SnCl4-mediated oxidative reaction for formation of binaphthoquinone and dinaphthofuran frameworks and its application to natural product synthesis

Article abstract of DOI:10.1016/S0040-4039(03)00213-2

A simple method was developed for the direct synthesis of 2,2′-binaphthoquinones and dinaphtho[1,2-b;2′,1′-d]furans, utilizing an oxidative reaction via electron donor-acceptor complexes of 1-naphthols with SnCl₄ in the absence or presence of dioxygen. As an application of this method to natural product synthesis, we describe a facile biomimetic synthesis of several binaphthoquinones, 3,3′-bijuglone, 3,3′-biplumbagin and elliptinone.

Full text of DOI:10.1016/S0040-4039(03)00213-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2041–2044
SnCl -mediated oxidative reaction for formation of
4
binaphthoquinone and dinaphthofuran frameworks and its
application to natural product synthesis
Tokutaro Ogata, Iwao Okamoto, Hirohisa Doi, Eiichi Kotani and Tetsuya Takeya*
Showa Pharmaceutical University, 3-3165 Higashi-tamagawagakuen, Machida, Tokyo 194-8543, Japan
Received 26 December 2002; revised 17 January 2003; accepted 17 January 2003
Abstract—A simple method was developed for the direct synthesis of 2,2%-binaphthoquinones and dinaphtho[1,2-b;2%,1%-d]furans,
utilizing an oxidative reaction via electron donor–acceptor complexes of 1-naphthols with SnCl in the absence or presence of
4
dioxygen. As an application of this method to natural product synthesis, we describe a facile biomimetic synthesis of several
binaphthoquinones, 3,3%-bijuglone, 3,3%-biplumbagin and elliptinone. © 2003 Elsevier Science Ltd. All rights reserved.
The biaryl substructure is a central building block in a
very large number of natural products. Among natu-
ral biaryls, binaphthoquinones including 3,3%-bijuglone
with SC. During further investigations to characterize
the range of applicability of SC, we have found a novel
method for synthesizing 2,2%-binaphthoquinones (7;
BNAPQ) and also dinaphtho[1,2-b;2%,1%-d]furans (8;
DNF) from the corresponding 4 in one-pot by utilizing
an oxidative reaction with SC in the presence or
1
a
(
1), 3,3%-biplumbagin (2) and elliptinone (3), dinaphtho-
furans such as balsaminones A, and binaphthols such
as michellamine A have been isolated from several
plants and show various biological activities. We are
1
absence of dioxygen (O ). Here, we describe these
2
results, including a possible reaction mechanism, and
their application to the biomimetic synthesis of natu-
rally occurring 1, 2 and 3.
interested in the oxidative reactions of 1-naphthols
(
NAP) in order to develop a method for constructing
these biaryl substructures, aiming at biomimetic synthe-
sis of the above natural products. In general, the syn-
thesis of binaphthoquinone and dinaphthofuran
frameworks requires two steps, namely, the biaryl cou-
pling of NAP and subsequent oxidation or ring closure
of the resulting 2,2%-binaphthols (BNAP). Although a
number of authors have obtained the above frame-
2
works in two steps, only few examples of synthesis in
one step have been recorded. On the other hand,
3
stannic chloride (SnCl ; SC) is used extensively in
4
organic synthesis as a Lewis acid for enhancing a
variety of organic reactions. However, there have been
only a few reports to date on oxidative reactions with
4
SC.
5
In the preceding paper, we reported the direct synthe-
sis of BNAP 5 utilizing the biaryl coupling reaction via
the electron donor–acceptor (EDA) complex of NAP 4
Figure 1.
5,6
Firstly, we investigated the reaction of 4 with the
SC/O system in the presence of O with the aim of
2
2
achieving a facile synthesis of naturally occurring 1 and
under various conditions; the results are shown in
Table 1 and Scheme 1. The best yields were obtained
when using NAP (1 mmol) with a catalytic amount of
Keywords: naphthoquinones; dinaphthofurans; oxidation; tin and
compounds; dioxygen.
2
*
Corresponding author. Tel./fax: +81-42-721-1579; e-mail: takeya@
ac.shoyaku.jp
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00213-2

2
042
T. Ogata et al. / Tetrahedron Letters 44 (2003) 2041–2044
a
Table 1. Oxidative reactions of naphthols with SnCl in the presence of O2
4
Entry
Naphthol
Solvent
Temp. (°C)
Time (h)
Product (isolated yield, %)
Recovered (%)
d
d
5
6
7
9
4
1
2
3
4
5
6
4a
4a
4b
4b
4d
4e
4a
4d
4e
5a
5a
CH Cl
23
23
1
20
7
71
1
5
48
1
75
–
52
–
20
8
82
20
5
5
22
Trace
Trace
–
–
4
–
–
76
–
48
–
–
–
–
–
–
24
19
–
16
–
–
15
–
20
–
33
53
8
47
60
–
2
2
2
CH Cl
2
MeNO2
MeNO2
MeNO2
MeNO2
CH Cl
100
100
100
100
23
100
100
23
b
7
2
2
2
b
8
MeNO2
MeNO2
CH Cl
–
–
7
1
b
9
5
1.5
0.5
–
37
46
1
1
0c
1
49
41
2
2
2
CH Cl
23
–
–
2
a
General procedure: the reactions of naphthols (1 mmol) with SnCl4 (0.25 equiv.) were carried out using dioxygen (O )-saturated solvent in a
2
sealed tube with stirring under normal laboratory light. Similar results were obtained in the dark.
This reaction was carried out with TiCl4 (0.25 equiv.) in place of SnCl4 under the same conditions as above.
b
c
With 30% H O2 (1 mmol) in place of dioxygen.
2
d
Structures 9 were elucidated by analyses of IR, 1H, 13C NMR spectra, with the aid of 2D NMR spectral analyses, and transformation to the
corresponding 10.
SC (0.25 equiv.) in all cases. We found that the nature
of the major products changed drastically with the
passage of time at room temperature. In the cases of
To throw light on the formation of 6 and 7 in the
reactions of 4a–b with the SC/O system in CH Cl , the
2
2
2
reactions of 5a with SC/H O (hydrogen peroxide) and
2
2
4
a–b, BNAP 5a–b were obtained as major products in
SC/O reagent systems in CH Cl were examined. In
2
2
2
the reaction for a short time (1–7 h) (entries 1, 3).
Prolonged reaction (60–75 h) under the same conditions
selectively afforded BNAPQ 7a–b, synthetic intermedi-
ates to the desired natural products, in one-step from
both cases, 6 and 7 were obtained (Table 1, entries
4
a–b (entries 2, 4). The reaction of 4a with SC in the
absence of O did not afford 7a (refer to Table 2, entry
2
3
). We also used another Lewis acid for the formation
of 7. However, the reaction of 4a with the TiCl /O
4
2
system in CH Cl gave 7a in very low yield (entry 7).
2
2
Although oxidative coupling reactions of naphthols by
1
g,7
several metal oxidants
2
in the absence or presence of
O have been studied extensively, mixtures of dimeric,
polymeric and quinoid compounds were usually gener-
ated. In addition, regioselective ortho/ortho coupling is
more difficult to control in the reaction of NAPs such
as 4d–e.
In order to obtain 5e as a synthetic intermediate for 3,
the ortho/ortho coupling reaction of 4e with SC in the
absence of O was tried first, but the yield of 5e was
2
very low, partly because of the formation of a poly-
meric mixture. We then studied the reaction with the
SC/O2 system. In this reaction, ortho/ortho-coupled
BNAP 5e was obtained along with the trimeric furan 9e
without the formation of BNAPQ (entry 6). A notewor-
thy feature in the reaction with the SC/O system is the
2
difference between 4a and 4d, i.e., the formation of 7a
and 9d having different frameworks. It appears that the
formation of 7 requires electron-rich NAP having a
methoxy group on ring A, as shown in Figure 1.
Thus, we have established a facile and biomimetic
1d
1c
synthesis of 3,3%-bijuglone (1), 3,3%-biplumbagin (2)
1
e
and elliptinone (3) from the corresponding 7a, 7b and
e, prepared by means of the reactions of NAPs with
the SC/O system.
5
Scheme 1.
2

T. Ogata et al. / Tetrahedron Letters 44 (2003) 2041–2044
2043
5
1
0–11). These results suggest the participation of H O
In our previous paper, the formation of 8a via 5a was
2
2
or O . In addition, the present reaction of 4 (1 mmol)
observed in the reaction of 4a with SC using MeNO as
2
2
involves a catalytic cycle of SC (0.25 equiv.) in all cases.
Furthermore, there are a number of reports on the
generation of H O via hydroperoxy radical (HO ·) by
a solvent (Scheme 3 and Table 2, entry 1). This result
prompted us to seek a simple method for DNF frame-
work synthesis from NAP in one step.
2
2
2
reduction of O in aprotic solvents in the presence of
Brønsted acids such as phenol and perchloric acid
2
In the second stage, further investigation of the reaction
5,6
(
HClO ) as proton sources by means of chemical and
of 4, as well as 5 and 9, with SC in the absence of O2
was carried out. As shown in Table 2, 8a and 8c were
obtained in satisfactory to high yield by the reactions of
4a and 4c with SC (1.3 equiv.) in CH Cl , and the
4
8
electrochemical methods.
The above results and information allow us to propose
a mechanism for the oxidative reaction of NAP with
2
2
reaction can be conveniently performed with NAP hav-
ing a methoxy group on ring A. In addition, the
transformations of 4 to 8 proceeded stoichiometrically
overall, while the formation of 5 proceeded with a
catalytic cycle of SC (entries 2, 3). On the other hand,
no reaction of 4d, which lacks the methoxy group,
occurred in CH Cl . In contrast with this result, the
SC in the presence of O as illustrated in Scheme 2.
2
Here, O can be incorporated into complex A to form
2
the complex B in which the O site receives one-electron
2
transfer from the anion radical species (SC-·) and sub-
sequent one-proton transfer from the NAP site to
reform SC with the generation of the radical C and
HO ·. Then, H O and O are generated by the dispro-
2
2
reaction of 4d in MeNO in place of CH Cl gave 8d
2
2
2
2
2
2
2
9
portionation of two HO ·. The complex E or F can be
formed by the interaction of H O or O with complex
and 5d (entry 6), and similar reaction for 15 h afforded
10, with the disappearance of 4d (entry 7). However,
polymeric compounds were formed in both cases. The
trimer 9 could be easily converted to the corresponding
furans 10 (Scheme 1). The different yields of 8 in the
present reactions are a consequence of the structural
differences between 4a–c and 4d: a methoxy group on
ring A is considered to activate the NAP ring for the
formation of the EDA complex with SC, and to inhibit
the production of polymeric compounds, including 10,
2
2
2
2
D generated from SC and 5, which is produced by
ortho/ortho radical coupling of two C. In the reaction
of 4a–b, the quinones 6 can be formed via Fenton-like
10b
1
0a
reaction of E or radical autoxidation type reaction
of F. The formation of BNAPQ 7 takes place similarly
route c). On the other hand, the trimeric furan 9 is
(
formed by para/ortho radical coupling between G and
C, followed by oxidative radical chain reaction in the
case of 4d–e (route d).
2
by reaction at the substituted position (R position).
SC plays an important role in the oxidative reactions of
The formation mechanism of 8 from 4 can be rational-
ized in terms of the biaryl coupling of two radicals C
NAP in the presence of O . That is, it acts not only as
2
5
a characteristic Lewis acid catalyst, but also as a medi-
ator for 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 NAP,
and as an oxygen source for the formation of BNAPQ.
via the EDA complex A of 4 with SC and subsequent
ring closure of the resulting 5 to form the DNF ring,
based on the results listed in entries 8–10 (refer to
Scheme 2). Although the exact mode of ring closure is
not clear, we believe that it may proceed via radical
Scheme 2. Proposed mechanism for oxidative reaction of 1-naphthols with SnCl in the presence of O .
4
2

2
044
T. Ogata et al. / Tetrahedron Letters 44 (2003) 2041–2044
Table 2. Reactions of naphthols with SnCl in the absence
product, while the reaction of 4a with SC gave DNF 8a
in high yield. The SC-mediated oxidative reactions of
4
of O2a
NAP in the presence or absence of O made it possible
to control the synthesis in the direction of either the
BNAPQ or the DNF framework.
2
Entry
Naphthol
Solvent
Time
Product (yieldb)
10
8
1
2
3
4a
4a
4a
4c
4d
4d
4d
5a
5c
5d
9d
9e
MeNO2
CH Cl
0.8
56
96
65
65
4.3
15
2
63
7
1.5
1
60
92
14
84
–
–
–
–
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2
2
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f
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5
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1
1
pathways involving the formation of H O (the naph-
2
thoxy radical-induced reaction), rather than nonradical
2
b
pathways
(the Lewis acid-promoted dehydration
pathway) because of the formation of polymeric com-
pounds (including 10) as described above (entries 6, 7
and 10). In this step, SC is inactivated by the resulting
8
H O. In the oxidative reaction with SC in the absence
2
of O , CH Cl and MeNO used as solvents also act as
2
2
2
2
12
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3
775.
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9
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2
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2
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In conclusion, the above reactions of NAP 4 proceed
differently in the SC/O system and SC system. For
2
example, the reaction of 4a with the SC/O2 system
afforded BNAPQ 7a in good yield as the major