Concise formal total synthesis of hybocarpone and related naturally occurring naphthazarins

Article abstract of DOI:10.1021/jo0519561

A concise formal total synthesis of the cytotoxic bisnaphthazarin derivative hybocarpone has been completed through the development of routes to the synthetic precursor, 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4- naphthoquinone. The oxidation of 3-e

Full text of DOI:10.1021/jo0519561

Concise Formal Total Synthesis of Hybocarpone and Related
Naturally Occurring Naphthazarins
Christina L. L. Chai,* John A. Elix, and Felicity K. E. Moore
Department of Chemistry, Australian National UniVersity, Canberra ACT 0200, Australia
christina_chai@ices.a-star.edu.sg
ReceiVed September 18, 2005
A concise formal total synthesis of the cytotoxic bisnaphthazarin derivative hybocarpone has been com-
pleted through the development of routes to the synthetic precursor, 3-ethyl-2-hydroxy-5,7,8-trimethoxy-
6-methyl-1,4-naphthoquinone. The oxidation of 3-ethyl-1,2,4,5,7,8-hexamethoxy-6-methylnaphthalene
under Rapoport conditions gave 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4-naphthoquinone in
modest yields after basic hydrolysis. In addition, treatment of 3-ethyl-1,2,4,5,7,8-hexamethoxy-6-meth-
ylnaphthalene with boron tribromide provided access to the naturally occurring naphthazarin, boryquinone.
The analogous oxidative demethylation of 3,6-dimethyl-1,2,4,5,7,8-hexamethoxynaphthalene and 3-ethyl-
1,2,4,5,7,8-hexamethoxynaphthalene resulted in the synthesis of 2,5,7,8-tetrahydroxy-3,6-dimethyl-1,4-
naphthoquinone (aureoquinone) and 3-ethyl-2,5,7,8-tetrahydroxy-1,4-naphthoquinone, respectively. An
alternative selective synthetic route to 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4-naphthoquinone
was also developed utilizing an intramolecular Claisen condensation of methyl 2-butyryl-3,5,6-trimethoxy-
4-methylphenylacetate with concomitant in situ aerial oxidation.
Introduction
demonstrated, and it has exhibited antiviral, antiproliferative,
anti-inflammatory, and antiprotozoal activity.2 Polyporic acid
(2), isolated from the lichen Pseudocyphellaria colensoi, has
been shown to inhibit the enzyme dihydroorotate dehydrogenase,
inhibitors of which have been linked to antitumor and immu-
nomodulating effects.3 The activity of polyporic acid (2) against
leukemia L-1210 cells has also been investigated.4,5 A sulfate
of a partially acetylated â(1 f 6) glucan from the lichen
Umbilicaria esculenta was shown to inhibit the cytopathic effect
A number of interesting secondary metabolites containing
various carboskeletons have been isolated from lichens.1 The
routine screening of lichen cultures for bioactive compounds
has facilitated the discovery of new lead compounds with
agricultural or therapeutic applications. For instance, compounds
isolated from lichens have displayed activity as plant-growth
regulators, and the common dibenzofuranoid derivative usnic
acid (1; Figure 1) has been shown to be particularly active. In
addition, the cytotoxicity of usnic acid (1) has been well-
(2) Ingolfsdottir, K. Phytochemistry 2002, 61, 729-736.
(3) Kraft, J.; Bauer, S.; Keilhoff, G.; Miersch, J.; Wend, D.; Riemann,
D.; Hirschelmann, R.; Holzhausen, H.-J.; Langner, J. Arch. Toxicol. 1998,
72, 711-721.
(4) Burton, J. F.; Cain, B. F. Nature 1959, 184, 1326-1327.
(5) Zee-Cheng, K.-Y.; Cheng, C.-C. J. Med. Chem. 1970, 13, 264-268.
* Corresponding author. Current address: Institute of Chemical and Engineer-
ing Sciences, 1 Pesek Road, Jurong Island, Singapore 627833. Phone: + (65)
67963902. Fax: + (65) 63166184.
(1) Muller, K. Appl. Microbiol. Biotechnol. 2001, 56, 9-16.
10.1021/jo0519561 CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/30/2005
992
J. Org. Chem. 2006, 71, 992-1001

Synthesis of Hybocarpone and Related Naphthazarins
the oxidative dimerization of naphthoquinone 6 to afford a
diastereomeric bisnaphthoquinonoid mixture after a water-
mediated ring closure. The diastereomers were subjected to
epimerization under acidic conditions, and O-methyl ether
cleavage gave racemic hybocarpone (4).
Our synthetic approaches to naphthoquinone 6 differ sig-
nificantly from that described by Nicolaou and Gray. This
paper details the development and implementation of two
alternative synthetic routes to naphthoquinone 6, which ef-
fectively comprise the formal total synthesis of hybocarpone
(4). In addition, the synthetic pathways developed allow access
to various naturally occurring naphthazarins and are readily
amenable to the synthesis of structural analogues of hybocarpone
(4). These results have been previously reported in a com-
munication.13
FIGURE 1. Selected bioactive secondary metabolites isolated from
lichen sources.
of the HIV virus in vitro,6 while it has been demonstrated that
the scabrosin diesters (3), isolated from the lichen Xantho-
parmelia scabrosa, are potent inhibitors of cellular proliferation.7
In 1999, we isolated and identified hybocarpone (4), a novel
pentacyclic oxido-bridged naphthazarin derivative, from lichen
mycobiont cultures of Lecanora hybocarpa Tuck.8 The anti-
proliferative activity of hybocarpone (4) was measured against
murine and human cancer cell lines, and the cytotoxicity
observed was consistently in the micromolar range. As the
exploitation of new lichen metabolites for therapeutic purposes
was central to our research interests, hybocarpone (4) was
regarded as an exciting lead compound. While a number of
monomeric and simple dimeric naphthazarins have been isolated
from plants, lichen, and marine invertebrates,9 the dinaphtho-
furantetraone carbon skeleton of hybocarpone (4) is highly
unusual and has not been previously observed in nature. Given
the potent cytotoxicity of hybocarpone (4), we wanted to further
investigate its biological properties and ascertain the structural
features responsible for the observed bioactivity. A total
synthesis of hybocarpone (4) would provide access to this unique
natural product and allow for further biological studies, given
the paucity of material isolated from natural sources.
Nicolaou and Gray have recently reported the only known
total synthesis of hybocarpone (4) to date.10,11 The Nicolaou
and Gray synthesis of hybocarpone (4) involves two key
synthetic steps: the formation of the bicyclic core and the
oxidative dimerization of the 2-hydroxy-1,4-naphthoquinone
precursor (Scheme 1). The construction of the bicyclic frame-
work was initially achieved via a photochemical Diels-Alder
reaction of the known 2,4,5-trimethoxy-3,6-dimethylbenzalde-
hyde (5)12 to access the appropriately functionalized key
precursor 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4-
naphthoquinone (6), following functional group installations and
interconversions. The next pivotal synthetic sequence involved
Results and Discussion
Retrosynthetic Analysis. An initial retrosynthetic analysis
revealed that naphthoquinone 6 may be derived through the
structural elaboration of a bicyclic carbon framework, such as
that of the relatively simple 2,5,7-trialkoxy-1,4-naphthoquinone
(Scheme 2). The regioselective functionalization of the 2,5,7-
trialkoxy-1,4-naphthoquinone may then allow for the synthesis
of a symmetrically substituted naphthalene, which would
represent the simplest system that contains all of the appropriate
oxygen functionality appended to the bicyclic core. The ap-
propriate alkylation and oxidative deprotection of this naph-
thalene may then give rise to the desired naphthoquinone 6.
Alternatively, a second synthetic route could utilize suitably
functionalized benzene derivatives as precursors to naphtho-
quinone 6. This approach involves the construction of the
bicyclic naphthoquinone system in the final stage of the
synthesis via an intramolecular Claisen condensation, followed
by in situ aerial oxidation.
Synthesis of Naphthoquinone 6 via Route 1. The develop-
ment of route 1 required synthetic access to an appropriate 2,5,7-
trialkoxy-1,4-naphthoquinone. Numerous methods for the syn-
thesis of the O-methyl ether derivative have been previously
reported,14-16 and our studies were based on the high-yield
procedure developed by Brassard and Savard.17 The Diels-
Alder reaction of 2,5-dichloro-1,4-benzoquinone with 1,3-di-
methoxy-1-trimethylsilylbuta-1,3-diene gave 2-chloro-5-hydroxy-
7-methoxy-1,4-naphthoquinone (7), which was subsequently
O-methylated through treatment with silver(I) oxide and methyl
iodide to give 2-chloro-5,7-dimethoxy-1,4-naphthoquinone (8)
in 58% yield (Scheme 3). The O-methylation of naphthoquinone
7 necessitated the use of large amounts of expensive silver(I)
oxide. In an effort to circumvent the use of silver(I) oxide,
naphthoquinone 7 was treated with ethereal diazomethane.
O-Methylation at the phenolic C5 position of naphthoquinone
7 did not occur under these conditions; however, 5-hydroxy-
7-methoxy-1-methyl-1H-benzo[f]indazole-4,9-dione (9) was iso-
lated as the major product. The tricyclic pyrazole derivative 9
presumably results from a 1,3-dipolar cycloaddition reaction
between diazomethane and naphthoquinone 7, with subsequent
(6) Hirabayashi, K.; Iwata, S.; Ito, M.; Shigeta, S.; Narui, T.; Mori, T.;
Shibata, S. Chem. Pharm. Bull. 1989, 37, 2410-2412.
(7) Ernst-Russell, M. A.; Chai, C. L. L.; Hurne, A. M.; Waring, P.;
Hockless, D. C. R.; Elix, J. A. Aust. J. Chem. 1999, 52, 279-283.
(8) Ernst-Russell, M. A.; Elix, J. A.; Chai, C. L. L.; Willis, A. C.;
Hamada, N.; Nash, T. H., III. Tetrahedron Lett. 1999, 40, 6321-6324.
(9) Thomson, R. H. Naturally Occurring Quinones III: Recent AdVances;
Chapman and Hall: London, 1987.
(10) Nicolaou, K. C.; Gray, D. Angew. Chem., Int. Ed. 2001, 40, 761-
763.
(11) Nicolaou, K. C.; Gray, D. L. F. J. Am. Chem. Soc. 2004, 126, 607-
612.
(13) Chai, C. L. L.; Elix, J. A.; Moore, F. K. E. Tetrahedron Lett. 2001,
42, 8915-8917.
(14) Birch, A. J.; Donovan, F. W. Aust. J. Chem. 1955, 8, 529-533.
(15) Davies, J. E.; King, F. E.; Roberts, J. C. J. Chem. Soc. 1955, 2782-
2786.
(12) Iyer, S.; Liebeskind, L. S. J. Am. Chem. Soc. 1987, 109, 2759-
2770.
(16) Bycroft, B. W.; Roberts, J. C. J. Chem. Soc. 1962, 2063-2064.
(17) Savard, J.; Brassard, P. Tetrahedron 1984, 40, 3455-3464.
J. Org. Chem, Vol. 71, No. 3, 2006 993

Chai et al.
SCHEME 1. Key Features of the Total Synthesis of rac-Hybocarpone (4)
SCHEME 2. Retrosynthesis of Naphthoquinone 6
SCHEME 3 a
potassium carbonate in acetone to give naphthalene 11 in
comparable yield (Scheme 4).
Following the synthesis of the appropriately O-protected
naphthalene 11, attempts were made to functionalize the C8
position. The Vilsmeier-Haack formylation of naphthalene 11
gave the corresponding aromatic 1,2,4,5,7-pentamethoxynaph-
thalene-8-aldehyde (12) in 83% yield when 5 equiv of phos-
phoryl chloride and 4 equiv of dimethylformamide in anhydrous
dichloromethane were used. The definitive requirements for the
stoichiometry of the reagents cannot be easily rationalized. The
transformation of the newly installed aldehyde functionality into
a hydroxyl group, under Baeyer-Villiger conditions, was then
sought so as to achieve the desired oxygenation pattern around
the naphthalene core. Unfortunately, attempts to oxidize naph-
thaldehyde 12 with meta-chloroperbenzoic acid to give the
corresponding formate ester were repeatedly unsuccessful.
Matsumoto et al. reported the synthesis of phenols from
aromatic aldehydes in excellent yields using acidic hydrogen
peroxide.18 When aldehyde 12 was treated with hydrogen
peroxide in acidic methanol under analogous conditions, 2,5,7,8-
tetramethoxy-1,4-naphthoquinone (13) was isolated. The spec-
troscopic and physical data of the naphthoquinone were identical
with literature data reported by Natori et al. for naphthoquinone
13, which they synthesized in low yields through the O-meth-
ylation of mompain, a naturally occurring naphthazarin.19 The
yields of the hydrogen peroxide oxidation of naphthaldehyde
12 on scales greater than 100 mg were found to be variable.
The poor reproducibility of this reaction may be due to incon-
sistencies in the concentration of hydrogen peroxide and the
consequent overoxidation of naphthaldehyde 12. With naph-
thoquinone 13 in hand, reductive methylation gave the key
symmetrical intermediate 1,2,4,5,7,8-hexamethoxynaphthalene
(14) in 74% yield (Scheme 4).
a (i) Ag2O, MeI, CHCl3, 58%; (ii) K2CO3, MeOH, 84%; and (iii) CH2N2,
Et2O, 80%.
N-methylation. The regioisomeric structure of naphthoquinone
9 was confirmed by X-ray analysis of a single crystal of the
corresponding O-methyl ether derivative. The conversion of
naphthoquinone 8 to 2,5,7-trimethoxy-1,4-naphthoquinone (10)
was then achieved in 84% yield using anhydrous potassium
carbonate in anhydrous methanol (Scheme 3).
Synthesis of Symmetrical Naphthalene 14. Following the
successful synthesis of naphthoquinone 10, the structural elab-
oration of the bicyclic framework was investigated. The
synthesis of the desired symmetrically substituted naphtha-
lene from naphthoquinone 10 required the installation of func-
tionality at the C8 position. As further functionalization would
be most readily achieved from a fully aromatic system, naph-
thoquinone 10 was converted to 1,2,4,5,7-pentamethoxynaph-
thalene (11) in 81% yield using dimethyl sulfate, potassium
hydroxide, and sodium dithionite in aqueous THF with tetrabu-
tylammonium bromide as a phase-transfer agent. Alternatively,
the reductive methylation of naphthoquinone 10 could be
achieved using sodium dithionite with dimethyl sulfate and
(18) Matsumoto, M.; Kobayashi, H.; Hotta, Y. J. Org. Chem. 1984, 49,
4741-4743.
(19) Natori, S.; Inouye, Y.; Nishikawa, H. Chem. Pharm. Bull. 1967,
15, 380-390.
994 J. Org. Chem., Vol. 71, No. 3, 2006

Synthesis of Hybocarpone and Related Naphthazarins
SCHEME 4 a
a (i) Na2S2O4, Me2SO4, KOH, NBu4Br, THF, H2O, rt, 18 h, 81% or K2CO3, Me2SO4, Na2S2O4, DMF, Me2CO, reflux, 18 h, 79%; (ii) POCl3, DMF,
CH2Cl2, then NaOAc, H2O, 83%; (ii) H2O2, H2SO4, MeOH, 72%; and (iv) Na2S2O4, Me2SO4, KOH, NBu4Br, THF, H2O, rt, 18 h, 74%.
SCHEME 5 a
a (i) BnBr, Ag2O, CHCl3, 54%; (ii) K2CO3, MeOH, 93%; (iii) Na2S2O4, Me2SO4, KOH, NBu4Br, THF, H2O, rt, 18 h, 56%; (iv) H2, 10% Pd on C, EtOAc,
80%; and (v) Fremy’s salt, MeCN, KH2PO4, quantitative.
SCHEME 6 a
Alternative Synthesis of Naphthoquinone 13. As naphtho-
quinone 13 could only be accessed in small quantities, alterna-
tive synthetic approaches to this naphthoquinone were explored.
It is known that the oxidation of an appropriately substituted
naphthol can give rise to the corresponding 1,4-naphthoquinone
in good yield. A synthetic route to the appropriately substituted
naphthol 5-hydroxy-1,2,4,7-tetramethoxynaphthalene (15) was,
therefore, designed with the purpose of carrying out an oxidation
to afford naphthoquinone 13 (Scheme 5). The first step in this
synthetic sequence involved the O-benzylation of naphthoqui-
none 7, a reaction which proceeded smoothly on treatment with
silver(I) oxide and benzyl bromide to give 5-benzyloxy-2-chloro-
7-methoxy-1,4-naphthoquinone (16) in 54% yield. The treatment
of naphthoquinone 16 with potassium carbonate in methanol
afforded 5-benzyloxy-2,7-dimethoxy-1,4-naphthoquinone (17)
a (i) 1.2 equiv n-BuLi, 1.2 equiv TMEDA, THF, excess EtI; (ii) 1.2 equiv
with excellent regioselectivity and yield. Naphthoquinone 17
n-BuLi, 1.2 equiv TMEDA, THF, excess MeI, 95%; and (iii) 4.5 equiv
n-BuLi, 4.5 equiv TMEDA, THF, excess MeI, quantitative.
was reductively methylated, and the resultant 5-benzyloxy-
1,2,4,7-tetramethoxynaphthalene (18) was subjected to hydro-
genolysis using 10% palladium on carbon as a catalyst in a hy-
drogen atmosphere to give naphthol 15 in 80% yield. Oxidation
of naphthol 15 with Fremy’s salt gave naphthoquinone 13 in
quantitative yield (Scheme 5). We have, therefore, successfully
developed two viable synthetic routes to naphthoquinone 13,
each comprised of five synthetic steps from naphthoquinone 7.
C Alkylation of the Symmetrical Naphthalene 14. Once
synthetic access to naphthalene 14 was achieved, various
C-alkylated naphthalenes were identified as appropriate synthetic
targets en route to both naphthoquinone 6 and several naturally
occurring naphthazarins. Our strategy toward the alkylation of
the aromatic naphthalene core involved metalation with an
J. Org. Chem, Vol. 71, No. 3, 2006 995

Chai et al.
SCHEME 7 a
a (i) AgO, HNO3, H2O, dioxane, sonication, quantitative and (ii) MeOH, 10% NaOH, reflux, 0.5 h, then H2SO4, quantitative.
SCHEME 8 a
organolithium reagent, followed by the alkylation of the resultant
aryllithium with an alkyl halide. Initial attempts to C alkylate
naphthalene 14 using a slight excess of n-butyllithium and a
large excess of ethyl iodide failed to yield any of the desired
product, and only starting material was recovered from the
reaction mixture. The alkylation was also unsuccessful when
other bases such as tert-butyllithium and the “superbase”
LiCKOR were employed.20
The use of basic additives such as tetramethylethylenediamine
(TMEDA) to de-aggregate n-butyllithium hexamers and, thereby,
a (i) BBr3, CH2Cl2, -78°C for 1 h, then 72 h at rt, quantitative.
increase the basicity of n-butyllithium is well-documented.21
When naphthalene 14 was treated with 1.2 equiv of n-butyl-
lithium and 1.2 equiv of TMEDA, followed by a large excess
of ethyl iodide, 3-ethyl-1,2,4,5,7,8-hexamethoxynaphthalene (19)
was isolated in 55% yield together with 23% of the dialkylated
material and recovered starting material (Scheme 6). The
ethylnaphthalene 19 displayed a characteristic proton singlet at
6.62 ppm in the aromatic region of the proton NMR spectrum.
Naphthalene 19 can be further C alkylated in excellent yield
under comparable reaction conditions using methyl iodide to
give the mixed dialkylated 3-ethyl-1,2,4,5,7,8-hexamethoxy-6-
methylnaphthalene (20). The proton NMR spectrum of naph-
thalene 20 lacks the aromatic proton signal, and typical C-methyl
and C-ethyl resonances were observed. When naphthalene 14
was treated with 4 equiv of n-butyllithium and a large excess
of methyl iodide, 3,6-dimethyl-1,2,4,5,7,8-hexamethoxynaph-
thalene (21) was isolated in excellent yield.
mixture of products. The use of various acylation conditions
and reagents including AlCl3/AcCl, FeCl3/Ac2O, and I2/Ac2O
resulted in either complex product mixtures or the recovery of
starting material. An alternative approach to C acylation would
involve the in situ generation of the appropriate aryl lithio
species followed by the reaction of the anion thus formed with
a suitable electrophile. When naphthalene 14 was treated with
n-butyllithium at -78 °C followed by the addition of N,N-
dimethylacetamide, however, a complex mixture of products
was obtained. Given the difficulties encountered, the selective
installation of C-acetyl functionality on the symmetrical naph-
thalene 14 was not pursued further.
Synthesis of Naphthoquinone 6 and Naturally Occurring
Naphthazarins. The treatment of the naphthalene 20 under the
classic Rapoport oxidation conditions,22 using freshly prepared
silver(II) oxide and nitric acid, gave a mixture of four products
in overall quantitative yield. These reaction products were
identified spectroscopically as the two isomeric naphthoquinones
22 and 23 and the corresponding hydrolysis products naphtho-
quinones 24 and 6. Treatment of the product mixture under basic
conditions gave an inseparable mixture of the 2-hydroxy
naphthoquinones 24 and 6 in a 1:1 ratio in quantitative yield
(Scheme 7). Given that naphthoquinone 6 was the key precursor
used by Nicolaou and Gray, a nonselective formal synthesis of
hybocarpone (4) was thereby accomplished.
Although the C-alkylation strategy discussed above was
successful and allows for the introduction of the alkyl substit-
uents present in the target compounds, the inherent competition
between mono- and dialkylation leads to moderate yields of
the desired naphthalene 19 at best. An alternative synthesis of
naphthalene 19, involving the acetylation of the naphthalene
core followed by the reduction of the acetyl group to the desired
ethyl substituent, was also investigated. In principle, the selective
installation of the carbon functionality could be achieved via a
Friedel-Crafts acylation of naphthalene 14, given that multiple
acylation is unlikely as a result of the deactivation of the
resulting aromatic species to further electrophilic aromatic
substitution. Initial attempts to perform a Friedel-Crafts acety-
lation on the symmetrical naphthalene 14 using acetic anhydride
and a catalytic amount of perchloric acid resulted in a complex
In principle, O-methyl cleavage of the various hexamethoxy-
naphthalenes could also lead to a number of naturally occurring
naphthazarins, depending on the selectivity of the ether cleavage.
The alkylated naphthalenes 19, 20, and 21 were each treated
with boron tribromide over 3 days. Under these conditions, the
(20) Schlosser, M.; Strunk, S. Tetrahedron Lett. 1984, 25, 741-744.
(21) Snieckus, V. Chem. ReV. 1990, 90, 879-933.
(22) Snyder, C. D.; Rapoport, H. J. Am. Chem. Soc. 1972, 94, 227-
231.
996 J. Org. Chem., Vol. 71, No. 3, 2006

Synthesis of Hybocarpone and Related Naphthazarins
SCHEME 9 a
a (i) Hippuric acid, Ac2O, NaOAc, 30%; (ii) 10% NaOH, H2O2, 55%; (iii) H2SO4, MeOH, 77%; (iv) NaBH4, THF, MeOH, 99%; (v) SOCl2, 96%; (vi)
DMSO, KCN, 99%; and (vii) H2SO4, AcOH, H2O, 86%.
SCHEME 10 a
a (i) Butyric anhydride, HClO4, 85% and (ii) NaOEt, EtOH, air bubbler, 48 h, H2SO4, 56%.
methyl ethers were completely cleaved and in situ aerial oxi-
dation ensued to give the deep-purple-red naphthazarins, 3-ethyl-
2,7-dihydroxy-1,4-naphthazarin (25),23 3-ethyl-2,7-dihydroxy-
6-methyl-1,4-naphthazarin (26),24 and aureoquinone (27),25
respectively, in quantitative yield (Scheme 8). Two tautomeric
forms for each of the isolated naphthazarins are possible, but
we were unable to make definitive structural assignments;
however, the spectroscopic data were consistent with those
reported in the literature for each naphthazarin.13
Synthesis of Naphthoquinone 6 via Route 2. As previously
outlined, our primary objective was to access synthetic hy-
bocarpone (4) for further biological testing, and given that pure
naphthoquinone 6 was required, a selective route to naphtho-
quinone 6 was also developed.
Synthesis of Phenylacetate Derivative 31. The alternative
synthesis of naphthoquinone 6 utilized the suitably substituted
known 2,3,5-trimethoxy-4-methylbenzaldehyde (28)26 for chain
homologation studies. The benzaldehyde 28 was treated with
hippuric acid in the presence of acetic anhydride to give
azlactone 2-phenyl-4-(2,3,5-trimethoxy-4-methylbenzylidene)-
4H-oxazol-5-one (29) in poor yield. Subsequent hydrolysis and
oxidation of azlactone 29 gave 2,3,5-trimethoxy-4-methylphen-
ylacetic acid (30), which was then converted into methyl 2,3,5-
trimethoxy-4-methylphenylacetate (31) through treatment with
acidic methanol in 66% yield over the two steps (Scheme 9).
Unfortunately, the yields for the azlactone 29 formation were
consistently low, and the subsequent rearrangement reaction to
phenylacetic acid 30 was not reproducible. This prompted an
investigation into an alternative synthesis of the methyl phen-
ylacetate 31.
An aldehyde can be readily converted into the corresponding
aliphatic acid via a one-carbon chain homologation using the
cyanide ion as the source of the carbon atom. To examine this
approach, it was necessary to reduce benzaldehyde 28 to access
the corresponding 2,3,5-trimethoxy-4-methylbenzyl alcohol (32).
This reduction was achieved in excellent yields using sodium
borohydride. The resulting benzyl alcohol 32 was then converted
into 2,3,5-trimethoxy-4-methylbenzyl chloride (33) through
treatment with thionyl chloride. Benzyl chloride 33 was then
treated with potassium cyanide to give 2,3,5-trimethoxy-4-
methylbenzyl cyanide (34), which was subsequently hydrolyzed
under acidic conditions to give the corresponding phenylacetic
acid 30 in excellent yield. Phenylacetic acid 30 was then
esterified under the acidic conditions previously developed to
give methyl phenylacetate 31 in good yield (Scheme 9).
(23) Moore, R. E.; Singh, H.; Chang, C. W. J.; Scheuer, P. J. Tetrahedron
1967, 23, 3271-3305. Moore, R. E.; Singh, H.; Chang, C. W. J.; Scheuer,
P. J. J. Org. Chem. 1966, 31, 3638-3645 and references therein. Moore,
R. E.; Singh, H.; Scheuer, P. J. J. Org. Chem. 1966, 31, 3645-3650.
(24) Huneck, S.; Ahti, T. Pharmazie 1982, 37, 302.
(25) Berg, A.; Gorls, H.; Dorfelt, H.; Walther, G.; Schlegel, B.; Grafe,
U. J. Antibiot. 2000, 53, 1293-1295.
(26) Kitahara, Y.; Nakahara, S.; Numata, R.; Kubo, A. Chem. Pharm.
Bull. 1985, 33, 2122-2128.
Synthesis of Naphthoquinone 6. Treatment of methyl ester
31 with butyric anhydride and perchloric acid gave methyl
2-butyryl-3,5,6-trimethoxy-4-methylphenylacetate (35) in good
yield. A Claisen condensation reaction was then performed
under basic conditions to give naphthoquinone 6 after aerial
oxidation and acidic workup (Scheme 10). The spectroscopic
data for naphthoquinone 6 were identical to those reported by
J. Org. Chem, Vol. 71, No. 3, 2006 997

Chai et al.
was removed under reduced pressure to give the crude O-methyl
derivative, 5,7-dimethoxy-1-methyl-1H-benzo[f]indazole-4,9-dione.
The resulting residue was purified by flash column chromatography
(Rf 0.30, 50% ethyl acetate/petroleum spirits) to give the product
in 76% yield as an orange solid; mp 201-202 °C. IR νmax (KBr):
3433 (s), 1681 (m), 1589 (w), 1496 (m), 1427 (w), 1272 (m), 1218
(m), 1157 (m), 1041 (s), 802 (w) cm-1. 1H NMR (CDCl3): δ 3.94
(s, 3H), 3.96 (s, 3H), 4.26 (s, 3H), 6.75 (d, J ) 3 Hz, 1H), 7.37 (d,
J ) 3 Hz, 1H), 7.95 (s, 1H). 13C NMR (CDCl3): δ 39.0, 56.0,
56.5, 104.3, 104.7, 115.5, 125.0, 136.1, 137.1, 137.7, 162.8, 164.3,
175.8, 178.6. MS (m/z): 272 (100%, M+•), 243 (86%, M+• - NMe),
241 (52%), 225 (31%). HRMS (m/z, M+•): calcd for C14H12N2O4,
272.0797; found, 272.0799. The structural assignment was con-
firmed by X-ray crystallographic analysis.
1,2,4,5,7-Pentamethoxynaphthalene (11). To a stirred solution
of 2,5,7-trimethoxy-1,4-naphthoquinone (10;17 0.5 g, 2.01 mmol)
in THF (50 mL) at 0 °C was added tetrabutylammonium bromide
(1.7 g, 5.27 mmol). This was followed by the addition of aqueous
sodium dithionite (100 mL, 30% w/v), dimethyl sulfate (23.8 mL,
0.25 mol), and finally, the slow addition of aqueous potassium
hydroxide (150 mL, 26% w/v) over a period of 5 min. The reaction
mixture was stirred at room temperature for 16 h, followed by the
addition of ethyl acetate (100 mL). The aqueous layer was isolated
and extracted with ethyl acetate (2 × 100 mL). The organic layers
were combined, dried with magnesium sulfate, and filtered, and
the solvent was removed under reduced pressure. The resultant
crude residue was purified by flash column chromatography on SiO2
using 25% ethyl acetate/petroleum spirits (Rf 0.3) to give naphtha-
lene 11 in 81% yield as a white solid; mp 118 °C. IR νmax (KBr):
2960 (w), 1625 (s), 1614 (m), 1592 (m), 1473 (w), 1379 (m), 1351
(s), 1262 (s), 1231 (s), 1205 (s), 1044 (s), 819 (m) cm-1. 1H NMR
(CDCl3): δ 3.87 (s, 3H), 3.91 (s, 6H), 3.92 (s, 3H), 3.96 (s, 3H),
6.36 (d, J ) 3 Hz, 1H), 6.49 (s, 1H), 6.96 (d, J ) 3 Hz, 1H). 13C
NMR (CDCl3): δ 55.2, 56.1, 56.6, 57.0, 60.5, 91.7, 94.6, 97.0,
108.9, 132.8, 135.9, 149.0, 154.3, 158.5, 158.7. MS (m/z): 278
(56%, M+•), 263 (100%, M+• - Me), 235 (37%), 220 (11%), 192
(13%), 148 (22%). HRMS (m/z, M+•): calcd for C15H18O5,
278.1154; found, 278.1150. Anal. Calcd for C15H18O5: C, 64.74;
H, 6.52. Found: C, 64.34; H, 6.54.
1,2,4,5,7-Pentahydroxynaphthalene-8-aldehyde (12). N,N-
Dimethylformamide (7.23 mmol, 0.56 mL) was added dropwise at
0 °C to a stirred solution of freshly distilled phosphoryl chloride
(9.44 mmol, 0.88 mL) in dry dichloromethane (40 mL). The solution
was stirred for 0.5 h at 0 °C, after which time 1,2,4,5,7-
pentamethoxynaphthalene (11; 0.5 g, 1.80 mmol) was added, and
the reaction mixture was stirred for an additional 0.5 h at 0 °C.
The reaction mixture was allowed to warm to room temperature,
and stirring was continued for an additional 16 h. The reaction was
then quenched by the addition of aqueous sodium acetate (50 mL,
10% w/v). The aqueous layer was extracted exhaustively with ethyl
acetate (3 × 50 mL), the combined organic fractions were dried
with magnesium sulfate and filtered, and the solvent was removed
in vacuo to give the aldehyde 12 as the crude product. The aldehyde
12 was purified by flash column chromatography on SiO2 using
80% ethyl acetate/petroleum spirits (Rf 0.50) to give the product
as a white microcrystalline powder (83%); mp 158-159 °C. IR
Nicolaou and Gray, and this, therefore, constitutes a selective
formal total synthesis of hybocarpone (4). In principle, numerous
analogues of hybocarpone (4) can be accessed by varying the
anhydride used to form the aryl ketone. This synthetic step could
potentially be utilized to access a number of bisnaphthazarin
derivatives.
Conclusion
In conclusion, we have successfully accessed naphthoquinone
6 via two synthetic pathways and thus completed the formal
total synthesis of the cytotoxic bisnaphthazarin hybocarpone (4).
Furthermore, our synthetic routes also allowed for the synthesis
of a number of naturally occurring monomeric naphthazarins
and, in principle, are amenable to the synthesis of structural
analogues of hybocarpone (4).
Experimental Section
Commercially available reagents were used throughout without
further purification, unless otherwise stated. All solvents were AR
grade, purified by literature procedures, and stored over freshly
activated molecular sieves where appropriate. All reactions were
carried out under an atmosphere of dry, oxygen-free nitrogen unless
otherwise specified. Reactions that involved moisture-sensitive
compounds were carried out using oven-dried apparatus and dry
solvents. IR spectra were recorded in the range 4000-800 cm-1
.
1H and 13C NMR spectra were recorded at 300 and 75 MHz,
respectively; J values were recorded in hertz. UV/vis spectra were
recorded using spectroscopic grade solvents, and the data are
displayed as the λmax wavenumber of the absorption peak in
nanometers. Melting points are uncorrected. For general reaction
protocols, representative procedures are given.
5-Hydroxy-7-methoxy-1-methyl-1H-benzo[f]indazole-4,9-di-
one (9). Diazomethane was prepared according to the procedure
described by Furniss et al.27 using diazald glassware. The use of
glassware containing ground glass joints should be avoided when
preparing and handling solutions of diazomethane, and the use of
a blast shield is recommended. To a solution of potassium hydroxide
(0.505 g, 9 mmol) in water (1 mL) was added 96% ethanol (2.5
mL), and the solution was warmed to 60-65 °C. A solution of
N-methyl-N-nitroso-p-toluenesulfonamide (2.165 g, 10 mmol) in
diethyl ether (25 mL) was then added dropwise, followed by the
addition of diethyl ether (30 mL). The ethereal diazomethane
solution thus generated was distilled into a solution of 2-chloro-
5,7-dimethoxy-1,4-naphthoquinone (7; 200 mg, 0.8 mmol) in diethyl
ether (10 mL). The reaction vessel was stoppered with a rubber
bung and left to stand overnight at room temperature. The solvent
was then allowed to evaporate at atmospheric pressure. The resulting
residue was purified by flash column chromatography (Rf 0.30, 50%
ethyl acetate/petroleum spirits), and 5-hydroxy-7-methoxy-1-methyl-
1H-benzo[f]indazole-4,9-dione (9) was isolated in 80% yield. MS
(m/z): 258 (100%, M+•), 229 (6%, M+• - NMe), 215 (21%), 187
(23%), 159 (22%), 132 (21%). HRMS (m/z, M+•): calcd for
C14H12N2O4, 258.0641; found, 258.0645. To a solution of 5-hy-
droxy-7-methoxy-1-methyl-1H-benzo[f]indazole-4,9-dione (9; 100
mg, 0.4 mmol) in acetone (2 mL) was added dry potassium
carbonate (80 mg, 0.6 mmol) with stirring. Methyl iodide (0.2 mL,
excess) was added to the reaction mixture, and the solution was
heated at reflux overnight. The reaction mixture was concentrated
in vacuo, and the residue was partitioned between 5% HCl (25
mL) and dichloromethane (25 mL). The aqueous layer was washed
with dichloromethane (2 × 25 mL). The combined organic extracts
were dried with magnesium sulfate and filtered, and the solvent
ν
max (KBr): 2936 (w), 2839 (w), 1682 (s), 1585 (s), 1516 (w), 1466
(s), 1400 (m), 1369 (m), 1335 (s), 1277 (m), 1215 (s), 1045 (s)
cm-1. H NMR (CDCl3): δ 3.55 (s, 3H), 3.85 (s, 3H), 3.86 (s,
1
3H), 3.91 (s, 6H), 6.36 (s, 1H), 6.47 (s, 1H), 10.41 (s, 1H). 13C
NMR (CDCl3): δ 56.1, 56.2, 56.7, 59.8, 91.7, 94.5, 108.3, 112.2,
130.8, 135.5, 150.4, 154.9, 156.4, 160.8, 192.4. MS (m/z): 306
(85%, M+•), 291 (93%, M+• - Me), 275 (100%, M+• - CHO),
263 (27%), 248 (19%), 219 (15%). HRMS (m/z, M+•): calcd for
C16H18O6, 306.1103; found, 306.1099. Anal. Calcd for C16H18O6:
C, 62.74; H, 5.92. Found: C, 63.02; H, 6.04.
2-Chloro-5-benzyloxy-7-methoxy-1,4-naphthoquinone (16).
To a solution of 2-chloro-5-hydroxy-7-methoxy-1,4-naphthoquinone
(7; 0.85 g, 3.6 mmol) in chloroform (10 mL) was added benzyl
(27) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R.
Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman
Scientific and Technical: London, 1989.
998 J. Org. Chem., Vol. 71, No. 3, 2006

Synthesis of Hybocarpone and Related Naphthazarins
bromide (0.85 mL, excess) and silver(I) oxide (3.3 g, 14.2 mmol),
and the reaction mixture was stirred at room temperature for 96 h.
The suspension was filtered through Celite, and the solvent was
removed in vacuo to give the crude product mixture, which was
then purified by column chromatography (Rf 0.42, 15% ethyl
acetate/petroleum spirits) to give 2-chloro-5-benzyloxy-7-methoxy-
1,4-naphthoquinone (16) in 54% yield; mp 121.5-123.5 °C. IR
M+• - Me), 217 (24%), 207 (16%), 189 (11%), 149 (12%). HRMS
(m/z, M+•): calcd for C14H14O6, 278.0790; found, 278.0785.
3-Ethyl-1,2,4,5,7,8-hexamethoxynaphthalene (19). To a solu-
tion of 1,2,4,5,7,8-hexamethoxynaphthalene (14; 0.1 g, 0.32 mmol)
in dry THF (2 mL) in a two-necked 10 mL round-bottomed flask
was added n-butyllithium (0.21 mL, 0.38 mmol) under nitrogen at
-78 °C with stirring. TMEDA (0.06 mL, 0.38 mmol) was added
to the reaction mixture, and the solution was stirred for 1 h at -78
°C before it was warmed to room temperature. The reaction mixture
was then cooled to -78 °C, ethyl iodide (0.1 mL, excess) was
added, and the solution was stirred for 1 h at -78 °C, followed by
warming to room temperature. The reaction mixture was quenched
with water and extracted with ethyl acetate (3 × 10 mL). The
organic extracts were combined, dried with magnesium sulfate, and
filtered, and the solvent was removed in vacuo to afford the crude
monoethylated naphthalene 19, diethylated naphthalene, and
1,2,4,5,7,8-hexamethoxynaphthalene (14) in a 5:4:1 ratio. The
compounds were then separated using flash column chromatography
with 15% ethyl acetate/petroleum spirits (Rf 0.28, 0.81, and 0.1,
respectively). Naphthalene 19 was isolated in 55% yield as a white
solid; mp 45 °C. 1H NMR (CDCl3): δ 1.21 (t, J ) 7 Hz, 3H), 2.77
(q, J ) 7 Hz, 2H), 3.76 (s, 3H), 3.83 (s, 3H), 3.85 (s, 3H), 3.97 (s,
3H), 3.98 (s, 3H), 4.00 (s, 3H), 6.62 (s, 1H). 13C NMR (CDCl3):
δ 15.5, 17.9, 56.7, 56.9, 61.1, 61.6, 61.8, 62.5, 95.9, 114.4, 125.1,
128.4, 136.6, 143.3, 149.2, 150.3, 150.9, 152.7. MS (m/z): 336
(100%, M+•), 321 (83%, M+• - Me), 306 (21%, M+• - Et), 289
(40%), 275 (12%), 263 (27%), 233 (11%). HRMS (m/z, M+•): calcd
for C18H24O6, 336.1573; found, 336.1570.
νmax (KBr): 3418 (w), 1682 (s), 1645 (s), 1593 (s), 1556 (w), 1454
(w), 1438 (w), 1385 (w), 1328 (s), 1302 (m), 1294 (m), 1276 (s),
1259 (s), 1233 (m), 1203 (m), 1170 (m), 1141 (m), 1090 (w), 1027
(w) cm-1. 1H NMR (CDCl3): δ 3.89 (s, 3H), 5.23 (s, 2H), 6.77 (d,
J ) 3 Hz, 1H), 7.04 (s, 1H), 7.29 (d, J ) 3 Hz, 1H), 7.30-7.55
(m, 5H). 13C NMR (CDCl3): δ 56.0, 70.8, 105.0, 105.9, 114.0,
126.5, 127.9, 128.6, 135.0, 135.6, 138.0, 142.6, 160.8, 164.5, 178.3,
180.4. MS (m/z): 330 (2%, M+•, C18H13O437Cl), 328 (16%, M+•,
C18H13O435Cl), 272 (31%), 238 (34%), 203 (19%), 175 (7%), 91
(100%). HRMS (m/z, M+•): calcd for C18H13O437Cl, 330.0473;
found, 330.0473. HRMS (m/z, M+•): calcd for C18H13O435Cl,
328.0502; found, 328.0500. Anal. Calcd for C18H13ClO4: C, 65.76;
H, 3.99. Found: C, 65.33; H, 4.11.
5-Hydroxy-1,2,4,7-tetramethoxynaphthalene (15). To a solu-
tion of 1,2,4,7-tetramethoxy-5-benzyloxynaphthalene (18; 115 mg,
0.33 mmol) in ethyl acetate (20 mL) was added 10% palladium on
carbon (12 mg), and the reaction mixture was stirred at room
temperature under a hydrogen atmosphere for 96 h. The resultant
suspension was then poured through Celite, and the volume of the
filtrate was concentrated under reduced pressure. The crude product
was purified chromatographically (Rf 0.19, 15% ethyl acetate/
petroleum spirits) to give the desired naphthol 15 in 80% yield;
mp 115-116 °C. IR νmax (KBr): 3384 (s), 2935 (m), 1634 (s),
1618 (m), 1452 (m), 1384 (s), 1354 (m), 1274 (m), 1229 (w), 1150
3-Ethyl-2,5,7,8-tetrahydroxy-6-methyl-1,4-naphthoquinone
(Boryquinone, 26). To a stirred solution of 3-ethyl-1,2,4,5,7-
pentamethoxy-6-methylnaphthalene (20; 10 mg, 0.03 mmol) in dry
dichloromethane (1 mL) was added boron tribromide (1 M in
dichloromethane, 0.15 mL) at -78 °C. The reaction mixture was
warmed to room temperature and stirred for 48 h, after which time
water (2 mL) was added prior to the addition of dichloromethane
(5 mL). The organic layer was isolated, and the aqueous layer was
extracted with dichloromethane (2 × 5 mL). The organic layers
were combined, dried with magnesium sulfate, and filtered, and
the solvent was removed in vacuo to afford the deep-red-purple
solid; mp 178-181 °C (lit. 180-184 °C).24 IR νmax (KBr): 3395
(s), 2932 (s), 1735 (s), 1658 (s), 1388 (s), 1303 (s), 1180 (s), 1095
1
(s), 1122 (m), 1109 (m), 1043 (m) cm-1. H NMR (CDCl3): δ
3.86 (s, 3H), 3.88 (s, 3H), 3.96 (s, 3H), 4.01 (s, 3H), 6.41 (d, J )
2 Hz, 1H), 6.43 (s, 1H), 6.88 (d, J ) 2 Hz, 1H), 9.21 (s, 1H). 13
C
NMR (CDCl3): δ 55.3, 56.3, 57.1, 60.6, 91.9, 92.8, 100.2, 106.9,
132.5, 136.9, 148.6, 153.1, 155.9, 159.8. MS (m/z): 264 (68%,
M+•), 249 (100%, M+• - Me), 221 (33%, M+• - Me - CO), 206
(14%), 175 (6%), 135 (7%). HRMS (m/z, M+•): calcd for C14H16O5,
264.0998; found, 264.1001. Anal. Calcd for C14H16O5: C, 63.63;
H, 6.10. Found: C, 63.64; H, 6.00.
1
(m), 810 (w) cm-1. H NMR (CDCl3): δ 1.14 (t, J ) 8 Hz, 3H),
2,5,7,8-Tetramethoxy-1,4-naphthoquinone (13). Method 1: To
a stirred solution of 1,2,4,5,7-pentahydroxynaphthalene-8-aldehyde
(12; 0.2 g, 0.65 mmol) in methanol (35 mL) was added 30%
aqueous hydrogen peroxide (1 mL) and concentrated sulfuric acid
(6.5 µL). The reaction mixture was stirred at room temperature for
1.5 h, poured onto cold, aqueous sodium bicarbonate (50 mL, 10%
w/v), and immediately extracted with ethyl acetate (3 × 50 mL).
The combined organic extracts were dried with magnesium sulfate
and filtered, and the solvent was removed in vacuo to give the deep
red naphthoquinone 13. Recrystallization from ethyl acetate/
petroleum spirits then gave naphthoquinone 13 as red needles in
72% yield. Method 2: To a solution of 5-hydroxy-1,2,4,7-
tetramethoxynaphthalene (15; 26 mg, 0.1 mmol) in acetonitrile (2
mL) was added potassium nitrosodisulfonate (30 mg, 0.11 mmol)
and potassium dihydrogen phosphate (15 mg, 0.11 mmol) in water
(0.5 mL) with sonication. The reaction mixture was then stirred
for 16 h without further sonication at room temperature. The solvent
was concentrated in vacuo, and the resultant residue was purified
by flash column chromatography (Rf 0.12, 80% ethyl acetate/
petroleum spirits) to give naphthoquinone 13 in quantitative yield;
mp 162 °C (lit. 169-171 °C).18 IR νmax (KBr): 3418 (w), 2946
(w), 2848 (w), 1679 (m), 1644 (s), 1627 (s), 1578 (w), 1548 (m),
1471 (m), 1353 (m), 1313 (m), 1259 (s), 1233 (s), 1216 (s), 1177
(m), 1094 (m), 1034 (s), 846 (m) cm-1. 1H NMR (CDCl3): δ 3.80
(s, 3H), 3.84 (s, 3H), 3.95 (s, 3H), 3.96 (s, 3H), 5.96 (s, 1H), 6.75
(s, 1H). 13C NMR (CDCl3): δ 56.1, 56.2, 56.7, 61.2, 102.1, 110.8,
112.5, 125.6, 144.1, 157.4, 158.7, 159.1, 179.6, 183.6. MS (m/z):
280 (22%, M+• + 2H), 278.0 (100%, M+•), 265 (31%), 263 (17%,
2.14 (s, 3H), 2.67 (q, J ) 8 Hz, 2H), 6.66 (s, 1H), 6.68 (s, 1H),
11.74 (s, 1H), 13.47 (s, 1H). MS (m/z): 264 (100%, M+•), 249.0
(15%), 221.0 (52%), 190 (6%), 167 (5%), 137 (10%). HRMS
(m/z, M+•): calcd for C13H12O6, 264.06334; found, 264.06335.
UV/vis (MeOH): 234, 264, 323, 510, 550 nm.
2-Phenyl-4-(2,3,5-trimethoxy-4-methylbenzylidene)-4H-oxazol-
5-one (29). A stirred mixture of 2,3,5-trimethoxy-4-methylbenzal-
dehyde (28;28 0.5 g, 2.70 mmol), hippuric acid (0.51 g, 2.85 mmol),
acetic anhydride (0.82 mL), and anhydrous sodium acetate (0.24
g, 2.90 mmol) was heated at 100 °C for 2 h. The reaction mixture
was cooled to room temperature, diluted with ethanol (4 mL), and
filtered to give an orange/yellow powder (0.190 g, 0.53 mmol).
The solid was purified by recrystallization from ethyl acetate/
petroleum spirits to give the desired compound in 30% yield as
yellow prisms; mp 153-154 °C. IR νmax (KBr): 3441 (s), 2939
(w), 1790 (m), 1651 (s), 1589 (s), 1458 (w), 1411 (w), 1319 (w),
1280 (s), 1226 (w), 1164 (m), 1110 (s), 1018 (w) cm-1 1H NMR
.
(CDCl3): δ 2.19 (s, 3H), 3.82 (s, 3H), 3.88 (s, 3H), 3.95 (s, 3H),
7.46-7.62 (m, 3H), 7.69 (s, 1H), 8.06-8.12 (m, 2H), 8.24 (s, 1H).
13C NMR (CDCl3): δ 9.5, 55.6, 60.4, 61.9, 107.6, 124.7, 125.7,
126.5, 127.6, 128.0, 128.9, 132.4, 133.1, 148.6, 151.7, 154.3, 162.7,
167.7. MS (m/z): 353 (58%, M+•), 205 (2%), 167 (3%), 105
(100%), 77 (32%). HRMS (m/z, M+•): calcd for C20H19NO5,
353.1263; found, 353.1266.
2,3,5-Trimethoxy-4-methylbenzyl Alcohol (32). To a solution
of 2,3,5-trimethoxy-4-methylbenzaldehyde (28;26 1.17 g, 5.90
mmol) in dry THF (15 mL) and methanol (4 mL) was added sodium
J. Org. Chem, Vol. 71, No. 3, 2006 999

Chai et al.
borohydride (0.06 g, 1.59 mmol) at 0 °C. The solution was stirred
for 1.5 h at 0 °C prior to the addition of 5% aqueous hydrochloric
acid (50 mL) and ethyl acetate (50 mL). The organic layer was
isolated, and the aqueous layer was extracted with ethyl acetate (2
× 50 mL). The combined organic extracts were dried with
magnesium sulfate and filtered, and the solvent was removed in
vacuo to givea pale oil in 99% yield. IR νmax (KBr): 3392 (s),
2937 (s), 1486 (s), 1463 (s), 1407 (s), 1242 (m), 1130 (s), 1091
Methyl 2-Butyryl-3,5,6-trimethoxy-4-methylphenylacetate (35).
To a solution of methyl 2,3,5-trimethoxy-4-methylphenyl acetate
(31; 0.92 g, 3.60 mmol) in butyric anhydride (3.5 mL) was added
40% aqueous perchloric acid (0.02 mL) at room temperature. The
reaction mixture was stirred for 16 h prior to the slow addition of
5% sodium hydrogen carbonate (10 mL). The solution was extracted
with ethyl acetate (3 × 20 mL), the combined organic extracts were
dried with magnesium sulfate and filtered, and the solvent was
removed under reduced pressure to give the crude product. The
residue was then purified using column chromatography (Rf 0.34,
50% ethyl acetate/petroleum spirits) to give the ketone 35 as a
colorless oil in 85% yield. IR νmax (KBr): 1740 (s), 1697 (m), 1578
(w), 1458 (m), 1404 (m), 1327 (w), 1261 (m), 1169 (s), 1092 (s),
1018 (m) cm-1. 1H NMR (CDCl3): δ 0.94 (t, J ) 7 Hz, 3H), 1.66
(h, J ) 7 Hz, 2H), 2.16 (s, 3H), 2.80 (t, J ) 7 Hz, 2H), 3.61 (s,
3H), 3.66 (s, 2H), 3.77 (s, 3H), 3.79 (s, 3H), 3.81 (s, 3H). 13C NMR
(CDCl3): δ 13.7, 17.1, 31.6, 46.6, 52.0, 60.2, 60.3, 60.4, 62.2, 123.0,
125.3, 132.2, 148.2, 151.6, 152.3, 171.8, 207.4. MS (m/z): 324
(84%, M+•), 288 (42%), 281 (67%), 265 (35%), 254 (92%), 253
(100%), 239 (43%). HRMS (m/z, M+•): calcd for C17H24O6,
324.1573; found, 324.1575.
(m), 1033 (m) cm-1 1H NMR (CDCl3): δ 2.10 (s, 3H), 3.77
.
(s, 3H), 3.79 (s, 3H), 3.82 (s, 3H), 4.64 (s, 2H), 6.58 (s, 1H). 13C
NMR (CDCl3): δ 8.7, 55.5, 60.1, 60.7, 60.8, 105.1, 120.0, 131.2,
144.4, 151.4, 154.0. MS (m/z): 212 (100%, M+•), 197 (48%, M+•
- Me), 182 (23%), 154 (40%), 137 (89%), 122 (15%), 109 (30%).
HRMS (m/z, M+•): calcd for C11H16O4, 212.1049; found, 212.1047.
Anal. Calcd for C11H16O4: C, 62.25; H, 7.60. Found: C, 62.41; H,
7.54.
2,3,5-Trimethoxy-4-methylbenzyl Chloride (33). A solution of
2,3,5-trimethoxy-4-methylbenzyl alcohol (32; 1.24 g, 5.84 mmol)
in thionyl chloride (1.36 g, 11.43 mmol) was stirred at room
temperature for 1 h. The excess thionyl chloride was removed in
vacuo, and the residue was partitioned between ethyl acetate (20
mL) and water (20 mL). The organic layer was separated and
washed with brine (20 mL). The organic fraction was then dried
with magnesium sulfate and filtered, and the solvent was removed
in vacuo. The product was purified by flash column chromatography
(Rf 0.42, 15% ethyl acetate/petroleum spirits) to give 2,3,5-
trimethoxy-4-methylbenzyl chloride (33) as a pale yellow oil in
96% yield. IR νmax (KBr): 2938 (s), 2836 (m), 1486 (s), 1464 (s),
1407 (s), 1336 (m), 1266 (m), 1243 (m), 1132 (s), 1091 (s), 1033
(s) cm-1. 1H NMR (CDCl3): δ 2.11 (s, 3H), 3.79 (s, 3H), 3.80 (s,
3H), 3.87 (s, 3H), 4.62 (s, 2H), 6.59 (s, 1H). 13C NMR (CDCl3):
δ 9.0, 41.5, 55.6, 60.2, 61.2, 106.6, 122.0, 128.1, 145.3, 151.9,
154.1. MS (m/z): 232 (33%, C11H537ClO3), 230 (100%, M+•,
2-Ethyl-3-hydroxy-5,6,8-trimethoxy-7-methyl-1,4-naphtho-
quinone (6). Method 1: To a solution of 3,6-diethyl-1,2,4,5,7,8-
hexamethoxynaphthalene (20; 67 mg, 0.2 mmol) in freshly distilled,
dry dioxane (2 mL) was added freshly prepared silver(II) oxide
(0.185 g, 0.8 mmol). The reaction mixture was then subjected to
sonication for 1 min to give a uniform dispersal of the oxidant. A
freshly prepared solution of aqueous 6 N nitric acid (0.2 mL) was
added, and the reaction mixture was stirred at room temperature
for 2 min. A chloroform/water (8:2 mL) mixture was then added.
The organic layer was isolated, washed with further aliquots of
water (2 × 5 mL), dried with magnesium sulfate, and filtered, and
the solvent was removed in vacuo. The residue was dissolved in
methanol (2 mL), and 10% aqueous sodium hydroxide (1 mL) was
added. The reaction mixture was stirred at room temperature for
0.5 h, after which time the solution was acidified with 5%
hydrochloric acid. The reaction mixture was then extracted with
ethyl acetate (3 × 5 mL), the combined organic extracts were dried
with magnesium sulfate and filtered, and the solvent was removed
under reduced pressure to give an inseparable mixture of naph-
thoquinone 6 and the regioisomeric 7-ethyl-3-hydroxy-5,6,8-tri-
methoxy-2-methyl-1,4-naphthoquinone (24) in equimolar amounts.
Method 2: A solution of ketone 35 (0.40 g, 1.25 mmol) in dry
ethanol (7 mL) was added dropwise to a solution of sodium
ethoxide, freshly prepared from sodium (0.067 g, 2.91 mmol) in
ethanol (7 mL). The reaction mixture was heated to reflux and
stirred for 20 min. Air was then bubbled through the solution
overnight at room temperature, after which the solvent was re-
moved in vacuo. To the residue was added 1 N sulfuric acid (5
mL) and ethyl acetate (10 mL). The organic layer was then
separated, and the aqueous layer was washed with aliquots of ethyl
acetate (2 × 10 mL). The combined organic extracts were dried
with magnesium sulfate and filtered, and the solvent was re-
moved under reduced pressure to give the orange naphthoqui-
none 6. The quinone was then purified by flash column chroma-
tography (Rf 0.35, 25% ethyl acetate/petroleum spirits) to give the
product in 56% yield; mp 105-106 (lit. 106-108 °C).11,12 IR νmax
(KBr): 3338 (m), 2935 (m), 1661 (m), 1644 (s), 1634 (m), 1461
(s), 1402 (s), 1352 (s), 1283 (s), 1255 (m), 1154 (m), 1024 (w)
C11H535ClO3), 215 (86%, M+• - Me), 195 (27%, M+•
-
35Cl), 180
(38%), 165 (9%), 152 (23%). HRMS (m/z, M+•): calcd for
C11H15O337Cl, 230.0680; found, 232.0681. HRMS (m/z, M+•): calcd
for C11H15O335Cl, 230.0710; found, 230.0713. Anal. Calcd for
C11H15ClO3: C, 57.27; H, 6.55. Found: C, 57.26; H, 6.48.
2,3,5-Trimethoxy-4-methylphenylacetic Acid (30). Method
1: A solution of 2-phenyl-4-(2,3,5-trimethoxy-4-methylbenzylidene)-
4H-oxazol-5-one (29; 1.26 g, 3.57 mmol) in 10% sodium hydroxide
(7.7 mL) was heated under reflux for 5 h. The reaction mixture
was cooled to 0 °C and diluted with ice-cold 10% sodium hydroxide
(2 mL). To this was added, with stirring, a 15% hydrogen peroxide
solution (1 mL), and the temperature was kept below 15 °C. The
reaction mixture was stirred at room temperature overnight and then
acidified with concentrated hydrochloric acid. The resultant solution
was then extracted with ethyl acetate (3 × 10 mL). The combined
organic extracts were dried with magnesium sulfate and filtered,
and the solvent was removed in vacuo to give the crude phenylacetic
acid 30. Method 2: To a solution of 2,3,5-trimethoxy-4-methyl-
benzyl cyanide (34; 1.27 g, 5.74 mmol) in glacial acetic acid (25
mL) was added water (8 mL) and concentrated sulfuric acid (2.5
mL). The reaction mixture was heated to reflux for 22 h and then
cooled to room temperature. The mixture was poured onto ice, and
the resultant solution was extracted with ethyl acetate (3 × 25 mL).
The combined organic extracts were washed with brine (30 mL),
dried with magnesium sulfate, and filtered, and the solvent was
concentrated under reduced pressure prior to purification via flash
column chromatography (Rf 0.46, 25% ethyl acetate/petroleum
spirits) to give phenylacetic acid 30 as a colorless oil in 86% yield.
IR νmax (KBr): 3456 (w), 2939 (s), 1736 (s), 1597 (s), 1466 (s),
1
cm-1. H NMR (CDCl3): δ 1.10 (t, J ) 8 Hz, 3H), 2.26 (s, 3H),
2.54 (t, J ) 8 Hz, 2H), 3.80 (s, 3H), 3.88 (s, 3H), 3.91 (s, 3H),
7.37 (s, 1H). 13C NMR (CDCl3, 500 MHz): δ 10.1, 12.7, 16.8,
60.9, 61.1, 61.2, 121.1, 121.4, 125.2, 136.9, 150.6, 151.6, 156.2,
157.1, 180.4, 183.7. MS (m/z): 306 (100%, M+•), 291 (59%, M+•
- CH2), 277 (38%, M+• - Et), 263 (26%), 233 (19%), 168 (22%),
168 (23%). HRMS (m/z, M+•): calcd for C16H18O6, 306.1103;
found, 306.1101. Anal. Calcd for C16H18O6: C, 62.74; H, 5.92.
Found: C, 62.57; H, 6.23.
1404 (s), 1319 (m), 1242 (s), 1126 (s), 1087 (m), 1034 (m) cm-1
.
1H NMR (CDCl3): δ 2.09 (s, 3H), 3.64 (s, 2H), 3.76 (s, 3H), 3.79
(s, 3H), 3.80 (s, 3H), 6.45 (s, 1H). 13C NMR (CDCl3): δ 9.0, 35.8,
55.8, 60.3, 60.7, 107.3, 120.6, 124.3, 145.3, 151.8, 154.0, 177.8.
MS (m/z): 240 (100%, M+•), 225 (90%, M+• - Me), 207 (24%),
181 (68%), 137 (38%), 109 (26%). HRMS (m/z, M+•): calcd for
C12H16O5, 240.0998; found, 240.1000.
1000 J. Org. Chem., Vol. 71, No. 3, 2006

Synthesis of Hybocarpone and Related Naphthazarins
naphthoquinone (13), 1,2,4,5,7,8-hexamethoxynaphthalene (14),
5-hydroxy-1,2,4,7-tetramethoxynaphthalene (15), 2-chloro-5-ben-
zyloxy-7-methoxy-1,4-naphthoquinone (16), 3,6-diethyl-1,2,4,5,7,8-
hexamethoxynaphthalene, 3-ethyl-1,2,4,5,7,8-hexamethoxy-6-meth-
ylnaphthalene (20; 1H NMR only), 3,6-dimethyl-1,2,4,5,7,8-hexa-
methoxynaphthalene (21; 1H NMR only), 2,3,5-trimethoxy-4-
methylbenzaldehyde (28),26 2-phenyl-4-(2,3,5-trimethoxy-4-meth-
ylbenzylidene)-4H-oxazol-5-one (29), 2,3,5-trimethoxy-4-methyl-
benzyl alcohol (32), 2,3,5-trimethoxy-4-methylbenzyl chloride (33),
methyl 2-butyryl-3,5,6-trimethoxy-4-methylphenylacetate (35), and
2-ethyl-3-hydroxy-5,6,8-trimethoxy-7-methyl-1,4-naphthoquino-
ne (6). Crystallographic data was obtained to support the structural
assignment of 5,7-dimethoxy-1-methyl-1H-benzo[f]indazole-4,9-
dione. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. The authors wish to express their grati-
tude toward Dr. Alison Edwards for assistance with the X-ray
crystallographic analysis of 5,7-dimethoxy-1-methyl-1H-benzo-
[f]indazole-4,9-dione.
Supporting Information Available: Experimental details for
the preparation of 1,2,4,5,7,8-hexamethoxynaphthalene (14), 5-ben-
zyloxy-2,7-dimethoxy-1,4-naphthoquinone (17), 5-benzyloxy-
1,2,4,7-tetramethoxynaphthalene (18), 3-ethyl-1,2,4,5,7,8-hexa-
methoxy-6-methylnaphthalene (20), 3,6-dimethyl-1,2,4,5,7,8-hexa-
methoxynaphthalene (21), 3-ethyl-2,5,7,8-tetrahydroxy-1,4-naph-
thoquinone (25), 2,5,7,8-tetrahydroxy-3,6-dimethyl-1,4-naphtho-
quinone (27), 2,3,5-trimethoxy-4-methylbenzyl cyanide (34), and
methyl 2,3,5-trimethoxy-4-methylphenylacetate (31) and 1H and 13
C
NMR spectra for 1,2,4,5,7-pentamethoxynaphthalene (11), 8-formyl-
1,2,4,5,7-pentahydroxynaphthalene (12), 2,5,7,8-tetramethoxy-1,4-
JO0519561
J. Org. Chem, Vol. 71, No. 3, 2006 1001