Synthesis of cuparene and herbertene via a common intermediate

Article abstract of DOI:10.1039/a904068d

A dihydroisobenzofuran (13) was obtained from β-ionone in 10 steps via an intramolecular Diels-Alder reaction. On further oxidation, reductive ring opening and decarboxylation, cuparene and'herbertene were synthesized.

Full text of DOI:10.1039/a904068d

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Synthesis of cuparene and herbertene via a common intermediate
Tse-Lok Ho* and May-Hua Chang
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan,
Republic of China. E-mail: tlho@cc.nctu.edu.tw
Received (in Cambridge, UK) 20th May 1999, Accepted 20th July 1999
A dihydroisobenzofuran (13) was obtained from β-ionone in 10 steps via an intramolecular Diels–Alder reaction.
On further oxidation, reductive ring opening and decarboxylation, cuparene and herbertene were synthesized.
Introduction
Cuparene (1) and herbertene (2) are prototypes of sesquiterpe-
nes containing interconnecting cyclopentane and benzene rings.
Cuparene occurs in the heartwood of conifers (e.g., Chamae-
1
cyparis thyoides) while herbertene is a constituent of the leafy
2
liverwort Herberta adunca. The popularity of cuparene, α-
cuparenone, β-cuparenone, and herbertene as synthetic targets
3
is attested by the appearance of more than sixty publications,
addressing various aspects of establishing two adjacent quater-
nary carbon atoms. Because the only difference between
cuparene and herbertene is the substitution pattern of the ben-
zene nucleus, any synthetic method relying on the construction
of the cyclopentane unit is applicable to a synthesis of the
Results and discussion
other sesquiterpene when a route to either cuparene or
herbertene has been delineated. However, we were intrigued by
the possibility of accessing both types of terpenes from a com-
mon intermediate for the sake of attendant synthetic economy.
Necessarily it entails the employment of a tetrasubstituted
cyclopentane derivative for attachment or de novo construction
of the aromatic ring. The divergent strategy is indicated in
eqn. (1).
We chose to start our synthesis from diketone 3, which is readily
accessible from β-ionone by epoxidation at the cyclic double
bond, boron trifluoride induced rearrangement (ring contrac-
4
5
tion), and hydrogenation. The cyclocondensation of 3 with
hydrazine hydrate in refluxing ethanol in the presence of oxygen
yielded pyridazine 4 (Scheme 1). The idea was to functionalize
the benzylic methyl group for a subsequent introduction of
a pendant group, and execution of an intramolecular Diels–
Alder reaction. At this point, we investigated the dienophilicity
of 4, but failed to observe any reaction with either methyl vinyl
ketone or acrylic acid. Eventually, the conversion of 4 to the N-
oxide 5 with MCPBA was effected. Because of the differential
steric shieldings, the regioisomer 6 was not formed in the
oxidation.
Next, 5 was treated with acetic anhydride at refluxing tem-
perature to provide the Polonovski product 7 which was then
saponified. Several attempts at using the benzylic alcohol 8 thus
obtained did not succeed to generate the desired products. For
example, pyrolysis of the silyl ether 9 furnished aldehyde 10,
presumably due to intervention of a more favorable retroene
decomposition [eqn. (3)]. Finally, 8 was derivatized to the prop-
To maximize the strategic advantage, we considered the
preparation of a series of intermediates possessing a local
symmetry. In other words, we desired to delay the branching of
the route as long as feasible in order to save our efforts. To that
end we focused on a dihydroisobenzofuran, with the expec-
tation that oxidation would give rise to two isomeric phthalides
which would be processed further to complete the synthesis of
cuparene and herbertene [eqn. (2)].
argyl ether (prop-2-ynyl ether) 11, subjected to isomerization
with t-BuOK to afford an allenyl ether 12 which was directly
thermolyzed in a sealed tube at 200 ЊC. The Diels–Alder reac-
tion followed by elimination of dinitrogen occurred, albeit in
low yield.
6
At this point we had passed the most crucial stage of our
J. Chem. Soc., Perkin Trans. 1, 1999, 2479–2482
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Scheme 1 Reagents and conditions: (i) N H ؒH O, EtOH (AIR), ∆; (ii) 3-ClC H CO H, CH Cl , 0 ЊC; (iii) Ac O, ∆; (iv) NaOH, EtOH, H O, ∆;
2
4
2
6
4
3
2
2
2
2
(
2
v) HC᎐CCH Br, 50% NaOH–CH Cl , n-Bu NBr; (vi) t-BuOK, t-BuOH, ∆; (vii) PCC, THF, ∆; (viii) Me SiCl, NaI, MeCN, ∆; (ix) Cu, quinoline,
20 ЊC.
2
2
2
4
3
8
synthesis, and were ready to process the dihydroisobenzofuran
3 by oxidation. This desymmetrization operation was carried
cyclohexadienol. Unfortunately, the photoinduced cyclization
was not realized.
1
out with PCC in 1,2-dichloroethane at 80–90 ЊC. An equimolar
mixture of phthalides 14 and 15 was obtained. We were rather
disappointed at not having been able to separate the lactone
mixture but even in capillary GC they elute very closely.
Our previous efforts in demonstrating the power of sym-
9
metry considerations in facilitating synthesis design has
10
resulted in an elaboration of α-cuparenone. The present work
illustrates a new approach to both the cuparane and herbertane
series while taking advantage of local symmetry of an inter-
Accordingly, these lactones were submitted to a reductive ring
7
11
cleavage using Me SiCl–NaI in refluxing acetonitrile. The two
mediate. Subesquent to our preliminary report, a paper
3
different carboxylic acids 16 and 17 thus produced were now
separable by recrystallization in hexane. Our use of the pro-
cedure was based on a conjecture that the benzylic iodides
would undergo reduction under the reaction conditions.
The final step was decarboxylation. Thus, in heating with
copper powder in quinoline each acid provided the correspond-
ing hydrocarbon in reasonable yield. The products were identi-
fied as cuparene and herbertene.
describing the creation of the aromatic ring of herbertene in
emulation of our Diels–Alder method has appeared. Their
modification, however, could not reach cuparene.
12
Experimental
IR spectra were measured on a JASCO FT/IR-200 spectro-
1
13
photometer. H- and C-NMR spectra were recorded in
As a corollary, we should mention an assay for a short
approach to cuparene using the enedione derived from
rearrangement of β-ionone monoepoxide. This route involves a
Wittig reaction with ethylidenetriphenylphoshorane to give the
dienone and a subsequent electrocyclization to afford a
CDCl on a Varian UNITY-300 instrument at 300 and 75 MHz
3
respectively; chemical shifts are reported in parts per million
downfield from TMS. The EIMS were obtained from a TRIO-
2000 mass spectrometer. Melting points were uncorrected.
Merck Kieselgel 60 of 70–230 mesh was used for column
2
480
J. Chem. Soc., Perkin Trans. 1, 1999, 2479–2482

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chromatography. During workup of reactions, organic solu-
[6-(1,2,2-Trimethylcyclopentan-1-yl)pyridazin-3-yl]methyl prop-
tions were dried over anhydrous Na SO4 and evaporated
2-ynyl ether (11)
2
in vacuo using a Buchi rotary evaporator.
A mixture of alcohol 9 (0.58 g, 2.6 mmol), aqueous 50% NaOH
(
1 mL), prop-2-ynyl bromide (0.74 g, 7.9 mmol), and a catalytic
3
-Methyl-6-(1,2,2-trimethylcyclopentan-1-yl)pyridazine (4)
amount of tetrabutylammonium bromide in dichloromethane
(8 mL) was stirred at room temperature for 10 h. Water was
added and the mixture was extracted with CH Cl . The organic
A solution of hydrazine hydrate (0.6 g, 18.7 mmol) in 95%
ethanol (20 mL) was mixed with diketone 3 (2.0 g, 9.4 mmol)
while being stirred magnetically. The mixture was refluxed for
2
2
layer was washed with brine, dried over anhydrous Na SO and
2
4
3
1 h, cooled to room temperature, and the solvent was removed
concentrated in vacuo. Silica gel chromatography using EtOAc–
n-hexane (1:10) as eluent furnished the prop-2-ynyl ether 11
(0.51 g, 75%). IR (film) 3307, 2960, 2873, 1438, 1464, 1368,
in vacuo. The crude product was chromatographed over silica
gel using EtOAc–n-hexane (1:8) as eluent to give pyridazine 4
Ϫ1
1
Ϫ1 1
(
1.6 g, 84%). IR (film): 2873, 2858, 1460, 1424 cm ; H-NMR
1111 cm ; H-NMR δ 7.50 (d, 1H, J = 9 Hz), 7.46 (d, 1H, J = 9
Hz), 4.91 (s, 2H), 4.28 (d, 2H, J = 2.4 Hz), 2.90–2.70 (m, 1H),
2.47 (d, 1H, J = 2.4 Hz), 1.90–1.50 (m, 5H), 1.36 (s, 3H), 1.14 (s,
δ 7.27 (d, 1H, J = 9 Hz), 7.14 (d, 1H, J = 9 Hz), 2.80–2.60 (m,
1
0
5
2
H), 2.58 (s, 3H), 1.82–1.40 (m 5H), 1.27 (s, 3H), 1.06 (s, 3H),
13
13
.49 (s, 3H); C-NMR δ 165.5 (s), 157.0 (s), 125.8 (d), 124.6 (d),
3H), 0.55 (s, 3H); C-NMR δ 167.5 (s), 157.2 (s), 125.2 (d),
1.8 (s), 44.7 (s), 39.8 (t), 35.9 (t), 25.8 (q), 24.2 (q), 23.4 (q),
124.6 (d), 79.0 (d), 75.1 (s), 70.8 (t), 58.2 (t), 52.2 (s), 45.0 (s),
ϩ
1.6 (q), 19.7 (t); MS m/z 204 (M , 18), 187 (17), 163 (11);
39.9 (t), 36.1 (t), 26.0 (q), 24.4 (q), 23.5 (q), 19.8 (t); MS m/z 258
ϩ
HRMS (EI) 204.1617 (204.1628 Calcd for C H N ).
(M , 13), 190 (14), 189 (100); HRMS (EI) 258.1718 (258.1733
13
20
2
Calcd for C H ON ).
16
22
2
3
-Methyl-6-(1,2,2-trimethylcyclopentan-1-yl)pyridazine N-oxide
(
5)
5-(1,2,2-Trimethylcyclopentan-1-yl)-1,3-dihydroisobenzofuran
13)
(
To an ice-cooled solution of pyridazine 4 (1.3 g, 6.4 mmol) in
CH Cl (20 mL) at 0 ЊC was added MCPBA (1.5 g, 8.8 mmol).
After removal of the ice bath, the mixture was stirred for 4 h,
and filtered. The filtrate was washed with saturated NaHCO3
and water, dried over Na SO and concentrated in vacuo. The
A solution of ether 11 (0.51 g, 2 mmol) and potassium tert-
butoxide (1.0 g, 8.9 mmol) in tert-butanol (2-methylpropan-2-
ol) (10 mL) was refluxed for 5 h. After cooling to room temper-
ature, the solvent was removed in vacuo. The crude product was
2
2
2
4
crude product was chromatographed over silica gel using
diluted with ether, washed with NH Cl, dried over anhydrous
4
CH Cl as eluent to give 5 (1.18 g, 83.2%). IR (film): 2964, 2870,
Na SO , and concentrated in vacuo to give the crude allenyl
2
2
2
4
Ϫ1
1
1
560, 1446, 1370 cm ; H-NMR δ 7.45 (d, 1H, J = 8.4 Hz), 7.01
ether 12 (0.5 g). This product and a catalytic amount of hydro-
quinone (0.01 g) in benzene (6 mL) was heated to 200 ЊC for
20 h. The cooled mixture was diluted with ether and acidified
with 10% HCl. After stirring for 1 h at room temperature, the
organic layer was separated and dried over Na SO . Concen-
(
(
d, 1H, J = 8.4 Hz), 2.70–2.60 (m, 1H), 2.48 (s, 3H), 1.85–1.45
m, 5H), 1.29 (s, 3H), 1.11 (s, 3H), 0.63 (s, 3H); C-NMR
13
δ 166.7 (s), 140.4 (s), 133.1 (d), 115.5 (d), 51.9 (s), 44.7 (s), 39.7
t), 35.5 (t), 26.0 (q), 24.0 (q), 23.2 (q), 19.3 (t), 17.7 (q); MS m/z
(
2
2
4
ϩ
20 (M , 8), 204 (15), 203 (100), 151 (87), 135 (15).
tration in vacuo and chromatography furnished 13 (0.1 g,
Ϫ1 1
2
2.4%). IR (film) 2970, 1693, 1457, 1380, 1047 cm ; H-NMR
3
-Acetoxymethyl-6-(1,2,2-trimethylcyclopentan-1-yl)pyridazine
δ 7.27 (d, 1H, J = 8 Hz), 7.21 (s, 1H), 7.13 (d, 1H, J = 8 Hz), 5.09
(
7)
(s, 4H), 2.60–2.40 (m, 1H), 1.80–1.40 (m, 5H), 1.27 (s, 3H), 1.07
13
(
s, 3H), 0.56 (s, 3H); C-NMR δ 147.0 (s), 138.5 (s), 136.0 (s),
A solution of the N-oxide 6 (1.38 g, 6.3 mmol) in Ac O (6 mL)
was refluxed for 12 h, cooled to room temperature, and evapor-
ated to dryness. The residue was redissolved in CH Cl ,
washed with aqueous NaHCO , and dried over Na SO . After
removal of the solvent in vacuo and chromatography (silica gel,
n-hexane–EtOAc 8:1) the pyridazine (7) was obtained (1.1 g,
4
2
1
3
1
26.1 (d), 119.8 (d), 119.3 (d), 73.6 (t), 73.4 (t), 50.6 (s), 44.1 (s),
9.6 (t), 37.0 (t), 26.4 (q), 24.6 (q), 19.6 (t); MS m/z 230 (M ,
8), 229 (100), 97 (11), 71 (16); HRMS (EI) 230.1626 (230.1671
ϩ
2
2
3
2
4
Calcd for C H O).
16
22
6
1
-(1,2,2-Trimethylcyclopentan-1-yl)-1,3-dihydroisobenzofuran-
-one (14) and 5-(1,2,2-trimethylcyclopentan-1-yl)-1,3-
Ϫ1
1
6.2%). IR (film): 2980, 1710 cm ; H-NMR δ 7.45 (d, 1H,
J = 13.8 Hz), 7.42 (d, 1H, J = 13.8 Hz), 5.38 (s, 2H), 2.90–2.70
dihydroisobenzofuran-1-one (15)
(
m, 1H), 2.13 (s, 3H), 1.90–1.40 (m, 5H), 1.34 (s, 3H), 1.13 (s,
13
3
1
2
6
H), 0.54 (s, 3H); C-NMR δ 170.6 (s), 167.8 (s), 155.4 (s),
A mixture of the dihydroisobenzofuran 13 (1.27 g, 5.5 mmol),
PPC (2.38 g, 16.2 mmol) in dry THF (10 mL) was heated at 60–
70 ЊC for 20 h. After cooling to room temperature, the mixture
was filtered, and the filtrate was concentrated. The crude prod-
uct was chromatographed over silica gel using EtOAc–n-hexane
25.1 (d), 124.7 (d), 65.2 (t), 52.3 (s), 45.0 (s), 39.9 (t), 36.1 (t),
6.0 (q), 24.3 (q), 23.5 (q), 20.8 (q), 19.8 (t); MS m/z 262 (M ,
ϩ
), 247 (21), 193 (100); HRMS (EI) 262.1689 (262.1682 Calcd
for C H N O ).
15
22
2
2
(
1:1) as eluent to give a mixture of lactones 14 and 15 (1.1 g,
Ϫ1 1
3
-Hydroxymethyl-6-(1,2,2-trimethylcyclopentan-1-yl)pyridazine
81.8%). IR (film) 2970, 2877, 1772, 1616, 1458 cm ; H-NMR
δ 7.90–7.30 (m, 6H), 5.28 (s, 2H), 2.60–2.40 (m, 2H), 1.95–1.40
(
8)
(
m, 10H), 1.31 (s, 6H), 1.11 (s, 3H), 1.09 (s, 3H), 0.56 (s, 3H),
To aqueous NaOH (0.23 g, 2.9 mmol in 5 mL H O) was added a
2
13
0
1
1
.54 (s, 3H); C-NMR δ 171.7 (s), 171.2 (s), 155.3 (s), 149.4 (s),
46.4 (s), 143.7 (s), 133.2 (d), 128.2 (d), 125.2 (s), 124.5 (d),
23.8 (d), 123.0 (s), 121.0 (d), 120.2 (d), 69.7 (t), 69.5 (t), 51.4
solution of pyridazine 7 (0.76 g, 2.9 mmol) in ethanol (0.45
mL). The mixture was refluxed for 4 h, ethanol was evaporated,
water was added, and the mixture was extracted with ether,
which was dried over Na SO4 and concentrated in vacuo.
The crude product was chromatographed over silica gel using
(
s), 50.9 (s), 44.5 (s), 44.3 (s), 39.6 (t), 39.5 (t), 37.0 (t), 26.3 (q),
2
ϩ
2
(
2
4.5 (q), 24.2 (q), 24.1 (q), 19.6 (t); MS m/z 244 (M , 25), 175
25), 174 (98), 162 (72), 145 (100), 143 (27); HRMS (EI)
44.1459 (244.1464 Calcd for C H O ).
16 20 2
n-hexane–EtOAc (1:1) as eluent to afford 8 (0.58 g, 92%). IR
Ϫ1
1
(
(
(
film) 3350, 2964, 2888, 1464, 1369, 1083 cm ; H-NMR δ 7.48
d, 1H, J = 9 Hz), 7.42 (d, 1H, J = 9 Hz), 4.91 (s, 2H), 2.80–2.60
m, 1H), 1.85–1.40 (m, 5H), 1.28 (s, 3H), 1.07 (s, 3H), 0.48 (s,
2
-Methyl-5-(1,2,2-trimethylcyclopentan-1-yl)benzoic acid (16)
and 2-methyl-4-(1,2,2-trimethylcyclopentan-1-yl)benzoic acid
17)
13
3
(
1
H); C-NMR δ 167.3 (s), 159.1 (s), 125.6 (d), 124.2 (d), 63.2
t), 52.1 (s), 44.8 (s), 39.8 (t), 35.9 (t), 25.9 (q), 24.2 (q), 23.4 (q),
9.7 (t); MS m/z 220 (M , 25), 189 (34), 151 (100), 124 (16);
(
ϩ
To a stirred mixture of 14–15 (0.24 g, 1 mmol) and sodium
HRMS (EI) 220.1573 (220.1577 Calcd for C H ON ).
iodide (3 g, 20 mmol) in dry acetonitrile (10 mL) was added
13
20
2
J. Chem. Soc., Perkin Trans. 1, 1999, 2479–2482
2481

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chlorotrimethylsilane (2.16 g, 20 mmol) slowly. The reaction
mixture was refluxed for 4 days. The cooled reaction mixture
was taken up in ether, washed with water, sodium thiosulfate
solution (10%), and brine. The ether solution was dried, evap-
orated, and the residue (0.19 g, 78.4%) was recrystallized from
n-hexane to separate the less soluble 16 from the more soluble
Herbertene (2)
A mixture of acid 17 (80.5 mg, 0.3 mmol), Cu powder (57 mg,
.9 mmol) and quinoline (8 mL) was manipulated in the same
0
manner as above. The crude product was chromatographed
over silica gel using n-hexane as eluent to give herbertene (2)
1
(
51.4 mg. 73.7%). H-NMR 7.20–7.10 (m, 3H), 7.00–6.90 (m,
1
7.
Compound 17 mp: 190–192 ЊC; IR (film) 3340, 2970, 2870,
672, 1458 cm ; H-NMR δ 7.91 (d, 2H, J = 7.8 Hz), 7.30–7.10
1
3
1
H), 2.60–2.40 (m, 1H), 2.34 (s, 3H), 1.80–1.40 (m, 5H), 1.25 (s,
H), 1.05 (s, 3H), 0.55 (s, 3H); C-NMR δ 147.5 (s), 136.6 (s),
27.8 (d), 127.4 (d), 126.1 (d), 124.1 (d), 50.5 (s), 44.4 (s), 39.9
13
Ϫ1
1
1
(
1
m, 2H), 2.59 (s, 3H), 2.50–2.40 (m, 1H), 1.80–1.40 (m, 5H),
.21 (s, 3H), 1.01 (s, 3H), 0.50 (s, 3H); C-NMR δ 171.2 (s),
(
t), 36.9 (t), 26.6 (q), 24.5 (q), 24.4 (q), 21.9 (q), 19.8 (t).
13
1
53.4 (s), 140.5 (s), 131.0 (d), 130.7 (d), 125.3 (d), 124.7 (s), 50.8
(
s), 44.5 (s), 39.7 (t), 26.7 (t), 26.4 (q), 24.3 (q), 24.2 (q), 22.6 (q),
Acknowledgements
ϩ
1
9.7 (t); MS m/z 246 (M , 73), 245 (100), 176 (90), 145 (39), 115
We thank the National Science Council, ROC, for financial
support. Identification of cuparene and herbertene by com-
parison was carried out with the help of Professor C.-L. Wu of
Tamkang University and is also gratefully acknowledged.
(
19); HRMS (EI) 246.1630 (246.1620 Calcd for C H O ).
16
22
2
Compound 16 mp: 156–158 ЊC; IR (film) 3340, 2970, 2870,
Ϫ1
1
1
1
2
(
1
2
672, 1458 cm ; H-NMR δ 8.06 (d, 1H, J = 2.7 Hz), 7.43 (dd,
H, J = 2.7, 8.4 Hz), 7.17 (d, 1H, J = 8.4 Hz), 2.64 (s, 3H), 2.50–
.40 (m, 1H), 1.80–1.40 (m, 5H), 1.27 (s, 3H), 1.01 (s, 3H), 0.55
13
s, 3H); C-NMR δ 173.4 (s), 145.3 (s), 138.2 (s), 131.7 (d),
References
31.2 (d), 130.1 (d), 127.3 (s), 50.3 (s), 44.3 (s), 39.7 (t), 26.8 (t),
1
2
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ϩ
6.3 (q), 24.2 (q), 21.6 (q), 19.7 (t); MS m/z 246 (M , 38), 245
(
(
100), 176 (95), 145 (60), 115 (21); HRMS (EI) 246.1615
246.1620 Calcd for C H O ).
3 T.-L. Ho, Evolution of Synthetic Pathways. Parallax and Calibration,
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16
22
2
4
B. Frei, H. Eichenberger, B. von Wartburg, H. R. Wolf and O. Jeger,
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5
6
W. Skorianetz and G. Ohloff, Helv. Chim. Acta, 1974, 57, 2439.
S. Nagashima and K. Kanematsu, Yuki Gosei Kagaku Kyokaishi,
A mixture of 16 (50 mg, 0.2 mmol), Cu powder (38 mg, 0.6
mmol) and quinoline (6 mL) was stirred at 220 ЊC for 6 h. On
cooling to room temperature, the mixture was filtered and the
filtrate was washed with ice-cold HCl (8 mL, 20 g ice), and the
organic layer was dried over Na SO and concentrated in vacuo.
(
J. Synth. Org. Chem. Jpn.), 1993, 51, 608.
7
G. A. Olah, S. C. Narang, B. G. B. Gupta and R. Malhotra,
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2
4
8 V. Ramamurthy and R. S. H. Liu, Tetrahedron Lett., 1973, 441.
9 T.-L. Ho, Symmetry. A Basis for Synthesis Design, Wiley, New York,
The crude product was chromatographed over silica gel using
1
995.
n-hexane as eluent to give cuparene (1) (29.5 mg, 71.9%).
1
10 T.-L. Ho and M.-H. Chang, Can. J. Chem., 1997, 75, 621.
H-NMR δ 7.22 (d, 2H, J = 8.1 Hz), 7.06 (d, 2H, J = 8.1 Hz),
1
1
1 T.-L. Ho and M.-H. Chang, Tetrahedron Lett., 1994, 27, 4819.
2 M. Tori, T. Miyake, T. Hamaguchi, M. Sono, Tetrahedron:
Asymmetry, 1997, 8, 2731.
2
1
.60–2.40 (m, 1H), 2.34 (s, 3H), 1.80–1.40 (m, 5H), 1.23 (s, 3H),
.07 (s, 3H), 0.57 (s, 3H); C-NMR δ 144.5 (s), 134.6 (s), 128.2
13
(
(
d), 126.9 (d), 50.3 (s), 44.3 (s), 39.8 (t), 36.8 (t), 27.0 (q), 24.4
q), 20.9 (q), 19.8 (q).
Paper 9/04068D
2
482
J. Chem. Soc., Perkin Trans. 1, 1999, 2479–2482