An efficient and practical total synthesis of (±)-α-cuparenone

Article abstract of DOI:10.1055/s-2007-990899

A one-pot cyclopentannulation approach as the key step for the total synthesis of (±)-α-cuparenone is described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-990899

PAPER
3827
An Efficient and Practical Total Synthesis of ( )-a-Cuparenone
Total
S
ynthesi
u
s
of
(
)-
a
-Cu
b
parenone hash P. Chavan,* Abasaheb N. Dhawane, Uttam R. Kalkote
Division of Organic Chemistry: Technology, National Chemical Laboratory, Pune 411 008, India
E-mail: sp.chavan@ncl.res.in
Received 14 August 2007; revised 11 September 2007
As a part of our ongoing interest towards the synthesis of
naturally occurring cyclopentane rings,8 we undertook the
synthesis of a-cuparenone (1). We have recently accom-
plished a short synthesis of a-cuparenone (1) employing
an acetophenone gem-diallylation strategy in which the
carbon-bearing aryl group served as an electrophile. Here-
in we wish to report a complimentary strategy leading to
a very simple and short synthesis of a-cuparenone (1), in
which the carbon-bearing aryl group acts as a nucleophile,
and which involves simple dialkylation to construct a cy-
clopentene ring as the key step. As is obvious from the ret-
rosynthesis (Scheme 1), our strategy was to prepare key
intermediate 5 by dialkylation and reduction of 4-methyl-
benzyl cyanide (7).
Abstract: A one-pot cyclopentannulation approach as the key step
for the total synthesis of ( )-a-cuparenone is described.
Key words: alkylation, annulation, reduction, hydroboration–
oxidation
a-Cuparenone (1), b-cuparenone (2), and cuparene (3)
(Figure 1) are bicyclic sesquiterpenes belonging to the cu-
parene family. Compounds 1 and 2 were isolated from
Thuja orientalis (Mayurpankhi) by Sukhdev and co-
workers in 19641 and cuparene (2) was isolated by Erdt-
man and co-workers in 1958.2 These compounds pose a
synthetic challenge to synthetic organic chemists, because
of the presence of two contiguous quaternary centers in a
cyclopentane ring, which has been constructed in a variety
of ways.3
O
O
O
O
1
4
5
1
2
3
CN
CN
Figure 1
6
7
Many studies towards the synthesis of a-cuparenone (1)4
and b-cuparenone (2),5 either in racemic or optically ac-
tive form, have been reported. The reported syntheses in-
volve either construction of the quaternary center starting
from a framework that contains the aromatic ring, or addi-
tion of the p-tolyl moiety to a cyclopentane ring deriva-
tive.6 Other methods build up the aromatic ring starting
from a cyclopentane ring.7 Other ways have also been
used for the synthesis, for example cyclopentannulation of
an open-chain intermediate that has one of the quaternary
centers.5
Scheme 1
The synthetic sequence as depicted in Scheme 2 shows
the synthesis starting at the dialkylation of commercially
available 4-methylbenzyl cyanide (7) by use of sodium
hydride and cis-1,4-dichlorobut-2-ene in tetrahydrofuran
to furnish cyclopentene derivative 6 along with side prod-
uct 8 in a 2:1 ratio as an inseparable mixture. We went
ahead with this mixture, in the hope that it would separate
at the hydroboration–oxidation step. Thus, the mixture of
6 and 8 was subjected to reduction with diisobutylalumi-
num hydride to furnish aldehydes 9 and 10, which were
further reduced under Huang-Minlon9 reaction conditions
without further purification to furnish the required olefin
intermediate 5 containing one of the quaternary centers
(Scheme 2). Formation of a single product after Huang-
Minlon reduction was initially surprising, but we thought
that the cyclopropane product may have undergone a
vinyl–cyclopropane rearrangement.10
Most of the reported synthesis have drawbacks such as
lengthy routes, expensive starting materials, tedious reac-
tion conditions, or low overall yield. To overcome these
problems, it is still necessary to develop a simple and ef-
ficient synthesis of a-cuparenone (1).
SYNTHESIS 2007, No. 24, pp 3827–3830
x
x.
x
x
.
2
0
0
7
To study the fate of the vinyl–cyclopropane rearrange-
ment, we prepared compound 8 (Scheme 3) in pure form
Advanced online publication: 15.11.2007
DOI: 10.1055/s-2007-990899; Art ID: Z18507SS
© Georg Thieme Verlag Stuttgart · New York

3828
S. P. Chavan et al.
PAPER
H
of the cyclopropane ring, and that only aldehyde 9 was
converted into olefin intermediate 5 in 60% isolated yield
(Scheme 2). Since direct formation of the cyclopentene
functionality led to the mixture of isomers, an alternative
approach was attempted to overcome this problem. Ac-
cordingly, 7 was dialkylated by use of allyl bromide and
sodium hydride in tetrahydrofuran (Scheme 4); subse-
quent ring-closing metathesis of diallyl 11 in the presence
of the Grubbs first-generation catalyst afforded the re-
quired compound 6 (Scheme 4).
i
N
+
N
N
7
6
8
H
ii
iii
CHO
CHO
+
9
10
O
O
v
ii
iv
i
N
N
N
7
11
6
5
4
1
O
vi
Scheme 4 Reagents and conditions: (i) NaH, AllBr, THF, r.t., 86%;
(ii) Grubbs first-generation catalyst, CH2Cl2, 3 h, 92%.
vii
Olefin 5 was subjected to a sequence of hydroboration11
and oxidation12 by use of the borane–dimethyl sulfide
complex, hydrogen peroxide/sodium hydroxide, and 2-io-
doxybenzoic acid (IBX); this gave cyclopentanone 4 in al-
most 85% yield (Scheme 2). Finally, ketone 4 on selective
dialkylation in the presence of freshly prepared lithium
hexamethyldisilazide and methyl iodide13 furnished a-cu-
parenone (1) (Scheme 2).14
12
Scheme 2 Reagents and conditions: (i) NaH, cis-1,4-dichlorobut-
2-ene, THF, r.t., 8 h, 80%; (ii) DIBAL-H, CH2Cl2, –78 °C, 1 h; (iii)
H2NNH2·H2O, NaOH, diethylene glycol, 180 °C, 8 h, 60%; (iv)
BH3·SMe2, THF, H2O2, 3 N NaOH, IBX, DMSO, 12 h, 85%; (v)
LiHMDS, MeI, DME, HMPA, 3 h, 70%; (vi) PDC, DMF, 100 °C, 8
h; (vii) ref. 8e.
Alternatively, 5 was readily converted into a-cuparenone
(1) via enone 12, furnished by allylic oxidation of 5 with
pyridinium dichromate in N,N-dimethylformamide at
100 °C (Scheme 2). Enone 12, on bis-methylation with
sodium hydride and methyl iodide, followed by the reduc-
tion of the double bond by palladium on carbon, afforded
a-cuparenone (1) (Scheme 2).8e
by alkylation of benzyl cyanide 7 with trans-1,4-dibro-
mobut-2-ene in the presence of sodium hydride in tetrahy-
drofuran in 85% yield. With pure compound 8 in hand, we
continued its investigation. Reduction of compound 8
with diisobutylaluminum hydride in dichloromethane at
–78 °C furnished aldehyde 10 (Scheme 3), which was fur-
ther subjected to Huang-Minlon9 conditions. Unfortunate-
ly, the required olefin intermediate 5 was not observed,
even after two to three trials (Scheme 3).
In conclusion, a very efficient and practical synthesis of a-
cuparenone (1) has been accomplished, in which all the
reaction sequences are easy to perform, are essentially
mild, and proceed in excellent yields. One of the shortest
and most efficient syntheses of a-cuparenone (1) has been
achieved in five steps in 28% overall yield.
It was therefore concluded that when a mixture of com-
pounds 9 and 10 was subjected to Huang-Minlon condi-
tions, compound 10 decomposed as a result of the strain
All solvents were freshly distilled before use. IR spectra were re-
corded on a Perkin-Elmer 68B or a Perkin-Elmer 1615 FT infrared
spectrophotometer. 1H (200 MHz) and 13C (50 MHz) NMR spectra
were recorded on a Bruker AC-200 spectrometer. The carbon reso-
nances were assigned by use of DEPT experiments. Mass spectra
were recorded at an ionization energy of 70 eV on a Finnigan MAT-
1020 spectrometer and on an API Q STARPULSAR spectrometer
using electrospray ionization. Microanalytical data were obtained
on a Carlo-Erba CHNS-O EA 1108 elemental analyzer. Progress of
the reactions was monitored by TLC on Merck silica gel 60 F254 pre-
coated plates, and compounds were visualized by fluorescence
quenching, by use of I2, or by charring after treatment with a p-anis-
aldehyde–AcOH–H2SO4 mixture in EtOH. Column chromatogra-
phy was performed on flash silica gel (230–400 mesh size).
H
ii
i
N
N
7
8
H
iii
CHO
10
5
Scheme 3 Reagents and conditions: (i) NaH, trans-1,4-dibromo-
but-2-ene, THF, r.t., 8 h, 85%; (ii) DIBAL-H, CH2Cl2, –78 °C, 1 h;
(iii) H2NNH2·H2O, NaOH, diethylene glycol, 180 °C.
1-p-Tolylcyclopent-3-ene-1-carbonitrile (6)
By Method A (Scheme 2): 4-Methylbenzyl cyanide (7; 5 g, 38
mmol) in anhyd THF (10 mL) was added to a 60% soln of NaH
Synthesis 2007, No. 24, 3827–3830 © Thieme Stuttgart · New York

PAPER
Total Synthesis of ( )-a-Cuparenone
3829
[3.053 g, 76 mmol; washed with anhyd PE (2–3 ×)] in anhyd THF
(15 mL) at 0 °C, and the mixture was stirred for 30 min. Dropwise
addition of cis-1,4-dichlorobut-2-ene (4.17 g, 38 mmol) in anhyd
THF (10 mL) over 20 min followed, and the mixture was stirred at
r.t. for 8 h. On completion of the reaction, the mixture was quenched
by the addition of sat. aq NH4Cl (15 mL), extracted with EtOAc (3
× 40 mL), and washed with brine (2 × 20 mL). The combined organ-
ic layers were dried (Na2SO4), filtered, and concentrated under re-
duced pressure, and the residue was purified by flash column chro-
matography (silica gel, EtOAc–PE, 3:97); this afforded a mixture of
compounds 6 and 8 (2:1) as a yellow oil.
forded aldehyde 9 and 10 as an inseparable mixture, which was used
without further purification.
To a stirred soln of the crude mixture of aldehyde 9 and 10 (4.1 g,
22 mmol) in diethylene glycol (20 mL) was added hydrazine mono-
hydrate (3.85 mL, 88 mmol) and NaOH (3.52 g, 88 mmol). The
mixture was heated to reflux for 8 h, and after completion of the re-
action it was diluted with H2O (10 mL) and extracted with EtOAc
(3 × 25 mL). The combined organic layers were then washed with
H2O (3 × 20 mL) and brine (1 × 15 mL), dried (Na2SO4), filtered,
and concentrated under reduced pressure. The residue was purified
by flash column chromatography (silica gel, EtOAc–PE, 2:98); this
afforded 5.
Yield: 5.52 g (80%).
By Method B (Scheme 4): The Grubbs first-generation catalyst
(194 mg, 10 mol%) was added to a degassed homogeneous soln of
11 (synthesis described further below; 500 mg, 2.36 mmol) in an-
hyd CH2Cl2 (50 mL) under argon. The mixture was stirred at r.t. for
4 h. On completion of the reaction, the solvent was removed under
vacuum and the residue was purified by flash column chromatogra-
phy (silica gel, EtOAc–PE, 2:98); this gave 6 as a thick, colorless
oil.
Yield: 2.2 g (60%).
IR (neat): 816, 1215, 1650, 2926, 3020 cm–1.
1H NMR (200 MHz, CDCl3): d = 1.38 (s, 3 H), 2.30 (s, 3 H), 2.45
(d, J = 14.2 Hz, 2 H), 2.73, (d, J = 14.2 Hz, 2 H), 5.71 (s, 2 H), 7.11
(d, J = 8.34 Hz, 2 H), 7.20 (d, J = 8.34 Hz, 2 H).
13C NMR (50 MHz, CDCl3): d = 20.8, 31.3, 45.7, 47.7, 125.8, 128.8,
129.3, 134.6, 148.6.
MS (EI, 70 eV): m/z = 173.27 [M+ + 1].
Yield: 390 mg (92%).
IR (neat): 817, 1514, 1640, 2238, 3029 cm–1.
Anal. Calcd for C13H16: C, 90.64; H, 9.36. Found: C, 90.75; H, 9.48
1H NMR (200 MHz, CDCl3): d = 2.39 (s, 3 H), 2.98 (d, J = 15.0 Hz,
2 H), 3.33 (d, J = 15.0 Hz, 2 H), 5.86 (s, 2 H), 7.22 (d, J = 8.3 Hz, 2
H), 7.39 (d, J = 8.3 Hz, 2 H).
13C NMR (50 MHz, CDCl3): d = 20.8, 44.5, 48.1, 124.6, 125.4,
128.3, 129.4, 134.2, 138.3.
3-Methyl-3-p-tolylcyclopentanone (4)
BH3·SMe2 (0.331 mL, 3.4 mmol) was added to a soln of olefin 5
(300 mg, 1.7 mmol) in anhyd THF at 0 °C. The mixture was al-
lowed to warm to r.t. and stirred for 4 h. It was then cooled to 0 °C,
and treated with aq NaOH (2.8 mL, 8.7 mmol) and 30% H2O2 (0.98
mL, 8.7 mmol). The mixture was allowed to warm to r.t. After 3 h,
the volatile materials were removed under reduced pressure and the
residue was dissolved in EtOAc (15 mL) and washed with H2O (3 ×
10 mL) and brine (1 × 10 mL). The organic layer was dried
(Na2SO4), filtered, and concentrated under reduced pressure; this af-
forded an alcohol, which was used without further purification.
1-p-Tolyl-2-vinylcyclopropane-1-carbonitrile (8)
4-Methylbenzyl cyanide (7; 5 g, 38 mmol) in anhyd THF (25 mL)
was added to a 60% soln of NaH [3.6 g, 76 mmol; washed with an-
hyd PE (2–3 ×)] in anhyd THF (10 mL) at 0 °C, and the mixture was
stirred for 30 min. Then trans-1,4-dibromobut-2-ene (8.16 g, 38
mmol) in anhyd THF (10 mL) was added dropwise over 20 min, and
the mixture was stirred at r.t. for 8 h. On completion of the reaction,
the mixture was quenched by the addition of sat. aq NH4Cl (15 mL),
extracted with EtOAc (3 × 40 mL), and washed with brine (2 × 20
mL). The combined organic layers were dried (Na2SO4), filtered,
and concentrated under reduced pressure, and the residue was puri-
fied by flash column chromatography (silica gel, EtOAc–PE, 3:97);
this afforded compound 8 as a yellow oil.
To a stirred soln of this alcohol in anhyd DMSO (10 mL) was added
IBX (732 mg, 2.6 mmol), and the mixture was stirred at r.t. for 4 h.
After completion of the reaction, the mixture was diluted with H2O
(15 mL) and extracted with Et2O (3 × 20 mL). The combined organ-
ic layers were then washed with H2O (3 × 10 mL) and brine (1 × 15
mL), dried (Na2SO4), filtered, and concentrated under reduced pres-
sure. The residue was purified by flash column chromatography
(silica gel, EtOAc–PE, 5:95); this afforded 4.
Yield: 5.8 g (85%).
IR (neat): 817, 1216, 1514, 1640, 2237, 3019 cm–1.
1H NMR (200 MHz, CDCl3): d = 1.69–1.83 (m, 2 H), 2.09–2.21 (m,
1 H), 2.39 (s, 3 H), 5.35–5.41 (m, 2 H), 5.71–5.88 (m, 1 H), 7.09–
7.25, (m, 4 H).
13C NMR (50 MHz, CDCl3): d = 20.9, 21.4, 22.9, 33.5, 118.4, 120.1,
125.4, 129.5, 132.6, 134.3, 137.3.
MS (EI, 70 eV): m/z = 207.07 [M+ + Na].
Yield: 0.276 g (85%); white solid; mp 58–59 °C.
IR (CHCl3): 815, 1246, 1246, 1614, 1720, 3019 cm–1.
1H NMR (200 MHz, CDCl3): d = 1.39 (s, 3 H), 2.21–2.29 (m, 2 H),
2.35 (s, 3 H), 2.25–2.37 (m, 2 H), 2.46 (d, J = 17.7 Hz, 1 H), 2.65
(d, J = 17.7 Hz, 1 H), 7.22–7.32 (m, 4 H).
13C NMR (50 MHz, CDCl3): d = 20.9, 29.5, 35.9, 36.7, 43.5, 52.8,
96.2, 125.5, 129.2, 135.7, 145.4.
MS (EI, 70 eV): m/z = 189.27 [M+ + 1].
Anal. Calcd for C13H13N: C, 85.21; H, 7.15; N, 7.64. Found: C,
84.99; H, 7.05; N, 7.32.
2,2,3-Trimethyl-3-p-tolylcyclopentanone (1)
LiHMDS (371 mg, 2.2 mmol) and HMPA (cat.) were added to a
stirred soln of 4 (200 mg, 1.0 mmol) in anhyd DME, and the mixture
was stirred for a few min before MeI (0.325 mL, 5.0 mmol) in anhyd
DME was added dropwise. The mixture was stirred for 3 h,
quenched with sat. aq NH4Cl, and extracted with EtOAc (3 × 50
mL). The extracts were washed with brine (1 × 20 mL) and the com-
bined organic layers were dried (Na2SO4), filtered, and concentrated
under reduced pressure. The residue was purified by flash column
chromatography (silica gel, EtOAc–PE, 3:97).
1-Methyl-4-(1-methylcyclopent-3-enyl)benzene (5)
A mixture of 6 and 8 (4 g, 21 mmol) was taken up in anhyd CH2Cl2
under argon, and the temperature was lowered to –78 °C. A 2 M
soln of DIBAL-H in toluene (20 mL, 43.5 mmol) was added drop-
wise, and the mixture was stirred at the same temperature until com-
pletion of the reaction. It was then quenched at –78 °C by the
dropwise addition of 2 N HCl and subsequently warmed to r.t. The
organic layer was separated, and the aqueous layer was extracted
with CH2Cl2 (3 × 20 mL). The combined organic layers were dried
(Na2SO4), filtered, and concentrated under reduced pressure; this af-
Yield: 152 mg (70%); colorless oil.
IR (neat): 815, 1460, 1510, 1735, 2960 cm–1.
Synthesis 2007, No. 24, 3827–3830 © Thieme Stuttgart · New York

3830
S. P. Chavan et al.
PAPER
1H NMR (200 MHz, CDCl3): d = 0.6 (s, 3 H), 1.20 (s, 3 H), 1.30 (s,
3 H), 1.90 (m, 1 H), 2.30 (s, 3 H), 2.50 (m, 2 H), 2.60 (m, 1 H), 7.20–
7.30 (m, 4 H).
13C NMR (50 MHz, CDCl3): d = 18.4, 20.8, 22.1, 25.9, 29.7, 33.8,
48.3, 53.2, 126.4, 128.9, 135.8, 141.9, 222.7.
Ogasawara, K. Tetrahedron Lett. 2000, 41, 2639.
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MS (EI, 70 eV): m/z (%) = 216 (75) [M+].
2-Allyl-2-p-tolylpent-4-enenitrile (11)
4-Methylbenzyl cyanide (7; 1.0 g, 7.6 mmol) in anhyd THF (10 mL)
was added to a soln of 60% NaH [0.68 g, 16.7 mmol; washed with
anhyd PE (2–3 ×)] in anhyd THF (15 mL) at 0 °C. The mixture was
stirred for 30 min and then AllBr (1.49 mL, 16 mmol) in anhyd THF
(5 mL) was added dropwise over 5 min and the mixture was stirred
at r.t. for a further 8 h. On completion of the reaction, the mixture
was quenched by the addition of sat. aq NH4Cl soln (10 mL), ex-
tracted with EtOAc (3 × 20 mL), and washed with brine (1 × 20
mL). The combined organic layers were dried (Na2SO4), filtered,
and concentrated under reduced pressure, and the residue was puri-
fied by flash column chromatography (silica gel, EtOAc–PE, 3:97).
(5) For recent synthesis, see: Kulkarni, M. G.; Davawala, S. I.;
Shinde, M. P.; Dhondge, A. P.; Borhade, A. S.; Chavhan, S.
W.; Gaikawad, D. D. Tetrahedron Lett. 2006, 47, 3027; and
references cited therein.
(6) (a) Parker, W.; Ramage, R. A. J. Chem. Soc. 1962, 1558.
(b) Nakatani, H.; So, T.; Ishibashi, H.; Ikeda, M. Chem.
Pharm. Bull. 1990, 38, 1233.
(7) (a) Abad, A.; Agullo, C.; Arno, M.; Cunat, A. C.; Garcia, M.
T.; Zaragoza, R. J. J. Org. Chem. 1996, 61, 5916. (b) Abad,
A.; Agullo, C.; Cunat, A. C.; Perni, R. H. J. Org. Chem.
1999, 64, 1741.
(8) (a) Chavan, S. P.; Ethiraj, K. S. Tetrahedron Lett. 1995, 36,
2281. (b) Chavan, S. P.; Ravindranathan, T.; Patil, S. S.;
Dhondge, V.; Dantale, S. W. Tetrahedron Lett. 1996, 37,
2629. (c) Chavan, S. P.; Thakkar, M.; Kharul, R. K.; Pathak,
A. B.; Bhosekar, G. V.; Bhadbhade, M. M. Tetrahedron
2005, 61, 3873. (d) Chavan, S. P.; Kharul, R. K.; Kale, R.
R.; Khobragade, D. A. Tetrahedron 2003, 59, 2737.
(e) Chavan, S. P.; Dhawane, A. N.; Kalkote, U. R.
Tetrahedron Lett. 2007, 48, 965.
Yield: 1.3 g (86%); yellowish oil.
IR (neat): 815, 1642, 1514, 2237, 2982 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.40 (s, 3 H), 2.71 (m, 2 H), 5.18
(m, 4 H), 5.69 (m, 2 H), 7.2 (d, J = 16 Hz, 2 H), 7.3 (d, J = 16 Hz, 2
H).
13C NMR (50 MHz, CDCl3): d = 20.9, 44.12, 47.2, 119.9, 121.5,
126.1, 129.4, 131.7, 134.6, 137.4.
Acknowledgment
A.N.D. thanks CSIR, New Delhi for the award of a fellowship. Fun-
ding from DST (CSIR), New Delhi to S.P.C. is gratefully acknow-
ledged.
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Synthesis 2007, No. 24, 3827–3830 © Thieme Stuttgart · New York