J., et al. Gaussian 09 (Rev. B.01); Gaussian, Inc.,
Wallingford, CT, 2009 (see the Supporting Information for full
reference).
(16) See the Supporting Information.
1
(17) H NMR studies at low temperature (∼−40 °C) did not show
any obvious change.
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