LETTER
2537
An Enantiospecific Approach to Irregular Ligusticum grayi Sesquiterpenes:
Synthesis of cis-Preisothapsa-2,8(12)-diene
Synthesi ofcis-Preisothapsa-2Srikrishna,*
A.
,8(12)-dien
K. Mahesh
Abstract: Enantiospecific syntheses of 1-epi- (or cis-)-preisothapsa-2,8(12)-diene and 1-epi- and 1,8-diepipreisothapsa-2-en-12-ols,
starting from the readily available monoterpene (R)-carvone, have
been accomplished.
Key words: thapsanes, preisothapsane, enantiospecific synthesis,
irregular sesquiterpenes, carvone
The Apiaceae family is known to be a rich source of novel
natural products, particularly several sesqui- and diterpenoids containing irregular carbon frameworks (incompatible with biosynthetic mechanisms involving farnesyl
diphosphate or geranylgeranyl diphosphate). In the 1980s,
the research group of Grande had reported the isolation
of a series of natural products from Thapsia villosa L.
containing a unique, sterically crowded, irregular sesquiterpene carbon framework, cis-1,2,2,6,8,9-hexamethylbicyclo[4.3.0]nonane, named as thapsanes. Biosynthesis of
thapsanes involves head-to-head coupling of geranyl
diphosphate with dimethylallyl diphosphate,1c followed
by cyclisation and a number of 1,2-migrations, involving
a series of pentamethylbicylo[4.3.0]nonane carbocations
(each of which could potentially lead to a new sesquiterpene framework). The roots of the plant Ligusticum grayi
(grows in Cascade mountains of California, Nevada, Oregon, and Washington) were used by native Americans for
medicinal purposes.
Recently, Cool and co-workers chemically analysed the
essential oil of L. grayi root and reported the isolation and
structure elucidation of 17 new sesquiterpenes 1–17
(Figure 1), belonging to eight new sesquiterpene frameworks in addition to cis- and trans-thapsanes (14–16),
originated most likely through a common biogenesis. The
new sesquiterpene groups are named as prethapsane (1,
2), isothapsane (3–5), ligustigrane (6, 7), isoligustigrane
(8, 9), preisothapsane (10, 11), allothapsane (12), isoprethapsane (13), and oshalagrane (17) on the basis of their
anticipated biogenetic relationship to thapsanes (14–16).
The sterically crowded frameworks of these new irregular
Ligusticum grayi sesquiterpenes 1–17 make them attractive synthetic targets. It is also quite interesting to note
that both cis and trans ring junctions of the bicycSYNLETT 2011, No. 17, pp 2537–2540xx. 201
Advanced online publication: 22.09.2011
DOI: 10.1055/s-0030-1260326; Art ID: D22111ST
© Georg Thieme Verlag Stuttgart · New York
lo[4.3.0]nonanes were encountered in these compounds
unlike the earlier reported cis-thapsanes.1 So far there is
no report in the literature on the synthesis of any of these
irregular sesquiterpenes 1–17, either in racemic or enantioselective manner. Earlier, we have developed a synthesis
of thapsanes, both in racemic as well as enantiospecific
manner.3a,h In continuation of our interest in the chiralpool-based enantiospecific synthesis of natural products
starting from the readily available monoterpene (R)-carvone (18),3 we have initiated a synthesis of the irregular
Ligusticum grayi sesquiterpenes 1–17, and herein we report an enantiospecific approach to cis- (or 1-epi)-preisothapsa-2,8(12)-diene (19).
HO
HO
R
H
1Δ
2 Δ4
6 Δ5
7 Δ6
3 Δ9(10), R = =CH2
4 Δ9(10), R = CH2OH
5 Δ8, R = CH2OH
3
13
HO
HO
4
8
HO
6
1
12
H
11
10
8 Δ5
9 Δ6
14
15
12
10 Δ1
11 Δ2
HO
R
HO
H
13
14 Δ9(10), α-Me, R = CH2OH
15 Δ9(10), β-Me, R = =CH2
16 Δ8, α-Me, R = CH2OH
17
Figure 1
It was contemplated (Scheme 1) that preisothapsadiene
(19) could be obtained from the monoprotected bicyclic
dione 20. The ketoketal 20 could be obtained from the tricyclic ketone 21 via degradation of the isopropenyl side
chain, followed by cyclopropane ring cleavage. Synthesis
of the tricyclic ketone 21 starting from the readily available monoterpene (R)-carvone (18) via the g,d-unsaturated acid 22 has been developed earlier in our laboratory.4
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Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
Fax +91(80)3600683; E-mail: askiisc@gmail.com
Received 11 July 2011
Dedicated to Professor D. Basavaiah on the occasion of his 60th birthday
LETTER
A. Srikrishna, K. Mahesh
O
O
19
O
O
H
H
20
HOOC
H
O
H
18
22
21
Scheme 1
X
O
Y
a
a
O
X
H
21
18 X, Y = O
23 X = OH, Y = H
25 X = OEt
22 X = OH
24 X = CHN2
O
+ 28
b
63%
c
O
H
27
O
68%
O
H
X
(4:1)
26 X = CH2
28 X = O
d 75%
e
O
O
OH
H
29
85%
O
H
31
O
f
O
O
H
O
H
30
R
20 R = H
32 R = Me
h
67%
g, 82%
O
97%
O
O
i
O
O
H
O
81%
H
33
j
OH
34
65%
k
O
69%
H
H
19
35
Scheme 2 Reagents and conditions: (a) ref. 4; (b) NaH, THF, DMF,
MeI, 0 °C to 40 °C, 24 h; (c) i. O3/O2, CH2Cl2–MeOH (4:1), –70 °C;
ii. Ac2O, Et3N, DMAP, C6H6, reflux, 6 h; (d) K2CO3, MeOH, r.t., 6 h;
(e) PDC, CH2Cl2, r.t., 5 h; (f) (CH2OH)2, PTSA, C6H6, reflux (Dean–
Stark), 6 h; (g) LDA, THF, –70 °C; MeI, r.t., 12 h; (h) H2 (1 atm), 10%
Pd/C, EtOAc, 1 h; (i) MeMgCl, Et2O, 0 °C to r.t., 4 h; (j) POCl3, pyridine, CH2Cl2, 0 °C, 1 h; (k) Ph3P+Me Br–, t-AmOK, C6H6, r.t., 4 h.
Synlett 2011, No. 17, 2537–2540
© Thieme Stuttgart · New York
The synthetic sequence starting from (R)-carvone 18 is
depicted in Scheme 2. The requisite key intermediate of
the sequence, tricyclic ketone 21 was obtained from (R)carvone 18 as described earlier4 via the Johnson’s orthoester Claisen rearrangement5 of carveol (23) and copper–
copper
sulfate
catalysed
intramolecular
cyclopropanation6 of the diazoketone 24 obtained from
the ester 25. The second quaternary carbon atom was created by one-step dialkylation of the tricyclic ketone 21
with sodium hydride and methyl iodide in THF and DMF
to furnish the alkylated ketone 26 in 63% yield.7 Next,
degradation of the isopropenyl into a ketone group was
addressed by employing a one-step ozonation–Criegee rearrangement.8 Thus, ozonation of the tricyclic ketone 26
in dichloromethane–methanol (4:1) at low temperature,
followed by treatment of the resultant methoxyhydroperoxide with acetic anhydride and triethylamine in refluxing
benzene furnished a 4:1 mixture of the tricyclic keto acetate 27 and the normal ozonolysis product, the diketone 28
in 68% yield, which were separated by column chromatography on silica gel. Hydrolysis of the acetate with potassium carbonate in methanol transformed the tricyclic
ketoacetate 27 into the hydroxyketone 29 in 75% yield.
Oxidation of the hydroxy group in 29 with pyridinium
dichromate (PDC) in dichloromethane at room temperature furnished directly the bicyclic enedione 30, in 85%
yield, via simultaneous cleavage of the cyclopropane ring
(or uncaging) in the resultant tricyclic dione 31. The structure of the enedione 30 was established from its spectral
data.7 The presence of carbonyl absorption bands at 1737
and 1681 cm–1 due to the cyclopentanone and cyclohexenones, respectively, in the IR spectrum, two doublets at
d = 6.61 and 5.92 (J = 10.2 Hz) ppm due to the b- and aolefinic protons, respectively, of cyclohexenone in the 1H
NMR spectrum, and two quaternary carbon resonances at
d = 218.8 and 196.3 ppm due to the cyclopentanone and
cyclohexenone ketone carbons, respectively, and two methines at d = 155.4 and 128.6 ppm due to the b- and a-carbons, respectively, of cyclohexenone in the 13C NMR
spectrum established the structure of the enedione 30. The
cis ring junction is a consequence of the stereospecific cyclopropanation of the diazoketone 24 via the syn face of
the side chain in the generation of the tricyclic ketone 26,
which has already been well established.4 To avoid regiochemical problems in subsequent reactions, the cyclopentanone in the enedione 30 was protected as its ethylene
ketal by treating with 1.2 equivalents of ethanediol in the
presence of a catalytic amount of 4-toluenesulfonic acid
(PTSA) in refluxing benzene under Dean–Stark conditions to furnish the keto ketal 20, in 67% yield, in a highly
regioselective manner. Next, introduction of carbons 13
and 14 were taken up. Kinetic alkylation of the enone 20
with lithium diisopropylamide (LDA) and methyl iodide
furnished the enone 32, in 82% yield,7 in a highly stereoselective manner. The stereochemistry of the secondary
methyl group was assigned on the basis of the approach of
the electrophile from the less hindered face of the bicyclic
system as one face is blocked by the methyl group on the
C-9 carbon atom of the bicyclic system.9
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2538
Synthesis of cis-Preisothapsa-2,8(12)-diene
Hydrogenation of the enone 32 in ethyl acetate using 10%
palladium over carbon as the catalyst at 1 bar pressure of
hydrogen furnished the saturated ketone 33 in near quantitative yield. Grignard reaction of the ketone 33 with methylmagnesium chloride in diethyl ether generated the
tertiary alcohol 34, in 81% yield, in a highly stereoselective manner. The stereochemistry of the alcohol in 34 was
tentatively assigned, and was confirmed by its facile dehydration to furnish the tetrasubstituted olefin in the next
reaction. Treatment of the tertiary alcohol 34 with phosphorus oxychloride and pyridine in dichloromethane followed by workup directly furnished the bicyclic
norpreisothapsenone (35), in 65% yield,7 in a highly regioselective manner via dehydration and hydrolysis of the
ketal moiety. Wittig reaction of the bicyclic ketone 35
with methyltriphenylphosphonium bromide and tert-amyl
oxide in benzene furnished cis- (or 1-epi)-preisothapsa2,8(12)-diene (19) in 69% yield, whose structure was established from its spectroscopic data.7
a or b
H
19
HO
H
H
36a α-H
36b β-H
Scheme 3 Reagents and conditions: (a) i. NaBH4, BF3·OEt2, THF,
0 °C to r.t., 1 h; ii. 3 N NaOH, 30% H2O2, r.t., 1 h, 36a/36b (2:1); (b)
i. 9-BBN, 0 °C to r.t., 1 h; ii. 3 N NaOH, 30% H2O2, r.t., 1 h; 53%.
Regioselective hydroboration of the exo-methylene with
in situ generated diborane, followed by oxidation transformed 1-epipreisothapsadiene (19) into a 2:1 epimeric
mixture
of
1-epipreisothaps-2-en-12-ols
(36a,b,
Scheme 3). On the other hand hydroboration with 9-BBN,
followed by oxidation furnished predominantly 1,8diepipreisothaps-2-en-12-ol (36b), via preferential hydroboration from the exo face of the cis-hydrindane system.10
In conclusion, we have developed an enantiospecific approach to 1-epipreisothapsa-2,8(12)-diene (19), 1-epiand 1,8-diepipreisothapsa-2-en-12-ols (36a,b), starting
from the readily available monoterpene (R)-carvone.
Criegee rearrangement and cyclopropane ring cleavage in
a tricyclic system were employed as the key strategy. Extension of the methodology for the enantiospecific synthesis of the natural irregular Ligusticum grayi
sesquiterpenes 1–17 is currently in progress.
Acknowledgment
We thank the Department of Biotechnology, New Delhi for the financial support.
2539
References and Notes
(1) (a) de Pascual Teresa, J.; Moran, J. R.; Grande, M. Chem.
Lett. 1985, 865. (b) de Pascual Teresa, J.; Moran, J. R.;
Fernandez, A.; Grande, M. Phytochemistry 1986, 25, 703.
(c) van Clink, J. W.; Becker, H.; Perry, N. B. Org. Biomol.
Chem. 2005, 3, 542.
(2) Cool, L. G.; Vermillion, K. E.; Takeoka, G. R.; Wong, R. Y.
Phytochemistry 2010, 71, 1545.
(3) For recent examples, see: (a) Srikrishna, A.; Anebouselvy,
K. Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem.
2010, 49, 771. (b) Srikrishna, A.; Pardeshi, V. H.;
Satyanarayana, G. Tetrahedron Lett. 2007, 48, 4087.
(c) Srikrishna, A.; Ravi, G. Tetrahedron 2008, 64, 2565.
(d) Srikrishna, A.; Pardeshi, V. H.; Satyanarayana, G.
Tetrahedron: Asymmetry 2008, 19, 1984. (e) Srikrishna, A.;
Ravi, G.; Venkatasubbaiah, D. R. C. Synlett 2009, 32.
(f) Srikrishna, A.; Anebouselvy, K. Indian J. Chem., Sect. B:
Org. Chem. Incl. Med. Chem. 2009, 48, 413. (g) Srikrishna,
A.; Babu, R. R.; Beeraiah, B. Tetrahedron 2010, 66, 852.
(h) Srikrishna, A.; Anebouselvy, K. Tetrahedron Lett. 2002,
43, 2765.
(4) Srikrishna, A.; Reddy, T. J.; Nagaraju, S. Tetrahedron Lett.
1996, 37, 1679.
(5) Johnson, W. S.; Werthemann, L.; Bartlett, W. R.; Brocksom,
T. J.; Li, T.-t.; Faulkner, D. J.; Petersen, R. J. Am. Chem. Soc.
1970, 92, 741.
(6) (a) Stork, G.; Ficini, J. J. Am. Chem. Soc. 1961, 83, 4678.
(b) Burke, S. D.; Grieco, P. A. Org. React. 1979, 26, 361.
(7) Yields refer to isolated and chromatographically pure
compounds. All the compounds exhibited spectral data [IR,
HRMS, 1H (400 MHz) and 13C NMR (100 MHz) in 1:1
mixture of CDCl3 and CCl4] consistent with their structures.
Selected Spectral Data for (1R,2S,4S,6S,9R)-4-Isopropenyl-1,7,7-trimethyltricyclo[4.3.0.02,9]nonan-8-one
(26)
[a]D22 –139.6 (c 1.4, CHCl3). IR (neat): nmax = 1721, 1376,
1199, 1154, 1091, 1012, 922, 887, 854 cm–1. 1H NMR: d =
4.61 (1 H, s) and 4.54 (1 H, s, C=CH2), 2.08 (1 H, d, J = 8.9
Hz), 2.10–1.90 (3 H, m), 1.86 (1 H, d, J = 9.6 Hz), 1.67 (3 H,
s, olefinic CH3), 1.51 (1 H, q, J = 8.6 Hz), 1.29 (1 H, dd, J =
14.3, 9.2 Hz), 1.35 (3 H, s), 1.09 (3 H, s, tert-CH3), 0.78 (3
H, s, tert-CH3), 0.52 (1 H, td J = 13.8, 7.3 Hz). 13C NMR:
d = 218.8 (C=O), 149.2 (C=CH), 108.5 (CH=C), 53.4 (C, C7), 45.8 (CH), 38.3 (CH), 38.2 (CH), 31.6 (CH), 28.0 (CH3),
27.6 (C, C-1), 25.4 (CH2), 24.0 (CH2), 22.0 (CH3), 21.4
(CH3), 18.4 (CH3). HRMS: m/z calcd for C15H22O [M + H]:
219.1749; found: 219.1746.
(1S,6R)-6,9,9-Trimethylbicyclo[4.3.0]non-4-en-3,8-dione
(30)
[a]D23 –201.4 (c 1.6, CHCl3). IR (neat): nmax = 1737, 1681,
1388, 1244, 1120, 787 cm–1. 1H NMR: d = 6.61 (1 H, d, J =
10.2 Hz, H-4), 5.92 (1 H, d, J = 10.2 Hz, H-5), 2.74 (1 H, dd,
J = 17.9, 6.9 Hz), 2.57 (1 H, d, J = 18.8 Hz), 2.50 (1 H, d,
J = 17.9 Hz), 2.39 (1 H, d, J = 18.8 Hz), 2.45–2.25 (1 H, m),
1.42 (3 H, s), 1.05 (3 H, s, tert-CH3), 0.88 (3 H, s, tert-CH3).
13
C NMR: d = 218.8 (C, C-8), 196.3 (C, C-3), 155.4 (CH, C5), 128.6 (CH, C-4), 52.0 (CH), 50.4 (CH2), 49.3 (C), 37.6
(C), 33.9 (CH2), 28.0 (CH3), 25.9 (CH3), 20.8 (CH3). HRMS:
m/z calcd for C12H16O2 [M + Na]: 215.1048; found:
215.1045.
(1S,2S,6S)-2,6,9,9-Tetramethylbicyclo[4.3.0]nonanspiro[8.2¢]-1,3-dioxalan-3-one (33)
[a]D25 +47.5 (c 2.7, CHCl3). IR (neat): nmax = 1718, 1307,
1255, 1170, 1160, 1081, 1060, 1018, 1090, 951, 850 cm–1.
1
H NMR: d = 4.00–3.80 (4 H, m, OCH2CH2O), 2.70–2.50 (1
H, m), 2.45–2.05 (3 H, m), 1.78 (1 H, d, J = 13.2 Hz), 1.69
(1 H, dd, J = 13.4, 6.4 Hz), 1.57 (1 H, d, J = 13.2 Hz), 1.38
Synlett 2011, No. 17, 2537–2540
© Thieme Stuttgart · New York
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LETTER
A. Srikrishna, K. Mahesh
(1 H, d, J = 10.6 Hz), 1.05 (3 H, d, J = 8.7 Hz, CH3), 1.06
(3 H, s), 1.00 (3 H, s, tert-CH3), 0.95 (3 H, s, tert-CH3).
13
C NMR: d = 215.0 (C, C=O), 118.2 (C, C-8), 65.2 (CH2,
OCH2CH2O), 64.3 (CH2, OCH2CH2O), 60.1 (CH, C-2), 48.3
(C), 46.7 (CH2, C-7), 42.9 (CH, C-1), 35.9 (C), 35.7 (CH2),
35.1 (CH2), 28.9 (CH3), 23.1 (CH3), 20.4 (CH3), 14.1 (CH3).
HRMS: m/z calcd for C15H24O3Na [M + Na]: 275.1623;
found: 275.1624.
(1R,6S)-2,3,6,9,9-Pentamethylbicyclo[4.3.0]nonan-2-en8-one (35)
[a]D21 –62.3 (c 2.4, CHCl3). IR (neat): nmax = 1739, 1378,
1178, 1076 cm–1. 1H NMR: d = 2.34 (1 H, d J = 17.0 Hz),
2.09 (1 H, d J = 17.0 Hz), 2.05 (1 H, br s), 1.89 (1 H, dd
J = 16.9, 5.7 Hz), 1.69 (3 H, s, olefinic CH3), 1.66 (3 H, s,
olefinic CH3), 1.40–1.20 (3 H, m), 1.21 (3 H, s), 1.07 (3 H,
s, tert-CH3), 0.90 (3 H, s, tert-CH3). 13C NMR: d = 222.8 (C,
C=O), 126.2 (C, C-2), 123.5 (C, C-3), 59.6 (CH, C-1), 52.9
(CH2), 48.2 (C, C-9), 35.4 (C, C-6), 31.0 (CH2), 30.5 (CH3),
28.8 (CH2), 25.1 (CH3), 21.9 (CH3), 19.9 (CH3), 18.9 (CH3).
HRMS: m/z calcd for C14H22ONa [M + Na]: 229.1568;
found: 229.1566.
(1R,6S)-2,3,6,9,9-Pentamethyl-8-methylenebicyclo[4.3.0]non-2-ene [cis-preisothapsa-2,8 (12)-diene (19)]
[a]D21 +2.8 (c 2.8, CHCl3). IR (neat): nmax = 1654, 1458,
1374, 879 cm–1. 1H NMR: d = 4.78 (1 H, s), 4.74 (1 H, s,
C=CH2), 2.37 (1 H, d, J = 14.8 Hz), 2.05 (1 H, d, J = 14.8
Hz), 2.00–1.70 (3 H, m), 1.67 (3 H, s, olefinic CH3), 1.64 (3
H, s, olefinic CH3), 1.48 (1 H, td, J = 12.7, 6.2 Hz), 1.08–
1.04 (1 H, m), 1.23 (3 H, s), 0.93 (6 H, s, tert-CH3). 13C
NMR: d = 162.8 (C, C-8), 125.5 (C), 124.6 (C), 104.2 (CH2),
62.6 (CH, C-1), 48.8 (CH2, C-7), 44.3 (C), 38.7 (C), 35.0
(CH3), 29.5 (CH2), 29.1 (CH2), 26.2 (CH3), 24.9 (CH3), 20.2
(CH3), 18.9 (CH3).
(8) (a) Schreiber, S. L.; Liew, W.-F. Tetrahedron Lett. 1983, 24,
2363. (b) Criegee, R. Ber. Dtsch. Chem. Ges. 1944, 77, 722.
Synlett 2011, No. 17, 2537–2540
© Thieme Stuttgart · New York
LETTER
(9) No attempt was made to confirm the stereochemistry of the
secondary methyl group in 33 as the carbon would become a
sp2 centre subsequently.
(10) Formation of a single isomer by employing bulky
hydroborating agent assisted in assigning the
stereochemistry of the alcohols 36a and 36b. Quite
expectedly, the 1H and 13C NMR spectra in C6D6 reported2
for the natural preisothaps-2-en-12-ol(11), which was
assigned to have the trans ring junction, did not match with
signals due to either isomer of the mixture of the alcohols
36a,b recorded in C6D6. IR (neat): nmax = 3350, 2954, 2925,
2868, 1596, 1462, 1370, 1020, 1000 cm–1. 1H NMR (400
MHz, C6D6): d [signals due to 1-epipreisothaps-2-en-12-ol
(36a)] = 3.60 (1 H, dd, J = 10.3, 7.0 Hz), 3.42 (1 H, dd, J =
10.2, 7.0 Hz), 2.20–1.80 (4 H, m), 1.80 (1 H, br s), 1.74 (3 H,
s, olefinic CH3), 1.71 (3 H, s, olefinic CH3), 1.57 (1 H, dd,
J = 12.1, 6.2 Hz), 1.40–1.43 (3 H, m), 1.15 (3 H, s), 1.09 (3
H, s, tert-CH3), 1.03 (3 H, s, tert-CH3); d [signals due to
1,12-diepipreisothaps-2-en-12-ol (36b)] = 3.63 (1 H, dd, J =
10.4, 5.4 Hz), 3.35 (1 H, dd J = 10.4, 8.2 Hz), 2.20–1.80 (4
H, m), 1.76 (3 H, s, olefinic CH3), 1.74 (3 H, s, olefinic CH3),
1.70–1.50 (1 H, m), 1.50–1.20 (4 H, m), 1.29 (3 H, s), 1.09
(3 H, s, tert-CH3), 0.72 (3 H, s, tert-CH3). 13C NMR (100
MHz, C6D6): d [signals due to 1-epipreisothaps-2-en-12-ol
(36a)] = 125.6 (C), 124.9 (C), 64.0 (CH2), 63.7 (CH3), 50.5
(CH), 43.8 (CH2), 41.8 (C), 39.2 (C), 30.0 (CH2), 29.4
(CH2), 28.2 (CH3), 27.9 (CH3), 25.7 (CH3), 19.9 (CH3), 17.9
(CH3); d [signals due to 1,12-diepipreisothaps-2-en-12-ol
(36b)] = 125.8 (C), 64.6 (CH), 63.7 (CH2), 50. 6 (CH), 44.6
(C), 43.3 (CH2), 38.0 (C), 36.5 (CH2), 31.4 (CH3), 30.3
(CH3), 29.4 (CH2), 19.9 (CH3), 19.6 (CH3), 17.9 (CH3). The
signal due to one of the olefinic carbons is merged in the
solvent signals. Spectrum recorded in CDCl3 exhibited the
two olefinic carbon resonances at d = 128.3 and 125.5 ppm.
MS: m/z (%) = 222 (10) [M+], 123 (100), 107 (90), 91 (45).
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2540