Synthesis of a new class of furan-fused tetracyclic compounds using o-quinodimethane chemistry and investigation of their antiviral activity

Article abstract of DOI:10.1021/jo0486995

The synthesis and evaluation of antiviral activity of new furan-fused tetracyclic compounds are described. The syntheses were satisfactorily achieved on the basis of o-quinodimethane chemistry, using furan-containing benzocyclobutene derivatives as a substrate, in high generality and stereoselectivity. The various derivatives thus synthesized were examined on their inhibitory activity on virus growth using a hemagglutinin (HA) method, leading to a discovery of promising candidates for new antiviral drugs having high activity and good therapeutic index.

Full text of DOI:10.1021/jo0486995

Synthesis of a New Class of Furan-Fused Tetracyclic Compounds
Using o-Quinodimethane Chemistry and Investigation of Their
Antiviral Activity
Yuji Matsuya,†,§ Kazushige Sasaki,† Mamiko Nagaoka,†,§ Hiroko Kakuda,† Naoki Toyooka,†,§
Nobuko Imanishi,‡ Hiroshi Ochiai,‡,§ and Hideo Nemoto*,†,§
Faculty of Pharmaceutical Sciences, and Department of Human Sciences and Fundamental Nursing,
Faculty of Medicine, Toyama Medical and Pharmaceutical University, 2630 Sugitani,
Toyama 930-0194, Japan
nemotoh@ms.toyama-mpu.ac.jp
Received July 28, 2004
The synthesis and evaluation of antiviral activity of new furan-fused tetracyclic compounds are
described. The syntheses were satisfactorily achieved on the basis of o-quinodimethane chemistry,
using furan-containing benzocyclobutene derivatives as a substrate, in high generality and
stereoselectivity. The various derivatives thus synthesized were examined on their inhibitory activity
on virus growth using a hemagglutinin (HA) method, leading to a discovery of promising candidates
for new antiviral drugs having high activity and good therapeutic index.
SCHEME 1
Introduction
Naturally occurring furan-fused polycyclic compounds
have been reported to exhibit significant and intriguing
biological activities,1 which include antibiotic, cardiotonic,
protein tyrosine kinase inhibitory, and antiviral activities
as seen in halenaquinone and related natural com-
pounds.2 In our previous reports on the short-step
synthesis of a model core structure associated with such
natural products,3 we revealed that the furan-fused
tetracyclic compound 1, which was concisely synthesized
on the basis of o-quinodimethane chemistry (Scheme 1),4
possessed a notable antiviral activity. This new finding
inspired us to examine the structure-activity relation-
ships of its congeners, aiming at the discovery of new
candidates for antiviral drugs. In this paper, we describe
the syntheses of various derivatives of 1 as a potential
lead compound using thermal cleavage of a benzo-
cyclobutene ring and subsequent intramolecular cyclo-
addition, and the results of the assay for their antiviral
activity.
† Faculty of Pharmaceutical Sciences, Toyama Medical and Phar-
maceutical University.
‡ Department of Human Sciences and Fundamental Nursing, Fac-
ulty of Medicine, Toyama Medical and Pharmaceutical University.
§ TOMECS (Toyama Medicinal Chemistry Society), Japan.
(1) (a) Lee, R. H.; Slate, D. L.; Moretti, R.; Alvi, K. A.; Crews, P.
Biochem. Biophys. Res. Commun. 1992, 184, 765-772. (b) Alvi, K. A.;
Diaz, M. C.; Crews, P.; Slate, D. L.; Lee, R. H.; Moretti, R. J. Org.
Chem. 1992, 57, 6604-6607.
Results and Discussion
Modification of the A-Ring. First, modification was
performed on the A-ring. The acetal structure at the
1-position seems to be a scaffold for modifications of the
A-ring. However, generation of the carbonyl group by
deacetalization of 1 under acid-catalyzed conditions was
found to be difficult because competitive cleavage of the
enol ether structure on the E-ring occurred to afford a
complex mixture. Accordingly, new substrates for the
thermal cycloaddition containing an oxygen function were
synthesized as shown in Scheme 2. Known furanyl
homoallyl alcohol 35 was prepared from 3-furancarb-
(2) (a) Roll, D. M.; Scheuer, P. J.; Matsumoto, G. K.; Claedy, J. J.
Am. Chem. Soc. 1983, 105, 6177-6178. (b) Kobayashi, M.; Shimizu,
N.; Kyogoku, Y.; Kitagawa, I. Chem. Pharm. Bull. 1985, 33, 1305-
1038. (c) Kobayashi, M.; Shimizu, N.; Kitagawa, I.; Kyogoku, Y.;
Harada, N.; Uda, H. Tetrahedron Lett. 1985, 26, 3833-3836. (d)
Nakamura, H.; Kobayashi, J.; Kobayashi, M.; Ohizumi, Y.; Hirata, Y.
Chem. Lett. 1985, 713-716. (e) Schmits, F. J.; Bloor, S. J. Org. Chem.
1988, 53, 3922-3925.
(3) (a) Toyooka, N.; Nagaoka, M.; Kakuda, H.; Nemoto, H. Synlett
2001, 1123-1124. (b) Toyooka, N.; Nagaoka, M.; Sasaki, E.; Qin, H.;
Kakuda, H.; Nemoto, H. Tetrahedron 2002, 58, 6097-6101. (c) Mat-
suya, Y.; Qin, H.; Nagaoka, M.; Nemoto, H. Heterocycles 2004, 62, 207-
211.
(5) Racherla, U. S.; Brown, H. C. J. Org. Chem. 1992, 57, 6614-
6617.
(4) Nemoto, H.; Fukumoto, K. Tetrahedron 1998, 54, 5425-5464.
10.1021/jo0486995 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/16/2004
J. Org. Chem. 2004, 69, 7989-7993
7989

Matsuya et al.
SCHEME 2
FIGURE 1. ORTEP drawing of compound 9a.
SCHEME 4
SCHEME 3
aldehyde (2) by the Grignard addition. This alcohol was
converted into silyl ethers 4a and 4b, which were
subjected to oxidative cleavage followed by reduction with
sodium borohydride to give the primary alcohols 6a and
6b. Treatment with tetrabromomethane and triphen-
ylphosphine afforded the corresponding bromides 7a and
7b, and the reaction of the bromides with a lithiated
benzocyclobutene derivative proceeded successfully to
furnish the required substrates 8a and 8b for the next
thermal reaction.
Thermal cycloaddition of 8a and 8b was performed in
refluxing o-dichlorobenzene for 2 h, and the results are
summarized in Scheme 3. In both cases, expected tetra-
cyclic products 9a and 9b were obtained concomitant
with olefin 10, which might be formed as a result of
â-elimination of the silyloxy group. The other stereoiso-
mers could not be detected, implying that the cycloaddi-
tion proceeded via the endo transition state as shown in
Scheme 1, exclusively. Also, it is suggested that the major
precursor of 10 is the epimeric product 9′,6 which can be
rationally explained by the fact that the silyloxy group
in 9′ has axial orientation, facilitating its elimination
much more than that of 9 because of 1,3-diaxial repulsion
or the presence of an adjacent anti-hydrogen for E2
elimination. The derivative 9c having a free hydroxyl
group was also prepared by desilylation of 9a. The
stereostructure of 9a was confirmed by X-ray crystal-
lographic analysis (Figure 1),7 and the structure of 9b
was presumed by the similarity of the NMR spectra to
that of 9a. In addition, conversion of 9c to 10 in refluxing
o-dichlorobenzene was observed, which supported the
stereostructure of 10.
Next, we investigated the synthesis of oxygen-contain-
ing derivatives at the 15-position on the A-ring according
to Scheme 4. Benzocyclobutene derivative 11 was lithi-
ated with LDA and was allowed to react with racemic
glycidyl tosylate to give the epoxide 12, which was
subjected to oxirane-ring opening with 3-lithiofuran,
prepared from 3-bromofuran and n-butyllithium, to afford
a requisite substrate 13.
(6) The diastereomers of the substrate 8a (or 8b) were indistin-
guishable in the spectral data (1H and 13C NMR) and on the TLC, so
the ratio of the diastereomers could not be estimated. It should be noted
that the diastereomeric ratio of the starting materials 8 does not reflect
the product ratio of 9 and 9′ (or 10) because the chiral center bearing
the cyano group in 8 is canceled out during formation of the o-
quinodimethane intermediate.
It was found that thermolysis of 13 gave rise to a novel
pentacyclic product 15 in addition to a normal cycload-
(7) Details of the crystallographic data are provided in the Support-
ing Information.
7990 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis of Furan-Fused Tetracyclic Compounds
SCHEME 5
FIGURE 2. ORTEP drawing of compound 15.
Modification of the C-Ring. The C-ring of the lead
compound 1 consists of an aromatic ring bearing a
methoxy group at the 9-position. Several variants on this
position were synthesized according to Schemes 6 and
7. The benzocyclobutene derivative 11 was demethylated
by treatment with trimethylsilyl iodide, and the phenolic
compound 22 thus obtained was alkylated with an alkyl
bromide having a three-carbon chain to afford 23. Treat-
ment of the phenol 23 with TBSOTf gave the correspond-
ing silyl ether 24 in good yield. After removal of THP
group, the resulting alcohol 25 was oxidized with PDC
and subsequently treated with 3-lithiofuran to give the
alcohol 26, which was converted into the ketone 27 by
reoxidation. The compound 28 obtained by ketalization
of the ketone was subjected to thermal cycloaddition to
furnish new tetracyclic derivative 29a having a silyloxy
group at 9-position with an exclusive stereoselectivity.
A free-hydroxy derivative 29b and TIPS derivative 29c
were also synthesized from 29a. A 9-unsubstituted
derivative 33 was synthesized as shown in Scheme 7. A
commercially available benzocyclobutene 30 was coupled
with a known furan-containing alkyl bromide 313b to give
32, the thermolysis of which afforded 33 as a sole
stereoisomer again. The 1H NMR spectra of the products
29a-c and 33 showed good similarity to that of the lead
compound 1.
Evaluation of the Bioactivity. The antiviral activity
of the furan-fused tetracyclic compounds obtained in this
study was surveyed using the assay method of hemag-
glutinin (HA) titers.8 HVJ in LLC-MK2 cells was used
for the assay, and the inhibitory activity on the virus
growth was assessed as minimum inhibitory concentra-
tions (MIC), which are summarized in Table 1.
As can be seen in the table, most of the test compounds
more or less exhibited the antiviral activity, and some of
them were much more potent than the lead compound
1. On the other hand, the compounds 15, 20, and 21
showed no or much less activity as compared to 1, which
suggested the importance of the ethylene ketal at the
1-position, the dihydrofuran structure at the E-ring, and
the presumably rigid open-cage conformation of the
molecule. Among all of the congeners examined in this
FIGURE 3. Coupling constant correlations of products 17 and
18.
FIGURE 4. Presumed process for the formation of the cyclic
acetal 15.
duct 14. The structure of 15 was unambiguously deter-
mined by X-ray crystallographic analysis (Figure 2).7 On
the other hand, the MOM-protected substrate 16 afforded
two tetracyclic adducts 17 and 18, the stereochemistry
of which were presumed by the coupling constant cor-
relations in their 1H NMR spectra (Figure 3). The
1
structure of 14 was supported by the fact that the H
NMR spectra of 14 and 17 were very much alike in
coupling patterns and chemical shifts. As shown in
Figure 4, the formation of 15 would be rationalized
considering that the isomer of 14 having an axial hy-
droxyl group readily underwent cyclization due to the
proximity of the free hydroxyl group to the enol ether
structure on the E-ring.
Modification of the E-Ring. Our next subject is
modification of the E-ring composed of the dihydrofuran
structure. Transformation of the lead compound 1 to a
corresponding tetrahydrofuran derivative was first ex-
amined by means of catalytic hydrogenation. This trans-
formation was achieved in a stereoselective manner using
palladium hydroxide as a catalyst. The stereostructure
of the product 19 was determined by NOE experiment
as shown in Scheme 5, and the result implied that the
hydrogenation occurred from the less hindered convex
face of the substrate having a cage-type conformation.
Compound 19 was further transformed into ketone 20
by treatment with p-TsOH in acetone. In addition to this
reductive transformation, aromatization of the dihydro-
furan system of 1 was also achieved in a three-step
sequence, providing 21, which has already been described
in our previous paper.3b
(8) Ochiai, H.; Sakai, S.; Hirabayashi, T.; Shimizu, Y.; Terasawa,
K. Antiviral Res. 1995, 27, 425-430.
J. Org. Chem, Vol. 69, No. 23, 2004 7991

Matsuya et al.
SCHEME 6
TABLE 1. The Effect of Synthesized Furan-Fused Tetracyclic Compounds on the Growth of HVJ in LLC-MK2 Cellsa
HA titers in culture supernatants in the presence of
therapeutic
the indicated sample concentrations (µg/mL)b
MIC (R)c
(µg/mL)
MCC (â)d
(µg/mL)
index
(R/â)
compound
2000
1000
500
250
125
62.5
1
16
64
4
32
64
4
32
64
4
32
64
32
<4
4
128
4
4
16
512
32
64
64
<4
16
128
4
64
64
64
<4
64
128
8
2000
62.5
32
9b
9c
10
14
15
17
18
19
20
21
29a
29b
29c
33
500
12.5
125
125
4
<4
64
<4
<4
4
4
64
4
<4
4
62.5
62.5
250
4
64
4
64
4
128
>2000
>2000
125
<0.125
<0.5
2
256
512
256
64
32
256
64
<4
8
32
64
32
1000
250
<4
32
<4
<4
<4
<4
32
therapeutic
index
sample concentrations (µg/mL)
MIC (R)c
MCC (â)d
(µg/mL)
compound
5
2.5
1.25
4
0.6
8
0.3
32
0.15
128
(µg/mL)
(R/â)
29c
<4
<4
0.6
125
0.0048
a The virus was adsorbed on the medium 1 h before the sample was added and was cultured for 24 h at 37 °C. b In the absence of the
sample (control experiment), the HA titers were 128. c The minimum inhibitory concentrations (MIC) of compounds more active than the
lead compound (1) are shown. d The maximum cytotoxic concentrations (MCC) of the compounds having less toxicity than 1 are shown.
study, we found the triisopropylsilyloxy derivative 29c
to have the most potent inhibitory activity for the virus
growth. Accounts of the substituent effects toward the
bioactivity are currently under consideration. The cyto-
toxic assay of several compounds was performed using
the MTT method,9 and the results are summarized as
the maximum cytotoxic concentrations (MCC) as well as
the calculated therapeutic indexes. Thus, we could find
several derivatives having more potent antiviral activity
than the lead compound 1, and, especially, the TIPS
derivative 29c was revealed to have the lowest MIC value
and good therapeutic index.
Conclusion
In this paper, we have demonstrated that the intra-
molecular [4+2] cycloaddition of o-quinodimethane gen-
erated by the thermal ring opening of benzocyclobutene
derivatives is a promising method for constructing a new
class of antiviral furan-fused tetracyclic compounds. This
synthetic methodology has the advantages of conciseness,
short-step sequence, high generality, and high endo-
(9) Mosmann, T. J. Immunol. Methods 1983, 65, 55-63.
7992 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis of Furan-Fused Tetracyclic Compounds
SCHEME 7
efforts to search for more efficient derivatives are cur-
rently ongoing in our laboratory.
Acknowledgment. This work was partially sup-
ported by a Grant-in-Aid (No. 12672094) for Scientific
Research from the Ministry of Education, Culture,
Sports, Science, and Technology of the Japanese Gov-
ernment.
Supporting Information Available: Synthetic procedure
and characterization data for all new compounds, including
1H and 13C NMR spectra, crystallographic data for 9a and 15,
and procedure for the bioassay. This material is available free
of charge via the Internet at http://pubs.acs.org.
selectivity. Evaluation of the antiviral activity of new
derivatives mentioned above gave several findings re-
garding the structure-activity relationships. Further
JO0486995
J. Org. Chem, Vol. 69, No. 23, 2004 7993