Enantiospecific first total synthesis of ent-Allothapsenol

Article abstract of DOI:10.1055/s-0031-1290527

The enantiospecific first total synthesis of the enantiomer of the irregular sesquiterpene from Ligusticumgrayi allothapsenol, starting from the readily available monoterpene (R)-carvone, is described, which confirmed the assumed absolute configuration of

Full text of DOI:10.1055/s-0031-1290527

LETTER
▌1021
letter
Enantiospecific First Total Synthesis of ent-Allothapsenol
Enantiospecific First Total Synthesis of ent-Allothapsenol
A. Srikrishna,* K. Mahesh
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
Fax +91(80)3600683; E-Mail: askiisc@gmail.com
Received: 15.01.2012; Accepted after revision: 14.02.2012
Dedicated to Professor A. J. Rao on the occasion of his 70th birthday
It was contemplated (Scheme 1) that the synthesis of allo-
thapsenol (1) could be accomplished via the hydrindane-
dione (3).The tricyclic ketone 4 containing two vicinal all-
carbon quaternary carbon atoms, a key intermediate used
in the synthesis of valerane sesquiterpenes,4 was consid-
ered as the ideal precursor for the synthesis of 3, via deg-
radation of the isopropenyl group followed by
cyclopropane ring cleavage. The ketone 4 could readily be
prepared from the monoterpene carvone (5) via 3-methyl-
carvone (6) and the γ,δ-unsaturated acid 7.
Abstract: The enantiospecific first total synthesis of the enantio-
mer of the irregular sesquiterpene from Ligusticumgrayi allo-
thapsenol, starting from the readily available monoterpene (R)-
carvone, is described, which confirmed the assumed absolute con-
figuration of the natural product.
Key words: allothapsane, enantiospecific synthesis, irregular ses-
quiterpenes, carvone
Allothapsenol (1) is one of the 17 irregular sesquiterpenes
recently isolated1 by Cool and co-workers from the root
oil of Ligusticumgrayi, which belongs to the Apiaceae
family and grows in Cascade mountains of California,
Nevada, Oregon, and Washington. The Apiaceae family is
known to be a rich source of novel natural products, par-
ticularly, several sesqui- and diterpenoids containing ir-
regular carbon frameworks. The roots of the plant
Ligusticumgrayi were used by native Americans for me-
dicinal purposes. The structure of allothapsenol (1, Figure
1) was assigned on the basis of spectroscopic data, in par-
ticular the 1D and 2D NMR, and the absolute configura-
tion was tentatively assigned in analogy to that of
thapsanes, such as 2, isolated from Thapsia villosa.2
OH
O
O
O
ent-allothapsenol
3
4
O
O
COOH
7
5
6
Scheme 1
13
H
4
HO
8
6
The synthetic sequence starting from (R)-carvone (5) is
depicted in Scheme 2 and Scheme 3. To begin with (R)-
carvone (5) has been transformed into (S)-3-methylcar-
vone (6) via an alkylative 1,3-enone transposition strate-
gy,5 which was then transformed into the γ,δ-unsaturated
acid 7 via Johnson’s orthoester Claisen rearrangement of
the alcohol 8. The requisite key intermediate of the se-
quence, the tricyclic ketone 4 was regio- and stereospecif-
ically obtained from the acid 7 via the anhydrous copper
sulfate catalysed intramolecular cyclopropanation of the
diazo ketone 9. The third quaternary carbon atom was cre-
ated by a one-step dialkylation of the tricyclic ketone 4
with sodium hydride and methyl iodide in THF to furnish
the alkylated ketone 10 in 78% yield, whose structure was
established from its spectral data.6 Next, degradation of
the isopropenyl into a ketone group was addressed by em-
ploying a one-step ozonation–Criegee rearrangement.7
Thus, ozonation of the tricyclic ketone 10 in dichloro-
methane–methanol (4:1) at low temperature, followed by
treatment of the resultant methoxy hydroperoxide with
acetic anhydride and triethylamine in refluxing benzene
furnished the tricyclic keto acetate 11 in 43% yield.8 Hy-
drolysis of the acetate with potassium carbonate in meth-
1
12
O
H
11
15
10
14
OH
1
2
Figure 1
The sterically crowded framework containing three con-
tiguous all-carbon quaternary centers made the allothap-
sanes attractive synthetic targets. So far there is no report
in the literature on the synthesis, either in racemic or en-
antiopure form, of any allothapsane. In continuation of
our interest in the chiral-pool-based enantiospecific syn-
thesis of natural products starting from the readily avail-
able monoterpenes,3 we have initiated the synthesis of the
irregular Ligusticumgrayi sesquiterpenes, and herein we
report the enantiospecific first total synthesis of the enan-
tiomer of allothapsenol (1), which also confirmed the ab-
solute configuration of the natural allothapsenol.
SYNLETT 204012, 237, 1021–1024
Advanced online publication: 29.03.2012
DOI: 10.1055/s-0031-1290527; Art ID: ST-2012-D0038-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

1022
A. Srikrishna, K. Mahesh
LETTER
anol transformed the tricyclic keto acetate 11 into the
hydroxy ketone 12 in 86% yield, which was found to exist
(>85%) as the hemiacetal 12a. Oxidation of the hydroxy
group in 12 with pyridinium chlorochromate (PCC) and
silica gel in dichloromethane at room temperature, fol-
lowed by treatment with p-toluenesulfonic acid (PTSA)
furnished the bicyclic enedione 3 in 86% yield, via cleav-
age of the cyclopropane ring (or uncaging) in the resultant
tricyclic dione 13, whose structure was established from
X
O
O
a
b
Y
X
5
6 X, Y = O
8 X = H, Y = OH
7 X = OH
9 X = CHN2
c
1
its spectral data in particular the IR, H NMR, and 13C
NMR.6
d
O
O
O
To avoid regiochemical problems in subsequent reactions
(Scheme 3), the cyclopentanone in the enedione 3 was
protected as its ethylene ketal by treating with 1.2 equiva-
lents 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 14 in 76%
yield, in a regioselective manner. Kinetic alkylation of the
enone 14 with lithium hexamethyldisilazide (LiHMDS)
and methyl iodide furnished the norallothapsane deriva-
tive 15 in 68% yield, in a highly stereoselective manner as
it is both kinetic and thermodynamically preferred. The
stereochemistry of the secondary methyl group was as-
signed 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.
10
4
e
HO
12
f
O
O
O
11
O
OH
12a
g
O
Hydrogenation of the enone 15 in ethyl acetate using 10%
palladium over carbon as the catalyst at one atmospheric
pressure of hydrogen furnished the saturated ketone 16 in
near quantitative yield. Reduction of the ketone 16 with
lithium aluminum hydride (LAH) in diethyl ether generat-
ed the secondary alcohol 17 in 82% yield, in a highly ste-
reoselective manner. The stereochemistry of the alcohol
in 17 was assigned based on the 1H NMR spectrum (broad
singlet due to CHOH established the axial orientation of
the hydroxy group), and was confirmed by its facile dehy-
dration as it is axial orientated and hence nicely suited for
a facile E2 elimination. It was contemplated to dehydrate
the secondary alcohol in 17 via elimination of the corre-
sponding mesylate. Treatment of the alcohol 17 with
methanesulfonyl chloride and pyridine followed by work-
up directly furnished the norallothapsenone 18 in 70%
yield, in a highly regioselective manner via formation as
well as elimination of the mesylate and hydrolysis of the
ketal moiety. Since the conventional Wittig reaction of the
ketone 18 was found to be inefficient, Yan’s protocol9 was
employed. Thus, treatment of the norketone 18 with mag-
nesium, dichloromethane, and titanium tetrachloride in
THF furnished allothapsa-2,8(12)-diene (19) in 71%
O
O
O
3
13
Scheme 2 Reagents and conditions: (a)4 i. MeMgI, Et2O; ii. PCC,
silica gel, CH2Cl2; iii. LAH, Et2O; (b)4 i. MeC(OEt)3, EtCOOH, temp;
ii. NaOH, MeOH, H2O; iii. (COCl)2, C6H6; iv. CH2N2, Et2O; (c)4 an-
hyd CuSO4, c-C6H12; (d) NaH, THF, MeI, r.t., 18 h, 78%; (e) i. O3/O2,
CH2Cl2–MeOH (4:1), NaHCO3; –70 °C; ii. Ac2O, Et3N, DMAP,
C6H6, reflux, 6 h; 43%; (f) K2CO3, MeOH, r.t., 6 h, 86%; (g) i. PCC,
silica gel, CH2Cl2, r.t., 8 h; PTSA, CH2Cl2, 1 h; 86%.
duced prior to the generation of the trisubstituted olefin.
Thus, hydrolysis of the ketal group in 17 generated the hy-
droxy ketone 21. Treatment of the hydroxy ketone 21 with
magnesium, dichloromethane, and titanium tetrachloride
furnished allothaps-8(12)-en-3-ol (22) in 58% yield. Re-
action of allothapsenol 22 with an excess of in situ gener-
ated borane–THF followed by oxidation with alkaline
hydrogen peroxide, as expected, furnished a 5:2 mixture
of the exo-23a and endo-23b diols in 74% yield, which
were separated by column chromatography on silica gel.
The stereochemistry of the hydroxymethyl group in 23a
and 23b were tentatively assigned and was confirmed by
the conversion of the major isomer into ent-allothapsenol
(1). In order to avoid a regiochemical problem, the prima-
ry alcohol in 23a was protected as its TBDMS ether by
treating with one equivalent of tert-butyldimethylsilyl
chloride and imidazole in the presence of a catalytic
amount of 4-N,N-dimethylpyridine (DMAP) to furnish
1
yield, whose structure was established from its H NMR
and 13C NMR spectral data.6
Since regioselective hydroboration of the exo-methylene
in the diene 19 under a variety of conditions was found to
be inefficient, and also bulky dialkylboranes are expected
to generate the endo-hydroxymethyl group leading to epi-
allothapsenol (20), the sequence was modified, and it was
considered that the primary hydroxy group may be intro-
Synlett 2012, 23, 1021–1024
© Georg Thieme Verlag Stuttgart · New York

LETTER
Enantiospecific First Total Synthesis of ent-Allothapsenol
1023
the ether 24a. Dehydration of the secondary alcohol in data6 in C6D6 identical to that of natural allothapsenol (1),
24a was effected by reacting with methanesulfonyl chlo- whereas the sign of the optical rotation was found to be
ride and pyridine to furnish allothapsenyl TBDMS ether opposite to that natural product, confirming the assigned
25a, which on deprotection with tetrabutylammonium absolute configuration of the natural product.
fluoride (TBAF) generated ent-allothapsenol (1). In a sim-
ilar manner, the minor diol 24b was transformed into epi-
allothapsenol 20. Structures of ent-1 and 20 were assigned
In conclusion, we have developed the enantiospecific first
total synthesis of the enantiomers of allothapsa-2,8(12)-
diene (19) and allothapsenol (1), starting from the readily
on the basis of the spectral data. Synthetic ent-allo-
available monoterpene (R)-carvone. A combination of the
1
thapsenol (1) exhibited H NMR and 13C NMR spectral
Criegee rearrangement and the regiospecific cyclopro-
pane ring cleavage were conveniently employed for the
generation of the key intermediate of the sequence.
O
a
O
O
O
O
Acknowledgment
3
14
We thank Dr. L. G. Cool for providing the copies of the 1H NMR
and 13C NMR spectra of natural allothapsenol, and the Department
of Science and Technology, New Delhi and Department of Biotech-
nology, New Delhi for the financial support.
b
O
O
O
O
c
O
O
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.onmornifIatngonimSorfniturItagponiSutpor
16
15
d
References
O
e
(1) Cool, L. G.; Vermillion, K. E.; Takeoka, G. R.; Wong, R. Y.
Phytochemistry 2010, 71, 1545.
X
O
HO
(2) (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) Srikrishna, A.; Anebouselvy, K. Tetrahedron Lett. 2002,
43, 5261.
17
18 X = O
19 X = CH2
f
g
(3) For recent examples, see: (a) Srikrishna, A.; Nagaraju, G.
Synlett 2012, 23, 123. (b) Srikrishna, A.; Gowri, V. Synlett
2011, 2652. (c) Srikrishna, A.; Mahesh, K. Synlett 2011,
2537. (d) Srikrishna, A.; Dethe, D. H. Indian J. Chem., Sect.
B: Org. Chem. Incl. Med. Chem. 2011, 50, 1092.
h
O
HO
HO
21
22
i
(e) Srikrishna, A.; Anebouselvy, K. Indian J. Chem., Sect. B:
Org. Chem. Incl. Med. Chem. 2010, 49, 771. (f) Srikrishna,
A.; Pardeshi, V. H.; Mahesh, K. Tetrahedron: Asymmetry
2010, 21, 2511. (g) Srikrishna, A.; Pardeshi, V. H.;
Satyanarayana, G. Tetrahedron: Asymmetry 2010, 21, 746.
(h) Srikrishna, A.; Ravi, G.; Venkatasubbaiah, D. R. C.
Synlett 2009, 32. (i) Srikrishna, A.; Ravi, G. Tetrahedron
2008, 64, 2565.
OR
OR
HO
HO
(5:2)
23a R = H
24a R = TBDMS
23b R = H
24b R = TBDMS
(4) Srikrishna, A.; Viswajanani, R.; Dinesh, C. Indian J. Chem.,
Sect. B: Org. Chem. Incl. Med. Chem. 2004, 42, 1265.
(5) (a) Buchi, G.; Egger, B. J. Org. Chem. 1971, 36, 2021.
(b) Srikrishna, A.; Hemamalini, P. Indian J. Chem., Sect. B:
Org. Chem. Incl. Med. Chem. 1990, 29, 152.
j, k
j, k
OR
OR
(6) Yields refer to isolated and chromatographically pure
compounds. All the compounds exhibited spectral data [IR,
HRMS, 1H NMR (400 MHz) and 13C NMR (100 MHz)]
consistent with their structures.
25b R = TBDMS
20 R = H
25a R = TBDMS
ent-1 R = H
Selected Spectral Data for (1R,2R,4R,6R,9S)-4-
Isopropenyl-1,6,7,7-tetramethyltricyclo[4.3.0.02,9]-
nonan-8-one (10): [α]D23 +116.9 (c 3.7, CHCl3). IR (neat):
νmax= 1721 (C=O), 1381, 1017, 885 cm–1. 1H NMR (400
MHz, CDCl3): δ = 4.65 (1 H, s) and 4.58 (1 H, s) [C=CH2],
2.10–1.95 (2 H, m), 1.87 (1 H, d, J= 10.1 Hz), 1.69 (3 H, s,
olefinic CH3), 1.75–1.25 (3 H, m), 1.28 (3 H, s), 1.17 (3 H,
s), 1.06 (3 H, s) and 0.81 (3 H, s) [4 × t-CH3], 0.68 (1 H, td,
J = 14.0, 6.4 Hz). 13C NMR (100 MHz, CDCl3): δ = 217.5
(C=O), 148.6 (C=CH2), 108.9 (C=CH2), 58.3 (C, C-7), 40.3
(C), 39.4 (CH), 38.5 (CH), 38.1 (CH2), 30.1 (CH), 28.9 (C),
Scheme 3 Reagents and conditions: (a) (CH2OH)2, PTSA, C6H6, re-
flux (Dean–Stark), 6 h, 76%; (b) LiHMDS, THF, –70 °C; MeI, r.t., 8
h, 68%; (c) H2 (1 atm), 10% Pd/C, EtOAc, 1 h, 98%; (d) LAH, Et2O,
0 °C to r.t., 1 h, 82%; (e) MsCl, pyridine, CH2Cl2, 0 °C, 1 h, 70%; (f)
Mg, CH2Cl2, TiCl4, THF, r.t., 1 h, 71%; (g) 3 M HCl, THF, r.t., 1 h,
81%; (h) Mg, CH2Cl2, TiCl4, THF, r.t., 1 h, 58%; (i) i. NaBH4,
BF3?OEt2, THF, 0 °C to r.t., 1 h; ii. 3 N NaOH, 30% H2O2, r.t., 1 h;
74%; (j) TBDMSCl, imidazole, DMAP, CH2Cl2, 24a 82%; 24b 87%;
(k) i. MsCl, pyridine, CH2Cl2, 0 °C, 1 h; ii. TBAF, THF, r.t., 3 h, 75%.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1021–1024

1024
A. Srikrishna, K. Mahesh
LETTER
26.3 (CH3), 24.7 (CH2), 24.1 (CH3), 21.3 (CH3), 19.9 (CH3),
18.6 (CH3). HRMS: m/z calcd for C16H24O [M + Na]:
255.1725; found: 255.1726.
C-8), 137.1 (C, C-2), 121.7 (CH, C-3), 104.6 (CH2, C=CH2),
51.4 (C, C-1), 47.2 (CH2), 46.7 (C), 41.2 (C), 32.3 (CH2),
31.5 (CH3), 30.3 (CH3), 22.1 (2 C, CH3 and CH2), 21.5
(CH3), 19.2 (CH3). MS: m/z (%) = 204 (100) [M+], 189 (12),
166 (22), 135 (18), 121 (100).
(1R,6S)-1,6,9,9-Tetramethylbicyclo[4.3.0]non-4-ene-3,8-
dione (3): [α]D23 –35.5 (c 2.4, CHCl3). IR (neat): νmax = 1737
(C=O), 1683 (C=O), 1458, 1390, 1379, 1267, 1117, 1087,
785 cm–1. 1H NMR (400 MHz, CDCl3 + CCl4): δ = 6.66 (1
H, d, J = 10.1 Hz, H-5), 5.98 (1 H, d, J = 10.1 Hz, H-4), 2.63
(1 H, d, J = 19.2 Hz), 2.47 (1 H, d, J = 16.4 Hz), 2.46 (1 H,
d, J = 19.2 Hz), 2.29 (1 H, d, J = 16.4 Hz), 1.34 (3 H, s), 1.10
(3 H, s), 1.06 (3 H, s), 1.01 (3 H, s) [4 × t-CH3]. 13C NMR
(100 MHz, CDCl3 + CCl4): δ = 219.9 (C, C-8), 198.8 (C, C-
3), 156.8 (CH, C-5), 126.5 (CH, C-4), 53.3 (C), 48.6 (CH2),
47.8 (C), 45.0 (CH2), 41.1 (C), 25.3 (CH3), 22.8 (CH3), 22.6
(CH3), 19.5 (CH3). HRMS: m/z calcd for C13H18O2 [M + Na]:
229.1204; found: 229.1206.
(1R,6R,8S)-1,2,6,9,9-Pentamethylbicyclo[4.3.0]nona-2-
ene-8-methanol [8-Epiallothapsenol (20)]: [α]D22 –5.4 (c
0.4, CHCl3). IR (neat): νmax = 3338 (OH), 1545, 1377, 1083,
1067, 1018, 818, 669 cm–1. 1H NMR (400 MHz, C6D6): δ =
5.46 (1 H, br s, olefinic H), 3.51 (1 H, dd, J = 10.2, 5.8 Hz),
3.23 (1 H, dd, J= 10.2, 7.9 Hz), 2.13–1.76 (2 H, m), 1.74–
1.68 (2 H, m), 1.67 (3 H, s, vinylic CH3), 1.33–1.30 (2 H, m),
1.23 (1 H, dd, J= 13.3, 9.8 Hz), 1.06 (3 H, s), 0.93 (3 H, s)
0.90 (3 H, s), 0.86 (3 H, s) [4 × t-CH3]. 13C NMR (100 MHz,
C6D6): δ = 138.7 (C, C-2), 124.2 (CH, C-3), 64.9 (CH2,
CH2OH), 53.6 (C), 49.2 (CH), 46.2 (C), 44.9 (CH2), 42.0
(C), 38.9 (CH2), 28.4 (CH3), 25.9 (CH3), 23.2 (CH2), 22.7
(CH3), 21.8 (CH3), 20.4 (CH3).
(1R,6R,8R)-1,2,6,9,9-Pentamethylbicyclo[4.3.0]nona-2-
ene-8-methanol [ent-Allothapsenol (ent-1)]: [α]D22 –28.5
(c 0.3, CHCl3). IR (neat): νmax = 3342 (OH), 1376, 1074,
1017, 817 cm–1. 1H NMR (400 MHz, C6D6): δ = 5.42 (1 H,
br s, olefinic H), 3.45 (1 H, dd, J = 10.4, 7.0 Hz), 3.26 (1 H,
dd, J = 10.3, 6.7 Hz), 2.10–1.75 (4 H, m), 1.61 (3 H, s,
vinylic CH3), 1.40 (1 H, br s), 1.35 (1 H, dd, J = 12.5, 5.3
Hz), 1.25 (1 H, t, J = 12.7 Hz), 1.12 (3 H, s), 0.96 (3 H, s),
0.86 (3 H, s), 0.85 (3 H, s) [4 × t-CH3]. 13C NMR (100 MHz,
C6D6): δ = 137.7 (C, C-2), 122.2 (CH, C-3), 64.6 (CH2,
CH2OH), 52.5 (C), 50.7 (CH), 44.2 (C), 42.8 (CH2), 42.5
(C), 33.6 (CH3), 32.9 (CH2), 23.5 (CH3), 23.3 (CH3), 22.7
(CH2), 21.7 (CH3), 20.1 (CH3).
(1R,2R,3S,6R)-3-Hydroxy-1,2,6,9,9-pentamethylbicyclo-
[4.3.0]nonan-8-one (21): [α]D22 –54.4 (c 4.0, CHCl3). IR
(neat): νmax = 3510 (OH), 1719 (C=O), 1381, 1246, 1227,
963 cm–1. 1H NMR (400 MHz, CDCl3):
δ = 3.84 (1 H, br s, CHOH), 2.66 (1 H, d, J = 17.6 Hz, H-7A),
1.95 (1 H, ddd, J = 14.2, 9.3, 7.5 Hz), 1.86 (1 H, d J = 17.6
Hz, H-7B), 1.74–1.68 (2 H, m), 1.60 (1 H, br s), 1.33–1.28
(1 H, m), 1.10–1.05 (15 H, m, 4 × t-CH3 and s-CH3). 13
C
NMR (100 MHz, CDCl3): δ = 224.4 (C, C=O), 72.0 (CH, C-
3), 54.1 (C), 48.3 (C), 47.9 (CH2), 40.1 (C), 38.1 (CH), 29.2
(CH2), 28.4 (CH2), 28.1 (CH3), 26.1 (CH3), 25.9 (CH3), 15.9
(CH3), 15.2 (CH3). HRMS: m/z calcd for C14H24O2Na [M +
Na]: 247.1674; found: 247.1672.
(1R,6R)-1,2,6,9,9-Pentamethyl-8-methylenebicyclo-
[4.3.0]non-2-ene [Allothapsa-2,8(12)-diene (19)]: [α]D
22
–11.5 (c 1.1, CHCl3). IR (neat): νmax = 3069, 1648, 1377,
1075, 879 cm–1. 1H NMR (400 MHz, CDCl3): δ = 5.40 (1 H,
br s, H-3), 4.84 (1 H, d, J= 2.2. Hz) and 4.79 (1 H, d, J = 2.3
Hz), 2.57 (1 H, dt, J = 15.6, 2.2 Hz) and 2.04 (1 H, d, J = 15.6
Hz) [H-7], 2.15–1.98 (1 H, m), 1.95–1.80 (1 H, m), 1.80–
1.65 (1 H, m), 1.66 (3 H, s, vinylic CH3), 1.15–1.00 (1 H, m),
1.12 (3 H, s), 1.11 (3 H, s), 0.99 (3 H, s) and 0.90 (3 H, s)
[4 × t-CH3]. 13C NMR (100 MHz, CDCl3): δ = 162.9 (C,
(7) (a) Schreiber, S. L.; Liew, W.-F. Tetrahedron Lett. 1983, 24,
2363. (b) Criegee, R. Ber. Bunsen-Ges. Phys. Chem. 1944,
77, 722.
(8) In addition to the acetate 11, ca. 10% of the ene-dione 3 was
also obtained.
(9) Yan, T.-H.; Tsai, C.-C.; Chen, C.-T.; Cho, C.-C.; Huang,
P.-C. Org. Lett. 2004, 6, 4961.
Synlett 2012, 23, 1021–1024
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