Convenient synthesis of labdane and drimane analogues with o-quinol functionality

Article abstract of DOI:10.1081/SCC-200036582

The Diels-Alder reactions of 4-carbomethoxy-o-benzoquinone with substituted-1,3-dienes give high yields of the drimane and labdane analogues containing an orthoquinol moiety.

Full text of DOI:10.1081/SCC-200036582

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Convenient Synthesis of
Labdane and Drimane
Analogues with o‐Quinol
Functionality
S. Meenakshi a , A. Sivaramkrishnan a , R.
Padmakumar a , Shreeshailkumar B. Hadimani a
Sujata V. Bhat a
&
a Department of Chemistry , Indian Institute of
Technology, Powai , Mumbai, 400 076, India
Published online: 10 Jan 2011.
To cite this article: S. Meenakshi , A. Sivaramkrishnan , R. Padmakumar ,
Shreeshailkumar B. Hadimani & Sujata V. Bhat (2004) Convenient Synthesis
of Labdane and Drimane Analogues with o‐Quinol Functionality, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:22, 4065-4076, DOI: 10.1081/SCC-200036582
To link to this article: http://dx.doi.org/10.1081/SCC-200036582
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 22, pp. 4065–4076, 2004
Convenient Synthesis of Labdane and
Drimane Analogues with o-Quinol
Functionality
S. Meenakshi, A. Sivaramkrishnan,
R. Padmakumar, Shreeshailkumar B. Hadimani, and
*
Sujata V. Bhat
Department of Chemistry, Indian Institute of Technology,
Powai, Mumbai, India
ABSTRACT
The Diels–Alder reactions of 4-carbomethoxy-o-benzoquinone with sub-
stituted-1,3-dienes give high yields of the drimane and labdane analogues
containing an orthoquinol moiety.
Key Words: Synthesis; Diels–Alder reaction; Labdane; Drimane
Analogues; o-Quinol functionality.
*
Correspondence: Sujata V. Bhat, Department of Chemistry, Indian Institute of
Technology, Powai, Mumbai 400 076, India; Fax: 91(022) 2573480; E-mail:
svbhat@chem.iitb.ac.in.
4065
DOI: 10.1081/SCC-200036582
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

4066
Meenakshi et al.
INTRODUCTION
There are many cyclic natural compounds with o-quinone moiety, which
exhibit interesting biological activities (Fig. 1). For example, the eremo-
philane class of sesquiterpenes (1) displays phytotoxic properties.[1]
Members of the quassinoid family (2) exhibit a gamut of biological activities,
which include antimalarial, antitumor and anti-inflammatory activities.[2,3]
Chinese drug Tanshen (Salviae miltiorrhizae) has been reported to have the
effect of promoting blood circulation, removing blood stasis, and draining
pus. A water-soluble derivative, sodium Tanshinone II-A sulfonate (3)[4]
showed protective action against anorexia, ischaemia, and platelet aggregation
in rats. In coronary patients, it reduced anginal pain and improved the
ischemic changes in the electrocardiogram. Polyhydroxylated limonoid with
decalin skeleton from neem[5] and drimanes from various plants display anti-
feedant activity.[6] The quinone methide, taxodione (4) from Tadodium
disticum, has antitumor activity.[7]
Figure 1. Examples of quinonoid natural products.

Synthesis of Labdane and Drimane Analogues
4067
We report herein the reaction of 4-carbomethoxy-o-benzoquinone with
substituted acyclic 1,3-butadienes to give high yields of drimane and
labdane analogues[8] with o-quinol-A ring.
Orthoquinones are readily generated in situ by oxidation of catechols. The
quinones, isolated in low yield by this procedure, are rather unstable to both
light and water.[9] Orthoquinones exhibit properties of both diene and dieno-
phile.[10,11] However, they had not received much attention as dienophiles,
until the synthesis of tanshinones. Lee et al. had used the cycloaddition reac-
tion of 3-methyl-4,5-benzofurandione, as the dienophile, with vinylcyclohex-
ene, as the diene, promoted by ultrasound in the synthesis of tanshindiol A
(Scheme 1).[12] Danishefsky et al. observed that the regioselectivity was influ-
enced by the reaction solvent.[13]
Ansell et al.[11] and Kraus and Taschner[14] had shown that o-quinones
bearing an electron withdrawing substituent at 4-position are more reactive
as dienophiles in cycloadditions with reactive acyclic dienes such as 2,3-
dimethyl-butadiene and vinyl-cyclohexene respectively.
RESULTS AND DISCUSSION
In continuation of the earlier work, we have studied the general appli-
cation of the Diels–Alder reaction of 4-carbomethoxy-orthoquinones with
acyclic 1- and/or 2-substituted-1,3-butadienes.
The 1.3-dienes 14, 19, and 24 were prepared from 3-methyl-2-butenol,
3,7-dimethyl-6-octenol (citronellol) and methylheptenone, respectively, in
three steps (Scheme 2).
For the synthesis of diene 14, 3-methyl-2-butenol (10) was first protected
as the benzyl ether (11) and was subjected to a modified Sharpless
oxidation[15] by using SeO2/TBHP in dichloromethane, which oxidizes the
Scheme 1. Synthesis of tanshinone skeleton through Diels–Alder reaction.

4068
Meenakshi et al.
Scheme 2. Synthesis of 1,2-disubstituted 1,3-dienes.
trans-methyl group to aldehyde 12. This compound was identified by the dis-
appearance of the methyl group and the appearance of peak at d 9.4 in the
1H NMR and the appearance of signal at 1705 cm21 for the C55O stretching
in the IR spectrum. The allylic alcohol 13, which was obtained as the copro-
duct in this reaction, was oxidized to the required aldehyde. The aldehyde 12
was treated with a Wittig reagent (methyl triphenylphosphonium iodide

Synthesis of Labdane and Drimane Analogues
4069
and n-BuLi) to give the required conjugated diene 14. The dienes 19 and 24
were similarly prepared by starting from citronellol (15) and methyl hepte-
none (20), respectively.
The Diels–Alder reactions of 1,2-disubstituted-1,3-dienes (8, 9, 14, 19,
and 24) and 4-carbomethoxy-o-benzoquinone (25) were carried out at 08C
room temperature for the time mentioned in Table 1 (Scheme 3). The reaction
was performed in a solvent mixture of benzene-diethyl ether (3 : 1) in the
absence of light. The products, substituted 1-hydroxy-2-oxo-5,8-dihydro-4a-
hydronaphthalenes (26–30) were characterized by MS, UV, IR, and NMR
spectra.
o-Quinones, being unstable, are prone to rapid dimerization.[12] To cir-
cumvent this problem and to increase yield of the cycloaddition reaction, gen-
erally, the reactions were carried out at 08C room temperature in the absence
of light.
The regioselectivity of cycloaddition in the reactions of unsymmetrically
substituted dienes determines the subsequent synthetic use of the cycloadduct
formed. In the cycloaddition between the unsymmetrical dienes and dieno-
phile, a possibility exists of forming at least two regioisomers.[16] All dienes
substituted at C-1 gave endo adducts as the sole product (Scheme 3). This
was based on spectral analysis. The IR spectra of cycloadducts showed
bands in the regions 3500, 1740, and 1680 cm21, indicating the presence of
1
hydroxyl, ester, and conjugated carbonyl groups. The H NMR spectra of
Table 1. Diels–Alder adducts of 1,3-dienes with 4-carbomethoxy-orthobenzo-
quinones.
Diene
R1
R2
Time (hr)
Adduct (Yield %)
8
9
Me
H
H
0.5
0.5
3.0
26 (85)
27 (80)a
28 (75)
Me
Me
14
19
24
Me
Me
3.0
8.0
29 (75)
30 (85)
aContains regioisomer 27a in almost equal amount.

4070
Meenakshi et al.
Scheme 3. Diels–Alder reaction of 1,3-dienes with 4-carbomethoxy-orthobenzo-
quinones.
the adducts showed a pair of doublets (1H each), around d 6.5 and d 6.9, with
coupling constant of around 9.9 Hz, assigned for ring-A 3-H and 4-H protons,
respectively, and a D2O exchangeable signal due to –OH group. The 6-H and
7-H olefinic protons resonate as a pair of multiplets around d 5.7 and 5.9. Thus,
the product was formed through Diels–Alder reaction followed by enolization
of the resulting a-diketone to the tautomeric diosphenol. This was confirmed
1
not only by the high-resolution H NMR spectrum but also by acetylation of
the enol hydroxy group. The formation of the enol acetate was confirmed by
1
the H NMR spectrum of the acetate (additional singlet at d 2.3, 3H) and IR
spectrum (additional signal at 1728 cm21). The regioselectivity of adducts
was further supported by NMR, NOESY experiment on adduct 26, which
showed connectivity between carbomethoxy and methyl groups.
No dimerization products of o-quinones were observed. Unlike earlier
reports, neither bis adduct[17] nor benzodioxane adduct[18] were observed
under the present reaction conditions.
CONCLUSIONS
Thus, the reaction of 4-carbomethoxybenzoquinone with substituted
1,3-butadienes gave drimane and labdane analogues with o-quinol-A ring.

Synthesis of Labdane and Drimane Analogues
4071
Various functionalities, such as a,b-unsaturated ketone, ester, trisubstituted
olefin, etc., can be exploited further in the synthesis of various analogues of
bioactive terpene natural products. The method, in general, which is carried
at ambient conditions with high yields of adducts, has the potential for
rapid assembly of decalin skeleton with an o-quinone moiety and can be
further exploited for synthesis of several analogues.
EXPERIMENTAL
IR spectra were recorded on a Perkin–Elmer model 681 spectrometer. 1H
NMR and 13C NMR, spectra were recorded on 300 MHz and 125 MHz spec-
trometers in CDCl3 with TMS as internal standard. Microanalysis was done on
Carlo Erba Strumentazinone 1106 elemental analyzer. Silica gel (100–200
mesh) used for column chromatography was activated by heating at 1208C
for 4 hr. Dienes 8 and 9 were obtained from Aldrich. Diene 24 was obtained
as described earlier.[19]
Synthesis of diene 14
Synthesis of 3-Methyl-2-buten-1-ol Benzyl Ether (11)
NaH (50% suspension in oil, 5.28 g, 110 mmol), taken in a flame-dried
three-necked, round-bottomed flask fitted with an addition funnel and a
reflux condenser, was washed with dry petroleum ether. Dry THF (35 mL)
was added to the flask and cooled to 08C. 3-Methyl-2-butenol (10, 7.95 g,
92.5 mmol) in THF (50 mL) was taken in the addition funnel and added drop-
wise to the suspension of NaH in THF. The reaction mixture was allowed to
reflux for an hour, cooled again to 08C, and a mixture benzyl bromide (18.8 g,
110 mmol) in dry THF (50 mL) was added to the reaction mixture via a
syringe. The reaction mixture was stirred at the same temperature for an
hour and then refluxed for 3 hr. It was then quenched with water, THF evap-
orated in vacuo and extracted with ethyl acetate, washed with brine, and dried
over anhydrous Na2SO4. The solvent was removed in vacuo, and the mixture
was purified over silica gel column to obtain the 3-methyl-butenyl-benzyl
ether (11, 13.98 g, 86%).
Oxidation of 3-Methylbutenyl Benzyl Ether (11)
SeO2 (2.28 g, 20.6 mmol), 70% aq. tert-butylhydropetoxide (TBHP,
20.5 mL, 160 mmol) in dichloromethane were stirred for 30 min at room

4072
Meenakshi et al.
temperature. To the reaction mixture, the 3-methylbutenyl benzyl ether (11,
13.2 g, 75 mmol) was added and stirred for 6 hr. Stirring was further continued
for 16 hr. Reaction mixture was quenched with 10% sodium bisulfite solution,
extracted with dichloromethane, washed with NaHCO3, water, and brine and
dried over anhydrous Na2SO4. The solvent was removed in vacuo and column
performed over silica gel to obtain the aldehyde 12 (7.84 g, 55%) and the
allylic alcohol 13 (2.74 g, 19%).
Wittig Reaction of the Aldehyde 12
Methyltriphenylphosphonium iodide (10.38 g, 25 mmol) was taken in a
flame-dried three-necked flask under nitrogen atmosphere in dry THF
(50 mL). The reaction mixture was cooled to 08C and n-BuLi (1.6 M in
hexane, 15.6 mL, 25 mmol) was added dropwise via syringe, and the reaction
was allowed to stir for 30 min at room temperature. The reaction mixture was
cooled to 08C again, and a solution of the aldehyde 12 (3.85 g, 20 mmol) in dry
THF (10 mL) was added dropwise through syringe and allowed to stir for 3 hr.
Reaction quenched with a slow addition of saturated NH4Cl solution, THF
evaporated in vacuum, and the residue was extracted with ethyl acetate,
washed with brine, and dried over anhydrous Na2SO4. The solvent was
removed in vacuum and the residue was purified over silica gel column to
give the 1,3-diene 14 (3.24 g, 86.2%).
IR (Neat, vmax): 3032, 2927, 2868, 1617, 1522, 1459, and 1374 cm21
.
Synthesis of 1,3-diene 19
Tetrahydropyranyl Ether of Citronellol (16)
Citronellol (15, 8 g, 51.2 mmol) was added to a solution of dihydropyran
(5.04 g, 60 mmol), ceric ammonium nitrate (catalytic amount) and acetonitrile
(80 mL) at 108C with stirring.[20] The stirring was continued for an hour at
room temperature. After the reaction was completed, the solvent was
removed in vacuum, the residue was extracted with ether, washed with
brine, dried over anhydrous Na2SO4, and solvent was removed in vacuum.
The residue was subjected to column chromatography over silica gel to give
the citronellolyl-tetrahydropyran ether (16, 10.22 g, 85%).
Oxidation of Citronellolyl-tetrahydropyran Ether (16)
Selenium dioxide (0.998 g, 9 mmol), 70% tert-butyl hydroperoxide
(9 mL, 70 mmol) were stirred in dry dichloromethane (35 mL) for 30 min.

Synthesis of Labdane and Drimane Analogues
4073
To this, a solution of the citronellolyl-tetrahydropyran ether (16, 8.4 g,
35 mmol) in dry dichloromethane (40 mL) was added and allowed to stir for
16 hr at room temperature. After the reaction was completed, the mixture
was quenched with 10% sodium bisulfite solution, extracted with dichloro-
methane. The organic layer was then washed with NaHCO3, water, and
brine and dried over anhydrous Na2SO4. The solvent was removed in
vacuum, and column chromatography was performed over silica gel to
obtain the aldehyde 17 (3.17 g, 36%) and the alcohol 18 (2.96 g, 33%).
Aldehyde 17: IR (Neat, vmax): 3028, 2917, 1692, 1594, 1459, and
1
1374 cm21. H NMR (CDCl3, 60 MHz): d 9.6 (1H, s), 6.6 (1H, t), 4.5 (1H,
m), 4.1–3.4 (4H, m), 2.2 (2H, m), 2.1 (3H, s), 1.7–1.3 (11H, m) and 0.95
(3H, d).
Wittig Reaction of Aldehyde 17
Methyltriphenylphosphonium iodide (4.14 g, 10 mmol) was taken in a
three-necked round-bottomed flask containing dry THF (30 mL). The flask
was cooled to 08C and to this was added n-BuLi (1.6 M in hexane, 6.6 mL,
10 mmol) in drops via syringe. The reaction mixture stirred at room tempera-
ture for 30 minutes, cooled again to 08C and a solution of the aldehyde 17
(2.0 g, 7.87 mmol) in 10 ml of dry THF was added via syringe. Reaction
was allowed to stir for 3 h at room temperature and quenched with saturated
NH4Cl. THF was evaporated in vacuum and the residue was extracted with
ethyl acetate, ethyl acetate extracts were washed with brine, dried over anhy-
drous Na2SO4. The solvent was removed under reduced pressure and the
column chromatography of the residue over silica gel using pet ether: ethyl
acetate as eluent gave 1,3-diene 19 (1.6 g, 81% yield).
1
IR (Neat, vmax): 2949, 2868, 1613, 1609, 1464 cm21. H NMR (CDCl3,
60 MHz): d 6.2 (1H, m), 5.3 (1H, m), 5.0 (2H, m), 4.5 (1H, m), 4.0–3.4
(4H, m), 1.9–1.6 (5H, m), 1.6–1.4 (11H, m), and 0.95 (3H, d).
General Procedure for the Diels–Alder Reaction of
4-Carbomethoxybenzoquinone with 1,3-Dienes
To a solution of methyl 3,4-dihydoxy-benzoate (25, 1.5 mmol), 1,3-diene
(8, 9, 14, 19 or 24, 1 mmol) cooled to 08C in benzene: ether (3 : 1), was added
freshly prepared Ag2O (2.0 mmol) and stirring continued for 3 hr at 08C and
then at room temperature for the period mentioned in Table 1. The reaction
mixture was filtered through celite, washed with ether, the filtrates were com-
bined, dried and evaporated. The column chromatography of the residue over
silica gel gave the products 26–30 in the yields mentioned in Table 1.

4074
Meenakshi et al.
Spectral data of the adducts, 26–30
5,8-Dihydro-1-hydroxy-4a-methoxycarbonyl-5-methyl-naphthalen-2-
(4aH)-one (26)
IR (Neat, vmax): 3450, 1740, and 1680 cm21. 1H NMR (CDCl3, 300 MHz):
d 6.9 (1H, d, J 9.88), 6.56 (1H, d, J 9.88), 5.84 (1H, m), 5.68 (1H, dt, J 13.2,
4.4), 3.7 (3H, s), 3.42 (1H, m), 3.3 (1H, m), 3.04 (1H, m), 1.78(1H, bs),
0.77(3H, d, J ¼ 7 Hz); MS: m/z 234 (Mþ); Analysis: C13H14O4 requires: C,
66.66%; H, 6.02%; Found C, 66.40%, H, 5.81%.
5,8-Dihydro-1-hydroxy-4a-methoxycarbonyl-6 (or 7)-methyl-
naphthalen-2-(4aH)-one (27 and 27a)
IR (Neat, vmax): 3400, 1740, and 1680 cm21. 1H NMR (CDCl3, 300 MHz):
d 6.97 and 6.93 (1H, d, J ¼ 9.9), 6.5 and 6.46 (1H, d, J ¼ 9.9), 5.45 (1H, m),
3.68 and 3.67 (3H, s), 3.48 (2H, d, Jgem ¼ 21.8), 3.1–3.0 (2H, m), 2.05 (1H,
m), 1.8 (3H, s); MS: m/z 234 (Mþ).
5-(Benzyloxymethyl)-5.8-Dihydro-1-hydroxy-4a-methoxycarbonyl-6-
methyl-naphthalen-2-(4aH)-one (28)
IR (Neat, vmax): 3490, 1750, and 1670 cm21. 1H NMR (CDCl3, 300 MHz):
d 7.33 (5H, m), 7.0 (1H, d, J ¼ 9.7), 6.62 (1H, brs), 6.46 (1H, d, J ¼ 9.7), 5.43
(1H, m), 4.32 (2H, m), 3.60 (3H, s), 3.31 (3H, m), 3.05 (2H, m), 1.78 (3H, s);
MS: m/z 340 (Mþ); Analysis: C20H22O5 requires: C, 71.17%; H, 6.26%;
Found: C, 70.81%; H, 6.26%.
5,8-Dihydro-1-hydroxy-4a-methoxycarbonyl-6-methyl-5-(30-methyl-
50-tetrahydro-pyranyloxy-pentan-10-yl)-naphthalen-2(4aH)-one (29)
IR (Neat, vmax): 3400, 1730, and 1650 cm21. 1H NMR (CDCl3, 300 MHz):
d 6.9 (1H, d, J ¼ 9.9), 6.5 (1H, d, J ¼ 9.9), 5.4 (1H, m), 4.8 (1H, brs,-O-CH-O-),
3.8 (3H, m), 3.65 (3H, s), 3.4 (2H, m), 3.0 (2H, m), 1.8 (3H, s), 1.9–1.4 (14H,
m), 0.8 (3H, d, J ¼ 6.6); MS: m/z 418 (Mþ); Analysis: C24H34O6 requires: C,
68.87%, H, 8.18%; Found: C, 68.64%; H, 8.25%.
5,8-Dihydro-1-hydroxy-4a-methoxycarbonyl-6-methyl-5-(30-
methylenedioxy-butan-10yl)-naphthalen-2-(4aH)-one (30)
1
IR (Neat, vmax): 3400, 1740, and 1650 cm21. H NMR (CDCl3): d 6.9
(1H, d, J ¼ 9.8), 6.6 (1H, brs,-OH), 6.5 (1H, d, J ¼ 9.8), 5.4 (1H, m), 3.8

Synthesis of Labdane and Drimane Analogues
4075
(4H, m), 3.6 (3H, s), 3.3 (1H, m), 3.0 (1H, m), 2.9 (1H, m), 1.8 (3H, s), 1.6 (2H,
m), 1.16 (2H, m), 1.1 (3H, s); MS: m/z 348 (Mþ); Anal. C19H24O6 requires: C,
65.50%; H, 6.94%; Found: C, 65.38%; H, 6.75%.
ACKNOWLEDGMENTS
The spectral assistance from R. S. I. C., Bombay and T. I. F. R. Mumbai
and sample of citronellol from S. H. Kelkar and Co., Mumbai are gratefully
acknowledged. We are thankful to Council of Scientific and Industrial
Research for financial assistance.
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Received in the USA July 12, 2004