5,6-Bis(trimethylsilyl)benzo[c]furan: An isolable versatile building block for linear polycyclic aromatic compounds

Article abstract of DOI:10.1016/S0040-4020(02)01219-X

Benzo[c]furans are a class of interesting and highly reactive compounds that readily undergo Diels-Alder cycloaddition with dienophiles to restore their aromaticity. Initially, the s-tetrazine approach established by Warrener was chosen for the synthesis of the title molecule. However, it was discovered that the rate of production of isobenzofuran from this approach was too slow to react with the fugitive arynes. Consequently, an alternative route was employed to realize the title molecule in a neat state. In this way, the reactions between the title molecule and arynes were successfully achieved. Herein, two synthetic approaches towards the title molecule and its further manipulation for the preparation of silylated linear polycyclic aromatic hydrocarbons (PAH) were reported.

Full text of DOI:10.1016/S0040-4020(02)01219-X

Tetrahedron 58 (2002) 9413–9422
5,6-Bis(trimethylsilyl)benzo[c]furan: an isolable versatile building
block for linear polycyclic aromatic compoundsq
*
Siu-Hin Chan, Chung-Yan Yick and Henry N. C. Wong
Department of Chemistry and Central Laboratory of the Institute of Molecular Technology for Drug Discovery and Synthesis,†
The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, People’s Republic of China
Dedicated to Professor Yao-Zeng Huang on the occasion of his 90th birthday
Received 10 June 2002; revised 3 September 2002; accepted 26 September 2002
Abstract—Benzo[c]furans are a class of interesting and highly reactive compounds that readily undergo Diels–Alder cycloaddition with
dienophiles to restore their aromaticity. Initially, the s-tetrazine approach established by Warrener was chosen for the synthesis of the title
molecule. However, it was discovered that the rate of production of isobenzofuran from this approach was too slow to react with the fugitive
arynes. Consequently, an alternative route was employed to realize the title molecule in a neat state. In this way, the reactions between the
title molecule and arynes were successfully achieved. Herein, two synthetic approaches towards the title molecule and its further
manipulation for the preparation of silylated linear polycyclic aromatic hydrocarbons (PAH) were reported. q 2002 Elsevier Science Ltd. All
rights reserved.
1. Introduction
groups have been shown to be very useful precursors in
many organic transformations.6,7 Furthermore, it is likely
that the silyl group can also increase the solubility of
polycyclic aromatic compounds obtained from Diels–Alder
reactions. It is therefore believed that the combined use of
the trimethylsilyl groups and the deoxygenation protocol
depicted in Scheme 1 can lead to the realization of silylated
linear polycylcic aromatic skeletons.
Benzo[c]furan (1), also known as isobenzofuran (IBF) was
postulated2 and detected3 as a reactive dienophile that
readily undergoes Diels–Alder reaction with alkynes and
other dienophiles to form the corresponding endoxide
adducts. It was isolated4 in the early seventies as a
crystalline solid, stable only at low temperature but readily
decomposed at room temperature. Noteworthy is that
isobenzofurans, with very few exceptions,5 are too unstable
to be isolated in the conventional sense. As part of our
continuing program concerning the use of silylated furans in
the regiospecific synthesis of polysubstituted furans,6,7 we
sought to extend the chemistry to include a silylated
benzo[c]furan, namely 5,6-bis(trimethylsilyl)benzo[c]furan
(2), which is expected to react with various dienophiles to
provide Diels–Alder adducts.
2. Result and discussion
5,6-Bis(trimethylsilyl)benzo[c]furan (2) was chosen as our
target molecule. For the generation of 2, we chose the
s-tetracene approach4,13 established by Warrener as the key
step. The starting material, 1,4-endoxy-1,4-dihydro-6,7-bis-
(trimethylsilyl)naphthalene (5), was synthesized through
three steps from commercially available 1,2,4,5-tetrachloro-
benzene14,15 as illustrated in Scheme 2. Thus, the reactions
of tetrachlorobenzene with magnesium and trimethylsilyl
chloride, gave 1,2,4,5-tetrakis(trimethylsilyl)benzene (3).
This was then transformed into the iodonium triflate 4 by
addition of triflic acid and iodobenzenediactetate.16 Upon
treatment of 4 with TBAF, a silylated benzyne was
presumably generated and was trapped by furan to afford 5.
The syntheses of several benzo[c]furans containing tri-
methylsilyl groups on the furan ring were reported by
Rickborn.8–11 However, in these endeavors, all silyl groups
in the isobenzofurans were used only as protecting groups
rather than functional groups. Due to the b-effect of
silicon,12 aromatic rings substituted with trimethylsilyl
q See Ref. 1.
The generation of 5,6-bis(trimethylsilyl)benzo[c]furan (2)
was successfully accomplished by treating 5 with 3,6-
di(pyridin-20-yl)-s-tetrazine (6) in a CHCl3 solution. In the
presence of various dienophiles, benzo[c]furan (2) produced
the expected Diels–Alder adducts 7–10 in yields from good
Keywords: isobenzofuran; Diels–Alder reaction; polycyclic aromatic
hydrocarbon.
*
Corresponding author. Tel.: þ852-2609-6329; fax: þ852-2603-5057;
e-mail: hncwong@cuhk.edu.hk
An Area of Excellence of the University Grants Committee (Hong Kong).
†
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01219-X

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S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
Scheme 1.
Scheme 2. Reagents and conditions: (i) Mg, Me3SiCl, HMPT, THF, reflux, 48%; (ii) Phl(OAc)2, CF3SO3H, i-Pr2NH, CH2Cl2, 61%; (iii) furan, n-Bu4NF, THF,
61%.
to excellent, as depicted in Scheme 3. Nitrile 10 could in
principle be dehydrated by treatment with a strong base like
bis(trimethylsilyl)amide, affording in fair yields the corre-
sponding naphthalenenitrile, a method commonly employed
in the preparation of naphthalocyanines.17 In our hands,
however, dehydration of compound 10 was smoothly
accomplished in an acceptable yield by reaction with a
mixture of lithium iodide and 1,8-diazabicyclo[5,4,0]undec-
1-ene in refluxing THF, leading to the naphthalenenitrile 11.
The most notable feature of our program is the manipulation
of trimethylsilyl substituents to realize linear polycyclic
aromatic hydrocarbon skeletons. In order to elongate the
benzo framework, 11 was allowed to react with iodobenze-
nediacetate in triflic acid, affording the iodonium triflate 18.
The iodonium salt 18 was then allowed to undergo an
elimination reaction with TBAF to presumably generate a
benzyne intermediate,16 which provided endoxide 19 upon
trapping with 3,4-bis(trimethylsilyl)furan.6 The final deoxy-
genating step was accomplished by employing TiCl4–
LiAlH4–Et3N in THF, furnishing anthracene 20 in a good
yield (Scheme 4).19
Benzo[c]furan 2 also reacted readily with quinones and the
results are summarized in Table 1. For naphthoquinone and
anthraoquinone,18 only the endo adducts 13 and 14 were
1
observed. This was confirmed by H NMR spectroscopy
A potential application of 11 is its transformation into
naphthalocyanine metal complexes. Due to their strong
absorption property in the near-IR region, naphthalo-
cyanines have been studied extensively in the field of
materials science.20 However, their strong tendency to
aggregate has given rise to severe solubility problems. It is
thus anticipated that by introducing appropriate bulky and
which showed two sets of doublet of doublets for the two
groups of aliphatic protons in the ring systems. While for
benzoquinone an exo–endo adduct 12 was obtained.
Adducts 12–14 were subjected to dehydration in refluxing
90% acetic acid, leading to aromatic compounds 15–17 as
bright yellow crystals.
Scheme 3. Reagents and conditions: (i) CHCl3; (ii) DMAD, 82%; (iii) dimethyl fumarate, 86%; (iv) N-phenyl-malemide, 76%; (v) fumaronitrile, 98%; (vi) Lil,
DBU, THF, reflux, 88%.

S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
Table 1. Diels–Alder adducts of 2 and their corresponding dehydration products
Entry Dienophile Diels–Alder adduct
9415
Yield(%)
Dehydration Product
Yield(%)
1
54
38
75
75
2
58
3
72
Scheme 4. Reagents and conditions: (i) Phl(OAc)2, CF3SO3H, CH2Cl2, 50%; (ii) 3,4-bis(trimethylsilyl) furan, n-Bu4NF, CH2Cl2, 62%; TiCl4, LiAlH4, Et3N,
THF, 78%.
lipophilic groups at the periphery of the ring, the molecule
would no longer aggregate as well as show better solubility
in organic solvents. Moreover, the silyl substituents can be
easily transformed into other functional groups. Thus, with a
sufficient amount of 11 in hand, the realization of a
naphthalocyaninozinc complex was accomplished in an
acceptable yield as a deep green powder (Scheme 5). The
UV–Vis absorption spectroscopy was studied, which
showed a typical absorption band pattern of naphthalo-
cyanines (Fig. 1). Furthermore, the molar absorption was
found to be concentration independent over the range of
4.2£1026–1.1£1024 in THF, a phenomenon that could be
ascribed to the existence of monomeric forms.
As mentioned before, the most challenging goal of the
project is the utilization of 5,6-bis(trimethylsilyl)benzo[c]-
furan (2) for the preparation of acenes. Our original idea, as
depicted in Scheme 1, required the reaction between two
reactive intermediates, namely isobenzofuran and an aryne.
As a preliminary study, we had attempted to carry out the
Scheme 5. Reagents and conditions: DBU, Zn(OAc)2, C6H13OH, reflux, 40%.

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S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
Figure 1. UV–Vis spectra of Nc sample.
reaction as usual employing isobenzofuran 2, generated as
before, and an aryne generated from phenyl[2-(trimethyl-
silyl)-1-phenyl]iodonium triflate16 and a solution of TBAF.
However, despite tremendous efforts, we failed to isolate
any cycloadduct from the aforementioned reaction mixture.
We believed that this was due to the slow generation of 2
from 5, and the unlikely reaction between two reactive
species under this condition. In view of this, we switched to
an alternative method for the generation of a pure solution
of isobenzofuran 2. 4,5-Bis(trimethylsilyl)-o-xylene (24)21
was the presursor of this new program. Starting from
o-xylene (22), bromination afforded dibromide 2321 which
was then transformed into 24 under a standard Grignard
condition. The whole process can be carried out in gram

S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
9417
Scheme 6. Reagents and conditions: (i) l2, Br2, 08C, 80%; (ii) Mg, Me3SiCl, HMPA, THF, 50%; (iii) NBS, AIBN, CCl4, reflux; (iv) CaCO3, Dioxane–H2O,
reflux; (v) p-TsOH, EtOH, 38% ((iii)–(v) 3-steps); (vi) LDA, THF, 08C.
scale without difficulties. Compound 24 was then bromi-
nated with 3 equiv. of NBS to afford the corresponding
tribromide. Hydrolysis with calcium carbonate furnished
the hemiacetal which was then transformed into 25
immediately in absolute ethanol with p-toluenesulfonic
acid as a catalyst (Scheme 6). Compound 25 was smoothly
converted to isolable 2 by reaction with lithium diiso-
propylamide (LDA) in THF.22
Before we proceeded to attempt the reaction between
isobenzofuran 2 and an aryne, it was necessary to assess the
1
stability of 2 by H NMR spectroscopic studies. Lithium
Figure 2. 1H NMR spectrum of 2 in CDCl3 recorded at room temperature at different times.

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S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
Scheme 7. Reagents and conditions: (i) Phenyl[2-(trimethylsilyl)-1-phenyl]idodonium triflate, TBAF, CH2Cl2, 80%; (ii) Phenyl[3-(trimethylsilyl)-2-
naphthyl]iodonium triflate, TBAF, CH2Cl2, 75%; (iii) TiCl4, LiAIH4, Et3N, THF, 88%.
diisopropylamide in THF was added to a solution of 25 in
THF at 08C under a nitrogen atmosphere (Scheme 6). The
generation of isobenzofuran 2 was monitored by TLC and
the reaction was eventually quenched by 1N HCl to remove
all the ammonium salts. The aqueous layer was removed
and the organic layer was washed with cold brine. After that
the organic solution was concentrated and then diluted with
deuterated chloroform to provide a neat solution of 2. The
1H NMR spectrum of 2 in CDCl3 was first recorded at room
temperature. From Fig. 1, absorptions at d 7.70 and 7.88
represented the benzo protons and the furan protons,
respectively. The spectrum clearly showed that the absorp-
tions at the aromatic region were still visible after 37 h, and
they only faded out after 56 h. It could therefore be
concluded that a CDCl3 solution of isobenzofuran 2 should
be stable at room temperature for up to at least 37 h (Fig. 2).
3. Experimental
3.1. General information
All reagents and solvents were reagent grade. Further
purification and drying by standard methods were employed
when necessary. All organic solvents were evaporated under
reduced pressure with a rotary evaporator. The plates used
for thin-layer chromatography (TLC) were E. Merck silica
gel 60F254 (0.25 mm thickness) precoated on aluminum
plates, and they were visualized under both long (365 nm)
and short (254 nm) UV light. Compounds on TLC plates
were visualized with a spry of 5% dodecamolybdophos-
phoric acid in ethanol and with subsequent heating. Column
chromatography was performed using E. Merck silica gel
(230–400 mesh).
After assessing the stability of 2, we then carried out the
reaction between 2 and an aryne. When 2 was allowed to
react with benzyne generated from (phenyl)[2-(trimethyl-
silyl)-1-phenyl]iodonium triflate16 and TBAF, cycloadduct
26 was obtained in a good yield. Further deoxygenation
under standard conditions was also accomplished to afford
anthracene 27. Furthermore, when 2 was allowed to react
with naphthalyne generated from (phenyl)[3-(trimethyl-
silyl)-2-naphthyl] iodonium triflate23 and TBAF, cyclo-
adduct 28 was obtained in excellent yield. Tetracene 29
was synthesized through a standard reduction of 28
(Scheme 7).
Melting points were measured on a Reichert Microscope
apparatus and were uncorrected. NMR spectra were
recorded on a Bruker DPX-300 spectrometer (300.13 MHz
1
for H and 75.47 MHz for 13C). All NMR measurements
were carried out at 300 K in deuterated chloroform solution
unless otherwise stated. Chemical shifts are reported as
parts per million (ppm) in d unit in the scale relative to the
resonance of CDCl3 (7.26 ppm in the 1H, 77.00 ppm for the
central line of the triplet in the 13C modes, respectively).
Coupling constants (J) are reported in Hz. Splitting patterns
are described by using the following abbreviations: s,
1
singlet; d, doublet; t, triplet; q, quartet; m, multiplet. H
NMR data is reported in this order: chemical shift;
multiplicity; coupling constant(s), number of proton. Mass
spectra (ERMS and HRMS) were obtained with a Thermo-
finnigan MAT95XL spectrometer and determined at an
ionized voltage of 70 eV unless otherwise stated. Relevant
data were tabulated as m/z. Elemental analyses were
performed at Shanghai Institute of Organic Chemistry, the
Chinese Academy of Sciences, China.
Besides reactions between 2 and the arynes, the cyclo-
additions between isobenzofuran 2 in a neat state and
dienophiles can also be accomplished to afford the
corresponding adducts 7 (75%), 8 (78%), 9 (70%) 10
(80%), 12 (54%), 13 (58%) and 14 (72%) with acceptable
yields compared with the yields obtained from the
s-tetrazine approach mentioned before.
In conclusion, we have successfully synthesized and trapped
5,6-bis(trimethylsilyl)benzo[c]furan (2) by two approaches.
With the s-tetrazine approach, we have synthesized
cycloadducts such as simple substituted endoxide, silylated
quinones to naphthalocyanine. On the other hand, a neat
solution of 5,6-bis(trimethylsilyl)benzo[c]furan (2) was also
obtained in a neat state by employing the acetal approach.
This method can allow the reaction between isobenzofuran
2 and elusive arynes to proceed. The latter approach will be
applied to the preparation of higher acene members.
3.1.1. 1,4-Epoxy-6,7-bis(trimethylsilyl)-1,4-dihydro-
naphthalene (5). A suspension of PhI(OAc)2 (1.1 g,
3.4 mmol) in CH2Cl2 (10 mL) was cooled at 08C and
treated dropwise with TfOH (0.6 mL, 7.0 mmol). After 1 h
at rt, the reaction was cooled to 08C again. Compound 3
(830 mg, 3.4 mmol) and diisopropylamine (1 mL,
7.0 mmol) in CH2Cl2 (10 mL) were added slowly. After
10 min, the solution was allowed to warm to rt. Furan
(0.8 mL, 12 mmol), followed by TBAF (1 M in THF, 5 mL,
5 mmol) were added. After a further 10 min, the mixture

S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
9419
was extracted with CH2Cl2 (3£10 mL). The combined
organic extracts were dried over MgSO4. Following
evaporation of solvent under reduced pressure, the residue
was chromatographed on silica gel (200 g, hexanes–ethyl
acetate 30:1) to afford 5 (600 mg, 2.07 mmol, 61%) as a
(167 mg, 0.49 mmol, 98%) as colorless crystals: mp 197–
1988C; 1H NMR (CDCl3) d 0.37 (s, 9H), 0.38 (s, 9H), 2.83
(d, J¼4.5 Hz, 1H), 3.54 (dd, J¼4.5, 4.5 Hz, 1H), 5.74 (d,
J¼4.5 Hz, 1H), 5.75 (s, 1H), 7.68 (s, 1H), 7.77 (s, 1H); 13C
NMR (CDCl3) d 1.9, 36.1, 37.1, 80.6, 83.0, 116.1, 118.3,
125.7, 127.9, 138.9, 140.6, 148.2, 148.3; MS m/z 340 (Mþ).
Anal. calcd for C18H24N2OSi2: C, 63.48; H, 7.10; N, 38.23.
Found: C, 63.59; H, 7.22; N, 8.20.
1
colorless solid: mp 90–918C; H NMR (CDCl3) d 0.37 (s,
18H), 5.73 (s, 2H), 7.03 (s, 2H), 7.61 (s, 2H); 13C NMR
(CDCl3) d 2.1, 82.3, 126.6, 142.8, 143.4, 148.3; MS m/z 288
(Mþ). Anal. Calcd for C16H24OSi2: C, 66.60; H, 8.38.
Found: C, 66.16; H, 8.31.
3.2.5. exo/endo-[5,14],[7,12]-Diendoxy-5,5a,6a,7,12,
12a,13a,14-octahydro-2,3,9,10-tetrakis(trimethylsilyl)-
pentacene-6,13-dione (12). This was prepared from 5
(170 mg, 0.5 mmol), p-benzoquinone (32 mg, 0.3 mmol)
and 6 (170 mg, 0.7 mmol) in chloroform (10 mL) in the
same manner as described above, yielding 12 (102 mg,
0.16 mmol, 54%) as colorless crystals: mp 110–1118C; 1H
NMR (CDCl3) d 0.20 (s, 18H), 0.31 (s, 18H), 1.75 (s, 2H),
3.76 (dd, J¼3.0, 2.1 Hz, 2H), 5.61 (s, 2H), 5.68 (dd, J¼3.0,
2.1 Hz, 2H), 7.51 (s, 2H), 7.55 (s, 2H); 13C NMR (CDCl3) d
2.0, 53.2, 55.0, 82.0, 83.7, 125.3, 128.0, 142.2, 142.8, 146.2,
206.4; MS m/z 262 (C14H22OSiþ). Anal. calcd for
C34H48O4Si4: C, 64.50; H, 7.64. Found: C, 64.90; H, 7.46.
3.2. General procedure for the preparation of Diels–
Alder adducts 7–10, 12–14 by s-tetrazine approach
3.2.1. 2,3-Dicarbomethoxy-1,4-epoxy-1,4-dihydro-6,7-
bis(trimethylsilyl)naphthalene (7). To a rapidly stirring
solution of 5 (150 mg, 0.5 mmol) and dimethyl acetylene-
dicarboxylate (0.07 mL, 0.6 mmol) in chloroform (5 mL)
was added 6 (140 mg, 0.06 mmol) in portions over 30 min.
The mixture was allowed to stir for a further 2 h. After
evaporation of solvent under reduced pressure, the residue
was subjected to chromatography on silica gel (20 g,
hexanes–ethyl acetate 10:1) to afford
7 (165 mg,
1
0.41 mmol, 82%) as a colorless solid: mp 99–1008C; H
NMR (CDCl3) d 0.36 (s, 18H), 3.81 (s, 6H), 5.96 (s, 2H),
7.74 (s, 2H); 13C NMR (CDCl3) d 2.0, 52.3, 84.8, 127.6,
144.9, 154.4, 150.9, 162.8; MS m/z 404 (Mþ). Anal. calcd
for C20H28O5Si2: C, 59.37; H, 6.98. Found: C, 59.18; H,
7.25.
3.2.6. endo-6,11-Endoxy-5a,6,11,11a-tetrahydro-8,9-
bis(trimethylsilyl)tetracene-5,12- dione (13). This was
prepared from 5 (150 mg, 0.5 mmol), 1,4-naphthoquinone
(110 mg, 0.7 mmol) and 6 (140 mg, 0.6 mmol) in chloro-
form (10 mL) in the same manner as described above,
yielding 13 (120 mg, 0.29 mmol, 58%) as colorless crystals:
mp 128–1298C; 1H NMR (CDCl3) d 0.14 (s, 18H), 3.78 (dd,
J¼3.6, 1.8 Hz, 2H), 5.84 (dd, J¼3.6, 1.8 Hz, 2H), 7.26 (s,
2H), 7.42 (dd, J¼4.2, 3.3 Hz, 2H), 7.62 (dd, J¼5.7, 3.3 Hz,
2H); 13C NMR (CDCl3) d 1.8, 50.1, 83.3, 126.2, 127.1,
133.7, 134.0, 140.5, 146.1, 194.3; MS m/z 262
(C14H22OSiþ). Anal. calcd for C24H28O3Si2: C, 68.53; H,
6.71. Found: C, 68.16; H, 6.79.
3.2.2. trans-2,3-Dicarbomethoxy-1,4-epoxy-1,2,3,4,-
tetrahydro-6,7-bis(trimethylsilyl) naphthalene (8). This
was prepared from 5 (150 mg, 0.5 mmol), dimethyl
fumarate (46 mg, 0.6 mmol) and 6 (140 mg, 0.6 mmol) in
chloroform (5 mL) in the same manner as described above,
yielding 8 (175 mg, 0.43 mmol, 86%) as a colorless solid:
1
mp 103–1048C; H NMR (CDCl3) d 0.32 (s, 9H), 0.35 (s,
9H), 3.05 (d, J¼4.2 Hz, 1H), 3.55 (s, 3H), 3.81 (s, 3H), 3.92
(dd, J¼5.4 Hz, 1H), 5.61 (d, J¼5.4 Hz, 1H), 5.67 (s, 1H),
7.52 (s, 1H), 7.63 (s, 1H); 13C NMR (CDCl3) d 2.0, 2.1,
49.1, 49.4, 51.9, 52.6, 80.3, 82.9, 125.6, 126.9, 141.6, 143.3,
145.7, 146.0, 170.2, 172.4; MS m/z 406 (Mþ). Anal. calcd
for C20H30O5Si2: C, 59.08; H, 7.44. Found: C, 58.99; H,
7.52.
3.2.7. endo-5,14-Endoxy-5,5a,13a,14-tetrahydro-2,3-
bis(trimethylsilyl)pentacene-6,13-dione (14). This was
prepared from 5 (150 mg, 0.5 mmol), 1,4-anthroquinone
(145 mg, 0.7 mmol) and 6 (140 mg, 0.6 mmol) in chloro-
form (10 mL) in the same manner as described above,
yielding 14 (169 mg, 0.36 mmol, 72%) as colorless crystals:
1
mp 195–1968C; H NMR (CDCl3) d 20.07 (s, 18H), 3.87
(dd, J¼3.6, 2.1 Hz, 2H), 5.87 (dd, J¼3.6, 2.1 Hz, 2H), 7.24
(s, 2H), 7.57 (dd, J¼6.3, 3.3 Hz, 2H), 7.84 (dd, J¼6.3,
3.3 Hz, 2H), 8.12 (s, 2H); 13C NMR (CDCl3) d 1.6, 50.7,
83.6, 127.2, 128.2, 129.2, 129.7, 130.3, 134.6, 140.7, 145.8,
194.5; MS m/z 262 (C14H22OSiþ). HRMS (FAB) Calcd for
C28H31O3Si2 (MHþ): 471.1806. Found: 471.1816.
3.2.3. 3a,4,9,9a-Tetrahydro-2-phenyl-6,7-bis(trimethyl-
silyl)-1H-benz[f]isoindole-1,3-(2H)-dione (9). This was
prepared from 5 (150 mg, 0.5 mmol), N-phenylmaleimide
(100 mg, 0.6 mmol) and 6 (140 mg, 0.6 mmol) in chloro-
form (5 mL) in the same manner as described above,
yielding 9 (165 mg, 0.38 mmol, 76%) as a colorless solid:
1
mp 2848C (decomp.); H NMR (CDCl3) d 0.39 (s, 18H),
3.2.8. 2,3-Dicyano-6,7-bis(trinethylsilyl)naphthalene
(11). A mixture of 10 (100 mg, 0.3 mmol), lithium iodide
(53 mg, 0.5 mmol) and DBU (0.1 mL, 5 mmol) in
anhydrous THF (3 mL) was heated at reflux for 3 h under
a nitrogen atmosphere. The mixture was poured into brine
(5 mL) and extracted with ether (3£5 mL). The combined
organic extracts were dried over MgSO4. After evaporation
of solvent under reduced pressure, the residue was
chromatographed on silica gel (20 g, hexanes–ethyl acetate
30:1) to afford 11 (85 mg, 0.26 mmol, 88%) as a colorless
3.15 (s, 2H), 6.00 (s, 2H), 7.33 (d, J¼7.2 Hz, 2H), 7.42–
7.52 (m, 3H), 7.71 (s, 2H); 13C NMR (CDCl3) d 2.0, 49.3,
82.0, 126.2, 126.6, 128.9, 129.2, 131.7, 142.8, 146.8, 175.5;
MS m/z 435 (Mþ). Anal. calcd for C24H29NO3Si2: C, 66.17;
H, 6.71; N, 3.21. Found: C, 65.91; H, 6.62; N, 3.13.
3.2.4. trans-2,3-Dicyano-1,4-epoxy-6,7-1,2,3,4,-tetra-
hydro-bis(trimethylsilyl) naphthalene (10). This was
prepared from
5 (150 mg, 0.5 mmol), fumaronitrile
1
(46 mg, 0.6 mmol) and 6 (140 mg, 0.6 mmol) in chloroform
(5 mL) in the same manner as described above, yielding 10
solid: 209–2108C; H NMR (CDCl3) d 0.47 (s, 18H), 8.22
(s, 2H), 8.32 (s, 2H); 13C NMR (CDCl3) d 1.7, 110.2, 115.9,

9420
S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
131.7, 135.2, 135.7, 149.8; MS m/z 323 (MHþ). HRMS
(FAB) Calcd for C18H23N2Si2 (MHþ): 323.1394. Found:
323.1403.
afford 19 (185 mg, 0.48 mmol, 60%) as a colorless solid: mp
1908C (decomp.); 1H NMR (CDCl3) d 0.20 (s, 18H), 6.00 (s,
2H), 7.61 (s, 2H), 8.19 (s, 2H); 13C NMR (CDCl3) d 20.1.
86.9, 110.3, 116.0, 117.9, 132.8, 135.1, 150.3, 162.9; MS
m/z 387 (M21þ). HRMS (FAB) calcd for C22H25N2OSi2
(MHþ): 389.1500. Found: 389.1502.
3.3. General procedure for the preparation of quinones
15–17
3.3.1. 2,3,9,10-Tetrakis(trimethylsilyl)pentacene-6,13-
dione (15). A suspension of 12 (60 mg, 0.09 mmol) in
90% acetic acid (3 mL) was heated at 1008C for 3 h under a
nitrogen atmosphere. After being cooled to rt, most of the
solvent was evaporated under vacuum. The residue was then
subjected to chromatography on silica gel (20 g, hexanes–
ethyl acetate 30:1) to afford 15 (21 mg, 0.034 mmol, 38%)
as a bright yellow solid: mp 210–2118C; 1H NMR (CDCl3)
d 0.49 (s, 36H), 8.40 (s, 4H), 8.90 (s, 4H); 13C NMR
(CDCl3) d 1.8, 129.5,131.1, 133.8, 137.1, 147.4, 182.8; MS
m/z 597 (MHþ). HRMS (FAB) Calcd for C34H45O2Si4
(MHþ): 597.2497. Found: 597.2467.
3.3.5. 2,3-Dicyano-7,8-bis(trimethylsilyl)anthrancene
(20). THF (0.3 mL) was added with stirring to titanium
(IV) chloride (0.13 mL, 1.2 mmol) at 08C. A solution of
LiAlH4 in THF (1 M, 0.5 mL) followed by Et3N (0.01 mL,
0.1 mmol) in THF (0.1 mL) was carefully introduced into
the above suspension. The mixture was refluxed at 658C for
0.5 h. After cooling to rt, 19 (15 mg, 0.04 mmol) was added.
The mixture was stirred at rt for 12 h and then poured into
20% aqueous NaHCO3 (5 mL). The resulting mixture was
extracted with CH2Cl2 (5£5 mL). The combined organic
solvent was dried over MgSO4 and concentrated under
reduced pressure. Chromatography on silica gel (5 g,
hexanes–ethyl acetate 15:1) afforded 20 (11 mg,
0.03 mmol, 78%) as a yellow solid: mp 2508C (decomp.);
1H NMR (CDCl3) d 0.49 (s, 18H), 8.38 (s, 2H), 8.52 (s, 2H),
8.53 (s, 2H); 13C NMR (CDCl3) d 1.5, 108.2, 116.2, 128.3,
129.9, 132.7, 135.9, 137.6, 145.9; MS m/z 372 (Mþ). HRMS
(ESI) Calcd for C22H24N2Si2Na: 395.1370. Found:
395.1366.
3.3.2. 8,9-Bis(trimethylsilyl)tetracene-5,12-dione (16).
This was prepared from 13 (42 mg, 0.1 mmol) in the same
manner as described above, yielding 16 (30 mg, 0.08 mmol,
1
75%) as a bright yellow solid: mp 172–1738C; H NMR
(CDCl3) d 0.48 (s, 18H), 7.82 (dd, J¼6.3, 3.3 Hz, 2H), 8.37
(s, 2H), 8.39 (dd, J¼6.3, 3.3 Hz, 2H), 8.80 (s, 2H); 13C
NMR (CDCl3) d 1.8, 127.5, 129.4, 130.2, 133.8, 134.1,
134.5, 137.2, 147.6, 183.0; MS m/z 402 (Mþ). Anal. Calcd
for C24H26O2Si2: C, 71.59; H, 6.51. Found: C, 71.11; H,
6.29.
3.3.6. [3,4,12,13,21,22,30,31-Octakis(trimethylsilyl)-2,3-
naphthalocyaninato]zinc (21). To a solution of 11
(200 mg, 0.6 mmol) in hexanol (6 mL) heated at 908C was
added Zn(OAc)2·2H2O (66 mg, 0.3 mmol) followed by
DBU (0.01 mL, 0.06 mmol). The mixture was then stirred at
908C under nitrogn for 3 h. After cooling to rt, the mixture
was poured into methanol–acetone (v/v 1:1, 30 mL). The
precipitate was collected by suction filtration and sub-
sequently washed with methanol (10 mL) and then dried in
vacuo. The green solid obtained was further purified with a
Soxhlet extractor using methanol–acetone (v/v 1:1, 60 mL).
After complete removal of the brown colored impurities, the
resulting green powder left was collected by extracting with
THF (50 mL). Evaporation of solvent under reduced
pressure provided 21 (80 mg, 0.24 mmol, 40%) as a green
3.3.3. 2,3-Bis(trimethylsilyl)pentacene-6,13-dione (17).
This was prepared from 14 (47 mg, 0.1 mmol) in the same
manner as described above, yielding 17 (34 mg, 0.08 mmol,
1
75%) as a bright yellow solid: mp 305–3068C; H NMR
(CDCl3) d 0.49 (s, 18H), 7.70 (dd, J¼6.3, 3.3 Hz, 2H), 8.11
(dd, J¼6.3, 3.3 Hz, 2H), 8.39 (s, 2H), 8.88 (s, 2H), 8.93 (s,
2H); 13C NMR (CDCl3) d 1.8, 129.4, 129.6, 129.8, 130.1,
130.6, 131.1, 133.9, 135.2, 137.2, 147.5, 183.0; MS m/z 452
(Mþ). HRMS (FAB) calcd for C28H29O2Si2 (MHþ):
453.1700. Found: 453.1706.
1
3.3.4. 7,8-Dicyano-1,4-epoxy-2,3-bis(trimethylsilyl)-1,4-
dihydroanthracene (19). TfOH acid (0.14 mL, 1.6 mmol)
was added dropwise to a suspension of iodobenzene
diacetate (250 mg, 0.8 mmol) in CH2Cl2 (2 mL) at 08C.
The resulting clear yellow solution was stirred at rt for 1 h. It
was then cooled again to 08C and a solution of 11 (250 mg,
0.8 mmol) in CH2Cl2 was added. After stirring for a further
1 h, the solvent was evaporated under reduced pressure and
the crude residue was triturated with Et2O (10 mL). The
iodonium salt 18 was collected by filtration as a white
powder and was used without further purification.
powder: H NMR (THF-d8) d 0.66 (s, 72H), 8.85 (s, 8H),
9.54 (br. s, 8H); UV–Vis [THF, 4.2£1026 M, lmax nm
(log e)]: 767 (5.90), 7.31 (5.10), 684 (5.12), 337 (5.36); MS
(LSI): an isotope cluster peaking at m/z 1353.46 (calcd for
C72H88N8Si8 (Mþ) 1353.45).
3.3.7. Ethoxy-5,6-bis(trimethylsilyl)-1,3-dihydroiso-
benzofuran (25). A mixture of 24 (250 mg, 1 mmol),
NBS (550 mg, 3.1 mmol) and AIBN (2 mg, 0.01 mmol) in
CCl4 (5 mL) was refluxed for 3 h. After cooling to rt, the
white solid was filtered off and the filtrate was washed with
sat. NaHCO3. After drying over MgSO4 and concentrated
under reduced pressure, the colorless oil obtained was
taken up with dioxane–water (v/v 3:1 10 mL). CaCO3
(900 mg, 9.0 mmol) was added to the solution and the
mixture was refluxed for 24 h. The mixture was allowed to
cool to rt and the CaCO3 was removed by filtration. Dioxane
was evaporated under vacuum. The residue left was
extracted with Et2O (4£5 mL). The combined organic
extract was dried over MgSO4 and concentrating under
reduced pressure. Chromatography on silica gel (20 g,
The white powder 18 obtained above was suspended in
CH2Cl2 (3 mL) followed by addition of 3,4-bis(trimethyl-
silyl)furan 7 (200 mg, 0.9 mmol). TBAF in THF (1 M,
0.8 mL) was added slowly to the above mixture at 08C.
After stirring for 0.5 h, the mixture was diluted with CH2Cl2
(5 mL) and wash with water (3 mL), and brine (3 mL)
successively. After drying over MgSO4 and concentrating
under reduced pressure, the crude product was chromato-
graphed on silica gel (20 g, hexanes–ethyl acetate 8:1) to

S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
9421
hexanes–ethyl acetate 5:1) provided a light yellow oil with
Rf of 0.2 which was dissolved in absolute ethanol (2 mL).
p-TsOH (2 mg, 0.01 mmol) was added and the solution was
stirred for 6 h. After addition of NaHCO3, the excess base
was filtered off and the filtrate was concentrated. Chroma-
tography on silica gel with 2% of triethylamine (10 g,
hexanes–ethylacetate 10:1) provided 25 (115 mg,
THF, 0.36 mL) was added dropwise under nitrogen. After
stirring for 10 min, the reaction was quenched with 1N HCl
(0.1 mL, 0.55 mmol). Then phenyl [2-(trimethylsilyl)-1-
naphthyl]iodonium triflate (100 mg, 0.18 mmol) was added.
To this suspension was then added TBAF (1 M in THF,
0.18 mL) dropwise. After stirring for a further 0.5 h, the
mixture was poured into water. The organic layer was
collected and the aqueous residue was extracted with Et2O
(3£5 mL). The combined organic extracts were dried over
MgSO4 and concentrated under reduced pressure. Chroma-
tography on silica gel (20 g, hexanes–ethyl acetate 30:1)
afforded 28 (56 mg, 0.29 mmol, 80%) as a colorless oil: 1H
NMR (CDCl3) d 0.35 (s, 18H), 6.20 (s, 2H), 7.43 (dd, J¼6.0,
3.3 Hz, 2H), 7.71–7.76 (m, 6H); 13C NMR (CDCl3) d 2.2,
82.4, 118.9, 126.2, 126.7, 128.2, 132.4, 143.9, 145.1, 146.6;
MS m/z 389 (MHþ). HRMS (FAB) Calcd for C24H29OSi2
(MHþ): 389.1751. Found: 389.1740.
1
0.38 mmol, 38%) as a colorless oil; H NMR (actone-d6)
d 0.39 (s, 18H), 1.17 (t, J¼4.8 Hz, 3H), 3.59–3.75 (m, 2H),
4.97 (d, J¼13.2 Hz, 1H), 5.11 (dd, J¼13.2, 1.8 Hz, 1H),
6.20 (d, J¼1.8 Hz, 1H), 7.71 (s, 2H); 13C NMR (acetone-d6)
d 2.2, 15.7, 63.2, 72.4, 107.4, 128.8, 130.3, 138.8, 140.9,
145.5, 147.5; MS m/z 306 (Mþ). HRMS (ESI) Calcd for
C16H28O2Si2Na: 331.1521. Found: 331.1519.
3.3.8. 5,10-Epoxy-5,10-dihydro-2,3-bis(trimethylsilyl)-
anthracene (26). A solution of 25 (56 mg, 0.18 mmol) in
anhydrous THF (3 mL) was cooled at 08C. LDA
(0.18 mmol, 1 M in THF) was added dropwise under
nitrogen. After stirring for 10 min, the reaction was
quenched with 1N HCl (0.1 mL, 0.55 mmol). Then phenyl
[2-(trimethylsilyl)-1-phenyl]iodonium triflate (90 mg,
0.2 mmol) was added. To this suspension was added
TBAF (1 M in THF, 0.2 mL) dropwise. After stirring for a
further 0.5 h, the mixture was poured into water. The
organic layer was collected and the aqueous residue was
extracted with Et2O (3£5 mL). The combined organic
extract was dried over MgSO4 and concentrated under
reduced pressure. Chromatography on silica gel (20 g,
hexanes–ethyl acetate 30:1) afforded 26 (48 mg,
3.3.11. 2,3-Bis(trimethylsilyl)naphthacene (29). THF
(0.7 mL) was introduced to stirred TiCl4 (0.18 mL,
1.67 mmol) at 08C under nitrogen. A suspension of
LiAlH4 (1 M in THF, 0.65 mL) was added carefully to the
above suspension. Et3N (0.04 mL, 0.38 mmol) in THF
(0.7 mL) was added. The mixture was stirred and refluxed at
658C for 0.5 h. It was allowed to cool to rt. Compound 28
(100 mg, 0.26 mmol) in THF (0.7 mL) was added dropwise.
The mixture was refluxed under nitrogen for 24 h. After
cooling to rt, it was then poured into 20% NaHCO3 solution
(5 mL) and filtered. The filtration cake was washed with
ether thoroughly. The filtrate was collected and the organic
layer was separated. The aqueous residue was extracted
with Et2O (3£5 mL). After drying over MgSO4 and
concentrated under reduced pressure, 29 was obtained
(85 mg, 0.23 mmol, 90%) with high purity as a colorless
1
0.14 mmol, 80%) as a colorless oil: H NMR (CDCl3) d
0.36 (s, 18H), 6.07 (s, 2H), 7.04 (dd, J¼5.1, 3.0 Hz, 2H),
7.37 (dd, J¼5.1, 3.0 Hz, 2H), 7.67 (s, 2H); 13C NMR
(CDCl3) d 2.1, 82.6, 120.5, 125.9, 126.6, 144.5, 147.3,
148.0; MS m/z 338 (Mþ). HRMS (FAB) Calcd for
C20H26OSi2: 338.1517. Found: 338.1531.
1
solid: H NMR (CDCl3) d 0.48 (s, 18H), 7.42 (dd, J¼6.9,
3.3 Hz, 2H), 8.05 (dd, J¼6.6, 3.3 Hz, 2H), 8.37 (s, 2H), 8.72
(s, 2H), 8.76 (s, 2H); 13C NMR (CDCl3) d 2.0, 125.2, 126.2,
126.5, 128.3, 130.4, 130.8, 131.5, 136.5, 140.6; MS m/z 372
(Mþ). HRMS (EI) Calcd for C24H28Si2: 372.1724. Found:
372.1719.
3.3.9. 2,3-Bis(trimethylsilyl)anthracene (27).24 THF
(0.2 mL) was introduced to stirred titanium (IV) Chloride
(0.08 mL, 0.7 mmol) at 08C under nirogen. A suspension of
LiAlH4 (1 M in THF, 0.25 mL) was added carefully to the
above suspension. Et3N (10 mg, 0.1 mmol) in THF (0.2 mL)
was added. The mixture was stirred and refluxed at 658C for
0.5 h. It was allowed to cooled to rt. Compound 26 (35 mg,
0.1 mmol) in THF (0.2 mL) was added dropwise. The
mixture was refluxed under nitrogen for 24 h. After cooling
to rt, it was then poured into 20% NaHCO3 solution (5 mL)
and filtered. The filtration cake was washed with ether
thoroughly. The filtrate was collected and the organic layer
was separated. The aqueous residue was extracted with Et2O
(3£5 mL). The combined organic extracts were dried over
MgSO4 and concentrated under reduced pressure. Chroma-
tography on silica gel (15 g, hexanes) afforded 27 (28 mg,
3.4. General procedure for the preparation of Diels–
Alder adducts 7–10, 12–14 from isobenzofuran 2
3.4.1. 2,3-Dicarbomethoxy-1,4-epoxy-1,4-dihydro-6,7-
bis(trimethylsilyl)naphthalene (7). A solution of 25
(150 mg, 0.5 mmol) in anhydrous THF (5 mL) was cooled
at 08C. LDA (1 M in THF, 0.5 mL) was added dropwise
under nitrogen. After stirring for 10 min at rt, the reaction
was quenched with 1N HCl (0.1 mL, 0.55 mmol). Then
dimethyl acetylenedicarboxylate (0.07 mL, 0.6 mmol) was
added. The solution was then allowed to stir for 2 h. After
evaporation of solvent under reduced pressure, the residue
was subjected to chromatography on silica gel (20 g,
1
0.88 mmol, 88%) as a colorless solid: H NMR (CDCl3) d
20.8 (s, 18H), 6.83 (s, 2H), 7.38 (dd, J¼5.1, 3.0 Hz, 2H),
7.77 (dd, J¼5.1, 3.0 Hz, 2H), 7.84 (s, 2H); 13C NMR
(CDCl3) d 2.1, 125.4, 126.1, 128.3, 130.6, 132.3, 136.2,
140.9; MS m/z 322 (Mþ). Anal. calcd for C20H26Si2: C,
74.46; H, 8.12. Found: C, 74.41; H, 8.15.
hexanes–ethyl acetate 10:1) to afford 7 (151 mg,
0.38 mmol, 75%) as a colorless solid: the spectroscopic
data were identical to an authentic sample prepared
previously.
3.4.2. trans-2,3-Dicarbomethoxy-1,4-epoxy-1,2,3,4,-
tetrahydro-6,7-bis(trimethylsilyl) naphthalene (8). This
was prepared from 25 (150 mg, 0.5 mmol), dimethyl
fumarate (46 mg, 0.6 mmol) and LDA (1 M in THF,
3.3.10. 5,12-Epoxy-5,12-Dihydro-2,3-bis(trimethylsilyl)-
naphthacene (28). A solution of 25 (110 mg, 0.36 mmol) in
anhydrous THF (5 mL) was cooled at 08C. LDA (1 M in

9422
S.-H. Chan et al. / Tetrahedron 58 (2002) 9413–9422
0.5 mL) in THF (5 mL) in the same manner as described
above, yielding 8 (158 mg, 0.39 mmol, 78%) as a colorless
solid: the spectroscopic data were identical to an authentic
sample prepared previously.
Special Administrative Region, China (project CUHK
4014/98P).
References
3.4.3. 3a,4,9,9a-Tetrahydro-2-phenyl-6,7-bis(trimethyl-
silyl)-1H-benz[f]isoindole-1,3-(2H)-dione (9). This was
prepared from 25 (150 mg, 0.5 mmol), N-phenylmaleimide
(100 mg, 0.6 mmol) and LDA (1 M in THF, 0.5 mL) in THF
(5 mL) in the same manner as described above, yielding 9
(152 mg, 0.35 mmol, 70%) as a colorless solid: the
spectroscopic data were identical to an authentic sample
prepared previously.
1. A preliminary communications has appeared: Yick, C. Y.;
Chan, S. H.; Wong, H. N. C Tetrahedron Lett. 2000, 41, 5957.
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hydro-bis(trimethylsilyl) naphthalene (10). This was
prepared from 25 (150 mg, 0.5 mmol), fumaronitrile
(46 mg, 0.6 mmol) and LDA (1 M in THF, 0.5 mL) in
THF (5 mL) in the same manner as described above,
yielding 10 (136 mg, 0.4 mmol, 80%) as a colorless solid:
the spectroscopic data were identical to an authentic sample
prepared previously.
6. Song, Z. Z.; Ho, M. S.; Wong, H. N. C. J. Org. Chem. 1994,
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12a,13a,14-octahydro-2,3,9,10-tetrakis(trimethylsilyl)-
pentacene-6,13-dione (12). This was prepared from 25
(170 mg, 0.5 mmol), p-benzoquinone (32 mg, 0.3 mmol)
and LDA (1 M in THF, 0.5 mL) in THF (5 mL) in the same
manner as described above, yielding 12 (102 mg,
0.16 mmol, 54%) as a colorless solid: the spectroscopic
data were identical to an authentic sample prepared
previously.
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3.4.6. endo-6,11-Endoxy-5a,6,11,11a-tetrahydro-8,9-
bis(trimethylsilyl)tetracene-5,12-dione (13). This was
prepared from 25 (150 mg, 0.5 mmol), 1,4-naphthoquinone
(110 mg, 0.7 mmol) and LDA (1 M in THF, 0.5 mL) in THF
(5 mL) in the same manner as described above, yielding 13
(120 mg, 0.29 mmol, 58%) as a colorless solid: the
spectroscopic data were identical to an authentic sample
prepared previously.
15. Winling, A.; Russell, R. A. J. Chem. Soc., Perkin Trans. 1
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3.4.7. endo-5,14-Endoxy-5,5a,13a,14-tetrahydro-2,3-
bis(trimethylsilyl)pentacene-6,13-dione (14). This was
prepared from 25 (150 mg, 0.5 mmol), 1,4-anthraoquinone
(145 mg, 0.7 mmol) and LDA (1 M in THF, 0.5 mL) in THF
(5 mL) in the same manner as described above, yielding 14
(169 mg, 0.36 mmol, 72%) as a colorless solid: the
spectroscopic data were identical to an authentic sample
prepared previously.
19. Xing, Y. D.; Huang, N. Z. J. Org. Chem. 1982, 47, 140.
20. Kaplan, M. L.; Lovinger, W. D.; Reents, Jr., W. D.; Schmidt,
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Acknowledgements
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63, 8579.
The work described in this project is fully supported by a
grant from the Research Grants Council of the Hong Kong
24. Wu, Y. M.; Wong, H. N. C. Unpublished results.