Synthesis and antitubercular activity of tricyclic analogs of puupehenone

Article abstract of DOI:10.1016/j.tet.2004.03.043

Tricyclic analogs 2a and 8 were prepared by four-step routes. The key step was an intermolecular hetero Diels-Alder reaction involving a quinone methide.

Full text of DOI:10.1016/j.tet.2004.03.043

Tetrahedron 60 (2004) 4223–4225
Synthesis and antitubercular activity of tricyclic analogs
of puupehenone
a
a
b
b
George A. Kraus,a Tuan Nguyen, Jaehoon Bae, Jesse Hostetter and Ed Steadham
,
*
aDepartment of Chemistry, Iowa State University, Ames, IA 50011, USA
bDepartment of Veterinary Pathology, Iowa State University, Ames, IA 50011, USA
Received 18 December 2003; revised 16 March 2004; accepted 16 March 2004
Abstract—Tricyclic analogs 2a and 8 were prepared by four-step routes. The key step was an intermolecular hetero Diels–Alder reaction
involving a quinone methide.
q 2004 Elsevier Ltd. All rights reserved.
Puupehenone (1a), a tetracyclic terpene bearing a quinone
methide subunit, was isolated from a deep water marine
sponge, Strongylophora hartmani.1 It exhibits 99%
inhibition of M. tuberculosis H37Rv at 12.5 mg/ml.2
Recently, O-methyl puupehenone (1b) was isolated and
found to inhibit lipoxygenase activity at the sub micromolar
level.3 As part of a program to identify useful antitubercular
agents for animal and human use,4 we decided to synthesize
and evaluate 2a, a tricyclic analog of 1a. Recently, a
tricyclic ortho-quinone related to 2b was synthesized by a
route very different from our pathway and was a more active
antitumor agent than 1a (Scheme 1).5
Our route began with commercially available 3,4-
dimethoxyphenol (3). Hydroxymethylation of phenols
under basic conditions provides 2-hydroxymethylphenols.10
When 3,4-dimethoxyphenol was treated with formalin
and aqueous calcium oxide, the desired alcohol 4 was
produced. Alcohol 4 was unstable to silica gel chromato-
graphy and decomposed in hours at ambient temperature.
In practice, freshly prepared 4 was added to 1-methyl-
cyclohexene and treated with trifluoroacetic acid using the
protocol of Pettus8 to afford the tricyclic benzopyran 5 in
45% yield over two steps from 3,4-dimethoxyphenol.
Compound 5 was produced as a single regioisomer. The
[4þ2] cycloaddition generated both the carbon–carbon and
carbon–oxygen bonds with a cis ring juncture, as
determined by a NOESY experiment. The demethylation
of 5 with boron tribromide afforded a catechol in 60%
yield. Previous syntheses of puupehenone utilized a
variety of oxidants to generate the quinone methide
moiety. The reaction of PDC afforded an inseparable
mixture of the desired quinone methide 2a plus some of
the ortho-quinone.
We envisioned the synthesis of 2a via a hetero-Diels–Alder
reaction involving a quinone methide. Quinone methides
were initially employed by Buchi in his elegant synthesis of
gymnitrol.6 The intramolecular version was used by Tius in
his innovative synthesis of canniboid natural products.7
Recently, Pettus and co-workers reported an interesting
alkylative variant.8 Quinone methides have been used in the
synthesis of a number of compounds.9
Scheme 1.
Keywords: Tricyclic analogs; Puupehenone; Strongylophora hartmani.
*
Corresponding author. Tel.: þ1-5152947794; fax: þ1-5152940105; e-mail address: gakraus@iastate.edu
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.03.043

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G. A. Kraus et al. / Tetrahedron 60 (2004) 4223–4225
Scheme 2.
The synthesis of the O-methyl analog was also achieved.
This synthesis began with the benzylation of isovanillin
followed by oxidation of the aldehyde to a formate and
hydrolysis of the formate with aqueous sodium hydroxide,
to produce the known phenol 6 in 80% yield.11 Hydroxy-
methylation of 6 with formalin and calcium oxide followed
by a hetero-Diels–Alder reaction afforded benzopyran 7 in
40% yield over two steps. Catalytic hydrogenation12 of
compound 7 followed by treatment with DDQ, afforded
product 8 in 95% isolated yield. The structure of 8 was
confirmed by 1H NMR, 13C NMR, HRMS, and UV
spectrometry (Scheme 2).
(s, 2H), 3.84 (s, 3H), 3.80 (s, 3H); 13C NMR 159.9, 149.5,
142.3, 116.7, 112.5, 101.4, 62.8, 56.8, 55.9. TLC (2:1
hexanes/ethyl acetate) Rf¼0.23.
1.1.2. 6,7-Dimethoxy-4a-methyl-2,3,4,4a,9,9a-hexa-
hydro-1H-xanthene (5). To compound
4
(0.6 g,
3.26 mmol) in 40 mL of CHCl3 at 0 8C, was added
1-methylcyclohexene (0.47 g, 4.9 mmol) followed by drop-
wise addition of trifluoroacetic acid (0.45 g, 3.91 mmol).
The mixture was boiled for 3 h. After cooling to rt, the
solution was diluted with water and extracted with CH2Cl2,
dried, concentrated and purified by sgc using 4:1 hexanes/
ethyl acetate to afford benzopyran 5 in 45% yield from 3,4-
dimethoxyphenol. The cis-ring juncture was assigned by
The combination of regioselective hydroxymethylation with
the hetero-Diels–Alder reaction provides a convenient
synthesis of tricyclic skeletons common to bioactive
terpenes. This pathway should be compatible with con-
siderable structural variation on both the phenol and alkene
units.
1
NOESY spectroscopy. H NMR (300 MHz, CDCl3) d 6.52
(s, 1H), 6.38 (s, 1H), 3.82 (s, 3H), 3.81 (s, 3H), 3.03 (dd,
J¼12, 6 Hz, 1H), 2.25 (d, J¼18 Hz, 1H), 1.94–1.90 (m,
1H), 1.67–1.24 (m, 8H), 1.19 (s, 3H); 13C NMR (300 MHz,
CDCl3) d 148.4, 146.9, 142.8, 112.8, 110.31 101.3, 74.9,
56.6, 56.0, 38.7, 37.2, 29.2, 28.7, 25.8, 25.6, 21.9. HRMS
(EI) calcd, for 262.1569, found 262.1572. TLC (2:1
hexanes/ethyl acetate) Rf¼0.72.
Compound 8 and the aminoglycoside antibiotic gentamycin
were tested in broth cultures of Mycobacterium avium
subspecies paratuberculosis (M. a. ptb). Broth were
sampled weekly and number of colony forming units were
determined for each time point. Values are expressed as the
percentage of the control treatment (bacteria in media
alone). Preliminary results indicated that with a concen-
tration of 8 at 0.625 mg/ml, after one week 43% of the
bacteria remained, compared to 0.1% remaining with
gentamycin.
1.1.3. 2-Hydroxy-10a-methyl-5,6,7,8,8a,10a-hexahydro-
xanthen-3-one (2a). To compound 5 (0.05 g, 0.19 mmol)
in 5 mL of CH2Cl2 at 278 8C, was added slowly 1.0 M
boron tribromide (0.57 mL, 0.57 mmol). After 30 min at
278 8C, the mixture was warmed to rt and stirred for 4 h.
The solution was diluted with CH2Cl2 and water was added
slowly. The aqueous layer was extracted with CH2Cl2 and
the combined organic layers were washed with saturated
NaHCO3, brine, dried, concentrated and purified by sgc
using 2:1 hexanes/ethyl acetate. 1H NMR (300 MHz,
CDCl3) d 6.69 (s, 1H), 6.37 (s, 1H), 3.00 (dd, J¼15, 6 Hz,
1H), 2.21 (d, J¼18 Hz, 1H), 1.91–1.10 (m, 9H), 1.21 (s,
3H); 13C NMR (300 MHz, CDCl3) d 147.2, 142.9, 137.0,
116.2, 111.9, 104.3, 74.9, 38.7, 37.1, 29.1, 28.6, 25.8, 25.6,
21.9; HRMS (EI) m/z calcd for 234.1256, found 234.1259.
TLC (2:1 hexanes/ethyl acetate) Rf¼0.37.
1. Experimental
1.1. General
1.1.1. 2-Hydroxymethyl-4,5-dimethoxy-phenol (4). To
3,4-dimethoxyphenol (1.00 g, 6.5 mmol) in 20 mL of
water at rt, was added 37% formalin (1.4 mL, 13.4 mmol)
and followed by calcium oxide (0.18 g, 3.3 mmol). After
one h, saturated aqueous ammonium chloride was added
and the organic layer was extracted with ether, dried,
concentrated and used immediately without purification. 1H
NMR (300 MHz, CDCl3) d 6.54 (s, 1H), 6.51 (s, 1H), 4.80
To the catechol (0.020 g, 0.085 mmol) in 2 mL of CH2Cl2 at
rt was added PDC (0.05 g, 0.13 mmol). After 4 h, the
mixture was filtered through Celite and concentrated to
afford 2a and the ortho-quinone.

G. A. Kraus et al. / Tetrahedron 60 (2004) 4223–4225
4225
1
Spectra for ortho-quinone. H NMR (300 MHz) 6.17(dd,
was added DDQ (0.022 g, 0.096 mmol). After 2 h, the
mixture was filtered through Celite, concentrated and
purified by sgc using 4:1 hexanes/ethyl acetate to afford 8
in 95% yield. 1H NMR (300 MHz, CDCl3) d 7.23 (bs, 1H),
6.46 (s, 1H), 6.18 (s, 1H), 3.89 (s, 3H), 2.22 (dd, J¼12, 3 Hz,
1H), 2.05–2.00 (m, 1H), 1.75–1.30 (m, 8H), 1.28 (s, 3H);
13C NMR (300 MHz, CDCl3) d 195.6, 156.4, 153.6, 142.0,
111.5, 106.8, 103.7, 79.8, 56.5, 52.5, 37.1, 26.9, 24.8, 24.2,
21.7. UV (CH2Cl2) lmax 240, 272, 335 nm; HRMS (EI) m/z
calcd for 246.1256, found 232.1258. TLC (2:1 hexanes/
ethyl acetate) Rf¼0.43.
J¼2.1, 0.9 Hz, 1H), 5.84(s, 1H), 3.09–3.16(m, 1H), 2.43(d,
J¼13.8 Hz, 1H), 1.96–2.04 (m, 1H), 1.43–1.89 (m, 1H),
1.40 (s, 3H).
1
Spectra for 2a. H NMR (300 MHz, CDCl3) d 7.21 (d,
J¼1.5 Hz, 1H), 6.35 (s, 1H), 5.95 (d, J¼3 Hz, 1H), 2.18–
2.10 (m, 1H), 1.98–1.21 (m, 8H), 1.19 (s, 3H); HRMS (EI)
m/z calcd for 232.1010, found 232.1102. TLC (2:1 hexanes/
ethyl acetate) Rf¼0.22.
1.1.4. 6-Benzyloxy-7-methoxy-4a-methyl-2,3,4,4a,9,9a-
hexahydro-1H-xanthene (7). To compound 6 (0.8 g,
3.5 mmol) in 20 mL of water at rt was added 37% formalin
(0.77 g, 7.35 mmol) followed by calcium oxide (0.098 g,
1.75 mmol). After an hour, saturated aqueous ammonium
chloride was added and the organic layer was extracted with
ether, dried, concentrated and used immediately without
Acknowledgements
We thank Iowa State University and The Healthy Livestock
Initiative Competitive Grants Program 2003, administered
by the ISU Veterinary College, for partial support of this
research.
1
purification. H NMR (300 MHz, CDCl3) d 7.43–7.30 (m,
5H), 6.56 (s, 1H), 6.51 (s, 1H), 5.10 (s, 2H), 4.76 (s, 2H),
3.82 (s, 3H); 13C NMR 149.2, 148.9, 142.9, 136.9, 128.8,
128.2, 127.7, 117.3, 113.4, 103.7, 71.1, 66.2, 57.1. TLC (2:1
hexanes/ethyl acetate) Rf¼0.13.
References and notes
To the crude benzyl alcohol (0.4 g, 1.53 mmol) in 20 mL of
CHCl3 at 0 8C, was added 1-methylcyclohexene (0.22 g,
2.3 mmol) followed by dropwise addition of trifluoroacetic
acid (0.035 g, 1.84 mmol). The mixture was boiled for 3 h.
After cooling to rt, the solution was diluted with water and
extracted with CH2Cl2, dried, concentrated and purified by
sgc using 4:1 hexanes/ethyl acetate to afford benzopyran 7
in 40% yield over two steps. 1H NMR (300 MHz, CDCl3) d
7.50–7.28 (m, 5H), 6.57 (s, 1H), 6.43 (s, 1H), 5.08 (s, 2H),
3.82 (s, 3H), 3.04 (dd, J¼15, 6 Hz, 1H), 2.26 (d, J¼15 Hz,
1H), 1.98–1.25 (m, 9H), 1.19 (s, 3H); 13C NMR (300 MHz,
CDCl3) d 148.0, 147.1, 143.5, 137.5, 128.7, 127.9, 127.6,
113.9, 111.2, 103.5, 74.9, 71.1, 57.1, 38.7, 37.2, 29.2, 28.7,
25.8, 25.6, 21.9. HRMS (EI) m/z calcd for 338.1882, found
338.1887. TLC (2:1 hexanes/ethyl acetate) Rf¼0.83.
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To the phenol (0.02 g, 0.08 mmol), in 5 mL of dioxane at rt