A vicarious synthesis of unsymmetrical meta- and para- terphenyls from 2H-pyran-2-ones

Article abstract of DOI:10.1016/j.tetlet.2005.10.018

An innovative synthesis of terphenyls functionalized with electron-withdrawing or -donating substituents is described and illustrated by carbanion-induced ring transformation of 2H-pyran-2-ones with 2-methoxyacetophenone in excellent yields. The existing

Full text of DOI:10.1016/j.tetlet.2005.10.018

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8487–8491
A vicarious synthesis of unsymmetrical meta- and para-
terphenyls from 2H-pyran-2-onesq
Atul Goel,* Deepti Verma and Fateh Veer Singh
Division of Medicinal and Process Chemistry, Central Drug Research Institute, Lucknow 226001, India
Received 16 August 2005;revised 28 September 2005;accepted 6 October 2005
Available online 21 October 2005
Abstract—An innovative synthesis of terphenyls functionalized with electron-withdrawing or -donating substituents is described and
illustrated by carbanion-induced ring transformation of 2H-pyran-2-ones with 2-methoxyacetophenone in excellent yields. The
existing protocols for the synthesis of terphenyls are generally inter- and intramolecular aryl–aryl cross couplings in the presence
of metal complexes. In this letter, we report a potentially useful alternative to conventional metal-catalyzed cross-coupling reactions.
Ó 2005 Elsevier Ltd. All rights reserved.
Symmetrical and unsymmetrical terphenyls functional-
ized with electron donor or acceptor groups are the
main constituents of a large number of mushrooms
belonging to the Thelephoraceae family.1 Recently,
numerous natural products having terphenyl architec-
ture such as thelephorin,2 terphenyllin,3 terferol4 and
terprenin5 have been reported to possess interesting bio-
logical properties. Several synthetic terphenyl deriva-
tives have been designed as selective inhibitors for
dihydroortate dehydrogenase6 and cyclooxygenase7
enzymes. Terphenyls containing acidic groups have
recently been found to be potent insulin sensitizers.8
Recently, Nozaki et al.9 have identified a terphenyl-
based novel auxin signaling inhibitor, terfestatin A,
from Streptomyces sp. F40. Owing to their interesting
optical10 and electrical11 properties, terphenyls find sev-
eral industrial applications as liquid crystals, conducting
polymers, heat storage and heat transfer agents, as
textile dye carriers and as a laser dye. In addition,
meta-terphenyls are ideal precursors in the design
of cyclophanes and are useful as tectons in crystal
engineering.12
tween electrophilic aromatic dihalides Ar(X)2 (X being
generally Br, I and OTf) and two equivalents of organo-
metallic species Ar–M (M being Mg, Ni, Zn, Sn and B)
is a versatile synthetic method for the preparation of
symmetrical and unsymmetrical terphenyls.13 Of the
various coupling reactions, the Pd-catalyzed Suzuki
couplings14 of a diverse array of haloarenes with aryl-
boronic acids has gained wide popularity due to the
commercial availability of several arylboronic acids
and innocuous nature of the latter, easy work-up, and
tolerance of the reactions to aqueous media. Symmetri-
cal terphenyls have been prepared by double coupling
reactions of aryl diboronic acids15 with aryl halides
and aryl distannanes16 with aryl bromides. The classical
approach for unsymmetrical terphenyls requires either
the reactions of biaryl boronic acids with aromatic
halides17 or the stepwise chemoselective cross-couplings
of aryl compounds containing two dissimilar reactive
halides or triflates.18 Recently non-C2-symmetric substi-
tuted triphenylene compounds have been prepared using
arylzinc halides.19 Despite the wide synthetic potential
of these metal-assisted cross-coupling reactions, they
suffer from the requirements for expensive organometal-
lic reagents/catalysts, harsh reaction conditions and
undesired by-products. Thus, there exists a need to de-
velop an expedient route for the synthesis of terphenyls
that does not require specialized reagents or catalysts
with flexibility of introducing the electron donor or
acceptor groups in their molecular makeup.
Transition metal-catalyzed aryl–aryl cross coupling
reactions are commonly employed for constructing biar-
yls. Palladium-catalyzed aryl–aryl cross-coupling be-
Keywords: Terphenyl;C–C Bond formation;Lactone;Pyran-2-one.
q C.D.R.I. Communication No. 6810.
*
Herein, we report a route for the synthesis of function-
Corresponding author. Fax: +91 522 2623405;e-mail: agoel13@
yahoo.com
alized terphenyls through reaction of 2H-pyran-2-ones
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.018

8488
A. Goel et al. / Tetrahedron Letters 46 (2005) 8487–8491
with 2-methoxyacetophenone in high yields without
using an organometallic reagent or a catalyst. The
advantage of the procedure lies in the creation of a Ômid-
dleÕ aryl ring through new carbon–carbon bond forma-
tion with flexibility of substituent variations in the
terphenyl framework.
terized as 20-methoxy-50-methylsulfanyl-[1,10;30,100]ter-
phenyl-40-carboxylic acid methyl esters 3a–d and the
minor polar compounds as 4-methoxy-3,6-diaryl-
pyrano[3,4-c]pyran-1,8-diones 4a–d by spectroscopic
analyses. In each case, compounds 3a–d and 4a–d were
very different in polarity and could be separated very
1
easily. The H NMR spectrum of 3a showed three sing-
Our approach to 3a–d is based on the ring transfor-
mation of 6-aryl-3-methoxycarbonyl-4-methylsulfanyl-
2H-pyran-2-ones 1a–d using 2-methoxyacetophenone 2
as a carbanion source. The 2H-pyran-2-ones 1a–d
precursors were prepared by the reaction of ethyl
2-carbomethoxy-3,3-di(methylsulfanyl)-acrylate20 with
substituted acetophenones under alkaline conditions in
high yields. Lactones, 1a–d have three electrophilic
centres: C2, C4 and C6 in which the latter position is
highly reactive towards nucleophiles due to the extended
conjugation and the presence of the electron-withdraw-
ing substitutent at position 3 of the pyran ring. Thus,
stirring an equimolar mixture of 1a–d, 2-methoxyace-
tophenone and powdered KOH in DMF for 9–12 h at
room temperature yielded two products 3a–d and 4a–d
(Scheme 1). The major nonpolar products were charac-
lets at d 2.50, 3.06 and 3.56 for the SCH3, OCH3 and
COOCH3 protons, respectively. Two multiplets at d
7.37–7.48 and 7.56–7.61 for nine and two protons,
respectively, were attributed to aromatic protons. The
IR spectrum showed a carbonyl peak at 1736 cmꢀ1
due to the presence of an ester group. The molecular
ion peak at 364 in the mass spectrum of 3a confirmed
the structure as 20-methoxy-50-methylsulfanyl-[1,10;30,100]-
terphenyl-40-carboxylic acid methyl ester. In the 1H
NMR spectrum of 4a two singlets at d 3.69 and 7.07
were attributed to OCH3 and aromatic methine protons,
respectively. The absence of peaks for SCH3 and
COOCH3 protons and the presence of two lactone–car-
bonyl peaks at 1703 and 1793 cmꢀ1 in the IR spectrum
confirmed the structure as 4-methoxy-3,6-diphenylpyr-
ano[3,4-c]pyran-1,8-dione.
The pyrano[3,4-c]pyran-1,8-dione ring system has not
been previously explored. Molecular orbital calcula-
tions, correlation of delocalization energies, p-bond
order and p-charge density of different theoretical
pyranopyrandiones have been reported.21 Some interest-
ing photochemical22 and luminescence properties23 have
been reported for compounds with similar molecular
architecture. Various natural and synthetic products
having the basic scaffold of pyranopyrandiones have
demonstrated anticancer24 and antibacterial25 activities.
The plausible reaction mechanisms for the formation
of terphenyls and pyrano[3,4-c]pyran-1,8-diones are
depicted in Scheme 1. The transformation of 6-aryl-
2H-pyran-2-ones 1a–d into terphenyls 3 is possibly
initiated by attack of the carbanion generated from
2-methoxyacetophenone at position C6 of lactone 1,
followed by intramolecular cyclization involving the car-
bonyl functionality of 2 and C3 of the pyranone ring
and elimination of carbon dioxide to yield 3a–d.
Similarly, the formation of pyrano[3,4-c]pyran-1,8-
diones could proceed through the attack of the carban-
ion at the less electrophilic position 4 with elimination of
methyl mercaptan followed by intramolecular cycliza-
tion involving the carbonyl group and the carbo-
methoxy functionality of the 2H-pyran-2-one and
elimination of methanol to yield products 4a–d in smal-
ler quantities.
In order to prepare terphenyls exclusively, we attempted
to reduce the electrophilicity at position 4 of lactone 1
by replacing the methyl sulfanyl group with a secondary
amine. With this consideration, we prepared 6-aryl-2-
oxo-4-piperidin-1-yl-2H-pyran-3-carbonitriles (5a–d) in
high yields by refluxing a solution of lactone 1 with
piperidine in methanol for 6–8 h. The reaction of
lactones 5a–d and 2-methoxyacetophenone under the
same reaction conditions as described in Scheme 1 gave
Scheme 1.

A. Goel et al. / Tetrahedron Letters 46 (2005) 8487–8491
8489
Scheme 2.
20-methoxy-50-piperidin-1-yl-[1,10;30,100]terphenyl-40-car-
bonitriles (6a–d) in high yields (Scheme 2). The terphe-
nyls are the sole products of the reaction and were
easily isolated by column chromatography and charac-
terized by spectroscopic means. The possible reaction
mechanism for the formation of terphenyls 6a–d is
depicted in Scheme 2.
A recent report by Luo and co-workers26 demonstrated
the potential applications of para-terphenyl derivatives
containing cyano groups in organic light emitting diode
(OLED) fabrication. It has been well documented that
the introduction of alkoxy substituents in p-conjugated
materials enhances the solubility of the polymer and
the presence of the cyano group influences photophysi-
cal and electroluminescent properties by lowering the
energy of the LUMO, thus exhibiting a relatively low
threshold voltage and high quantum efficiency in LED
devices.27 The paucity of synthetic methodology for
the preparation of para-terphenyls containing cyano
groups prompted us to exploit our methodology for pre-
paring para-terphenyls.
Scheme 3.
terphenyls through the carbanion-induced ring trans-
formation of 2H-pyran-2-ones, in good yields. The
methodology is very simple, economical and does not
require any specialized organometallic reagents or
catalysts. This methodology is a potentially useful alter-
native to conventional metal-catalyzed cross-coupling
reactions.
The starting material was synthesized by reacting an
equimolar mixture of methyl 2-cyano-3,3-di(methyl-
sulfanyl)acrylate20 7 and 3,4-dimethoxyphenylacetone
8 in dry DMSO to prepare 5-(3,4-dimethoxyphenyl)-6-
methyl-4-methylsulfanyl-2-oxo-2H-pyran-3-carbonitrile
9 in 89% yield (Scheme 3). The 2H-pyran-2-one 9 was
then reacted with 2-methoxyacetophenone in the pres-
ence of a base to afford 60,300,400-trimethoxy-50-methyl-
30-methylsulfanyl-[1,10;40,100]terphenyl-20-carbonitrile 10
in 56% yield. This is a unique methodology for the con-
struction of the para-terphenyl ring system under mild
conditions, without using any organometallic reagents
or catalysts. Data for all of the representative com-
pounds are incorporated in the reference section.28
Acknowledgements
This work was supported by the Council of Scientific
and Industrial Research, New Delhi, India. The authors
thank SAIF, CDRI, Lucknow for providing spectro-
scopic data of the synthesized compounds.
References and notes
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of 6-aryl-3-methoxycarbonyl-4-methylsulfanyl-2H-pyran-
2-one 1 (1 mmol), 2-methoxyacetophenone 2 (1.1 mmol)
and powdered KOH (1.2 mmol) in dry DMF (5 mL) was
stirred at room temperature for 9–12 h. The reaction
mixture was poured into ice water with vigorous stirring
and neutralized with dilute HCl. The solid thus obtained
was filtered and pure compounds were isolated by passing
through a silica gel column using chloroform–hexane (1:2)
as eluent;Compound 3a: white solid;yield 62%;mp 146–
148 °C; 1H NMR (200 MHz, CDCl3) d 2.50 (s, 3H, SCH3),
3.06 (s, 3H, OCH3), 3.56 (s, 3H, COOCH3), 7.37–7.48 (m,
5. (a) Stead, P.;Affleck, K.;Sidebottom, P. J.;Taylor, N. L.;
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9H, ArH), 7.56–7.61 (m, 2H, ArH);IR (KBr) 1736 cm
8. (a) von Geldern, T. W.;Brun, R. P.;Kalmanovich, M.;
Wilcox, D.;Jacobson, P. B. Synlett 2004, 1446–1448;(b)
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(CO);MS (FAB) 364 (M +);Anal. Calcd for C 22H20O3S:
C, 72.5;H, 5.5%. Found: C, 72.2;H, 5.9%. Compound
3b: White solid;yield 58%;mp 106–108 °C; 1H NMR
(200 MHz, CDCl3) d 2.32 (s, 3H, CH3), 2.40 (s, 3H,
SCH3), 2.98 (s, 3H, OCH3), 3.46 (s, 3H, COOCH3), 7.14–
ꢀ1
7.43 (m, 10H, ArH);IR (KBr) 1730 cm
(CO);MS
(FAB) 378 (M+). Compound 3c: White solid;yield 56%;
mp 148–150 °C; 1H NMR (200 MHz, CDCl3) d 2.50 (s,
3H, SCH3), 3.06 (s, 3H, OCH3), 3.55 (s, 3H, COOCH3),
7.34 (s, 1H, ArH), 7.36–7.40 (m, 5H, ArH), 7.47 (d, 2H,
J = 8.4 Hz, ArH), 7.57 (d, 2H, J = 8.4 Hz, ArH);IR
(KBr) 1727 cmꢀ1 (CO);MS (FAB) 444 (M ++2), 442 (M+).
Compound 3d: White solid;yield 60%;mp 154–156 °C; 1H
NMR (200 MHz, CDCl3) d 2.50 (s, 3H, SCH3), 3.06 (s,
3H, OCH3), 3.55 (s, 3H, COOCH3), 7.34–7.43 (m, 8H,
ArH), 7.51–7.56 (m, 2H, ArH);IR (KBr) 1727 cm ꢀ1 (CO);
MS (FAB) 398 (M+). Compound 4a: Yellow solid;yield
18%;mp 260–262 °C; 1H NMR (200 MHz, CDCl3) d 3.69
(s, 3H, OCH3), 7.07 (s, 1H, CH), 7.48–7.60 (m, 6H, ArH),
7.96–8.04 (m, 2H, ArH), 8.00–8.20 (m, 2H, ArH);IR
(KBr) 1703, 1793 cmꢀ1 (CO);MS (FAB) 347 (M ++1);
Anal. Calcd for C21H14O5: C, 72.8;H, 4.0%. Found: C,
72.3;H, 4.0%. Compound 4b: Yellow solid;yield 14%;mp
´
13. Hassan, J.;Se vignon, M.;Gozzi, C.;Schulz, E.;Lemaire,
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15. Todd, M. H.;Balasubramanian, S.;Abell, C. Tetrahedron
Lett. 1997, 38, 6781–6784.
1
219–220 °C; H NMR (200 MHz, CDCl3) d 2.45 (s, 3H,
CH3), 3.69 (s, 3H, OCH3), 7.01 (s, 1H, CH), 7.33 (d, 2H,
J = 8.0 Hz, ArH), 7.36–7.54 (m, 3H, ArH), 7.89 (d, 2H,
J = 8.0 Hz, ArH), 8.10–8.14 (m, 2H, ArH);IR (KBr)
1710, 1779 cmꢀ1 (CO);MS (FAB) 361 (M ++1). Com-
pound 4c: Yellow solid;yield 13%;mp 248–250 °C; 1H
NMR (200 MHz, CDCl3) d 3.69 (s, 3H, OCH3), 7.05 (s,
1H, CH), 7.50–7.56 (m, 3H, ArH), 7.68 (d, 2H, J = 8.6 Hz,
ArH), 7.87 (d, 2H, J = 8.6 Hz, ꢀA1rH), 8.10–8.16 (m, 2H,
ꢀ
16. Corsico, E. F.;Rossi, R. A. Synlett 2000, 230–232.
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Lando, V. R.;Dupont, J.;Monteiro, A. L.
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ArH);IR (KBr) 1718, 1769 cm
(CO);MS (FAB) 427,
19. (a) Borner, R. C.;Jackson, R. F. W. J. Chem. Soc., Chem.
Commun. 1994, 845;(b) Rottla ¨nder, M.;Knochel, P.
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425 (M++1). Compound 4d: Yellow solid;yield 18%;mp
1
258–260 °C; H NMR (200 MHz, CDCl3) d 3.69 (s, 3H,
OCH3), 7.04 (s, 1H, CH), 7.48–7.56 (m, 5H, ArH), 7.95 (d,
2H, J = 8.4 Hz, ArH), 8.11–8.16 (m, 2H, ArH);IR (KBr)
1721, 1770 cmꢀ1 (CO);MS (FAB) 383, 381 (M ++1).
Compound 6a: White solid;yield 74%;mp 150–152 °C; 1H
NMR (200 MHz, CDCl3) d 1.58–1.64 (m, 2H, CH2), 1.73–
1.82 (m, 4H, 2CH2), 3.03 (s, 3H, OCH3), 3.12–3.18 (m,
4H, 2CH2), 6.99 (s, 1H, ArH), 7.38–7.61 (m, 10H, ArH);
IR (KBr) 2217 cmꢀ1 (CN);MS (FAB) 368 (M +);Anal.
Calcd for C25H24N2O: C, 81.4;H, 6.5;N, 7.6%. Found: C,
81.0;H, 6.7;N, 7.5%. Compound 6b: White solid;yield
84%;mp 108–110 °C; 1H NMR (200 MHz, CDCl3) d
1.57–1.63 (m, 2H, CH2), 1.73–1.83 (m, 4H, 2CH2), 2.40 (s,
24. (a) Ikekawa, T.;Uehara, N.;Maeda, Y.;Nakanishi, M.;
Fukuoka, F. Cancer Res. 1969, 29, 734–735;(b) Mizuno,
T. Int. J. Med. Mushrooms 1999, 1, 9–29.

A. Goel et al. / Tetrahedron Letters 46 (2005) 8487–8491
8491
3H, CH3), 3.03 (s, 3H, OCH3), 3.11–3.17 (m, 4H, CH2),
6.98 (s, 1H, ArH), 7.22–7.26 (m, 2H, ArH), 7.39–7.51 (m,
CH2), 1.74–1.85 (m, 4H, 2CH2), 3.02 (s, 3H, OCH3), 3.12–
3.18 (m, 4H, 2CH2), 6.94 ꢀ(s1, 1H, ArH), 7.39–7.56 (m, 9H,
ꢀ1
7H, ArH);IR (KBr) 2216 cm
(CN);MS (FAB) 382
ArH);IR (KBr) 2216 cm
(CN);MS (FAB) 402 (M +).
(M+). Compound 6c: White solid;yield 67%;mp 163–
Compound 10: White solid;yield 56%;mp 133–134 °C; 1H
NMR (200 MHz, CDCl3) d 2.10 (s, 3H, CH3), 2.27 (s, 3H,
SCH3), 3.33 (s, 3H, OCH3), 3.90 (s, 3H, OCH3), 3.96
(s, 3H, OCH3), 6.72 (s, 1H, ArH), 6.74 (d, 1H, J = 8.0 Hz,
ArH), 6.98 (d, 1H, J = 8.0 Hz, ArH), 7.48–7.53 (m,
1
164 °C; H NMR (200 MHz, CDCl3) d 1.57–1.64 (m, 2H,
CH2), 1.73–1.80 (m, 4H, 2CH2), 3.02 (s, 3H, OCH3), 3.12–
3.18 (m, 4H, 2CH2), 6.94 (ꢀs,1 1H, ArH), 7.44–7.60 (m, 9H,
ArH);IR (KBr) 2221 cm
(CN);MS (FAB) 448, 446
(M+). Compound 6d: White solid;yield 80%;mp 164–
5H, ArH);IR (KBr) 2221 cm
ꢀ1
(CN);MS (FAB) 406
1
166 °C; H NMR (200 MHz, CDCl3) d 1.58–1.64 (m, 2H,
(M++1).