Aerobic oxidative dimerization of 1-naphthols to 2,2′- binaphthoquinones mediated by SnCl4 and its application to natural product synthesis

Article abstract of DOI:10.1016/j.tet.2004.07.073

We developed a simple method for the direct synthesis of 2,2′-binaphthoquinones, utilizing oxidative dimerization via electron donor-acceptor complex formation of 1-naphthols with SnCl₄ in the presence of dioxygen. This oxidation involves a catalytic cycle of SnCl ₄, and the reaction mechanism is discussed. As an application of this method to natural products synthesis, we describe facile biomimetic syntheses of the binaphthoquinones 3,3′-bijuglone, 3,3′-biplumbagin and elliptinone.

Full text of DOI:10.1016/j.tet.2004.07.073

Tetrahedron 60 (2004) 9049–9060
Aerobic oxidative dimerization of 1-naphthols to
2,20-binaphthoquinones mediated by SnCl4 and its application to
natural product synthesis
*
Tetsuya Takeya, Hirohisa Doi, Tokutaro Ogata, Iwao Okamoto and Eiichi Kotani
Showa Pharmaceutical University, 3-3165 Higashi-Tamagawagakuen, Machida, Tokyo 194-8543, Japan
Received 1 July 2004; revised 28 July 2004; accepted 28 July 2004
Available online 25 August 2004
Abstract—We developed a simple method for the direct synthesis of 2,20-binaphthoquinones, utilizing oxidative dimerization via electron
donor–acceptor complex formation of 1-naphthols with SnCl4 in the presence of dioxygen. This oxidation involves a catalytic cycle of SnCl4,
and the reaction mechanism is discussed. As an application of this method to natural products synthesis, we describe facile biomimetic
syntheses of the binaphthoquinones 3,30-bijuglone, 3,30-biplumbagin and elliptinone.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
reactions are not catalytic but require a more than
stoichiometric amount of oxidant. These methods suffer
from drawbacks such as the use of an expensive oxidant and
the production of copious amounts of heavy metal-contain-
ing wastes. In addition, they are not always suitable for
large-scale reaction.
Quinone derivatives, including biarylquinones, are not only
versatile compounds for organic synthesis, but also
intermediates in the biosynthesis of various natural
products, and several applications of quinone derivatives
as synthons for the synthesis of natural products have been
reported.1 Among natural biaryls, 2,20-binaphthoquinones,
including 3,30-bijuglone (1),1d–e 3,30-biplumbagin (2)1f–g
and biramentaceone (3),1h and 2,20-binaphthols, such as
elliptinone (4), gossypol1l–p and michellamine A,1q have
been isolated from several plants and show various
biological activities0. It is considered that the biogenetic
pathway to the 2,2 -binaphthoquinones (BNPQ) involves
oxidative biaryl coupling of 1-naphthols (NPOH) and
subsequent oxidation of the resulting 2,20-binaphthols
(BNPOH). We are interested in the oxidative reactions of
1-naphthols in order to develop a method for constructing
these biaryl substructures, aiming at biomimetic synthesis of
various natural products (Fig. 1).
The replacement of current stoichiometric oxidations for the
production of fine chemicals with environmentally benign
catalytic oxidations is one of the major goals of so-called
green chemistry. Dioxygen (O2) is an attractive oxidant and
the development of synthetic methodologies using dioxygen
is a rewarding goal from both economic and environmental
points of view. Thus, aerobic catalytic oxidative dimeriza-
tion with air or dioxygen has been extensively studied for
many years. For instance, several methods using the
combination of oxovanadium(IV)-amino acid complex,
Ru(II)-salen complex, Cu(II)-amine complex, Fe (III)
complex, VO(acac)2, CuSO4/Al2O3 or CuCl as a catalyst
and dioxygen or air as an oxidant have been developed for
oxidative coupling of 2-naphthols.4
In general, the synthesis of the BNPQ framework requires
two steps, namely, the biaryl coupling of NPOH and
subsequent oxidation of the resulting BNPOH. Although a
number of authors have obtained the above framework in
two steps,2 only a few examples of synthesis in one step
have been recorded.3 In most cases, however, the coupling
Alternatively, stannic chloride (SnCl4; SC) as an inorganic
metal halide is used extensively in organic synthesis as a
Lewis acid for enhancing a variety of organic reactions.5a–d
However, to our knowledge there have been only a few
reports to date on oxidative reactions using SC as a
catalyst.5e–f Recently, we reported that BNPOH 6 and
dinaphtho[1,2-b;20,10-d]furans (9; DNF) were obtained in
excellent yields through the catalytic oxidative coupling
reaction of NPOH 5 with the SC system in the absence of
Keywords: Naphthols; Binaphthoquinones; Dimerization; Tin chloride;
Dioxygen.
* Corresponding author. Tel.: C81-4-272-11579; fax: C81-4-272-11579;
e-mail: takeya@ac.shoyaku.ac.jp
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.07.073

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T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
Figure 1.
O2.6a,b Furthermore, in a preliminary communication,6c we
reported a simple and convenient method for oxidative
dimerization of 5 to the corresponding BNPQ 8 in one step,
without the formation of DNF 9, by using SC as a catalyst in
the presence of O2.
that the hydroxyl group in 5a,b is required for the coupling
reaction. Furthermore, in order to extend the range of
applicability of the SC/O2 system, the reaction of several
NPOHs under the same conditions was carried out. In the
case of 5c, which might be a precursor for the synthesis of 3,
the expected coupling product 8c was not obtained (entry 8).
These results showed that the present SC/O2 system has
different reactivity from the SC system6b reported pre-
viously by us. We now present a detailed study of the
aerobic oxidative reaction of 5 with the SC/O2 system,
including further additional experiments, and its application
to the biomimetic synthesis of naturally occurring 1, 2 and 4.
Next, the reaction of 5d with the SC/O2 system was
examined (Scheme 2). Surprisingly, the reaction for a short
time (0.5 h) did not afford BNPOH 6d at all, but gave the
O–Cpara coupled product, the novel dinaphthofuranone (11;
DNFO) in 88% yield (Table 2, entry 1). Prolonged reaction
(67 h) under the same conditions selectively afforded BNPQ
8d (59%), in one step from 5d (entry 2). We also used
another Lewis acid for the formation of 8d. The reaction of
5d with the TiCl4/O2 system gave 8d in 51% yield (entry 3).
The formation of 11 in entry 1 was different from the result
(entry 4) with the SC system. DNFO 11 is an analogous to
2. Results and discussion
2.1. Oxidative reactions of NPOH 5 with the SnCl4/O2
system
the skeleton of the known Pummerer’s ketone 14 (Cortho
–
Cpara coupled product).8 The oxidative Cortho–Cpara
coupling reaction of p-cresol having a methyl group at the
para position has been often cited as an analogue to usnic
acid and morphine biosyntheses (Scheme 3).9
First, we investigated the reaction of 57 with the SC/O2
system with the aim of achieving a facile synthesis of
naturally occurring 1–4 under various conditions. The
results are shown in Table 1 and Scheme 1. The optimum
conditions were NPOH (1 mmol) with a catalytic amount of
SC (0.25 equiv.). We found that the nature of the major
products changed drastically with the passage of time at
room temperature (23 8C). In the cases of 5a,b, BNPOH
6a,b were obtained as major products in the reaction for a
short time (1–7 h) (entries 3, 6). Prolonged reaction
(20–71 h) under the same conditions selectively afforded
BNPQ 8a,b, synthetic intermediates to the desired natural
products 1 and 2, in one step from 5a,b (entries 4, 7).
Furthermore, we examined the reaction of 5e, having a
methyl group in place of a methoxyl group, for comparison
with the above reaction of 5d (entries 6–8). The result
differed from that of 5d; only 6e (68%) was obtained,
without the formation of 8d and 15 (entry 6).
The structure of 11 was elucidated by analyses of the IR, 1H,
and 13C NMR spectra with the aid of 2D NMR analyses
(1H–1H COSY, 13C–1H COSY, HMBC and NOESY
experiments), and also by chemical transformation as
shown in Scheme 2. The formation of the dihydrofuran
ring in 11 was confirmed by observation of the long-range
correlations of 7-H and C-6a, and 6-H and C-6b in the
HMBC spectrum of 11 (Fig. 2). The stereochemistry of 11
was confirmed by the NOESY spectrum, showing that
structure 1 has the trans-configuration between C-13a and
C-6a (Fig. 3). Treatment of 11 in methanol (MeOH)
On the contrary, prolonged reaction of 5a with the SC
system did not afford 8a, but gave DNF 9a, as reported
previously by us6a,b (refer to Table 1, entry 2). The reason
for this is discussed below. We also used another Lewis acid
for the formation of 8. However, the reaction of 5a with the
TiCl4/O2 system in CH2Cl2 gave 8a in very low yield (entry
5). With the naphthol ethers 5i,j, the oxidation reaction
almost did not proceed (entries 9, 10). These results show

T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
9051
containing concentrated HCl gave the dinaphthofuran 12
(27%) and dinaphthofuranol 13 (24%).10 Furthermore, the
reaction of 11 with the SnCl4/O2 system afforded 8d in 36%
yield.
Although oxidative coupling reactions of naphthols using
several metal oxidants1q,11 in the absence or presence of O2
have been studied extensively, mixtures of dimeric,
polymeric and quinoid compounds were usually generated.
In addition, regioselective ortho/ortho coupling is more
difficult to control in the reaction of NPOHs such as 5f–h,
which lack a methoxyl group on ring A. In order to obtain 6g
as a synthetic intermediate for 4, the ortho/ortho coupling
reaction of 5f as a model substrate with SC in the absence of
O2 was tried first, but the yield of 6f was low,6b partly
because of the formation of a polymeric mixture. We then
studied the reaction of 5g with the SC/O2 system (Scheme 4,
Table 3). In this reaction, ortho/ortho-coupled BNPOH 6g
was obtained along with the trimeric furan 16g, without the
formation of BNPQ (Table 3, entry 3). In the case of 5h, the
desired dimer was obtained in a lower yield (entry 5).
Physical data for the compounds 16f and 16g were identical
with those of the corresponding compounds reported
previously by us.6b
The noteworthy features of the present reactions with the
SC/O2 system were as follows, as compared with the SC
system reported previously by us6b (Scheme 5).
(1) The prolonged catalytic reactions of NPOH 5a,b with
SC (0.25 equiv.) in the presence of dioxygen afforded
BNPQ 8a,b in one step via the formation of BNPOH
6a,b. The prolonged reaction of 5d under the same
conditions gave 8d via 11.
(2) The formation of 8 can be conveniently performed with
electron-rich NPOH 5 having a methoxyl group at the
R5 position on ring A. However, there is an exception:
in the case of 5c as a substrate, the coupling reaction to
6c did not take place. The reason for this may be that 5c
is much more oxidizable than the other compounds
because it has the lowest oxidation potential (0.82 V vs.
Ag/AgCl), as reported by us previously.6b
(3) NPOHs 5e, 5f and 5g having a methyl group on ring A
or B could not be converted to the corresponding
BNPQs 8e, 8f and 8g.
(4) When using 5d as a substrate, a particular difference
between oxidation with the SC/O2 system and the SC
system was observed. Among NPOHs 5, only oxidation
of 5d with the SC/O2 system for a short time (0.5 h)
afforded DNF 11 having a different framework from
other products, without the formation of the BNPOH
framework. In contrast with this result, the reaction with
the SC system gave only 6d. Although the reason for
this difference is unclear, it may be related to the
influence or participation of dioxygen (O2). Some
structural difference of the electron donor–acceptor
(EDA) complex formed between SC and 5d in the
presence of O2 or in absence of O2 in the reaction
process may be involved.
(5) In all reactions of 5 with the SC/O2 system, formation of
DNFs 9 obtained in the reaction in the SC system was
not observed at all. The reason for this is discussed
below.

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T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
Scheme 1.
Scheme 2.

T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
Table 2. Reactions of the naphthols 5d,e with the SnCl4/O2 system at 23 8Ca
9053
Entry
NPOH
Reagent
Time (h)
Product (%)b
Recovered
6
8
11
5 (%)
1
2
5d
5d
5d
5d
5e
5e
5e
5e
SnCl4/O2
SnCl4/O2
TiCl4/O2
SnCl4
SnCl4/O2
SnCl4/O2
SnCl4/O2
SnCl4
0.5
67
72
18
9
24
115
48
—
—
—
98
68
59
61
52
—
59
51
—
—
—
—
—
88
—
—
—
—
—
—
—
—
—
—
—
—
—
—
38
3c
4d
5
6
7
8d
a
The reactions of 5d,e (1 mmol) with SnCl4 (0.25 equiv.) were carried out using dioxygen (O2)-saturated solvent in a sealed tube with stirring under normal
laboratory light. Similar results were obtained in the dark.
Isolated yield.
This reaction was carried out with TiCl4 (0.25 equiv.) in place of SnCl4 under the same conditions as above.
SnCl4 (1.3 equiv.) at 100 8C. This result was reported previously by us (see Ref. 6b).
b
c
d
2.2. Proposed mechanism for oxidative reaction of
1-naphthols 5 with SnCl4 in the presence of O2
To throw light on the formation of 7 and 8 in the reactions of
5a,b with the SC/O2 system in CH2Cl2, the reactions of 6a
or 7a with the SC/H2O2 (hydrogen peroxide) and SC/O2
reagent systems in CH2Cl2 were examined. In the case of
6a, 7a and 8a were obtained with both reagent systems
(refer to Table 1, entries 11 and 12). The reaction of 7a
afforded 8a as a major product in both cases (entries 13 and
14). These results suggest the participation of H2O2 or O2. In
addition, the present reaction of NPOH 5 (1 mmol) involves
a catalytic cycle of SC (0.25 equiv.) in all cases.
Furthermore, there are a number of reports on the generation
of H2O2 via the hydroperoxy radical (HO2%) by reduction of
O2 in aprotic solvents in the presence of Brønsted acids such
as phenol and perchloric acid (HClO4) as proton sources, by
means of chemical and electrochemical methods.12
The above results and information are consistent with the
mechanism illustrated in Scheme 6 for the oxidative
reaction of NPOH 5 with SC in the presence of O2. This
reaction is initiated by the formation of the EDA complex B
via the complex A of NPOH 5 with SC (SnCl4) and O2. The
O2 site in the complex B receives one-electron transfer from
the anion radical species (SC–%) and subsequent one-proton
transfer from the NPOH site to reform SC, with the
generation of the radical C and HO2%. Then, H2O2 and O2
are generated by the disproportionation of two HO2%.13 The
complex E or F can be formed by the interaction of H2O2 or
O2 with complex D generated from SC and BNPOH 6,
which is produced by ortho/ortho radical coupling of two C
(route b). In the reaction of 5a,b, the quinones 7a,b can be
formed via Fenton-like reaction14a of E or radical
autoxidation type reaction14b of F. The formation of
BNPQ 8a,b takes place similarly (route e). On the other
hand, the trimeric furan 16 is formed by para/ortho radical
coupling between H and C, followed by oxidative radical
chain reaction in the case of 5f,g (route f).
Scheme 3.
Figure 2. The Main HMBC correlations of 11.
Although DNF 9a was obtained in the prolonged reaction of
NPOH 5a with the SC system in the absence of O2 (Table 1,
entry 2), the formation of 9 was not observed at all in the
case with the SC/O2 system in the presence of O2 (Table 1,
entries 3 and 4). The reason for this is considered to be as
follows. It is known that SC forms six-coordination
complexes (s-type EDA complexes) with hydroxyarenes,
Figure 3. The main NOESY correlations of 11.

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T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
Scheme 4.
Table 3. Reactions of naphthols 5f–h with the SnCl4/O2 system at 23 8Ca
Entry
NPOH
Reagent
Time (h)
Product (%)b
Recovered
6
16
8
5 (%)
1
2
5f
5f
5g
5g
5h
5f
SnCl4/O2
TiCl4/O2
SnCl4/O2
TiCl4/O2
SnCl4/O2
SnCl4
1
1
5
5
43
4.3
20
20
8
5
—
16
24
16
19
—
—
—
—
—
—
—
7
33
47
53
60
3
3
4c
5
6d
—
24
a
The reactions of naphthols (1 mmol) with SnCl4 (0.25 equiv.) were carried out using dioxygen (O2)-saturated solvent in a sealed tube with stirring under
normal laboratory light. Similar results were obtained in the dark.
Isolated yield.
This reaction was carried out with TiCl4 (0.25 equiv.) in place of SnCl4 under otherwise the same conditions as above.
The reaction with SnCl4 (1.3 equiv.) at 100 8C was carried out to afford 6f along with 1,5,8,12-tetramethoxydinaphtho[1,2-b:10,20-d]furan (8%). This result
was reported previously by us (see Ref. 6b).
b
c
d
aromatic aldehydes and 1,10-binaphthols as O-donors.15 In
of O2. However, in the case with the SC/O2 system, the
presence of O2 or H2O2 inhibits the formation of the
complex G. The formation of the complex E or F of O2 or
H2O2 with SC and 6 may be favored over that of the
our preceding paper,6b we proposed a mechanism for the
formation of 9a via six-coordination complexes G between
SC and two hydroxyl moieties of BNPOH 6 in the absence
Scheme 5.

T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
9055
Scheme 6. Proposed mechanism for oxidative reaction of 1-naphthols 5 with the SnCl4/O2 system.
Scheme 7. Proposed mechanism for the formation BNPQ 8d by the reaction of NPOH 5d with the SC/O2 system.

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T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
Scheme 8.
complex G. Accordingly, the DNF framework, such as 9a,
is not formed in the prolonged reaction of 5 with the SC/O2
system.6a,b
AlI316 followed by air auto-oxidation. Physical data for the
synthetic compounds 1,1d–e 21f–g and 41i–k were identical
with those of the corresponding natural products.
The formation mechanism of 8d via 11 (route c) by the
reaction of 5d with the SC/O2 system is postulated to
proceed as shown in Scheme 7, based on (i) Table 2 (entries
1, 2) and (ii) the formation of 8d by the reaction of trans-11
with the same reagent (refer to Scheme 2). The diastero-
meric ketals, cis-11 and trans-11 could be first formed by
O–Cpara coupling between two naphthoxyradicals C,
generated by the SC-mediated oxidative reaction of
NPOH 5d, followed by intramolecular cycloaddition of
20. Furthermore, the epimerization at C13a of cis-11 is
initiated by the opening of the furan ring induced by Lewis
acid (SnCl4). The formed zwitterion I can undergo ring
closure again by nucleophilic attack from the opposite side,
thus shifting the equilibrium towards the thermodynami-
cally more stable epimer (trans-11). BNPQ 8d could be
formed through aromatization of I followed by further
SC-mediated oxidative reaction of BNPOH 22 (refer to
Scheme 6).
3. Conclusion
In conclusion, we found a simple method for the synthesis
of BNPQ 8 in one step by means of aerobic oxidative
dimerization of NPOH 5 with the SC/O2 system. We have
established a facile and biomimetic synthesis of several
natural products, 3,30-bijuglone (1), 3,30-biplumbagin (2)
and elliptinone (4), by utilizing BNPQs 8a,b and BNPOH
6g prepared by means of the above reactions. The present
aerobic oxidative reaction provides a new methodology for
constructing 2,20-binapthoquinones with potential as inter-
mediates for biomimetic synthesis of natural products.
4. Experimental
4.1. General
SC plays an important role in the oxidative reactions of
NPOH in the presence of O2: it acts not only as a
characteristic Lewis acid catalyst, but also as a mediator
of the oxidative reaction. On the other hand, O2 acts as one-
electron acceptor from the anion radical species (SC–%) and
a one-proton acceptor from NPOH, and as an oxygen source
for the formation of BNPQ.
All melting points are uncorrected. Infrared (IR) spectra
were recorded with a JASCO IR-700 spectrometer, and H
1
and 13C NMR spectra with JEOL JNM-AL300 and JNM-
alpha 500 spectrometers, using tetramethylsilane as an
internal standard (CDCl3, CD3COCD3 and CD3SOCD3
solutions). Mass spectra were recorded on a JEOL JMS-
D300 or Shimadzu QP-5000 spectrometer. Elemental
analyses were done using a Yanaco CHN-MT-3 apparatus.
Merck Kieselgel 60 (230–400 mesh), Wako silica gel C-200
(200 mesh) and Merck Kieselgel 60 F254 were used for flash
column chromatography, column chromatography and thin-
layer chromatography, respectively. Each organic extract
was dried over MgSO4 or Na2SO4. Oxidation potential was
measured on a Yanaco P-1100 voltammetric analyzer by
cyclic voltammetry in argon-saturated CH2Cl2, MeNO2 or
MeCN solutions with an Ag/AgCl reference electrode.
Tetrabutylammonium perchlorate (0.1 M solution) was used
as a supporting electrolyte. Storage and handling of SnCl4:
SnCl4 should be stored in a container with silica gel, blue,
2.3. Biomimetic syntheses of natural products 1, 2 and 4
Fin0ally, we investigated the synthesis of 3,30-bijuglone (1),
3,3 -biplumbagin (2) and elliptinone (4) from the corre-
sponding 8a, 8b and 6g, prepared by means of the reactions
of NPOHs with the SC/O2 system as shown in Scheme 8.
7c
The demethylation of 8a and 8b with MgBr2 led to the
corresponding0 3,30-bijuglone (1; mp 265–267 8C) in 97%
yield and 3,3 -biplumbagin (2; mp 212–214 8C) in 98%
yield. Furthermore, elliptinone (4; mp 302–305 8C) was
synthesized in 64% yield by the demethylation of 6g with

T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
9057
medium granule to minimize exposure to moisture. The
container should be flushed with N2 or Ar and tightly sealed.
Perform all manipulations under N2 or argon. All reactions
were carried out in an anhydrous state.
dione (7a) and 5,50-dimethoxy-3,30-bi-1,4-naphtho-
quinone (8a). These compounds were obtained from 5a
by the above general procedure A (with the SnCl4/O2
system). The residue was subjected to flash column
chromatography on silica gel. The eluate with AcOEt–
hexane (1:6, v/v) gave 7a and 8a along with the recovered
5a. Yields are listed in Table 1. 6a: Colorless needles
(CHCl3–hexane), mp 207–209 8C. LR-MS m/z: 406 (MC).
4.2. Synthesis of 1-naphthols 5a–j
1-Naphthols 5a,7a,b 5b,7c 5c,7d–f 5e,6b 5f,7g and 5g7h were
synthesized according to the protocol reported previously,
and 5d and 5h are commercially available (Tokyo Kasei
Chemical Industries, Ltd, Japan).
1
IR, H and 13C NMR data were described previously by
us.6b 7a: Deep purple needles (hexane–AcOEt), mp 120–
1
122 8C. IR (KBr) cmK1: 3394, 1607. H NMR (CDCl3) d:
3.95 (3H, s, OMe), 3.99 (3H, s, OMe), 4.010(3H, s, OMe),
6.72 (1H, s, 30-H), 6.86 (1H, d, JZ7.9 Hz, 7 -H), 7.04 (1H,
s, 3-H), 7.30 (1H, d, JZ8.2 Hz, 7-H), 7.37 (1H, t, JZ7.9,
8.5 Hz, 60-H), 7.66 (1H, t, JZ7.6, 8.2 Hz, 6-H), 7.76 (1H,
dd, JZ0.9, 7.6 Hz, 5-H), 7.86 (1H, dd, JZ1.1, 8.5 Hz, 50-
H), 9.41 (1H, s, OH). 13C NMR (CDCl3) d: 55.9 (C40-OMe),
56.1 (C8-OMe), 56.5 (C80-OMe), 105.6 (C70), 107.1 (C30),
114.9 (C20), 115.3 (C8a0), 115.9 (C50), 117.8 (C7), 118.6
(C5), 121.1 (C8a), 126.3 (C60), 128.90(C4a0), 134.35 (C3),
134.38 (C6), 134.41 (C4a), 146.3 (C1 ), 147.8 (C40), 150.9
(C2), 156.4 (C80), 159.6 (C8), 183.2 (C1), 185.3 (C4). LR-
MS m/z: 390 (MC). HR-MS Calcd for C23H18O6: 390.1100,
Found: 390.1170. Anal. Calcd for C23H18O6: C, 70.76; H,
4.65. Found: C, 70.56; H, 4.62. 8a: yellow needles (CHCl3–
hexane), mp 202–204 8C. IR (KBr) cmK1: 1644, 1604,
1570. 1H NMR (CDCl3) d: 3.99 (6H, s, 5 and 50-OMe), 6.95
(2H, s, 2 and 20-H), 7.33 (2H, d, JZ8.2 Hz, 6 and 60-H),
7.71 (2H, t, JZ7.9, 8.2 Hz, 7 and 70-H), 7.76 (2H, d, JZ
7.9 Hz, 8 and 80-H). 13C NMR (CDCl3) d: 56.5 (C5 and C50-
OMe), 118.2 (C6 and C60), 119.1 (C8 and C80), 119.8 (C5a
and C50a), 134.2 (C8a and C8a00), 134.8 (C2 and C200), 135.2
(C7 and C70), 146.8 (C3 and C3 ), 160.1 (C5 and C5 ), 182.1
(C4 and C40), 184.4 (C1 and C10). LR-MS m/z: 374 (MC).
HR-MS Calcd for C22H14O6: 374.0790, Found: 374.0762.
Anal. Calcd for C22H14O6: C, 70.59; H, 3.77. Found: C,
70.49; H, 3.78.
4.3. General procedure A for oxidative reaction of
NPOHs 5a–j, DNF 11, BNPOHs 6a–7a with SnCl4/O2 or
TiCl4/O2 system in the presence of dioxygen
SnCl4 or TiCl4 (0.25 equiv.) was added to a solution of the
selected substrate (1 mmol) in dioxygen (O2)-saturated
solvent (CH2Cl2 or MeNO2; 20 ml) and the mixture was
stirred for 5 min with bubbling of O2 gas at room
temperature under normal laboratory light. Then, the
reaction mixture was stirred in a sealed tube until
disappearance of the substrate (1-naphthols and 2,20-
binaphthyls), except in cases where the starting material
was recovered. The reaction mixture was poured into ice
water, 10% HCl was added, and the mixture was extracted
with CHCl3. The organic layer was washed with H2O, then
dried and concentrated. The residue was purified by flash
column chromatography on silica gel.
4.4. General procedure B for oxidative reaction of 6a and
7a with the SnCl4/H2O2 system
SnCl4 (1 equiv.) was added to a solution of the selected
naphthol (1 mmol) and 30% H2O2 (1 mmol) in CH2Cl2
(20 ml) and the mixture was stirred for 10 min at room
temperature under normal laboratory light in an argon
atmosphere. Then, the reaction mixture was stirred for 1.5 h
in a sealed tube under the same conditions. The reaction
mixture was poured into ice water, 10% HCl was added, and
the mixture was extracted with CHCl3. The organic layer
was washed with H2O, then dried and concentrated. The
residue was subjected to flash column chromatography on
silica gel. The eluate with AcOEt–hexane (1:6, v/v) gave 7a
and 8a along with recovered 6a. Yields are listed in Table 1.
4.5.2. 1,5,8,12-Tetramethoxydinaphtho[1,2-b:10,20-
d]furan (9a). This compound was obtained as colorless
needles (CHCl3–hexane), mp over 300 8C from 5a by the
above general procedure A (with the SnCl4/O2 or TiCl4/O2
system). LR-MS m/z: 388 (MC). IR, 1H and 13C NMR data
were described previously by us.6b
4.5.3. 4,40,8,80-Tetramethoxy-3,30-dimethyl[2,20]di-1,10-
naphthol (6b), 10-hydroxy-8,40,80-trimethoxy-3,30-
dimethyl[2,20]binaphthalenyl-1,4-dione (7b) and 5,50-
dimethoxy-2,20-dimethyl[3,30]binaphthoquinone (8b).
These compounds were obtained from 5b by the above
general procedure A (with the SnCl4/O2 system). 6b:
Colorless needles (CHCl3–MeOH), mp 251–252 8C. LR-
MS m/z: 434 (MC). IR, 1H and 13C NMR data were
described previously by us.6b 7b: Orange amorphous
powder (CHCl3–MeOH), mp 261–263 8C. IR (KBr)
4.5. General procedure C for oxidative reactions of 1-
naphthols 5d and 5e with SnCl4 in the absence of
dioxygen
SnCl4 (1.3 equiv.) was added to a solution of the selected
naphthol (1 mmol) in CH2Cl2 (20 ml) and the mixture was
stirred for 20 min at room temperature under normal
laboratory light in an argon atmosphere. Then, the reaction
mixture was heated at 100 8C in a sealed tube with stirring.
The reaction mixture was poured into ice water, 10% HCl
was added, and the mixture extracted with CHCl3. The
organic layer was washed with H2O, then dried with
Na2SO4 and concentrated. The residue was purified by flash
column chromatography on silica gel.
1
cmK1: 3384, 1654, 1582. H NMR (CDCl3) d: 1.59 (3H,
s, Me), 2.17 (3H, s, Me), 3.84 (3H, s, OMe), 3.96 (3H, s,
OMe), 4.00 (3H, s, OMe), 6.78 (1H, d, JZ8.4 Hz, Ar-H),
7.29 (1H, dd, JZ1.1, 7.5 Hz, Ar-H), 7.37 (1H, t, JZ7.5 Hz,
Ar-H), 7.68 (1H, t, JZ8.4 Hz, Ar-H), 7.71 (1H, d, JZ
8.4 Hz, Ar-H), 7.84 (1H, dd, JZ1.1, 7.5 Hz, Ar-H), 9.29
(1H, s, OH). LR-MS m/z: 418 (MC). HR-MS Calcd for
C25H22O6: 418.1417, Found: 418.1441. Anal. Calcd for
4.5.1. 4,40,8,80-Tetramethoxy-2,20-di-1,10-naphthol (6a),
10-hydroxy-40,8,80-trimethoxy[2,20]binaphthalenyl-1,4-

9058
T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
C25H22O6: C, 71.76; H, 5.30. Found: C, 71.46; H, 5.32. 8b:
yellow needles (CHCl3–hexane), mp 261–263 8C. LR-MS
JZ1.3, 7.0, 8.4 Hz, 2 and 11-H, or 3 and 10-H), 8.37 (2H,
broad d, JZ8.5 Hz, 1 and 12-H, or 4 and 9-H), 8.47 (2H,
broad d, JZ7.7 Hz, 1 and 12-H, or 4 and 9-H). LR-MS m/z:
328 (MC). HR-MS Calcd for C22H16O3: 328.1095, Found:
328.1146. Anal. Calcd for C22H16O4: C, 80.47; H, 4.91.
Found: C, 80.57; H, 4.95. 13: Colorless needles (AcOEt–
1
m/z: 402 (MC). IR, H and 13C NMR data were described
previously by us.6b
4.5.4. 5-Methoxy-7-methyl-1,4-naphthoquinone (10c).
This compound was obtained as yellow needles (CHCl3–
hexane), mp 169.5–170 8C (lit.17a 166.5–167.5 8C) from 5c
by the above general procedure A (with the SnCl4/O2
system). LR-MS m/z: 202 (MC). IR, 1H and 13C NMR data
were described previously by us.6b
hexane), mp 222–223 8C (lit.10 307 8C). IR (KBr) cmK1
:
3316, 1633, 1599. 1H NMR (CDCl3) d: 4.01 (3H, s, 5-OMe),
7.41 (1H, s, 7-H), 7.43 (1H, s, 6-H), 7.44–7.65 (5H, m,
Ar-H), 8.23–8.38 (3H, m, Ar-H), 9.01 (1H, s, 8-OH). LR-
MS m/z: 314 (MC). HR-MS Calcd for C21H14O3: 314.33,
Found: 314.33. Anal. Calcd for C21H14O3: C, 80.24; H,
4.49. Found: C, 80.20; H, 4.50.
4.5.5. 8,13a-Dimethoxy-6a,13a-dihydro-6H-dinaphtho-
[1,2-b;20,10-d]furan-5-one (11). This compound was
obtained as yellowish-green amorphous powder (benzene–
hexane), mp 228–229 8C from 5d by the above general
4.6. Oxidation of 11 with the SC/O2 system
procedure A (with the SnCl4/O2 system). IR (KBr) cmK1
:
The reaction of 11 with the SC/O2 system was carried out at
23 8C for 12 h according to the above general procedure A
(with the SnCl4/O2 system) to give 8d (36%).
1
1686, 1598. H NMR (CDCl3) d: 2.63 (1H, dd, JZ2.6,
12.9 Hz, 6a-H), 2.76 (1H, dd, JZ3.7, 12.9 Hz, 6b-H), 3.69
(3H, s, 13a-OMe), 3.91 (3H, s, 8-OMe), 4.14 (1H, dd, JZ
2.6, 3.7 Hz, 6a-H), 6.71 (1H, s, 7-H), 7.43–7.52 (3H, m, Ar-
H), 7.71 (1H, dt, JZ1.5, 7.5 Hz, 2-H), 7.84 (1H, dd, JZ1.5,
7.7 Hz, 4-H), 7.96–8.05 (3H, m, Ar-H). 13C NMR (CDCl3)
d: 28.11 (C6), 46.93 (C6a), 49.66 (C13a–OMe), 55.55 (C8–
OMe), 97.23 (C13a), 103.59 (C7), 110.39 (C6b), 120.29 (C9
or C12), 121.17 (C9 or C12), 124.64 (C12a), 124.82 (C1),
125.29 (C8a), 125.48 (C4), 125.66 (C10 or C11), 126.03
(C10 or C11), 128.91 (C3), 129.24 (C12b), 134.03 (C2),
141.15 (C4a), 141.83 (C13b), 148.98 (C8), 193.64 (C5).
LR-MS m/z: 346 (MC). HR-MS Calcd for C22H18O4:
346.1200, Found: 346.1224. Anal. Calcd for C22H18O4: C,
76.29; H, 5.24. Found: C, 76.39; H, 5.27.
4.7. Methylation of 13
CH3I (12 ml, 0.02 mmol) was added to a solution of 13
(15 mg, 0.05 mmol) and anhydrous K2CO3 (66 mg) in dry
DMF (6 ml), and the solution was stirred at 23 8C for 4 h
with vigorous stirring. The reaction mixture was poured into
ice–water, neutralized with 10% HCl, and extracted with
ether. The organic layer was washed with H2O, dried and
concentrated. The residue was purified by recrystallization
from MeOH to yield 12 (91%).
4.7.1. 4,40-Dimethyl[2,20]binaphthalenyl-1,10-diol (6e).
This compound was obtained as a colorless amorphous
powder (CHCl3–hexane), mp 219–220 8C from 5e by the
above general procedure A (with the SnCl4/O2 system) or
the general procedure D (with SnCl4). LR-MS m/z: 314
(MC). IR, 1H and 13C NMR data were described previously
by us.6b
4.5.6. 2,20-Binaphthalenyl-1,4,10,40-tetraone (8d). This
compound was obtained as yellow needles (CHCl3), mp
288 8C (decomp) (lit.17b,17c 270 8C, decomp.) from 5d by
the above general procedure A (with the SnCl4/O2 system).
1
IR (KBr) cmK1: 1664, 1613, 1587. H NMR (CDCl3) d:
7.07 (2H, s, 3 and 30-H), 7.75–7.80 (4H, m, Ar-H), 8.12–
8.16 (4H, m, Ar-H). LR-MS m/z: 314 (MC). HR-MS Calcd
for C20H10O4: 314.0576, Found: 314.0561. Anal. Calcd for
C20H10O4: C, 76.43; H, 3.21. Found: C, 76.23; H, 3.19.
4.7.2. 5,50,8,80-Tetramethoxy[2,20]di-1,10-naphthol (6f)
and 12-(10-hydroxy-50,80-dimethoxy-naphthalen-20-yl)-
1,4,7,10-tetramethoxy-13-oxadibenzo[a,g]fluoren-11-ol
(16f). These compounds were obtained from 5f according to
the above general procedure A (with the SnCl4/O2 system).
6f: Colorless needles (hexane–AcOEt), mp 257–260.5 8C.
4.5.7. 4,40-Dimethoxy[2,20]binaphthalenyl-1,10-diol (6d).
This compound was obtained as colorless needles
(benzene), mp 223–224 8C from 5d by the above general
1
LR-MS m/z: 406 (MC). IR, H and 13C NMR data were
1
procedure D (with SnCl4). LR-MS m/z: 346 (MC). IR, H
described previously by us.6b 16f: Light brown amorphous
and 13C NMR data were described previously by us.6b
powder (CHCl3–MeOH), mp 263–265 8C. IR (KBr) cmK1
:
1
3346, 1612, 1449. H NMR (CD3SOCD3) d: 3.53 (3H, s,
7-OMe), 03.96 (3H, s, 10-OMe), 3.99 (3H, s, 50-OMe), 4.01
(3H, s, 8 -OMe), 4.10 (6H, s, 1- and 4-OMe), 6.87 (1H, d,
JZ8.5 Hz, 90-H), 6.89 (1H, d, JZ8.5 Hz, 80-H), 6.92 (1H, d,
JZ8.6 Hz, 6 -H), 6.96 (1H, d, JZ8.6 Hz, 7 -H), 7.14 (1H, d,
JZ8.6 Hz, 20-H), 7.20 (1H, d, JZ8.6 Hz, 30-H), 7.59 (1H, d,
JZ8.6 Hz, 3 -H), 7.74 (1H, d, JZ8.6 Hz, 4 -H), 8.06 (1H, d,
JZ9.2 Hz, 5-H), 8.71 (1H, d, JZ9.2 Hz, 6-H), 9.94 (1H, s,
10-OH), 10.63 (1H, s, 11-0OH). 13C NMR (CD3SOCD3) d:
55.34 (4-OMe), 55.64 (5 -OMe), 55.66 (10-OMe), 56.28
(7-OMe), 56.31 (80-OMe), 57.00 (1-OMe), 104.15 (C60 or
C70), 104.17 (C60 or C70), 104.19 (C9), 105.12 (C2), 106.74
(C3), 107.39 (C20), 107.53 (C8), 108.23 (C12), 111.03
(C40), 113.48 (C13b), 113.73 (C10a), 114.93 (C6c), 115.09
(C8a0), 115.78 (C5), 121.46 (C4a), 122.09 (C6a), 123.48
4.5.8. Reaction of 11 with c-HCl in MeOH. A solution of
11 (50 mg, 0.15 mmol) and conc. HCl (6.3 ml) in MeOH
(6.3 ml) was refluxed with stirring at 110 8C for 5 h. The
reaction mixture was poured into ice–water and extracted
with CH2Cl2. The organic layer was washed with H2O, then
dried and concentrated. The residue was subjected to flash
column chromatography on silica gel using AcOEt–hexane
(1:10,0v/v) as an eluent to give 5,8-dimethoxydinaphtho[1,2-
b;200,10-d]furan (12) (27%) and 8-methoxy-dinaphtho[1,2-
b;2 ,1 -d]furan-5-ol (13) (24%). 12: Pale blue needles
(benzene), mp 215.5–216.5 8C (lit.10 216–217 8C). IR
1
(KBr) cmK1: 1598, 1583. H NMR (CDCl3) d: 4.12 (6H,
s, 5 and 8-OMe), 7.23 (2H, s, 6 and 7-H), 7.54 (2H, ddd, JZ
1.3, 7.0, 8.4 Hz, 2 and 11-H, or 3 and 10-H), 7.67 (2H, ddd,

T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
9059
(C6), 123.52 (C6b), 127.19 (C4a0), 130.29 (C30), 148.28
(C7), 149.05 (C10), 149.27 (C50), 149.31 (C13a), 149.73
(C80), 150.09 (C4), 150.54 (C1), 151.20 (C11), 151.75
(C10), 154.08 (C12a). LR-MS m/z: 606 (MC). HR-MS
Calcd for C36H30O9: 606.1881, Found: 606.1887. Anal.
Calcd for C36H30O9: C, 71.28; H, 4.98. Found: C, 70.98; H,
5.05.
tetraone (2) was synthesized as yellow needles (CHCl3–
hexane), mp 212–214 8C (lit.1g 214–217 8C) from 8b in 98%
yield by a procedure similar to that used for 1. IR (KBr)
cmK1: 1665, 1645, 1455, 1280, 1244, 1200, 1057, 1023,
741. 1H NMR (CDCl3) d: 2.08 (6H, s, C3 and C30-Me), 7.29
(2H, dd, JZ1.3, 8.3 Hz, 7 and 70-H), 7.66 (2H, t, JZ8.3 Hz,
C 6 and C60 H), 7.73 (2H, dd, JZ1.3, 8.3 Hz, C5 and C50-
H), 11.73 (2H, s, 5 and 50-H). MS m/z: 374 (MC). HR-MS
Calcd for C22H14O6: 374.0790. Found 374.0783. Anal.
Calcd for C22H14O6: C, 70.59; H, 3.77. Found: C, 70.79; H,
3.75.
4.7.3. 5,50,8,80-Trimethoxy-6,60-dimethyl[2,20]di-1,10-
naphthol (6g) and 12-(1-hydroxy-5,8-dimethoxy-6-
methylnaphthalen-2-yl)-1,4,7,10-tetramethoxy-3,8-
dimethyl-13-oxadibenzo[a,g]fluoren-11-ol (16g). These
compounds were obtained from 5g by the above general
procedure A (with the SnCl4/O2 system). 6g: Colorless
needles (ether–hexane), mp 229–232 8C. LR-MS m/z: 434
(MC). IR, 1H and 13C NMR data were described previously
by us.6b16g: Pale green amorphous powder (CHCl3–
hexane), mp 249–251 8C. IR (KBr) cmK1: 3372, 1619,
4.7.6. Synthesis of elliptinone (4) from 6g. A solution16 of
Al powder (416.2 mg, 15.4 mmol) and I2 (2936 mg,
11.6 mmol) in absolute benzene (6.7 ml) was stirred under
an argon atmosphere for 3 h. A solution of 6g (21.7 mg,
0.05 mmol) in absolute benzene (16.7 ml) was added to the
above solution (AlI3) and the whole was stirred at ambient
temperature for 3 h, then further stirred under air for 1 h.
The reaction mixture was poured into ice water and acidified
with 10% HCl. The whole was extracted with CHCl3. The
organic layer was washed with H2O, then dried with
Na2SO4 and concentrated. The residue was subjected to
flash chromatography (AcOEt–hexaneZ1:2) to yield 12 mg
(64%) of0 1,10-dihydroxy-6,60-dimethyl[2,20]binaphthal-
enyl-5,8,5 ,80-tetraone (4) as orange red needles (CHCl3–
hexane), mp 302–305 8C (lit.1k 300–305 8C). IR (KBr)
cmK1: 3426, 1640, 1603, 1417, 1353, 1258. 1H NMR
(CDCl3) d: 2.23 (6H, d, C6 and C60-Me), 6.85 (2H, q, JZ
1.5 Hz, C7 and C70-H), 7.73 (4H, s, C 3, 4 and C30, 40-H),
12.50 (2H, s, C1 and C10-OH). 13C NMR (CDCl3) d: 16.5,
115.3, 118.7, 131.2, 131.9, 135.5, 137.7, 149.8, 158.9,
184.5, 190.5. MS m/z: 374 (MC). HR-MS Calcd for
C22H14O6: 374.0790. Found 374.0769. Anal. Calcd for
C22H14O6: C, 70.59; H, 3.77. Found: C, 70.49; H, 3.80.
1
1581. H NMR (CD3SOCD3) d: 2.45 (3H, s, 3-Me), 2.49
(3H, s, 60-Me), 2.62 (3H, s, 8-Me), 3.64 (3H, s, 1-OMe),
3.71 (3H, s, 7-OMe), 3.90 (3H, s, 4-OMe), 3.91 (3H, s, 50-
OMe), 4.08 (3H, s, 80-OMe), 4.17 (3H, s, 10-OMe), 6.85
(1H, s, 2-H), 6.93 (1H, s, 70-H), 7.12 (1H, s, 9-H), 7.63 (1H,
d, JZ8.5 Hz, 40-H), 7.66 (1H, d, JZ8.5 Hz, 30-H), 7.99 (1H,
d, JZ9.2 Hz, 5-H), 8.80 (1H, d, JZ9.2 Hz, 6-H), 9.73 (1H,
s, 10-OH),010.35 (1H, s, 11-OH). 13C NMR (CD3SOCD3) d:
15.50 (C6 -Me), 15.59 (C3-Me), 15.88 (C8-Me), 55.88 (C1-
OMe), 56.21 (C80-OMe), 56.82 (C10-OMe), 60.21 (C7 or
C50-OMe), 60.25 (C7 or C50-OMe), 60.48 (C4-OMe),
106.43 (C6b), 107.21 (C12), 107.31 (C70), 107.97 (C9),
109.92 (C2), 110.87 (C40), 0112.28 (C10a), 112.45 (C13b),
113.67 (C20), 114.10 (C8a ), 116.07 (C5), 120.36 (C6a),
123.67 (C6), 123.69 (C6c), 125.05 (C30), 125.43 (C600),
126.30 (C4a), 127.96 (C8), 129.77 (C4a ), 131.03 (C3 ),
146.84 (C4), 147.14 (C50), 147.81 (C7), 149.82 (C13a),
150.29 (C1), 151.41 (C11), 151.75 (C80), 152.10 (C10),
152.59 (C10), 154.48 (C12a). LR-MS m/z: 648 (MC). HR-
MS Calcd for C39H36O9: 648.2349, Found: 648.2386. Anal.
Calcd for C39H36O9: C, 72.21; H, 5.59. Found: C, 72.41; H,
5.60. Anal. Calcd for C39H36O9: C, 72.21; H, 5.59. Found:
C, 72.15; H, 5.61.
References and notes
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4.7.4. Synthesis of 3,30-bijuglone (1) from 8a. Magnesium
bromide (anhydrous)7c (2.2 g, 12 mmol) was added to a
solution of 8a (182 mg, 0.5 mmol) dissolved in absolute
toluene (50 ml) and the whole was refluxed for 12 h. The
reaction was quenched with cooled water and saturated
NH4Cl solution, and the whole was stirred for 30 min. The
mixture was extracted with CHCl3. The organic layer was
washed with H2O, dried with Na2SO4 and concentrated. The
residue was subjected to flash chromatography (CHCl3–
AcOEt–hexaneZ3:1:4) to yield 183 0mg (98%) of 8,80-
dihydroxy[2,20]binaphthalenyl-1,4,10,4 -tetraone (1) as an
orange amorphous powder (benzene), mp 265–267 8C
(decomp.) (lit.1e 270 8C), 173 mg (97%). IR (KBr) cmK1
:
1663, 1627, 1454, 1293. 1H NMR (CDCl3) d: 7.04 (2H, s, 2
and 20-H), 7.34 (2H, m, Ar-H), 7.70 (4H, m, Ar-H), 11.76
(2H, s, 5 and 50-OH). LR-MS m/z: 346 (MC). HR-MS Calcd
for C20H10O6: 346.0478, Found: 346.0472. Anal. Calcd for
C20H10O6: C, 69.37; H, 2.91. Found: C, 69.27; H, 2.90.
4.7.5. Synthesis of 3,30-biplumbagin (2) from 8b. 8,800-
Dihydroxy-3,30-dimethyl[2,20]binaphthalenyl-1,4,10,4 -

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T. Takeya et al. / Tetrahedron 60 (2004) 9049–9060
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