Oxadisilole fused triptycene and extended triptycene: Precursors of triptycyne and extended triptycyne

Article abstract of DOI:10.1021/jo1016025

With benzobisoxadisilole 1 as a 1,4-benzdiyne equivalent, oxadisilole fused triptycene 5 and extended triptycene 6 were synthesized. Triptycenes 5 and 6 are new precursors of triptycyne 7 and extended triptycyne 8 respectively via the phenyliodination/fluoride induced elimination protocol. Using these two arynes, a series of triptycene derivatives were synthesized.

Full text of DOI:10.1021/jo1016025

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Oxadisilole Fused Triptycene and Extended Triptycene:
Precursors of Triptycyne and Extended Triptycyne
Bao-Jian Pei, Wing-Hong Chan, and Albert W. M. Lee*
Department of Chemistry and Centre for Advanced Luminescence Materials,
Hong Kong Baptist University, Kowloon, Hong Kong SAR, China
alee@hkbu.edu.hk
Received August 16, 2010
With benzobisoxadisilole 1 as a 1,4-benzdiyne equivalent, oxadisilole fused triptycene 5 and extended
triptycene 6 were synthesized. Triptycenes 5 and 6 are new precursors of triptycyne 7 and extended
triptycyne 8 respectively via the phenyliodination/fluoride induced elimination protocol. Using these
two arynes, a series of triptycene derivatives were synthesized.
Introduction
including intramolecular charge transfer,4 atropisomerism
studies,5 ligands design,6 host-guest chemistry,7 supra-
molecular chemistry,8 molecular gear devices,9 and pharmaceutical
Iptycenes refer to a large family of compounds with unique
[2.2.2] bicyclic bridgehead ring systems built up by a number
of arene units. A prefix is used to further describe the number
of independent arenes.1 Triptycene, 9,10-dihydro-9,10[10,20]-
benzenoanthracene, is the most basic structure of the ipty-
cene family. It is named after “the triptych of antiquity” which
was a book with three leaves hinged on a common axis.2
Because of its unique geometrical, structural and electronic
characters such as rigidity, bulkiness, nonplanarity, and
π-electron richness, triptycene has been used as key building
block of molecules and polymers of both fundamental and
practical importance.3 A wide range of research fields
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Published on Web 10/14/2010
DOI: 10.1021/jo1016025
r
2010 American Chemical Society

Pei et al.
JOCArticle
SCHEME 1. Linear and Angular Benzobisoxadisilole as Synthetic Equivalents of 1,4- and 1,3-Benzdiyne
SCHEME 2. Synthesis of Oxadisilole Fused Triptycene 5 and Extended Triptycene 6
Reagents and conditions: (a) PhI(OAc)2/TfOH, CH2Cl2. 0 °C-rt, 3 h; (b) KF/TBAF, rt; (c) anthracene, 10 h, 82% from 1; (d) furan, 1 h, 88% from 1;
(e) anthracene, H2O, microwave, 135 °C, 40 min, 93%; (f) AcOH/Ac2O, 130 °C, 12 h, 81%.
designs10 have incorporated various triptycene structures
into their studies.
is the most widely used approach.3a In this paper, we would
like to report our efforts in using linear benzobisoxadisilole 1
as a synthon for the preparation of oxadisilole fused tri-
ptycene 5 and extended triptycene 6. Subsequently, these
oxadisilole fused triptycenes serve as new precursors of
triptycyne 7 and extended triptycyne 8 (Scheme 1).
Recently, we demonstrated that linear and angular benzo-
bisoxadisilole 1 and 2 can serve as the precursors for stepwise
generation of 1,4- and 1,3-benzdiyne 3 and 4 (Scheme 1).11
This chemistry has been applied to the synthesis of acenes,12
acenequinones,13and benzo[b]triphenylenes14 with interest-
ing photophysical properties. Isolatable oxadisilole fused
isobenzofuran12 and isoindole15 were also prepared and used
as building blocks for functional materials. Among the known
methods to prepare triptycenes, Diels-Alder cycloaddition
Results and Discussion
Preparation of Oxadisilole Fused Triptycene and Extended
Triptycene. The opening of one oxadisilole ring of 1 to
benzyne 10 via intermediate 9 proceeded smoothly under the
phenyliodination/fluoride induced elimination protocol.11,16
Trapping of oxadisilole fused benzyne 10 with anthracene
should afford one of the target molecules, oxadisilole fused
triptycene 5 (Scheme 2). However, with 4 equiv of anthracene
and tetrabutylammonium fluoride (TBAF, 2 equiv) as the
fluoride source, the desired product 5 could only be isolated
in 52% yield (Table 1, entry 1). Two major side products,
dimer 1111b and amine addition product 12, were obtained
with total yield of 30%. In our previous studies of trapping
benzyne 10 with other dienes, we observed that addition of a
(10) (a) Perchellet, E. M.; Wang, Y.; Lou, L.; Zhao, H.; Battina, S. K.;
Hua, D. H.; Perchellet, J.-P. H. Anticancer Res. 2007, 27, 3259. (b) Hua,
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A. W. M. J. Org. Chem. 2004, 69, 7190.
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A. W. M. J. Org. Chem. 2006, 71, 3512.
(13) Chen, Y. L.; Wong, M. S.; Wong, W. Y.; Lee, A. W. M. Tetrahedron
Lett. 2008, 49, 1257.
(14) Chen, Y. L.; Wong, M. S.; Wong, W. Y.; Lee, A. W. M. Tetrahedron
Lett. 2007, 48, 2421.
(15) Chen, Y. L.; Lee, M. H.; Wong, W. Y.; Lee, A. W. M. Synlett 2006,
2510.
(16) (a) Kitamura, T.; Y amane, M. J. Chem. Soc., Chem. Commun. 1995,
983–984. (b) Kitamura, T.; Yamane, M.; Inoue, K.; Todaka, M.; Fukatsu,
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J. Org. Chem. Vol. 75, No. 21, 2010 7333

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TABLE 1. Effect of Different Fluoride Sources in the Preparation of 5
yields of products
entry
fluoride source
TBAF (2 equiv)
TBAF (2 equiv)
KF (4 equiv)
CsF (4 equiv)
KF (4 equiv)/TBAF (0.3 equiv)
additive
5 (%)
11 (%)
12 (%)
1
2
3
4
5
NH(i-Pr)2 (3 equiv)
52
56
71
73
82
19
22
<5
<5
<5
11
-
-
-
-
-
-
-
-
SCHEME 3. Triptycyne 7 Generation and Trapping Experiments
bulky amine such as diisopropylamine was beneficial to the
reactions.11,17 However, in the present case, omission of the
amine did not affect the yield of 5 but dimer 11 was still the
major side product (entry 2). Since anthracene is not a very
reactive diene as compared to those we used previously, we
believed that the rate of the formation of benzyne 10 should
be slowed down so that the chance of the dimerization could
be reduced. The use of KF as the fluoride source, which is
only slightly soluble in CH2Cl2, solved the problem. The
yield of oxadisilole fused triptycene 5 jumped to 71% with
less than 5% of the dimer 11 (entry 3). Using CsF afforded a
similar result with 73% yield of 5 (entry 4). Finally, we found
that combination of KF and a catalytic amount of TBAF gave
the best result. Oxadisilole fused triptycene 5 could be obtained
in 82% isolated yield in a three-step reaction from 1 (entry 5).
For the extended triptycene series, furan was used to provide
the extra four carbons (Scheme 2).18 Reacting benzyne 10 with
furan afforded oxadisilole fused endoxide 13 in 88% overall yield
(18) (a) Hart, H.; Bashir-Hashemi, A.; Luo, J.; Meador, M. A. Tetra-
hedron 1986, 42, 1641. (b) Luo, J.; Hart, H. J. Org. Chem. 1987, 52, 4833.
(c) Thomas, S. W.; Long, T. M.; Pate, B. D.; Kline, S. R.; Thomas, E. L.;
Swager, T. M. J. Am. Chem. Soc. 2005, 127, 17976. (d) Chen, Z.; Amara, J. P.;
Thomas, S. W.; Swager, T. M. Macromolecules 2006, 39, 3202. (e) Rybacek,
J.; Zavada, J.; Holy, P. Synthesis 2008, 3615.
(17) Winling, A.; Russell, R. A. J. Chem. Soc., Perkin Trans. 1 1998, 3921.
7334 J. Org. Chem. Vol. 75, No. 21, 2010

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from 1.11 Diels-Alder reaction between 13 and anthracene was
first carried out in refluxing p-xylene for 2 days. Cycloadduct 14
was obtained in 55% yield. Finally, the yield of 14 could be
improved to 93% when the Diels-Alder reaction was carried
out in water under microwave irradiation for 40 min.19 To
complete the synthesis of oxadisilole fused extended triptycene
6, cycloadduct 14 was dehydrated. Typical acidic dehydrated
conditions found in the literature (TsOH, HCl and H3PO4)18
gave poor results. In most case, the unexpected retro-Diels-
Alder and desilylated products were obtained. Eventually, we
found that the use of AcOH/Ac2O (2: 1) mixture at reflux for
12 h could afford the target compound 6 in 81% yield.
Triptycyne 7 and Extended Triptycyne 8 from 5 and 6.
Oxadisilole fused triptycene 5 and oxadisilole fused extended
triptycene 6 are set up as the precursors of the corresponding
triptycynes 7 and 8. These triptycynes are versatile building
blocks for a series of triptycene derivatives via Diels-Alder
reactions (Schemes 3 and 5).
Without isolation, 15 was treated with KF and a catalytic
amount of TBAF to generate triptycyne 7. Trapping experi-
ments of 7 with furan, 2-methylfuran, oxadisilole fused iso-
benzofurans (16 and 17),12 cyclopentadiene, anthracene, and
N-tert-butoxycarbonylpyrrole are depicted in Scheme 3. Cyclo-
adducts 18-24 were obtained in moderate to good yields from
triptycyne precursor 5.
With the generation of triptycyne 7 from oxadisilole fused
triptycene 5, we have once again demonstrated that benzo-
bisoxadisilole 1 is a synthetic equivalent of 1,4-benzdiyne 3.
As depicted in Scheme 4, the stepwise generated 1,4-benzdiyne
was first reacted with anthracene and then with another diene
to build up a series of triptycene derivatives 18-24.
We then moved onto the chemistry of extended triptycyne
8. Under the standard protocol of phenyliodination/fluoride
induced elimination, extended triptycyne 8 was generated
from oxadisilole fused extended triptycene 6. Cycloaddition
reactions of 8 with furan, anthracene, and N-tert-butoxy-
carbonylpyrrole to cycloadducts 25-27 are summarized in
Scheme 5.
Phenyliodination of 5 with phenyliodonium diacetate (PhI-
(OAc)2) and trifluoromethanesulfonic acid (TfOH) to inter-
mediate 15 took place readily in CH2Cl2 at room temperature.
In summary, starting from benzobisoxadisilole 1, oxadi-
silole fused triptycene 5 and extended triptycene 6 were
synthesized with good overall yields. Triptycenes 5 and 6
are new precursors of triptycyne 7 and extended triptycyne 8
respectively via the phenyliodination/fluoride induced elim-
ination protocol. Cycloaddition of 7 and 8 with various
dienes afforded a series of triptycene derivatives 18-27. In
the overall transformations, benzobisoxadisilole 1 served as
a synthetic equivalent of 1,4-benzdiyne 3.
SCHEME 4. Benzobisoxadisilole 1 as 1,4-Benzdiyne Synthetic
Equivalent for the Preparation of Triptycene Derivatives 18-24
Experimental Section
Oxadisilole Fused Triptycene 5. Trifluoromethanesulfonic
acid (0.21 mL, 2.4 mmol) was added with a syringe to a stirred
solution of phenyliodonium diacetate (392 mg, 1.2 mmol) in
10 mL of CH2Cl2 at 0 °C under N2. The mixture was stirred for
0.5 h at 0 °C and for 2 h at room temperature. The clear yellow
solution was cooled again to 0 °C followed by dropwise addition
of a solution of benzobisoxadisilole 1 (338 mg, 1.0 mmol) in
4 mL of CH2Cl2. The mixture was warmed to room temperature
and stirred for 0.5 h. Anthracene (632 mg, 4.0 mmol) was added
followed by KF (232 mg, 4.0 mmol) and TBAF (1.0 M in THF,
0.3 mL, 0.3 mmol). After 10 h, water was added and the resulting
mixture was extracted with CH2Cl2. The organic extracts were
dried over anhydrous Na2SO4, filtered, and concentrated under
SCHEME 5. Generation of Extended Triptycyne 8 and Trapping Experiments
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reduced pressure. A mixture of CH2Cl2 (3 mL) and petroleum
ether (20 mL) was added to the residue and then filtered. The
filtrate was concentrated and purified by flash chromatography
on silica gel with 2% EtOAc in petroleum ether as eluent to
afford product 5 as white solid in 82% yield (315 mg). Mp
264-266 °C. 1HNMR (CDCl3, ppm): δ 0.29 (s, 12H), 5.44
(s, 2H), 7.00 (m, 4H), 7.40 (m, 4H), 7.57 (s, 2H). 13CNMR
(CDCl3, ppm): δ 1.1, 54.2, 123.7, 125.2, 125.9, 145.0, 145.3,
146.1; IR (KBr, cm-1): 3039, 2953, 1457, 1251, 1106, 936; MS m/z
385.1 (MHþ). HRMS (MALDI-TOF) for C24H25OSi2 [M þ H]þ
calcd 385.1443, found 385.1452.
Compound 12. As a side product, compound 12 is more polar
than compoud 5. White solid; mp 38-40 °C. 1HNMR (CDCl3,
ppm): δ 0.34 (s, 6H), 0.37(s, 6H), 1.29 (d, J = 6.8 Hz, 12H), 3.90
(m, 2H), 6.93 (dd, J = 2.4, 8.4 Hz, 1H), 6.99 (d, J = 2.4 Hz, 1H),
7.40 (d, J = 8.4 Hz, 1H). 13CNMR (CDCl3, ppm): δ 1.0, 1.5,
21.2, 47.2, 117.9, 118.0, 131.3, 133.1, 148.6, 148.7. IR (KBr, cm-1):
2985, 1252, 1124, 928. HRMS (MALDI-TOF) for C10H29NOSi2
[M þ H]þ calcd 308.1860, found 308.1873.
anthracene (1.6 mmol) or N-tert-butoxycarbonylpyrrole (4.0 mmol))
was added followed by KF (93 mg, 1.6 mmol) and TBAF (1.0 M in
THF, 0.12 mL, 0.12 mmol). The reaction mixture was stirred at
0 °C for 0.5 h then warmed to room temperature and stirred over-
night. Water was added and the resulting mixture was extracted
with CH2Cl2. The organic extracts were dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified by column chromatography on silica gel with
2 to 10% EtOAc in petroleum ether as eluent to afford products
18-24.
Cycloadduct 18. Sixty-four percent yield; white solid; mp
183-185 °C. 1HNMR (CDCl3, ppm): δ 5.34 (s, 2H), 5.60
(s,2H), 6.96 (m, 6H), 7.32 (s, 2H), 7.35 (m, 4H). 13CNMR
(CDCl3, ppm): δ 54.2, 82.2, 116.9, 123.4, 123.4, 125.0, 125.0,
143.1, 143.2, 145.3, 145.4, 146.6. IR (KBr, cm-1): 3014, 2954,
1457, 1279, 1191, 980. HRMS (MALDI-TOF) for C24H17O
[M þ H]þ: calcd 321.1274, found 321.1292.
Cycloadduct 19. Fifty-five percent yield; white solid; mp
1
128-130 °C. HNMR (CDCl3, ppm): δ 1.89 (s, 3H), 5.38 (s,
1H), 5.39 (s, 1H), 5.54 (d, J = 2.0 Hz, 1H), 6.71 (d, J = 5.2 Hz,
1H), 7.00 (m, 5H), 7.27 (s, 1H), 7.31 (s, 1H), 7.38 (m, 4H).
13CNMR (CDCl3, ppm): δ 15.2, 54.2, 54.3, 81.7, 89.3, 115.5,
116.6, 123.4, 123.4, 125.0, 125.0, 142.8, 143.0, 144.5, 145.4,
145.4, 145.5, 145.7, 148.2, 149.0. IR (KBr, cm-1): 3016, 2955,
1457, 1384, 1198, 1108. HRMS (MALDI-TOF) for C25H19O
[M þ H]þ: calcd 335.1430, found 335.1446.
Cycloadduct 14. Anthracene (106 mg, 0.6 mmol), endoxide 13
(137 mg, 0.5 mmol) and H2O (4 mL) were added into a 10 mL
glass vessel which was then sealed with rubber septa and put into
the Microwave Reactor (750W). The highest power (750 W) was
set at the beginning so the targeted temperature can be reached
within 4 min. During the reaction, the temperature (135 °C) and
the pressure (1.0-1.2 MPa) in the vessel were steady. The mix-
ture was stirred for 40 min at 135 °C and then cooled to room
temperature. The resulting mixture was extracted by CH2Cl2
(2 ꢀ 10 mL). The organic extracts were dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified by column chromatography on silica gel
with 25% CH2Cl2 in petroleum ether as eluent to afford product
14 as white solid in 93% yield (210 mg). Mp 205-206 °C.
1HNMR (400 MHz, CDCl3): δ 0.27 (s, 6H), 0.29 (s, 6H), 2.29
(s, 2H), 4.44(s, 2H), 4.95 (s, 2H), 7.00 (m, 2H), 7.16 (m, 2H), 7.22
(m, 2H), 7.31 (m, 4H). 13CNMR (100 MHz, CDCl3): δ 0.9, 1.0,
47.4, 48.9, 81.2, 120.8, 123.5, 123.8, 125.7, 126.0, 141.4, 144.2,
146.6, 148.0. IR (KBr, cm-1): 3062, 2951, 2927, 1464, 1456,
1251, 1101, 926. HRMS (MALDI-TOF) for C28H28O2Si2Na
[M þ Na]þ: calcd 475.1520, found 475.1522.
Cycloadduct 20. Forty-five percent yield; white solid; mp
1
248-250 °C. HNMR (CDCl3, ppm): δ 0.33 (s, 6H), 0.38 (s,
6H), 0.39 (s, 6H), 0.51 (s, 6H), 5.36 (s, 2H), 5.89 (s, 2H), 6.97 (m,
4H), 7.33 (s, 2H), 7.36 (m, 4H). 13CNMR (CDCl3, ppm): δ 1.42,
1.57, 2.24, 2.32, 54.2, 82.5, 116.9, 123.4, 123.5, 123.6, 125.1,
125.2, 139.9, 143.9, 145.2, 145.5, 149.7, 151.3. IR (KBr, cm-1):
2959, 1458, 1254, 942. HRMS (MALDI-TOF) for C36H38O3Si4
[M]þ: calcd 630.1892, found 630.1905.
Cycloadduct 21. Forty-nine percent yield; white solid; mp
225-227 °C. 1HNMR (CDCl3, ppm): δ 0.29 (s, 6H), 0.34
(s, 6H), 5.38 (s, 2H), 5.95 (s, 2H), 6.99 (m, 4H), 7.37 (m, 4H),
7.42 (s, 2H), 7.44 (s, 2H). 13CNMR (CDCl3, ppm): δ 0.8, 1.0,
54.2, 82.3, 116.9, 122.1, 123.4, 123.5, 125.1, 125.1, 143.8, 145.1,
145.2, 145.4, 146.6, 149.2. IR (KBr, cm-1): 2954, 1458, 1252,
1096, 935. HRMS (MALDI-TOF) for C32H29O2Si2 [M þ H]þ:
calcd 501.1700, found 501.1712.
Cycloadduct 22. Thirty-four percent yield; white solid; mp
>350 °C. 1HNMR (CDCl3, ppm): δ 5.33 (s, 4H), 6.93 (m, 8H),
7.31 (m, 8H), 7.46 (s, 2H). 13CNMR (CDCl3, ppm): δ 53.9,
119.7, 123.4, 125.0, 142.4, 145.3. HRMS for C34H22 [M]þ: calcd
430.1716, found 430.1732.
Oxadisilole Fused Extended Triptycene 6. Compound 14
(181 mg, 0.4 mmol) was dissolved in a mixture of AcOH (4.0
mL) and Ac2O (2.0 mL). The reaction mixture was stirred at
130 °C for 12 h and then poured into 10 mL ice-water. The
mixture was extracted by CH2Cl2 (3 ꢀ 10 mL) and the organics
were dried over anhydrous Na2SO4. After being filtered and
concentrated, the residue was purified by column chromatog-
raphy on silica gel with 20% CH2Cl2 in petroleum ether as eluent
to afford product 6 as white solid in 81% yield (140 mg). Mp
296-297 °C. 1HNMR (400 MHz, CDCl3): δ 0.38 (s, 12H), 5.51
(s, 2H), 7.02 (m, 4H), 7.42 (m, 4H), 7.78 (s, 2H), 7.92 (s, 2H).
13CNMR (100 MHz, CDCl3): δ 1.2, 53.7, 121.9, 123.7, 125.5,
130.7, 132.1, 142.7, 143.5, 144.4. IR (KBr, cm-1): 3065, 2960,
1456, 1251, 1096, 931. HRMS (MALDI-TOF) for C28H26OSi2
[M]þ: calcd. 434.1517, found 434.1496.
Cycloadducts 18-24 from Triptycyne 7 Generated from Oxadisi-
lole Fused Triptycene 5. Trifluoromethanesulfonic acid (0.088 mL,
1.0 mmol) was added with a syringe to a stirred solution of phenyl-
iodonium diacetate (164 mg, 0.5 mmol) in 6 mL of CH2Cl2 at 0 °C
under N2. The mixture was stirred for 0.5 h at 0 °C and for 2 h at
room temperature. The clear yellow solution was cooled again to
0 °C followed by dropwise addition of a solution of 5 (135 mg,
0.4 mmol in 3 mL of CH2Cl2). The mixture was warmed to room
temperature and stirred for 1 h. After cooled to 0 °C, the diene
(furan (4.0 mmol), 2-methylfuran (4.0 mmol), isobenzofuran 16
(1.0 mmol), isobenzofuran 17 (1.0 mmol), cyclopentadiene (4.0 mmol),
Cycloadduct 23. Forty-six percent yield; white solid; mp
1
220-222 °C. HNMR (CDCl3, ppm): δ 2.15 (d, J = 6.8 Hz,
1H), 2.25 (d, J = 6.8 Hz, 1H), 3.75 (s, 2H), 5.31 (s, 2H), 6.68
(s, 2H), 7.00 (m, 4H), 7.31 (s, 2H), 7.34 (m, 4H). 13CNMR
(CDCl3, ppm): δ 50.1, 54.3, 70.6, 118.0, 123.3, 123.3, 124.8,
124.8, 141.8, 143.1, 145.9, 146.0, 149.0. IR (KBr, cm-1): 3001,
2947, 1638, 1458, 1300, 1192, 935. HRMS (MALDI-TOF) for
C25H18 [M]þ: calcd 318.1403, found 318.1401.
Cycloadduct 24. Fifty-five percent yield; white solid; mp
1
135-137 °C. HNMR (CDCl3, ppm): δ 1.35 (s, 9H), 5.35 (s,
2H), 5.41 (m, 2H), 6.87 (br, 1H), 6.97 (m, 5H), 7.35 (m, 6H).
13CNMR (CDCl3, ppm): δ 28.1, 54.1, 65.9, 66,7, 80.5, 117.1,
117.6, 123.4, 124.9, 125.0, 142.5, 143.0, 144.0, 145.3, 145.4,
154.6. IR (KBr, cm-1): 2973, 1705, 1458, 1367, 1324, 1252,
1164. HRMS (MALDI-TOF) for C29H25NO2Na [M þ Na]þ:
calcd 442.1777, found 442.1784.
Cycloadducts 25-27 from Extended Triptycyne 8 Generated
from Oxadisilole Fused Extended Triptycene 6. The extended
triptycyne 8 was generated from 6 under the similar procedure
for triptycyne 5 generation. Triptycyne 8 was reacted with furan,
(19) Pei, B. J.; Lee, A. W. M. Tetrahedron Lett. 2010, 51, 4519.
7336 J. Org. Chem. Vol. 75, No. 21, 2010

Pei et al.
JOCArticle
anthracene and N-tert-butoxycarbonylpyrrole to afford the
cycloadducts 25-27.
Cycloadduct 25. Sixty-one percent yield; light yellow solid;
mp 191-193 °C. HNMR (CDCl3, ppm): δ 5.52 (s, 2H), 5.76
(s, 2H), 6.90 (s, 2H), 7.03 (m, 4H), 7.41(m, 4H), 7.47 (s, 2H), 7.68
(s, 2H). 13CNMR (CDCl3, ppm): δ 53.7, 81.8, 118.4, 122.2,
123.6, 123.7, 125.3, 125.4, 129.9, 141.7, 142.7, 144.3, 144.6,
144.8. HRMS (MALDI-TOF) for C28H18O [M]þ: calcd
370.1352, found 370.1352.
Cycloadduct 26. Twenty-seven percent yield; white solid;
mp >350 °C. 1HNMR (400 MHz, CDCl3): δ 5.44 (s, 4H),
6.96 (m, 8H), 7.35 (m, 8H), 7.62 (s, 4H). 13CNMR (100 MHz,
CDCl3): δ 53.7, 121.4, 123.6, 125.3, 129.7, 142.1, 144.6. HRMS
for C38H24 [M]þ: calcd 480.1872, found 480.1870.
5.50 (s, 2H), 5.54 (m, 2H), 6.85 (br, 1H), 7.02 (m, 5H), 7.42
(m, 6H), 7.66 (s, 2H). 13CNMR (100 MHz, CDCl3): δ 28.1, 53.7,
65.4, 66.1, 80.6, 122.1, 123.6, 123.7, 123.8, 125.3, 125.4, 125.7,
142.5, 143.9, 144.6, 144.7, 155.1. IR (KBr, cm-1): 2955, 1705,
1458, 1367, 1337, 1252, 1161. HRMS (MALDI-TOF) for
C33H27NO2Na [M þ Na]þ: calcd 492.1934, found 492.1928.
1
Acknowledgment. Financial support from the Hong Kong
Research Grants Council (HKBU 201307) is gratefully
acknowledged.
Supporting Information Available: General Methods sec-
tion and 1H and 13C NMR spectra of compounds 5, 6, 12, 14,
and 18-27. This material is available free of charge via the
Internet at http://pubs.acs.org.
Cycloadduct 27. Forty-five percent yield; light yellow solid;
1
mp 180-182 °C. HNMR (400 MHz, CDCl3): δ 1.34 (s, 9H),
J. Org. Chem. Vol. 75, No. 21, 2010 7337