Microwave-mediated claisen rearrangement followed by phenol oxidation: A simple route to naturally occurring 1,4-benzoquinones. The first syntheses of verapliquinones A and B and panicein A

Article abstract of DOI:10.1021/jo050336x

The naturally occurring 1,4-benzoquinones 2-methoxy-6-propyl-1,4- benzoquinone (1), 2-methoxy-6-pentyl-1,4-benzoquinone (primin 2), 2-methoxy-6-pentadecyl-1,4-benzoquinone (3), 2-methoxy-6-heptadecyl-l,4- benzoquinone (dihydroirisquinone, pallasone B; 4) were synthesized by a simple protocol involving microwave accelerated Claisen rearrangement of allyl ethers 10, followed by hydrogenation of the side chain alkene, and oxidation to the quinone. The Claisen-based methodology was extended to the first synthesis of the marine benzoquinones verapliquinones A and B (5 and 6), and panicein A (7). Isoarnebifuranone (9) was also synthesized by a similar strategy.

Full text of DOI:10.1021/jo050336x

Microwave-Mediated Claisen Rearrangement Followed by Phenol
Oxidation: A Simple Route to Naturally Occurring
1,4-Benzoquinones. The First Syntheses of Verapliquinones A and
B and Panicein A
Christopher J. Davis, Timothy E. Hurst, Aouregan M. Jacob, and Christopher J. Moody*
Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom
c.j.moody@ex.ac.uk
Received February 22, 2005
The naturally occurring 1,4-benzoquinones 2-methoxy-6-propyl-1,4-benzoquinone (1), 2-methoxy-
6-pentyl-1,4-benzoquinone (primin 2), 2-methoxy-6-pentadecyl-1,4-benzoquinone (3), 2-methoxy-
6-heptadecyl-1,4-benzoquinone (dihydroirisquinone, pallasone B; 4) were synthesized by a simple
protocol involving microwave accelerated Claisen rearrangement of allyl ethers 10, followed by
hydrogenation of the side chain alkene, and oxidation to the quinone. The Claisen-based methodology
was extended to the first synthesis of the marine benzoquinones verapliquinones A and B (5 and
6), and panicein A (7). Isoarnebifuranone (9) was also synthesized by a similar strategy.
Introduction
oxy-6-propyl-1,4-benzoquinone (1), 2-methoxy-6-pentyl-
1,4-benzoquinone (primin 2), 2-methoxy-6-pentadecyl-1,4-
benzoquinone (3), 2-methoxy-6-heptadecyl-1,4-benzo-
quinone (dihydroirisquinone, pallasone B) (4), verapli-
quinones A and B (5 and 6), panicein A (7), and
isoarnebifuranone (9) (Figure 1).3 Of these, the verapli-
quinones and panicein A have not previously been
synthesized.
Quinones, particularly the terpenoid benzoquinones,
are widespread in Nature,1 and are essential for many
life processes. Although quinones, the largest group of
natural pigments, do contribute to natural color, their
major role is in redox processes. For example, the
ubiquinones act as essential electron-transfer agents in
the respiratory chain, and pyrroloquinolinequinone (PQQ)
is a redox cofactor. Other quinone natural products also
possess potent biological activity: for example, adriamy-
cin (doxorubicin) is a front-line cancer chemotherapy
treatment. Although quinones such as these are struc-
turally quite complex, even the relatively simple terpe-
noid benzoquinones possess significant biological activity,
primin 2, for example, being a potent skin sensitizer (see
below). Although naturally occurring benzoquinones have
been synthesized by a variety of routes, there is no
general route to these natural products. We now report
a simple and versatile route to benzoquinones based on
the Claisen rearrangement,2 and illustrate it by the
synthesis of the naturally occurring benzoquinones 2-meth-
Results and Discussion
The 6-alkyl-2-methoxy-1,4-benzoquinones 1-4 are iso-
lated from a variety of natural sources. 2-Methoxy-6-
propyl-1,4-benzoquinone (1) is found in the fungus Ca-
marops microspora;4 the compound was originally synthe-
sized by Dean et al. in 1955,5 and more recently by Ko¨nig
et al.6 Primin (2-methoxy-6-pentyl-1,4-benzoquinone) 2,
originally isolated from the primose Primula obconica,7
(3) Preliminary communication: Davis, C. J.; Moody, C. J. Synlett
2002, 1874-1876.
(4) Volc, J.; Sedmera, P.; Roy, K.; Sasek, V.; Vokoun, J. Collect.
Czech. Chem. Commun. 1977, 42, 2957-2961.
(5) Dean, F. M.; Osman, A. M.; Robertson, A. J. Chem. Soc. 1955,
11-17.
(6) Ko¨nig, W. A.; Faasch, H.; Heitsch, H.; Colberg, C.; Hausen, B.
M. Z. Naturforsch. (B) 1993, 48, 387-393.
(7) Bloch, B.; Karrer, P. Vjschr. Naturforsch. Ges. Zu¨rich 1928, 72,
1; Chem. Abstr. 1928, 22, 2784.
* To whom correspondence should be addressed. Fax +44-1392-263-
434.
(1) Thomson, R. H. Naturally Occurring Quinones, 4th ed.; Black-
ie: London, UK, 1997.
(2) Castro, A. M. M. Chem. Rev. 2004, 104, 2939-3002.
10.1021/jo050336x CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/28/2005
4414
J. Org. Chem. 2005, 70, 4414-4422

Verapliquinones A and B and Panicein A
SCHEME 1. Retrosynthetic Analysis of
2,6-Disubstuted-1,4-benzoquinones
The requisite allyl aryl ethers 10 were prepared by
Mitsunobu reaction (Ph3P, DIAD) of 2-methoxyphenol
with the appropriate allylic alcohol, which was prepared
by addition of vinylmagnesium bromide to the corre-
sponding aldehyde, although 2-methoxyphenyl allyl ether
(10a) is commercially available (or readily prepared by
reaction of 2-methoxyphenol with allyl bromide). Al-
though the Claisen rearrangement of the allyl ether 10a
has been carried out by prolonged heating at 180 °C,18
we investigated a range of conditions using the allyl ether
10b as substrate, including microwave heating, condi-
tions that are reported to accelerate Claisen rearrange-
ments.19-25 Heating under reflux for 3 h in 1,2-dichlo-
robenzene resulted in isolation of the required rearrange-
ment product in 50% yield as a single (presumably E)
isomer. However, under controlled microwave irradiation
(focused reactor, 300 W, 180 °C), the allyl ether 10b
underwent rapid Claisen rearrangement in DMF as
solvent in excellent yield in 25 min. Application of these
conditions to the other allyl aryl ethers 10 resulted in
equally rapid reactions, and gave the rearrangement
products 11 in good yield (Table 1) as single geometric
isomers. In the case of 11c, the (E)-geometry of the double
FIGURE 1. Structures of some naturally occurring benzo-
quinones.
and more recently from wood of Miconia species,8 is the
most studied of the alkylbenzoquinones because of its
allergenic properties.9 It is a potent sensitizer and
induces contact dermitis. It was first prepared in 1967
by Schildknecht,10 and has been the subject of further
synthetic studies, including those of Ko¨nig et al. as part
of a study of the allergenic properties of 6-alkyl-2-
methoxy-1,4-benzoquinones,6 and more recently Mabic et
al.9 2-Methoxy-6-pentadecyl-1,4-benzoquinone (3) is iso-
lated from seed oils of various iris species (Iris pseuda-
corus, I. sibirica, I. missourensis),11 and has been syn-
thesized by Ko¨nig as part of the aforementioned study.6
The seed oil of I. pseudacorus also contains irisquinone,
hydrogenation of which gives dihydroirisquinone.12 This
dihydro derivative, also known as pallasone B, is isolated
from I. pallash fisch (var. chinensis fisch),13 and in trace
amounts from other iris species.11 Pallasone B has been
previously synthesized.14
Simple retrosynthetic analysis of the quinones 1-4
suggests that they could be obtained by oxidation of the
corresponding 2-methoxy-6-substituted phenols, them-
selves obtained by Claisen rearrangement of the ap-
propriately substituted allyl aryl ethers (Scheme 1). Since
its discovery almost a century ago, the aromatic Claisen
rearrangement has found a myriad of uses in organic
synthesis,2 and not surprisingly there are examples of
its use in the synthesis of benzoquinones, albeit few.5,15-17
1
bond was proved by H NMR spectroscopy; in the rear-
rangement products 11b and 11d, overlap of NMR
signals prevented unambiguous assignment of the alkene
geometry, but they are assumed to be (E). The next step
was to hydrogenate the double bond to give the 2-meth-
oxy-6-alkylphenols 12 in good yield. Finally, phenolic
oxidation gave the corresponding 2-methoxy-6-alkyl-1,4-
benzoquinones 1-4 (Scheme 2), whose properties matched
those described for the natural products. All oxidations
were initially carried out with Fremy’s salt (potassium
nitrosodisulfonate),26 but in cases where this proved
unsatisfactory, oxygen in the presence of salcomine was
used.27,28
To extend the microwave-mediated Claisen rearrange-
ment based methodology to more complex benzoquinones,
we considered the marine natural products verapliquino-
(15) Barnes, M. F.; Ollis, W. D.; Sutherland, I. O.; Gottlieb, O. R.;
Magalhaes, M. T. Tetrahedron 1965, 21, 2707-2715.
(16) Reinaud, O.; Capdevielle, P.; Maumy, M. Tetrahedron Lett.
1985, 26, 3993-3996.
(17) Tisdale, E. J.; Vong, B. G.; Li, H. M.; Kim, S. H.; Chowdhury,
C.; Theodorakis, E. A. Tetrahedron 2003, 59, 6873-6887.
(18) Capecchi, T.; de Koning, C. B.; Michael, J. P. J. Chem. Soc.,
Perkin Trans. 1 2000, 2681-2688.
(19) Giguere, R. J.; Bray, T. L.; Duncan, S. M.; Majetich, G.
Tetrahedron Lett. 1986, 27, 4945-4948.
(8) Marini-Bettolo, G. B.; Monache, F. D.; daLima, O. G.; Coelho, S.
D. Gazz. Chim. Ital. 1971, 101, 41-46.
(20) Bagnell, L.; Cablewski, T.; Strauss, C. R.; Trainor, R. W. J. Org.
Chem. 1996, 61, 7355-7359.
(9) Mabic, S.; Vaysse, L.; Benezra, C.; Lepoittevin, J. P. Synthesis
1999, 1127-1134.
(21) An, J. Y.; Bagnell, L.; Cablewski, T.; Strauss, C. R.; Trainor,
R. W. J. Org. Chem. 1997, 62, 2505-2511.
(10) Schildknecht, H.; Schmidt, H. Z. Naturforsch. 1967, 22b, 287-
294.
(22) Kumar, H. M. S.; Anjaneyulu, S.; Reddy, B. V. S.; Yadav, J. S.
Synlett 2000, 1129-1130.
(11) Marner, F. J.; Horper, W. Helv. Chim. Acta 1992, 75, 1557-
1562.
(23) Daskiewicz, J. B.; Bayet, C.; Barron, D. Tetrahedron Lett. 2001,
42, 7241-7244.
(12) Seki, K.; Kaneko, R. Chem. Ind. (London) 1975, 349-350.
(13) Wu, S. J.; Zhang, L.; Yang, X. X.; Li, D. Y. Acta Chim. Sin.
1981, 39, 767-770.
(24) Al-Maharik, N.; Botting, N. P. Tetrahedron 2003, 59, 4177-
4181.
(14) Tao, Z.; Liu, J. Zhongguo Yiyao Gongye Zazhi 1991, 22, 57-
59; Chem. Abstr. 1991, 115, 8400.
(25) Kotha, S.; Mandal, K.; Deb, A. C.; Banerjee, S. Tetrahedron Lett.
2004, 45, 9603-9605.
J. Org. Chem, Vol. 70, No. 11, 2005 4415

Davis et al.
NMR spectroscopy.29 Separation of the isomers was
reported to be extremely difficult even by HPLC, and only
enough pure verapliquinone A and verapliquinone B for
a UV spectrum was obtained. Panicein A was isolated
from the breadcrumb sponge Halichondria panicea iso-
lated in the Bay of Naples in 1973, and its structure
assigned by NMR spectroscopy,30 although the geometry
of the double bond was not reported. Panicein A, along
with the corresponding hydroquinone, has also been
isolated from the sponge Reniera mucosa by Salva and
co-workers,31 and the hydroquinone is also found in R.
fulva.32 Again, no conclusive proof of the alkene geometry
was offered, although nOe studies confirm the (E)-
geometry of the double bond in the hydroquinone.33
Therefore, we assume that panicein A has the structure
7.
SCHEME 2. Synthesis of
2-Methoxy-6-propyl-1,4-benzoquinone (1),
2-Methoxy-6-pentyl-1,4-benzoquinone (primin 2),
2-Methoxy-6-pentadecyl-1,4-benzoquinone (3), and
2-Methoxy-6-heptadecylyl-1,4-benzoquinone
(dihydroirisquinone, pallasone B; 4)a
Since the verapliquinones and panicein A contain a
trisubstituted alkene side chain, the Claisen rearrange-
ment precursor of necessity contains a quaternary center.
Therefore a slightly different strategy was required for
the preparation of the required allyl aryl ether. The
synthesis of the verapliquinones is shown in Scheme 3,
and starts from the known propargylic alcohol 3,7-
dimethyloct-6-en-1-yn-3-ol (13),34 prepared by the addi-
tion of ethynylmagnesium bromide to 6-methyl-5-hepten-
2-one in 69% yield. The propargyl alcohol 13 was then
coupled to commercially available 2-methoxyphenol by
using a method developed for tertiary propargyl ethers
that involves the copper(II)-catalyzed reaction of the
corresponding trifluoroacetate in the presence of DBU.35
This gave the aryl alkynyl ether 14 in 56% yield,
hydrogenation of which over Lindlar’s catalyst gave the
required allyl aryl ether 15. Microwave-mediated Claisen
rearrangement of ether 15 gave the required 2-(3,7-
dimethyl-octa-2,6-dienyl)-6-methoxyphenol (16) as a mix-
ture of E,Z-isomers in 83% yield. The two isomers of 16
were not separable by flash chromatography, so they
were taken as a mixture and oxidized to the 1,4-
benzoquinones with use of Fremy’s salt to give a 2:1
mixture of verapliquinone A (5) and verapliquinone B (6)
(Scheme 3). The major isomer of the synthetic material
was assigned as the E-alkene (verapliquinone A) by
a For compounds 10-12: a, R ) H; b, R ) C2H5; c, R ) C12H25
;
d, R ) C14H29
.
TABLE 1. Synthesis of
2-Methoxy-6-propyl-1,4-benzoquinone (1),
2-Methoxy-6-pentyl-1,4-benzoquinone (primin 2),
2-Methoxy-6-pentadecyl-1,4-benzoquinone (3), and
2-Methoxy-6-heptadecylyl-1,4-benzoquinone
(dihydroirisquinone, pallasone B; 4; a, R ) H; b, R )
C2H5; c, R ) C12H25; d, R ) C14H29
)
yield/%
12
1
comparison with the published H NMR spectroscopic
data of the natural product,29 although in common with
the original authors, we could not separate the E,Z-
isomers.
R
10
11
1-4
a
b
c
H
70
96
75
92
83
80
88
85
72a
79a
56b
48b
C2H5
49
44
74
C12H25
C14H29
The route to panicein A is similar in its requirement
for a tertiary allyl ether substrate for the Claisen
rearrangement. The starting material is the propargylic
alcohol, 5-(4-methoxy-2,3,6-trimethylphenyl)-3-methyl-
pentyn-3-ol (17), prepared by addition of ethynylmagne-
sium bromide to the known ketone 4-(4-methoxy-2,3,6-
trimethylphenyl)butan-2-one, obtained in four steps from
d
a Oxidation with Fremy’s salt. b Oxidation with salcomine/O2.
nes A and B (5 and 6) and panicein A (7) as suitable
targets, particularly since they have not been synthesized
to date. The verapliquinones A and B were isolated from
an Aplidium sp. (Ascidiacea) collected off the Breton
coast, and characterized as a mixture of E,Z-isomers by
(29) Guella, G.; Mancini, I.; Pietra, F. Helv. Chim. Acta 1987, 70,
621-626.
(30) Cimino, G.; DeStefano, S.; Minale, L. Tetrahedron 1973, 29,
2565-2570.
(26) Parker, K. A.; Su, D.-S. In Encyclopedia of Reagents for Organic
Synthesis; John Wiley & Sons Ltd: Chichester, UK, 1995; Vol. 6, pp
4271-4274.
(27) DeJong, C. R. H. I.; Hageman, H. J.; Hoentjen, G.; Mus, W. J.
Organic Syntheses; Wiley: New York, 1988; Collect. Vol. VI, pp 412-
413.
(31) Zubia, E.; Ortega, M. J.; Carballo, J. L.; Salva, J. Tetrahedron
1994, 50, 8153-8160.
(32) Casapullo, A.; Minale, L.; Zollo, F. J. Nat. Prod. 1993, 56, 527-
533.
(33) We thank Professor Javier Salva (University of Cadiz, Spain)
for information.
(28) Parker, K. A.; Taveras, D. In Encyclopedia of Reagents for
Organic Synthesis; John Wiley & Sons Ltd: Chichester, UK, 1995; Vol.
6, pp 4423-4425.
(34) Kahn, P. H.; Cossy, J. Tetrahedron Lett. 1999, 40, 8113-8114.
(35) Godfrey, J. D.; Mueller, R. H.; Sedergran, T. C.; Soundararajan,
N.; Colandrea, V. J. Tetrahedron Lett. 1994, 35, 6405-6408.
4416 J. Org. Chem., Vol. 70, No. 11, 2005

Verapliquinones A and B and Panicein A
SCHEME 3. Synthesis of Verapliquinones A and B
(5 and 6)
SCHEME 4. Synthesis of Panicein A (7; Ar )
4-methoxy-2,3,6-trimethylphenyl)
2,3,5-trimethylphenol.36 In this instance, copper(II)-
catalyzed coupling of the tertiary trifluoroacetate to a
phenol gave poor yields, so a modification using a
propargylic carbonate was used.17 This gave the required
4-methoxyphenyl propargyl ether (18) in modest yield
(Scheme 4). Lindlar reduction of 18 was followed by a
high-yielding microwave-mediated Claisen rearrange-
ment to give the phenol 20 as a 3:2 mixture of alkene
isomers. These E,Z-isomers proved inseparable so were
oxidized as a mixture with cerium(IV) ammonium nitrate
to give an inseparable 3:2 mixture of quinones 7. nOe
spectroscopic studies were conducted on the mixture of
quinones: preirradiation of the major ArCH2 signal at δ
3.17 resulted in 5.2% enhancement of the major alkene
methyl signal at δ 1.75, whereas preirradiation of the
minor alkene methyl signal at δ 1.90 resulted in 4.1%
enhancement of the minor alkene signal at δ 5.21. Hence
this confirmed that the major isomer had the natural E
geometry.
euchroma, and assigned as the E-alkene.37 Subsequent
synthetic studies by Moore and co-workers established
that the natural product had the Z-alkene geometry,
although the compound was unstable and readily isomer-
ized to the E isomer.38 Further studies confirmed the
facile double bond isomerization.39 Our Claisen rear-
rangement-based synthesis starts with the aldehyde 21,
prepared in three steps from furan-3-carboxaldehyde.40
Reaction with isopropenylmagnesium bromide gives the
known allylic alcohol 22,38 Mitsunobu reaction of which
with 2,3-dimethoxyphenol gave the Claisen precursor 23.
Microwave Claisen rearrangement results in formation
of the phenol as a 9:1 mixture of alkene isomers with
the more stable E isomer predominating. Oxidation of
this mixture of alkenes with oxygen in the presence of
(37) Yao, X. S.; Ebizuka, Y.; Noguchi, H.; Kiuchi, F.; Seto, H.;
Sankawa, U. Tetrahedron Lett. 1984, 25, 5541-5542.
(38) Foland, L. D.; Decker, O. H. W.; Moore, H. W. J. Am. Chem.
Soc. 1989, 111, 989-995.
(39) Yao, X. S.; Ebizuka, Y.; Noguchi, H.; Kiuchi, F.; Shibuya, M.;
Iitaka, Y.; Seto, H.; Sankawa, U. Chem. Pharm. Bull. 1991, 39, 2962-
2964.
(40) Weyerstahl, P.; Schenk, A.; Marschall, H. Liebigs Ann. 1995,
1849-1853.
Finally the synthesis of the furan-containing quinone
arnebifuranone was addressed. This natural product was
originally isolated in 1984 from the roots of Arnebia
(36) Samokhvalov, G. I.; Zakharova, N. I.; Sokolova, N. N.; Filippova,
T. M.; Raigorodskaya, O. I.; Murav’ev, V. V.; Tuguzova, A. M.;
Khristoforov, V. L. Pharm. Chem. J. 1994, 28, 343-348.
J. Org. Chem, Vol. 70, No. 11, 2005 4417

Davis et al.
cally homogeneous. IR spectra were recorded in the range
SCHEME 5. Synthesis of Isoarnebifuranone 9
4000-600 cm-1 1H and 13C NMR spectra were recorded at
.
300 or 400 MHz (1H frequencies, corresponding 13C frequencies
are 75 and 100 MHz); J values were recorded in Hz. In the
13C NMR spectra, signals corresponding to CH, CH2, or CH3
groups, as assigned from DEPT, are noted; all others are C.
Microwave reactions were carried out in a CEM Discover 300W
focused microwave reactor.
2-Allyl-6-methoxyphenol (11a). 1-Allyloxy-2-methoxyben-
zene (10a) (0.300 g, 6.1 mmol) in anhydrous DMF (4 mL) was
heated to 180 °C in a sealed tube in a microwave reactor (300
W) for 30 min. The mixture was then diluted with ether (20
mL), washed with water (6 × 10 mL), dried (MgSO4), filtered,
and evaporated in vacuo. The crude product was purified by
flash chromatography on silica, eluting with ethyl acetate-
light petroleum (1:19) to give the title compound 11a as a
colorless oil (0.210 g, 70%) (lit.18 clear oil); IR (film)/cm-1 3529
(O-H); 1H NMR (300 MHz; CDCl3) δ 6.84-6.74 (3H, m), 6.07-
5.98 (1H, m), 5.71 (1H, s), 5.12-5.05 (2H, m), 3.90 (3H, s),
3.44-3.42 (2H, d, J ) 6.5).
6-Methoxy-2-propylphenol (12a). 2-Allyl-6-methoxyphe-
nol (11a) (0.227 g, 1.38 mmol) in ethyl acetate (3 mL) was
added to 10% Pd/C in ethyl acetate (2 mL). The mixture was
stirred under a hydrogen atmosphere overnight (17 h). The
mixture was filtered through Celite, washed through with
ethyl acetate (20 mL), and evaporated in vacuo. The crude
product was purified by flash chromatography on silica, eluting
with ethyl acetate-light petroleum (1:19) to give the title
compound 12a as a colorless oil (0.109 g, 83%) (lit.6 bp 71-72
1
°C at 0.5 mmHg); IR (film)/cm-1 3549 (O-H); H NMR (300
MHz; CDCl3) δ 6.82-6.74 (3H, m), 5.72 (1H, s), 3.91 (3H, s),
2.69-2.63 (2H, m), 1.72-1.62 (2H, m), 1.00 (3H, t, J ) 7.1).
2-Methoxy-6-propyl-1,4-benzoquinone (1). To 6-meth-
oxy-2-propylphenol (12a) (0.178 g, 1.07 mmol) in acetone (60
mL) was added Fremy’s salt (1.15 g, 4.28 mmol) in sodium
hydrogen phosphate buffer (40 mL). The mixture was stirred
for 1.5 h at room temperature, then extracted with ethyl
acetate (3 × 60 mL), dried (MgSO4), filtered, and evaporated
in vacuo. The crude compound was purified by flash chroma-
tography on silica, eluting with ethyl acetate-light petroleum
(1:4) to give the title compound 1 as a yellow solid (0.139 g,
72%); mp 75-77 °C (from ethyl acetate/light petroleum) (lit.5
mp 78-79 °C; lit.4 mp 77-78 °C); IR (KBr)/cm-1 1690 (CdO),
1650 (CdO); 1H NMR (400 MHz; CDCl3) δ 6.43-6.42 (1H, m),
5.84 (1H, d, J ) 2.5), 3.77 (3H, s), 2.35 (2H, td, J ) 7.6, 1.4),
1.54-1.44 (2H, m), 0.93-0.89 (3H, m); 13C NMR (75 MHz;
CDCl3) δ 194.8 (C), 188.0 (C), 159.2 (C), 147.6 (C), 133.3 (CH),
107.5 (CH), 56.7 (Me), 31.0 (CH2), 21.3 (CH2), 14.1 (Me).
1-Methoxy-2-(pent-1-en-3-yloxy)benzene (10b). To a
solution of 2-methoxyphenol (3.00 g, 2.66 mL, 24.2 mmol),
pent-1-en-3-ol (2.48 mL, 2.08 g, 24.2 mmol), and triph-
enylphosphine (8.26 g, 31.5 mmol) in anhydrous toluene (70
mL) at 0 °C was slowly added diisopropyl azodicarboxylate
(DIAD) (6.20 mL, 6.37 g, 31.5 mmol). The reaction mixture
was then allowed to warm to room temperature and stirred
for 24 h under a nitrogen atmosphere. The mixture was
concentrated to a third of its original volume in vacuo and light
petroleum was added to precipitate triphenylphosphine oxide.
The precipitate was removed by filtration, washed with light
petroleum (30 mL), and evaporated in vacuo. The mixture was
then dissolved in ethyl acetate (50 mL), washed with NaOH
(2 M; 2 × 40 mL), water (2 × 40 mL), and brine (40 mL), dried
(MgSO4), filtered, and evaporated in vacuo. The crude product
was purified by flash chromatography on silica, eluting with
ethyl acetate-light petroleum (1:19) to give the title compound
10b (2.25 g, 49%) as a colorless oil; IR (film)/cm-1 1588 (Cd
C); 1H NMR (300 MHz; CDCl3) δ 6.94-6.84 (4H, m), 5.89-
5.85 (1H, m), 5.28-5.18 (2H, m), 4.55-4.48 (1H, m), 3.87 (3H,
s), 1.92-1.72 (2H, m), 1.03 (3H, t, J ) 7.2); 13C NMR (75 MHz;
CDCl3) δ 150.7 (C), 148.2 (C), 138.4 (CH), 121.8 (CH), 121.1
(CH), 117.2 (CH2), 116.9 (CH), 112.5 (CH), 82.5 (CH), 56.4
(Me), 28.8 (CH2), 10.2 (Me); MS (EI) 192 (M+, 19%), 153 (19),
salcomine gave isoarnebifuranone (9) as the major prod-
uct (Scheme 5).
The Claisen rearrangements described in Schemes 3-5
all result in mixtures of alkene isomers, and this is clearly
a limitation of the methodology. Despite its widespread
use in organic synthesis, this aspect of the aromatic
Claisen rearrangement, i.e., the question of alkene
geometry arising from rearrangement of substituted aryl
allyl ethers (Scheme 1, R2 * R3), surprisingly remains
almost unexplored, with only a handful of examples in
the literature.2 Studies are underway in our laboratory
to address this issue. Nevertheless, the present protocol
involving the formation and microwave-mediated Claisen
rearrangement of allyl aryl ethers, followed by phenol
oxidation results in the synthesis of a range of naturally
occurring benzoquinones. The method has been used to
effect the first synthesis of the verapliquinones A and B,
and panicein A.
Experimental Section
Commercially available reagents were used throughout
without further purification unless otherwise stated; solvents
were dried by standard procedures. Light petroleum refers to
the fraction with bp 40-60 °C and ether refers to diethyl ether.
Reactions were routinely carried out under a nitrogen atmo-
sphere. Fully characterized compounds were chromatographi-
4418 J. Org. Chem., Vol. 70, No. 11, 2005

Verapliquinones A and B and Panicein A
137 (13), 125 (100). Found: C, 75.4; H, 8.7. C12H16O2 requires
C, 75.0; H, 8.4. Found: M+, 192.1151. C12H16O2 requires
192.1150.
(3H, s), 2.42 (2H, td, J ) 7.6, 1.4), 1.44-1.43 (2H, m), 1.28-
1.23 (4H, m), 0.89 (3H, t, J ) 7.0).
1-Methoxy-2-(pentadec-1-en-3-yloxy)benzene (10c). To
a solution of 2-methoxyphenol (0.274 g, 2.21 mmol), pentadec-
1-en-3-ol (see the Supporting Information) (0.500 g, 2.21
mmol), and triphenylphosphine (4.430 g, 3.82 mmol) in
anhydrous toluene (5 mL) at -10 °C was added DIAD (0.87
mL, 0.894 g, 4.42 mmol). The solution was then allowed to
warm to room temperature and stirred under a nitrogen
atmosphere for 24 h. The solution was diluted with ethyl
acetate (10 mL) and washed with NaOH (2 M; 2 × 5 mL),
water (5 mL), and brine (5 mL), dried (MgSO4), filtered, and
evaporated in vacuo. The crude product was purified by flash
chromatography on silica, eluting with ethyl acetate-light
petroleum (1:99) to give the title compound 10c (0.323 g, 44%)
as a colorless oil; IR (film)/cm-1 1594 (CdC); 1H NMR (400
MHz; CDCl3) δ 6.70-7.10 (4H, m), 5.71-5.83 (1H, ddd, J )
17.2, J ) 10.5, J ) 6.6), 5.24 (1H, d, J ) 17.2), 5.19 (1H, d, J
) 10.5), 4.58 (1H, q, J ) 6.6), 3.87 (3H, s), 1.10-2.00 (22H,
m), 0.91 (3H, t, J ) 6.8); 13C NMR (100 MHz; CDCl3) δ 150.3
(C), 147.8 (C), 138.3 (CH), 121.4 (CH), 120.6 (CH), 116.5 (CH2),
116.4 (CH), 112.1 (CH), 80.8 (CH), 55.9 (Me), 35.5 (CH2), 31.9
(CH2), 29.8-29.4 (7 × CH2), 25.4 (CH2), 22.7 (CH2), 14.2 (Me);
MS (CI) 332 (M+, 16%), 286 (83), 153 (17), 125 (45), 124 (100).
Found: MH+, 333.2791. C22H36O2 + H requires 333.2794.
2-Methoxy-6-(pentadec-2-enyl)phenol (11c). 1-Methoxy-
2-(pentadec-1-en-3-yloxy)benzene (10c) (0.150 g, 0.45 mmol)
in anhydrous DMF (3 mL) in a sealed tube was heated to 220
°C for 1 h in a microwave reactor (300 W). The solution was
then diluted with ethyl acetate (15 mL), washed with water
(3 × 10 mL) and brine (10 mL), and dried (MgSO4), and the
solvent was removed under reduced pressure. The crude
product was purified by flash chromatography on silica, eluting
with ethyl acetate-light petroleum (1:99) to give the title
compound 11c (0.112 g, 75%) as a colorless oil; IR (film)/cm-1
3554 (O-H); 1H NMR (400 MHz; CDCl3) δ 6.65-6.85 (3H, m),
5.73 (1H, s), 5.50-5.66 (2H, m), 3.89 (3H, s), 3.38 (2H, d, J )
6.3), 2.01-2.07 (2H, m), 1.24-1.41 (20H, m), 0.91 (3H, t, J )
6.7); 13C NMR (100 MHz; CDCl3) δ 146.3 (C), 143.3 (C), 131.9
(CH), 127.7 (CH), 126.9 (C), 122.1 (CH), 119.3 (CH), 108.4
(CH), 55.9 (Me), 32.60 (CH2), 31.57 (CH2), 31.9 (CH2), 29.8-
29.2 (8 × CH2), 22.7 (CH2), 14.2 (Me); MS (CI) 333 (MH+, 56%),
332 (M+, 100%), 331 (39), 163 (12), 137 (74). Found: MH+,
333.2780. C22H36O2 + H requires 333.2794.
2-Methoxy-6-(pent-2-enyl)phenol (11b). (a) Microwave
Heating in DMF. 1-Methoxy-2-(pent-1-en-3-yloxy)benzene
(10b) (0.200 g, 1.05 mmol) was dissolved in DMF (3 mL) in a
sealed tube. The mixture was heated in a microwave reactor
(300 W) at 180 °C for 25 min. The mixture was diluted in ethyl
acetate (15 mL), washed with water (5 × 10 mL) and brine
(10 mL), dried (MgSO4), filtered, and evaporated in vacuo. The
crude product was then purified by flash chromatography on
silica, eluting with dichloromethane and light petroleum (1:
3) to give the title compound 11b (0.191 g, 96%) as a colorless
1
oil; IR (film)/cm-1 3539 (O-H); H NMR (300 MHz; CDCl3) δ
6.83-6.72 (3H, m), 5.70 (1H, s), 5.61-5.53 (2H, m), 3.89 (3H,
s), 3.36 (2H, d, J ) 5.0), 2.09-2.00 (2H, m), 0.99 (3H, t, J )
7.4); 13C NMR (75 MHz; CDCl3) δ 146.7 (C), 143.7 (C), 133.7
(CH), 127.3 (C), 127.2 (CH), 122.5 (CH), 119.7 (CH), 108.8
(CH), 56.4 (Me), 33.0 (CH2), 25.9 (CH2), 14.2 (Me); MS (CI)
193 (MH+, 19%), 137 (100). Found: MH+, 193.1225. C12H16O2
+ H requires 193.1229.
(b) Microwave Heating in Toluene. 1-Methoxy-2-(pent-
1-en-3-yloxy)benzene (10b) (0.200 g, 1.05 mmol) was dissolved
in toluene (3 mL) in a sealed tube. The mixture was heated in
a microwave reactor (300 W) at 180 °C for 2.5 h. The solvent
was evaporated in vacuo and the crude product was then
purified by flash chromatography on silica, eluting with
dichloromethane and light petroleum (1:3) to give the title
compound 11b (0.105 g, 53%) as a colorless oil; data as above.
(c) Microwave Heating in 1,2-Dichlorobenzene. 1-Meth-
oxy-2-(pent-1-en-3-yloxy)benzene (10b) (0.208 g, 1.09 mmol)
was dissolved in 1,2-dichlorobenzene (3 mL) in a sealed tube.
The mixture was heated in a microwave reactor (300 W) at
190 °C for 10 min. The crude product was then purified by
flash chromatography on silica, eluting with dichloromethane
and light petroleum (1:3) to give the title compound 11b (0.150
g, 72%) as a colorless oil; data as above.
(d) Heating under Reflux in 1,2-Dichlorobenzene.
1-Methoxy-2-(pent-1-en-3-yloxy)benzene (10b) (0.250 g, 1.31
mmol) was dissolved in 1,2-dichlorobenzene (3 mL). The
reaction mixture was heated under reflux for 3 h. The crude
product was then purified by flash chromatography on silica,
eluting with dichloromethane and light petroleum (1:3) to give
the title compound 11b (0.124 g, 50%) as a colorless oil; data
as above.
2-Methoxy-6-pentadecylphenol (12c). A solution of
2-methoxy-6-(pentadec-2-enyl)phenol (11c) (0.088 g, 0.26 mmol)
in ethyl acetate (2 mL) was added to Pd/C (10%; 0.009 g) in
ethyl acetate (1 mL). The solution was stirred under a
hydrogen atmosphere for 48 h at room temperature, then
filtered through Celite and washed through with ethyl acetate
(3 × 5 mL). The solvent was evaporated in vacuo and the crude
product was purified by flash chromatography on silica, eluting
with ethyl acetate-light petroleum (1:99) to give the title
compound 12c (0.076 g, 88%) as a colorless oil; IR (film)/cm-1
2-Methoxy-6-pentylphenol (12b). A solution of 2-meth-
oxy-6-pent-2-enylphenol (11b) (0.102 g, 0.53 mmol) in ethyl
acetate (1.5 mL) was added to Pd/C (10%; 0.010 g) in ethyl
acetate (0.5 mL). The reaction mixture was stirred at room
temperature under an atmosphere of hydrogen for 17 h. The
product was filtered through a short pad of Celite and washed
through with ethyl acetate (5 mL). The solvent was evaporated
in vacuo and the crude product purified by flash chromatog-
raphy on silica, eluting with ethyl acetate-light petroleum (1:
9) to give the title compound 12b (0.082 g, 80%) as a colorless
oil (lit.9 colorless liquid); IR (film)/cm-1 3539 (O-H), 2960,
1
3544 (O-H); H NMR (300 MHz; CDCl3) 6.72-6.58 (3H, m),
5.61 (1H, s), 3.77 (3H, s), 2.55 (2H, t, J ) 7.5), 1.60-1.20 (26H,
m), 0.81 (3H, t, J ) 5.9); 13C NMR (75 MHz; CDCl3) δ 146.6
(C), 143.8 (C), 129.1 (C), 122.7 (CH), 119.5 (CH), 108.5 (CH),
56.3 (Me), 32.3 (CH2), 30.3-29.8 (12 × CH2), 23.1 (CH2), 14.5
(Me); MS (CI) 335 (MH+, 81%), 334 (M+, 100), 333 (67).
Found: MH+, 335.2964. C22H38O2 + H requires 335.2950.
1
2935, 2848, 1614, 1593, 1475; H NMR (300 MHz; CDCl3) δ
6.83-6.72 (3H, m), 5.73 (1H, s), 3.89 (3H, s), 2.66 (2H, t, J )
7.5), 1.77-1.60 (2H, m), 1.40-1.35 (4H, m), 0.93 (3H, t, J )
6.0).
2-Methoxy-6-pentyl-1,4-benzoquinone (primin 2). To a
solution of 2-methoxy-6-pentylphenol (12b) (0.038 g, 0.20
mmol) in acetone (12 mL) was added Fremy’s salt (0.215 g,
0.8 mmol) in sodium hydrogen phosphate buffer (10 mL). The
reaction mixture was stirred for 3 h, then extracted with ethyl
acetate (2 × 20 mL). The organic layers were combined,
washed with brine (20 mL), dried (MgSO4), filtered, and
evaporated in vacuo to give the title compound 2 (0.033 g, 79%)
as a golden crystalline solid, mp 64.5-65.5 °C (from methanol)
(lit.8 mp 64-65 °C); IR (KBr)/cm-1 1680 (CdO), 1655 (CdO);
1H NMR (300 MHz; CDCl3) δ 6.48 (1H, s), 5.87 (1H, s), 3.81
2-Methoxy-6-pentadecyl-1,4-benzoquinone (3). 2-Meth-
oxy-6-pentadecylphenol (12c) (0.076 g, 0.23 mmol) was dis-
solved in anhydrous acetonitrile (2 mL) and salcomine catalyst
(0.014 g, 0.05 mmol) was added. Oxygen was bubbled through
the solution for 15 min and the reaction mixture was then
stirred in air at room temperature for 24 h. The reaction
mixture was then filtered through Celite and washed through
with ethyl acetate (3 × 10 mL) and the solvent was removed
under reduced pressure. The crude product was purified by
flash chromatography on silica, eluting with ethyl acetate-
light petroleum (1:9) to give the title compound 3 (0.045 g, 56%)
J. Org. Chem, Vol. 70, No. 11, 2005 4419

Davis et al.
as a yellow solid; mp 64-66 °C (from methanol) (lit.6mp 80-
82 °C); IR (KBr)/cm-1 1685 (CdO), 1653 (CdO); 1H NMR (400
MHz; CDCl3) δ 6.47 (1H, s), 5.87 (1H, s), 3.81 (3H, s), 2.42
(2H, t, J ) 7.6), 1.5-1.20 (26H, m), 0.87 (3H, t, J ) 6.8); 13C
NMR (75 MHz; CDCl3) δ 187.7 (C), 182.1 (C), 158.8 (C), 147.6
(C), 132.9 (CH), 107.1 (CH), 56.2 (Me), 31.9 (CH2), 29.7-29.0
(10 × CH2), 28.7 (CH2), 27.7 (CH2), 22.7 (CH2), 14.1 (Me); MS
(CI) 349 (MH+, 67%), 347 (27), 227 (26), 187 (18), 167 (25),
154 (27), 153 (100), 149 (37), 115 (40), 113 (14). Found: MH+,
349.2748. C22H36O3 + H requires 349.2743.
1-Methoxy-2-(heptadec-1-en-3-yloxy)benzene (10d). To
a solution of 2-methoxyphenol (0.43 mL, 0.49 g, 3.93 mmol),
heptadec-1-en-3-ol (see the Supporting Information) (1.66 g,
3.93 mmol), and triphenylphosphine (1.55 g, 5.90 mmol) in
anhydrous toluene (15 mL) was added DIAD (1.16 mL, 1.19
g, 5.90 mmol). The solution was then stirred at room temper-
ature for 24 h. The solution was diluted with ethyl acetate
(30 mL) and washed with NaOH (2 M; 20 mL), water (3 × 20
mL), and brine (20 mL), dried (MgSO4), filtered, and evapo-
rated in vacuo. The crude product was purified by flash
chromatography on silica, eluting with ethyl acetate-light
petroleum (1:19) to give the title compound 10d (1.05 g, 74%)
as a colorless solid; mp 42.5-43.5 °C (from ethyl acetate-light
petroleum); IR (film)/cm-1 1589 (CdC); 1H NMR (300 MHz;
CDCl3) δ 6.93-6.86 (4H, m), 5.89 (1H, ddd, J ) 17.3, 10.4,
6.8), 5.27-5.16 (2H, m), 4.57 (1H, q, J ) 6.8), 3.87 (3H, s),
1.95-1.85 (1H, m), 1.76-1.62 (1H, m), 1.55-1.41 (2H, m), 1.27
(22H, br), 0.92-0.87 (3H, m); 13C NMR (75 MHz; CDCl3) δ
150.1 (C), 147.7 (C), 138.2 (CH), 121.2 (CH), 120.5 (CH), 116.5
(CH2), 116.3 (CH), 112.0 (CH), 80.8 (CH), 55.9 (Me), 35.4 (CH2),
31.8 (CH2), 29.6-29.3 (9 × CH2), 25.3 (CH2), 22.6 (CH2), 14.1
(Me); MS (EI) 360 (M+, 50%), 137 (48), 124 (100), 109 (37).
Found: M+, 360.3018. C24H40O2 requires 360.3028.
2-Heptadec-2-enyl-6-methoxyphenol (11d). 1-Methoxy-
2-(heptadec-1-en-3-yloxy)benzene (10d) (0.300 g, 2.78 mmol)
in anhydrous DMF (4 mL) in a sealed tube was heated to 180
°C for 30 min in a microwave reactor (300 W). The solution
was diluted with ether (20 mL), washed with water (6 × 10
mL), dried (MgSO4), filtered, and evaporated in vacuo. The
crude product was purified by flash chromatography on silica,
eluting with ethyl acetate-light petroleum (1:19) to give the
title compound 11d (0.275 g, 92%) as a colorless solid; mp
42.5-43.5 °C (from ethyl acetate-light petroleum); IR (KBr)/
cm-1 3549 (O-H); 1H NMR (300 MHz; CDCl3) δ 6.81-6.73 (3H,
m), 5.69 (1H, s), 5.58 (2H, m), 3.39 (3H, s), 3.36 (2H, d, J )
5.9), 2.03-1.98 (2H, m), 1.34-1.27 (24H, br), 0.89 (3H, t, J )
6.5); 13C NMR (75 MHz; CDCl3) δ 146.7 (C), 143.7 (C), 132.4
(CH), 128.0 (CH), 127.3 (C), 122.5 (CH), 119.7 (CH), 108.8
(CH), 56.4 (Me), 33.0 (CH2), 32.3 (CH2), 30.1-29.6 (11 × CH2),
23.1 (CH2), 14.5 (Me); MS (CI) 361 (MH+, 100%), 285 (38), 137
(47). Found: MH+, 361.3098. C24H40O2 + H requires 361.3107.
2-Heptadecyl-6-methoxyphenol (12d). 2-Heptadec-2-
enyl-6-methoxyphenol (11d) (0.270 g, 0.75 mmol) in ethyl
acetate (4 mL) was added to Pd/C (10%; 0.027 g) in ethyl
acetate (1 mL). The solution was stirred under a hydrogen
atmosphere for 4 h, then filtered through Celite and washed
through with ethyl acetate (10 mL). The solvent was evapo-
rated in vacuo and the crude product was purified by flash
chromatography on silica, eluting with ethyl acetate-light
petroleum (1:19) to give the title compound 12d (0.230 g, 85%)
as a colorless solid; mp 44.5-45.5 °C (from ethyl acetate-light
petroleum) (lit.41 mp 52.1-52.7 °C); IR (KBr)/cm-1 3442 (O-
H); 1H NMR (300 MHz; CDCl3) δ 6.79-6.71 (3H, m), 5.68 (1H,
s), 3.89 (3H, s), 2.64 (2H, t, J, 7.7), 1.62-1.58 (2H, m), 1.27
(28H, br), 0.89 (3H, t, J ) 6.4); 13C NMR (75 MHz; CDCl3) δ
146.6 (C), 143.8 (C), 129.1 (C), 122.7 (CH), 119.5 (CH), 108.5
(CH), 56.3 (Me), 32.3 (CH2), 30.2-29.8 (14 × CH2), 23.1 (CH2),
14.5 (Me).
(12d) (0.200 g, 0.56 mmol) was dissolved in anhydrous aceto-
nitrile (4 mL) and salcomine catalyst (0.036 g, 0.11 mmol) was
added. Oxygen was bubbled through the solution for 15 min,
and the mixture was stirred in air at room temperature for
24 h. The solution was filtered through Celite, washed through
with dichloromethane (20 mL), and evaporated in vacuo. The
crude product was purified by flash chromatography on silica,
eluting with ethyl acetate-light petroleum (1:4) to give the
title compound 4 (0.102 g, 48%) as a yellow solid; mp 97.5-
98.5 °C (from ethyl acetate-light petroleum) (lit.12 mp 90.5-
1
91.0 °C); H NMR (300 MHz; CDCl3) δ 6.48 (1H, d, J ) 2.1),
5.88 (1H, d, J ) 2.1), 3.82 (3H, s), 2.43 (2H, t, J ) 7.0), 1.50-
1.47 (2H, m), 1.30-1.25 (28H, m), 0.88 (3H, t, J ) 6.2); 13C
NMR (75 MHz; CDCl3) δ 190.4 (C), 184.8 (C), 161.5 (C), 150.2
(C), 135.5 (CH), 109.7 (CH), 58.9 (Me), 34.6 (CH2), 32.4-31.9
(12 × CH2), 31.4 (CH2), 30.3 (CH2), 25.4 (CH2), 16.8 (Me).
1-(3,7-Dimethyloct-6-en-1-yn-3-yloxy)-2-methoxyben-
zene (14). To a solution of 3,7-dimethyloct-6-en-1-yn-3-ol (13)
(see the Supporting Information) (3.06 g, 20.1 mmol) in
anhydrous acetonitrile (11 mL) under nitrogen and cooled to
below -5 °C was added DBU (3.92 mL, 3.99 g, 26.2 mmol).
Trifluoroacetic anhydride (2.81 mL, 4.22 g, 20.1 mmol) was
added over a 25 min period while maintaining the temperature
below 2 °C. The resulting solution was allowed to stir at 0 °C
for 30 min before the addition to the 2-methoxyphenol solution
(described below).
To a solution of 2-methoxyphenol (1.92 mL, 2.17 g, 17.5
mmol) in anhydrous acetonitrile (11 mL) under nitrogen and
cooled to below -4 °C was added DBU (3.39 mL, 3.46 g, 22.7
mmol) and CuCl2‚2H2O (0.003 g). The solution of the trifluo-
roacetate (above), maintained at 0 °C, was added to the
2-methoxyphenol solution over a 40 min period while main-
taining the temperature below 0 °C. After being stirred for 5
h below 0 °C, the mixture was concentrated in vacuo and the
residue partitioned between water (50 mL) and ethyl acetate
(150 mL). The organic fraction was washed with HCl (2 M; 50
mL), NaOH (2 M; 50 mL), saturated NaHCO3 (50 mL), and
brine (50 mL) and dried (MgSO4), filtered, and evaporated in
vacuo. The crude product was purified by flash chromatogra-
phy on silica, eluting with ethyl acetate-light petroleum (1:
19) to give the title compound 14 as a colorless oil (2.55 g, 56%);
1
IR (film)/cm-1 3298 (C≡C-H); H NMR (300 MHz; CDCl3) δ
7.42 (1H, dd, J ) 7.8, 1.4), 7.08-7.02 (1H, m), 6.92-6.86 (2H,
m), 5.20-5.14 (1H, m), 3.82 (3H, s), 2.55 (1H, s), 2.43-2.22
(2H, m), 2.06-1.82 (2H, m), 1.70 (3H, d, J ) 1.1), 1.65 (3H, s),
1.58 (3H, s); 13C NMR (75 MHz; CDCl3) δ 153.0 (C), 144.7 (C),
132.0 (C), 123.9 (CH), 123.7 (CH), 123.4 (CH), 120.3 (CH),
112.2 (CH), 85.3 (C), 77.0 (C), 74.6 (CH), 55.7 (Me), 42.3 (CH2),
26.7 (Me), 25.7 (Me), 23.3 (CH2), 17.7 (Me); MS (EI) 258 (M+,
2%), 215 (12), 124 (100), 69 (68). Found: M+, 258.1630.
C17H22O2 requires 258.1620.
1-(3,7-Dimethylocta-1,6-dien-3-yloxy)-2-methoxyben-
zene (15). 1-(3,7-Dimethyloct-6-en-1-yn-3-yloxy)-2-methoxy-
benzene (14) (1.26 g, 4.92 mmol) was dissolved in anhydrous
methanol (15 mL), and Lindlar catalyst (0.047 g) and quinoline
(1 mL) were added. The mixture was evacuated and then
stirred under hydrogen for 4 h. The mixture was filtered
through Celite, washed through with methanol (30 mL), and
concentrated in vacuo. The residue was dissolved in dichlo-
romethane (30 mL), washed with HCl (2 M; 2 × 30 mL), water
(30 mL), and brine (30 mL), dried (MgSO4), filtered, and
evaporated in vacuo. The crude product was purified by flash
chromatography on silica, eluting with ethyl acetate-light
petroleum (1:19) to give the title compound 15 as a colorless
oil (0.77 g, 60%); IR (film)/cm-1 1588 (CdC); 1H NMR (300
MHz; CDCl3) δ 7.05 (1H, dd, J ) 7.9, 1.5), 7.01-6.95 (1H, m),
6.88 (1H, m), 6.84-6.78 (1H, m), 6.12 (1H, dd, J ) 17.9, 10.7),
5.19-5.12 (3H, m), 3.82 (3H, s), 2.16 (2H, q, J ) 7.9), 1.86-
1.77 (2H, m), 1.70 (3H, s), 1.62 (3H, s), 1.40 (3H, s); 13C NMR
(75 MHz; CDCl3) δ 153.0 (C), 145.1 (C), 143.4 (CH), 131.5 (C),
124.4 (CH), 123.00 (CH), 122.99 (CH), 120.2 (CH), 114.1 (CH2),
2-Heptadecyl-6-methoxy-1,4-benzoquinone (dihydroir-
isquinone, pallasone B; 4). 2-Heptadecyl-6-methoxyphenol
(41) Jinno, S.; Okita, T. Chem. Pharm. Bull. 1998, 46, 1688-1694.
4420 J. Org. Chem., Vol. 70, No. 11, 2005

Verapliquinones A and B and Panicein A
112.3 (CH), 82.8 (C), 55.8 (Me), 41.4 (CH2), 25.7 (Me), 22.7
(CH2), 22.1 (Me), 17.6 (Me).
the above carbonate in acetonitrile (4 mL) was then added
dropwise. The reaction mixture was stirred overnight at 0 °C,
then quenched with water (10 mL) and extracted into ether
(3 × 10 mL). The combined organic extracts were dried
(MgSO4) and concentrated in vacuo. The crude product was
purified by flash chromatography on silica, eluting with ethyl
acetate-light petroleum (1:19) to afford the title compound
18 as a pale oil (0.212 g, 49%); IR (CHCl3)/cm-1 3305 (C≡C-
H), 2121 (C≡C); 1H NMR (300 MHz; CDCl3) δ 7.18-7.13 (2H,
m), 6.84-6.79 (2H, m), 6.57 (1H, s), 3.79 (3H, s), 3.78 (3H, s),
2.98-2.87 (2H, m), 2.62 (1H, s), 2.33 (3H, s), 2.25 (3H, s), 2.14
(3H, s), 2.04-1.87 (2H, m), 1.59 (3H, s); 13C NMR (75 MHz;
CDCl3) 156.0 (C), 155.8 (C), 149.4 (C), 136.6 (C), 134.3 (C),
130.7 (C), 123.8 (CH), 123.2 (C), 114.3 (CH), 110.7 (CH), 85.6
(C), 76.3 (C), 75.5 (CH), 56.0 (2 × Me), 42.4 (CH2), 27.3 (Me),
2-(3,7-Dimethylocta-2,6-dienyl)-6-methoxyphenol (16).
1-(3,7-Dimethylocta-1,6-dien-3-yloxy)-2-methoxybenzene (15)
(0.400 g, 1.54 mmol) in anhydrous DMF (5 mL) was heated at
180 °C in a sealed tube in a microwave reactor (300 W) for 40
min. The mixture was diluted with ether (30 mL) and washed
with water (3 × 20 mL) and brine (20 mL), dried (MgSO4),
filtered, and evaporated in vacuo. The crude product was
purified by flash chromatography on silica, eluting with ethyl
acetate-light petroleum (1:19) to give the title compound 16
as a colorless oil (0.330 g, 83%) as a mixture of E,Z isomers;
IR (film)/cm-1 3544 (O-H); 1H NMR (300 MHz; CDCl3) δ 6.80-
6.72 (3H, m), 5.71 (1H, s), 5.36 (1H, td, J ) 7.3, 1.3), 5.19-
5.10 (1H, m), 3.89 (3H, s), 3.38 (2H, d, J ) 6.8), 2.19-2.06
(4H, m), 1.76-1.59 (9H).
2-(3,7-Dimethylocta-2,6-dienyl)-6-methoxy-1,4-benzo-
quinone (Verapliquinones A and B; 5/6). To a solution of
2-(3,7-dimethylocta-2,6-dienyl)-6-methoxyphenol (16) (mixture
of E,Z isomers) (0.247 g, 0.95 mmol) in acetone (60 mL) was
added Fremy’s salt (1.02 g, 3.8 mmol) in sodium hydrogen
phosphate buffer (40 mL). The reaction mixture was stirred
at room temperature for 1.5 h then extracted with ethyl acetate
(3 × 50 mL). The combined organic extracts were washed with
brine (50 mL), dried (MgSO4), filtered, and evaporated in
vacuo. The crude product was purified by flash chromatogra-
phy on silica, eluting with ethyl acetate-light petroleum (1:
9) to give the title compounds 5/6 as an orange oil (0.156 g,
60%) (lit.29 yellow oil) (2:1 mixture of geometric isomers; this
ratio was determined by 1H NMR integrations of methyl peaks
at δ 1.76 and 1.69; the major isomer was assigned as verapli-
quinone A (5) by comparison with published NMR data;29 IR
(film)/cm-1 1685 (CdO), 1655 (CdO); 1H NMR (400 MHz;
CDCl3) major isomer (verapliquinone A) δ 6.47-6.44 (1H, m),
5.87-5.86 (1H, d, J ) 2.1), 5.15 (1H, t, J ) 7.3), 5.08-5.06
(1H, m), 3.28 (3H, s), 3.14 (2H, d, J ) 7.3), 2.11-2.05 (4H, m),
1.69 (3H, s), 1.63 (3H, s), 1.60 (3H, s); 1H NMR (400 MHz;
CDCl3) minor isomer (verapliquinone B) δ 6.47-6.44 (1H, m),
5.87-5.86 (1H, d, J ) 2.1), 5.15 (1H, t, J ) 7.3), 5.08-5.06
(1H, m), 3.28 (3H, s), 3.14 (2H, d, J ) 7.3), 2.11-2.05 (4H, m),
1.76 (3H, s), 1.65 (3H, s), 1.59 (3H, s).
25.0 (CH2), 20.7 (Me), 16.1 (Me), 12.4 (Me); MS (CI) 370
+
(MNH4+, 5%), 353 (5), 231 (15), 163 (100). Found: M + NH4
370.2376. C23H28O3 + NH4 requires 370.2377.
,
5-(4-Methoxy-2,3,6-trimethylphenyl)-3-(4-methoxyphe-
noxy)-3-methylpentene (19). 5-(4-Methoxy-2,3,6-trimeth-
ylphenyl)-3-(4-methoxyphenoxy)-3-methylpentyne (18) (0.180
g, 0.511 mmol) was dissolved in anhydrous methanol (5 mL)
and Lindlar catalyst (0.020 g) and quinoline (0.25 mL) were
added. The mixture was then evacuated and stirred under a
hydrogen atmosphere overnight. The mixture was then filtered
through Celite, washed with methanol (20 mL), and concen-
trated in vacuo. The crude product was purified by flash
chromatography on silica, eluting with ethyl acetate-light
petroleum (1:9) to afford the title compound 19 as a pale oil
(0.123 g, 68%); IR (CHCl3)/cm-1 1595 (CdC); 1H NMR (300
MHz; CDCl3) δ 6.97-6.93 (2H, m), 6.79-6.75 (2H, m), 6.55
(1H, s), 6.21-6.12 (1 H, m), 5.24-5.16 (2 H, m), 3.78 (3H, s),
3.77 (3H, s), 2.75-2.69 (2H, m), 2.28 (3H, s), 2.19 (3H, s), 2.13
(3H, s), 1.85-1.73 (2H, m), 1.43 (3H, s); 13C NMR (75 MHz;
CDCl3) δ 155.7 (C), 155.3 (C), 150.0 (C), 143.9 (CH), 136.4 (C),
134.1 (C), 131.2 (C), 123.2 (CH), 115.0 (C), 114.3 (CH), 110.7
(CH), 81.8 (CH2), 56.0 (Me), 55.9 (Me), 41.6 (CH2), 24.4 (CH2),
22.8 (Me), 20.7 (Me), 16.1 (Me), 12.4 (Me); MS (CI) 372
+
(MNH4+, 30%), 355 (33), 248 (100). Found: M + NH4
372.2536. C23H30O3 + NH4 requires 372.2533.
,
4-Methoxy-2-[5-(4-methoxy-2,3,6-trimethylphenyl)-3-
methylpent-2-enyl]phenol (20). A solution of 5-(4-methoxy-
2,3,6-trimethylphenyl)-3-(4-methoxyphenoxy)-3-methylpen-
tene (19) (0.100 g, 0.282 mmol) in DMF (1 mL) in a sealed
tube was heated at 220 °C for 20 min in a microwave reactor
(300 W). The reaction mixture was then diluted with ethyl
acetate (8 mL), the organics separated, and the aqueous layer
further extracted with ethyl acetate (10 mL). The combined
organics were then dried (MgSO4) and concentrated in vacuo.
The crude product was purified by flash chromatography on
silica, eluting with ethyl acetate-light petroleum (1:4) to afford
the title compound 20 as a pale oil (0.087 mg, 87%), in a 3:2
(E:Z) mixture of geometric isomers; IR (CHCl3)/cm-1 3605 (O-
H); 1H NMR (300 MHz; CDCl3) δ 6.77-6.62 (3H, m), 6.57 (1H,
s), 6.56 (1H, m), 5.41 (0.6H, m), 5.37 (0.4H, m), 4.77 (0.6H, s),
4.71 (0.4H, s), 3.79 (3H, s), 3.74 (1.8H, s), 3.72 (1.2H, s), 3.37
(1.2H, d, J ) 7.0 Hz), 3.31 (0.8H, d, J ) 7.0 Hz), 2.75-2.68
(2H, m), 2.34 (1.2H, s), 2.31 (1.8H, s), 2.28-2.10 (2H, m), 2.26
(1.2H, s), 2.22 (1.8H, s), 2.11 (3H, s), 1.89 (1.2H, s), 1.87 (1.8H,
s); 13C NMR (75 MHz; CDCl3) δ 155.8 (C), 155.7 (C), 154.1 (C),
148.6 (C), 148.5 (C), 139.2 (C), 138.8 (C), 136.4 (C), 136.3 (C),
134.1 (C), 134.0 (C), 131.2 (C), 128.5 (C), 128.4 (C), 123.3 (C),
123.2 (C), 122.8 (CH), 121.8 (CH), 116.7 (CH), 116.6 (CH),
116.1 (CH), 112.5 (CH), 112.4 (CH), 110.8 (CH), 110.7 (CH),
56.1 (Me), 56.0 (Me), 40.0 (CH2), 32.5 (CH2), 30.2 (CH2), 29.8
5-(4-Methoxy-2,3,6-trimethylphenyl)-3-(4-methoxyphe-
noxy)-3-methylpentyne (18). (a) To a solution of 5-(4-
methoxy-2,3,6-trimethylphenyl)-3-methylpentyn-3-ol (17) (see
the Supporting Information) (0.520 g, 2.11 mmol) in THF at 0
°C was added dropwise n-butyllithium (2.5 M in hexanes; 1.27
mL, 3.17 mmol). The mixture was stirred at this temperature
for 30 min and methyl chloroformate (0.25 mL, 3.17 mmol)
was then added dropwise. The reaction mixture was allowed
to warm to room temperature, then stirred for a further 1.5
h. The reaction mixture was then partitioned between dichlo-
romethane (10 mL) and water (10 mL) and the organic layer
separated. The aqueous layer was further extracted with ether
(2 × 10 mL). The combined organic layers were dried (MgSO4)
and concentrated in vacuo. The crude product was purified by
flash chromatography on silica, eluting with ethyl acetate-
light petroleum (1:9) to afford 5-(4-methoxy-2,3,6-trimeth-
ylphenyl)-3-methoxycarbonyloxy-3-methylpentyne as a pale oil
(0.444 g, 69%), used without further purification; IR (CHCl3)/
1
cm-1 3305 (C≡C-H), 2112 (C≡C),1750 (CdO); H NMR (300
MHz; CDCl3) δ 6.56 (1H, s), 3.79 (6H, s), 2.87-2.80 (2H, m),
2.68 (1H, s), 2.33 (3H, s), 2.24 (3H, s), 2.14 (3H, s), 2.10-1.84
(2H, m), 1.80 (3H, s); 13C NMR (75 MHz; CDCl3) δ 153.4 (C),
151.6 (C), 134.0 (C), 131.8 (C), 127.5 (C), 120.9 (C), 108.3 (CH),
80.9 (C), 74.6 (C), 72.1 (CH), 53.5 (Me), 52.4 (Me), 38.8 (Me),
24.2 (Me), 22.2 (CH2), 18.2 (Me), 13.6 (Me), 9.9 (Me); MS (CI)
322 (MNH4+, 12%), 261 (12), 229 (94), 163 (100). Found: M +
NH4+, 322.2012. C18H24O4 + NH4 requires 322.2013.
(b) To a solution of 4-methoxyphenol (0.150 g, 1.22 mmol)
in acetonitrile (1 mL) at -20 °C was added anhydrous copper-
(II) chloride (0.160 mg, 0.1 mol %) and DBU (0.24 mL, 1.59
mmol). The mixture was stirred for 15 min and a solution of
(CH2), 29.3 (CH2), 28.5 (CH2), 24.0 (Me), 20.9 (Me), 20.8 (Me),
+
16.9 (Me), 16.2 (Me), 16.1 (Me),12.4 (Me); MS (CI) 372 (MNH4
10%), 355 (10), 219 (13), 163 (100), 151 (72). Found: M + NH4
372.2535. C23H30O3 + NH4 requires 372.2533.
,
,
+
2-[5-(4-Methoxy-2,3,6-trimethylphenyl)-3-methylpent-
2-enyl]-1,4-benzoquinone (Panicein A and isomer; 7). To
a solution of 4-methoxy-2-[5-(4-methoxy-2,3,6-trimethylphe-
J. Org. Chem, Vol. 70, No. 11, 2005 4421

Davis et al.
nyl)-3-methylpent-2-enyl]phenol (20) (0.074 g, 0.209 mmol) in
MeCN/H2O (2:1, 1.5 mL) at 0 °C was added cerium ammonium
nitrate (0.252 g, 0.460 mmol) in MeCN/H2O (2:1, 1.5 mL). The
reaction mixture was allowed to warm to room temperature,
stirred for 1 h, and partitioned between water (10 mL) and
ethyl acetate (10 mL). The organic layer was separated and
the aqueous layer further extracted with ethyl acetate (2 ×
10 mL). The combined organics were dried (MgSO4) and
concentrated in vacuo. The crude product was purified by flash
chromatography on silica, eluting with ethyl acetate-light
petroleum (1:9) to afford the title compound (0.046 g, 65%) as
a pale oil (lit.30 no data) in a 3:2 (E:Z) mixture of isomers; IR
(10 mL), and dried (MgSO4), and the solvent was removed
under reduced pressure. The crude product was purified by
flash chromatography on silica, eluting with ethyl acetate-
light petroleum (1:99) to give the title compound 24 (0.112 g,
75%) as a brown oil (mixture of isomers: cis:trans ) 1:9); IR
(film)/cm-1 3501 (O-H); 1H NMR (400 MHz; CDCl3) major
isomer δ 7.39-7.31 (1H, m), 7.21 (1H, s), 6.72 (1H, d, J ) 8.5),
6.42 (1H, d, J ) 8.5), 6.28 (1H, s), 5.85 (1H, s), 5.28-5.19 (1H,
m), 3.89 (3H, s), 3.85 (3H, s), 3.26 (2H, s), 2.49-2.45 (2H, m),
2.32-2.27 (2H, m), 1.58 (3H, s); 13C NMR (100 MHz; CDCl3)
major isomer δ 150.6 (C), 147.6 (C), 142.6 (CH), 138.9 (CH),
135.4 (C), 134.8 (C), 125.2 (CH), 124.9 (C), 124.5 (CH), 119.2
(C), 111.1 (CH), 103.3 (CH), 60.9 (Me), 55.8 (Me), 38.8 (CH2),
28.5 (CH2), 24.9 (CH2), 16.1 (Me); MS (CI) 303 (MH+, 17%),
302 (M+, 30), 167 (100), 149 (22). Found: MH+, 303.1611.
C18H22O4 + H requires 303.1596.
(E)-6-[5-(3-Furyl)-2-methylpent-2-enyl]-2,3-dimethoxy-
1,4-benzoquinone (isoarnebifuranone; 9). 6-[5-(3-Furyl-
2-methylpent-2-enyl]-2,3-dimethoxyphenol (24) (0.214 g, 0.71
mmol) was dissolved in anhydrous acetonitrile (5 mL) and
salcomine catalyst (0.046 g, 0.14 mmol) was added. Oxygen
was bubbled through the solution for 15 min and the reaction
mixture was then stirred in air at room temperature for 24 h.
The reaction mixture was then filtered through Celite and
washed through with ethyl acetate and the solvent was
removed under reduced pressure. The crude product was
purified by flash chromatography on silica, eluting with ethyl
acetate-light petroleum (1:19) to give the title compound 9
(0.118 g, 53%) as an orange oil (lit.38 red-orange oil) mixture
of isomers (cis:trans ) 1:9); IR (film)/cm-1 1659 (CdO); 1H
NMR (400 MHz; CDCl3) major isomer δ 7.34 (1H, s), 7.19 (1H,
s), 6.27 (1H, s), 6.25 (1H, s), 5.31-5.22 (1H, m), 4.02 (3H, s),
3.98 (3H, s), 3.06 (2H, s), 2.49-2.44 (2H, m), 2.31-2.26 (2H,
m), 1.55 (3H, s); 13C NMR (100 MHz; CDCl3) major isomer δ
184.4 (C), 184.1 (C), 145.9 (C), 144.9 (C), 144.6 (C), 142.7 (CH),
138.8 (CH), 130.9 (CH), 130.7 (C), 128.9 (CH), 124.5 (C), 110.9
(CH), 61.3 (Me), 61.2 (Me), 37.9 (CH2), 28.6 (CH2), 24.7 (CH2),
16.2 (Me); MS (CI) 317 (MH+, 16%), 183 (100). Found: MH+,
317.1398. C18H20O5 + H requires 317.1389.
1
(CHCl3)/cm-1 1660 (CdO); H NMR (300 MHz; CDCl3) δ 6.76
(0.6H, s), 6.73 (0.6H, d, J ) 2.3 Hz), 6.70 (0.4H, s), 6.68 (0.4H,
d, J ) 2.3 Hz), 6.57 (0.6H, s), 6.56-6.53 (0.6H, m), 6.48 (0.4H,
s), 6.42-6.40 (0.4H, m), 5.24 (0.6H, m), 5.18 (0.4H, m), 3.78
(1.8H, s), 3.74 (1.2H, s), 3.17 (1.2H, d, J ) 7.2 Hz), 3.04 (0.8H,
d, J ) 7.2 Hz), 2.73-2.65 (2H, m), 2.31 (1.8H, s), 2.29 (1.2H,
s), 2.23 (1.8H, s), 2.21 (1.2H, s), 2.19-2.12 (2H, m), 2.14 (1.8H,
s), 2.08 (1.2H, s), 1.90 (1.2H, s), 1.75 (1.8H, s); 13C NMR (100
MHz; CDCl3) δ 188.0 (C), 187.8 (C), 187.6 (C), 187.4 (C), 155.3
(C), 155.2 (C), 148.4 (C), 148.3 (C), 140.4 (C), 139.8 (C), 136.8
(CH), 136.6 (CH), 136.4 (CH), 136.3 (C), 136.0 (C), 135.9 (C),
133.8 (C), 133.6 (C), 132.3 (CH), 132.1 (CH), 130.6 (C), 130.3
(C), 122.8 (C), 122.8 (C), 118.6 (CH), 117.7 (CH), 110.3 (CH),
110.3 (CH), 55.5 (Me), 55.4 (Me), 39.6 (CH2), 31.8 (CH2), 28.7
(CH2), 27.8 (CH2), 27.4 (CH2), 27.1 (CH2), 23.7 (Me), 20.6 (Me),
20.4 (Me), 16.4 (Me), 15.8 (Me), 15.8 (Me), 12.0 (Me), 11.9 (Me);
MS (CI) 356 (MNH4+, 85), 341 (70), 269 (62), 256 (53), 238
(75), 233 (100). Found: M + NH4+, 356.2217. C22H26O3 + NH4
requires 356.2220.
3-[3-(2,3-Dimethoxyphenoxy)-4-methylpent-4-enyl]fu-
ran (23). DIAD (1.22 mL, 4.52 mmol) was added to a stirred
solution of triphenylphosphine (1.185 g, 4.52 mmol) in THF
(15 mL) at -10 °C and the resulting solution was stirred for
10 min. 5-(3-Furyl)-2-methylpenten-3-ol (22) (see the Support-
ing Information) (0.500 g, 3.01 mmol) was added and the
stirring continued for 10 min. 2,3-Dimethoxyphenol (0.26 mL,
2.01 mmol) was added and the solution was stirred for a
further 2 h at -10 °C then overnight at room temperature.
The reaction mixture was then diluted with ethyl acetate (20
mL) and washed with NaOH (2 M; 3 × 10 mL) and water (3
× 10 mL). The combined organic extracts were dried (MgSO4)
and filtered and the solvent was removed under reduced
pressure. The crude product was purified by flash chromatog-
raphy on silica, eluting with ethyl acetate-light petroleum (1:
99) to give the title compound 23 (0.244 g, 40%) as a colorless
Acknowledgment. We thank Paul Hafner for some
preliminary experiments during his undergraduate
project, Professor Javier Salva (University of Cadiz,
Spain) for information on the structure of panicein A
hydroquinone, the EPSRC and the EPSRC Mass Spec-
trometry Centre at Swansea, and the EPSRC Chemical
Database Service at Daresbury.42
1
oil; IR (film)/cm-1 1598 (CdC); H NMR (400 MHz; CDCl3) δ
7.36 (1H, s), 7.21 (1H, s), 6.89 (1H, m), 6.53 (2H, m), 6.28 (1H,
s), 4.98 (1H, m), 4.95 (1H, m), 4.56 (1H, m), 3.97 (3H, s), 3.85
(3H, s), 2.69-2.53 (2H, m), 2.22-2.10 (1H, m), 2.01-1.89 (1H,
m), 1.75 (3H, s); 13C NMR (100 MHz; CDCl3) δ 153.6 (C), 152.1
(C), 144.1 (C), 142.8 (CH), 139.0 (CH), 138.9 (C), 124.30 (C),
123.3 (CH), 113.3 (CH2), 110.9 (CH), 108.7 (CH), 105.2 (CH),
81.9 (CH), 60.8 (Me), 55.9 (Me), 34.3 (CH2), 20.9 (CH2), 17.4
(Me); MS (CI) 303 (MH+, 50%), 167 (21), 155 (49), 149 (100).
Found: MH+, 303.1589. C18H22O4 + H requires 303.1596.
6-[5-(3-Furyl)-2-methylpent-2-enyl]-2,3-dimethoxyphe-
nol (24). 3-[3-(2,3-Dimethoxyphenoxy)-4-methylpent-4-enyl]-
furan (23) (0.233 g, 0.77 mmol) in anhydrous DMF (4 mL) in
a sealed tube was heated to 180 °C for 1.75 h in a microwave
reactor (300 W). The solution was then diluted with ethyl
acetate (15 mL), washed with water (2 × 40 mL) and brine
Supporting Information Available: Copies of 1H and 13
C
NMR spectra of compounds 1-4, 5/6 (H only), 7-9, 10a-d,
11a (H only), 11b-d, 12a,b (H only), 12c,d, 13-15, 16 (H
only), 17-24; experimental details for the preparation of
pentadec-1-en-3-ol, heptadec-1-en-3-ol, 3,7-dimethyloct-6-en-
1-yn-3-ol (13), 5-(4-methoxy-2,3,6-trimethylphenyl)-3-methyl-
pentyn-3-ol (17), (E)-ethyl 3-(3-furyl)propenoate, ethyl 3-(3-
furyl)propanoate, 3-(3-furyl)propanal (21), and 5-(3-furyl)-2-
methylpenten-3-ol (22). This material is available free of
charge via the Internet at http://pubs.acs.org.
JO050336X
(42) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. J. Chem. Inf.
Comput. Sci. 1996, 36, 746-749.
4422 J. Org. Chem., Vol. 70, No. 11, 2005