Novel meso-Polyarylamine-BODIPY hybrids: Synthesis and study of their optical properties

Article abstract of DOI:10.1021/jo802519b

A series of 14 meso-polyarylamine-BODIPY (borondipyrromethene) hybrids of the general structure A were synthesized. Two methods were used to prepare them. The first protocol involved a direct Liebeskind-Srogl cross-coupling of thiomethylbodipys 1-2 with a

Full text of DOI:10.1021/jo802519b

Novel meso-Polyarylamine-BODIPY Hybrids: Synthesis and Study
of Their Optical Properties
Erik Lager,† Jianzhao Liu,§ Ange´lica Aguilar-Aguilar,† Ben Zhong Tang,*,§ and
Eduardo Pen˜a-Cabrera*,†
Departamento de Qu´ımica, UniVersidad de Guanajuato, Col. Noria Alta S/N, Guanajuato,
Gto. 36050 Mexico, and Department of Chemistry, The Hong Kong UniVersity of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China
eduardop@quijote.ugto.mx; tangbenz@ust.hk
ReceiVed NoVember 11, 2008
A series of 14 meso-polyarylamine-BODIPY (borondipyrromethene) hybrids of the general structure A
were synthesized. Two methods were used to prepare them. The first protocol involved a direct
Liebeskind-Srogl cross-coupling of thiomethylbodipys 1-2 with arylaminoboronic acids (4 examples,
75-98%). The second method involves a two-step sequence: a Liebeskind-Srogl reaction to prepare 6
meso-bromoarylbodipys (58-83%) followed by a Suzuki coupling of these Br-containing BODIPYs with
arylaminoboronic acids (10 examples, 44-84%). Seven of these derivatives displayed emission in the
near-infrared region. The optical properties of compound 18 were rationalized in terms of its crystal
structure.
Introduction
fluorescent switches, chemosensors, and laser dyes.6 For the
reasons mentioned above, hybrids that contain both polyary-
lamine and BODIPY moieties might display useful properties
such as Twisted Intramolecular Charge Transfer (TICT) be-
havior.7 This phenomenon is of particular importance in the
design of chemosensors since its understanding would allow
control of both the degree of electron transfer and the change
of molecular geometry.8 Prompted by our recent report on the
Owing to the many applications and interesting properties,
push-pull (also termed D-π-A systems, D ) donor, A )
acceptor) molecular systems have attracted significant attention.1
One of the more interesting properties that these compounds
may display is the emission in the near-IR (NIR) region, which
may offer potential application in medicine.2 In particular,
polyarylamine derivatives are attractive electron-donor moieties.
These compounds are standard hole-transport materials and
studies on magnetic materials have been done based on the
generation of high-spin triarylamine polyradicals.3 Similarly,
there are reports in the literature of polyarylamines used in the
fabrication of red OLEDs.4 With respect to the electron-
accepting fragment, boradiazaindacene (BODIPY)5 derivatives
were chosen in this work due to their well-documented
properties and applications which include labeling reagents,
(3) (a) Bellman, E.; Shaeen, S. E.; Thayurnanavan, S.; Barlow, S.; Grubbs,
R. H.; Marder, S. R.; Kippelen, B.; Peygharnbarian, N. Chem. Mater. 1998, 10,
1668. (b) Ito, A.; Ota, K.; Tanaka, K.; Yamabe, T.; Yoshizawa, K. Macromol-
ecules 1995, 28, 5618. (c) Wienk, M. M.; Janssen, R. A. J. Am. Chem. Soc.
1997, 119, 4492. (d) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994,
116, 11576. (e) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem.
1997, 62, 448. (f) Hauck, S.; Lakshmi, K. V.; Hartwig, J. F. Org. Lett. 1999, 1,
2057. (g) Promarak, V.; Ichikawa, M.; Maunmart, D.; Sudyoadsuk, T.;
Saengsuwan, S.; Keawin, T. Tetrahedron Lett. 2006, 47, 8949. (h) Murata, H.;
Lahti, P. M. J. Org. Chem. 2007, 72, 4974.
(4) Chen, C.-T. Chem. Mater. 2004, 16, 4389. and references cited therein.
(5) Haugland, R. P. Handbook of Fluorescent Probes and Research Chemi-
cals, 6th ed.; Molecular Probes: Eugene, OR, 1996.
(6) For excellent reviews of the synthesis and optical properties of BODIPY
dyes see: (a) Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891. (b) Ulrich,
G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184. (c) Ziessel,
R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31, 496.
† Universidad de Guanajuato.
§ The Hong Kong University of Science and Technology.
(1) Tong, H.; Hong, Y.; Dong, Y.; Ren, Y.; Hau¨ssler, M.; Lam, J. W. Y.;
Wong, K. S.; Tang, B. Z. J. Phys. Chem. B. 2007, 111, 2000. and references
cited therein.
(7) Grabowski, Z. R.; Rotkiewics, K.; Rettig, W. Chem. ReV. 2003, 103,
3899.
(2) Kiyose, K.; Kojima, H.; Nagano, T. Chem. Asian J. 2008, 3, 506.
10.1021/jo802519b CCC: $40.75
Published on Web 02/09/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 2053–2058 2053

Lager et al.
SCHEME 1. General Synthesis of 8-Substituted BODIPYs
with the Liebeskind-Srogl Cross-Coupling
SCHEME 2. Synthesis of Hybrids 3-6 via a Direct L-S
Reaction
synthesis of meso-substituted BODIPY dyes9 based on the
Liebeskind-Srogl (L-S) cross-coupling, we decided to prepare
a series of novel polyarylamine-BODIPY hybrids of the general
structure A and study their properties.
is important to emphasize that the commercial form of m-
aminophenylboronic acid used to prepare 5 is the monohydrate.
This result further demonstrated the high tolerance of the L-S
cross-coupling to the reaction conditions since 5 was produced
in 75% eVen in the presence of 1 equiV of water.
For the L-S/Suzuki approach, 1-2 were reacted with the
corresponding bromoarylboronic acids in order to prepare the
bromoarylbodipys 7-12 (Scheme 3).
Herein we report our results.
Results and Discussion
Building blocks 7-12 were uneventfully synthesized in
58-83% isolated yield providing a series of brominated
derivatives with an excellent handle for additional elaboration.
The series 7-10 will allow structure-properties studies with
systematic variations of not only the electronic features of the
different analogues, but also the steric aspects of them. It is
easily envisioned as well the possibility to prepare dendrimeric
materials starting from dibromoarylbodipy 11 via the rich
plethora of transition metal-catalyzed reactions. Similarly,
bromothienylbodipy 12 will serve to prepare other thiophene-
containing derivatives interesting for material science.12 Two
of such systems have been recently published.13
Once 7-12 were prepared, they were further functionalized
with amino-containing boronic acids under the Suzuki conditions
(Scheme 4).
Except for hybrids 13 (44% yield) and 16 (47% yield) which
were prepared in modest yields, the Suzuki coupling worked
efficiently for the rest of the derivatives yielding at the end a
family of 10 additional polyarylamine-BODIPY hybrids in
69-84% isolated yields.
Synthesis of the Polyarylamine-BODIPY Hybrids. The key
step relies on the cross-coupling reaction of thioorganics with
organoboron derivatives mediated by Cu(I) and catalyzed by
Pd(0) under neutral conditions (the Liebeskind-Srogl reaction)
(eq 1).10
R1-SR′ + R2-B(OH)2CuI) OCOR 8 R1-R2 + Cu-SR′ +
cat . Pd
RC(O)O-B(OH)2 (1)
As we have recently demonstrated, this protocol can be used
in a very efficient manner to prepare 8-subtituted BODIPYs
starting from 8-thiomethylbodipys 1-2 (Scheme 1).11
Accordingly, we envisioned that this process could be used
to prepare a family of novel polyarylamine-BODIPY hybrids
in two ways, either directly using the L-S cross-coupling, or
by a sequential L-S/Suzuki process. Thus, hybrids 3-6 were
prepared via a direct L-S cross-coupling according to
Scheme 2.
In this fashion, amino-containing commercially available
boronic acids were reacted with 1-2 following the literature
procedure to yield hybrids 3-6 in 75-98% isolated yields. It
Optical Properties of the Hybrids. Boradiazaindacene
(BODIPY) derivatives have found an array of applications in
molecular biology, as chemical sensors, and as molecular
devices (such as fluorescent switches) owing to their unique
optical properties. These include sharp absorption and fluores-
cence bands, high extinction coefficients, efficient light emission,
(8) Liu, Y.; Han, M.; Zhang, H.-Y.; Yang, L.-X.; Jiang, W. Org. Lett. 2008,
10, 2873.
(9) Pen˜a-Cabrera, E.; Aguilar-Aguilar, A.; Gonza´lez-Dom´ınguez, M.; Lager,
E.; Zamudio-Va´zquez, R.; Godoy-Vargas, J.; Villanueva-Garc´ıa, F. Org. Lett.
2007, 9, 3985.
(10) (a) Yang, H.; Hao, L.; Wittenberg, R.; Egi, M.; Huang, W.; Liebeskind,
L. S. J. Am. Chem. Soc. 2007, 129 (5), 1132. and references cited therein. (b)
Wittenberg, R.; Srogl, J.; Egi, M.; Liebeskind, L. S. Org. Lett. 2003, 5 (17),
3033. (c) Liebeskind, L. S.; Srogl, J. J. Am. Chem. Soc. 2000, 122 (45), 11260.
For a recent example of Pd-free, Cu(I)-mediated cross-coupling reaction see:
Villalobos, J. M.; Srogl, J.; Liebeskind, L. S. J. Am. Chem. Soc. 2007, 129,
15734.
(12) (a) Cremer, J.; Briehn, C. Chem. Mater. 2007, 19, 4155. (b) Kono, T.;
Kumaki, D.; Nishida, J.; Sakanoue, T.; Kakita, M.; Tada, H.; Tokito, S.;
Yamashita, Y. Chem. Mater. 2007, 19, 1218. (c) Ong, B. S.; Wu, Y.; Liu, P.;
Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378.
(13) (a) Bennston, A. C.; Copley, G.; Harriman, A.; Rewinska, D. B.;
Harrington, R. W.; Clegg, W. J. Am. Chem. Soc. 2008, 130, 7174. (b) Zrig, S.;
Re´my, P.; Andrioletti, B.; Rose, E.; Asselberghs, I.; Clays, K. J. Org. Chem.
2008, 73, 1563.
(11) 1 and 2 were prepared according to the following: Goud, T. V.; Tutar,
A.; Biellmann, J.-F. Tetrahedron 2006, 62, 5084.
2054 J. Org. Chem. Vol. 74, No. 5, 2009

NoVel Meso-Polyarylamine-BODIPY Hybrids
SCHEME 3. Synthesis of the Bromoarylbodipys 7-12
SCHEME 4. Synthesis of Hybrids 13-22
state, the molecule is almost planar, which corresponds to the
maximum conjugation between the dimethylamino group and
the phenyl ring. According to the Franck-Condon principle,
the molecule in the locally excited state (LE) is still planar, but
solvent relaxation takes place with a concomitant rotation of
the dimethylamino group until it is twisted at right angles and
the conjugation is lost. In the resulting TICT state, which is
stabilized by the polar solvent molecules, there is a total charge
separation between the dimethylamino group and the cyanophe-
nyl moiety. In addition to the fluorescence band due to emission
from the LE state, a largely red-shifted emission band corre-
sponding to the emission from the TICT state is observed. In
such systems, increasing the solvent polarity will lead to red
shifts of their absrptions (λab) and emissions (λem), its λem being
more sensitive to the solvent polarity. Normally, in nonpolar
solvent, there is only one emission peak from the LE state, while
two emission peaks appear in polar solvent from the LE and
TICT states, respectively, in which the latter peak is red-shifted
by up to 100 nm from the former. TICT state is in most cases
more likely to relax by nonradiative processes than by fluores-
cence emission, leading to the lower PL (photoluminescence)
intensity and ΦF (fluorescence quantum yield) value in polar
environments. Thus, the emission properties of this group of
molecules highly depend on the solvent polarity.
Figure 1 shows the UV absorption spectra of some BODIPYs
in cyclohexane (CH) and THF solutions. All the molecules
exhibit a similar spectral pattern and display a sharp absorption
peak at about 500 nm in both solvents, although the spectra of
6, 13, and 15 are slightly broader in THF than those in CH.
Due to the electron-donating ability, ethyl-substituted 4 and 14
exhibit better conjugation with the BODIPY core than the
(14) (a) Sunahara, H.; Urano, Y.; Kojima, H.; Nagano, T. J. Am. Chem. Soc.
2007, 129, 5597–5604. (b) Gabe, Y.; Urano, Y.; Kikuchi, K.; Kojima, H.;
Nagano, T. J. Am. Chem. Soc. 2004, 126, 3357–3367. (c) Li, Z.; Miller, E. W.;
Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10–11. (d)
Yamada, K.; Nomura, Y.; Citterio, D.; Iwasawa, N.; Suzuki, K. J. Am. Chem.
Soc. 2005, 127, 6956–6957. (e) Baruah, M.; Qin, W.; Vallee, R. A. L.; Beljonne,
D.; Rohand, W.; Bonens, N. Org. Lett. 2005, 7, 4377–4380. (f) Qi, X.; Jun,
E. J.; Xu, L.; Kim, S.-J.; Hong, J. S. J.; Yoon, Y. J.; Yoon, J. J. Org. Chem.
2006, 71, 2881–2884.
(15) (a) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J. J.
Phys. Chem. A 1998, 102, 10211–10220. (b) Qin, W.; Baruah, M.; Bonens, N.
J. Phys. Chem. A 2005, 109, 7371–7384. (c) Valeur, B. Molecular Fluorescence:
Principle and Applications; Wiley-VCH Verlag GmbH: Weinheim, Germany,
2001.
and high chemical and photolytic stability.14 We are interested
in investigating the optical properties of boradiazaindacenes with
electron-donating amine groups because their interaction with
the electron-withdrawing BODIPY group may enable them to
exhibit intriguing twisted intramolecular charge transfer (TICT)
behavior.15 TICT has been known to greatly affect the emission
properties of some chromophoric molecules containing donor
and acceptor groups. The prime example of great interest is
4-N,N-dimethylaminobenzonitrile (DMABN). In the ground
J. Org. Chem. Vol. 74, No. 5, 2009 2055

Lager et al.
FIGURE 1. UV absorption of the polyarylamine-bodipy hybrids in cyclohexane (CH) and THF at a concentration of 10 µM.
nonsubstituted 3 and 13, thus leading to a red shift of the
absorption by ∼10 nm. Dialkylamino groups in 6 and 15 have
poorer electronic communication with the BODIPY core than
the corresponding diphenylamino groups in 3 and 13, thus
resulting in slight blue shift of the absorption. Parts A and B of
Figure 2 show the PL spectra of the para-substituted BODIPYs
in CH and THF, respectively. Their emissions are strongly
dependent on solvent polarity. The spectra measured in nonpolar
CH display only one single sharp emission peak originating from
the so-called LE state. A new peak, however, emerged at longer
wavelengths in polar THF, which may be attributed to the
emission from the TICT state. Table S1 (see the Supporting
Information) shows the solvent polarity effect on the emission
of 4 as a demonstration, which is of a typical TICT feature.
Excitation of the molecules can lead to a fast transfer from the
LE state to a weakly emissive TICT state in polar solvents,
resulting in dual emission from both states. Compared with their
para-substituted counterparts, the emissions of meta- and ortho-
substituted amine-BODIPY hybrids in THF are only slightly
redder than those in CH (Figure 2, parts C and D), suggesting
that they exhibit little or no TICT behavior owing to the poorer
electronic communication of the amine groups with BODIPY
cores. Interestingly, 18 displays only one emission peak at 531
nm in THF, but two at 528 and 663 nm in CH. From the crystal
structure of 18 (see the Supporting Information), we can see
that it takes a very twisted conformation due to steric effects. It
can be envisioned that the TICT state of 18 can occur in both
nonpolar and polar solvents owing to its inherent twisted
structure. However, in polar THF, the emission intensity from
the TICT state is so low that it is not measurable, thus showing
one apparent emission peak from the LE state. The photoimages
of THF and CH solutions of some samples taken under normal
room illumination and UV light are shown in parts E and F of
Figure 2. The distinct emission color between the LE and ICT
states enables us to distinguish them with the naked eye. Such
dyes may thus be utilized as fluorescent probes for surface
polarity detection of biological species. The quantum yields of
these molecules in CH and THF are measured and given in
Table 1. Due to the formation of the TICT state of para-
substituted BODIPYs in THF, the values in this solvent are
10-50 times lower than those in CH. The quantum yields of
meta-substituted BODIPYs in THF are in the range of 0.5-0.7%,
which are 5-10 times lower than those in CH. For ortho-
substituted ones, the values are also low and almost the same
in both solvents. Among all the molecules, the values are in
the order of para- > meta- > ortho-substituted ones. As can be
seeen from both Figure 2 and Table 1, compounds 3, 4, 6, 13,
14, 18, and 22 display NIR emission.
Conclusions
Two general methods were developed to prepare polyary-
lamine-BODIPY hybrids. One involves the Liebeskind-Srogl
cross-coupling and the other one consist of a L-S/Suzuki
sequence. The optical properties of the hybrids were studied
and it was found that seven of these analogues displayed NIR
2056 J. Org. Chem. Vol. 74, No. 5, 2009

NoVel Meso-Polyarylamine-BODIPY Hybrids
FIGURE 2. Photoluminescence (PL) spectra (A-D) and some photoimages (E and F) of the polyarylamine-BODIPY hybrids in cyclohexane (CH)
and THF with a concentration of 10 µM.
J. Org. Chem. Vol. 74, No. 5, 2009 2057

Lager et al.
TABLE 1. Optical Properties of the Polyarylamine-BODIPY
Hybridsa
After flash-chromatography (SiO2-gel, EtOAc/hexanes gradient)
purification, followed by washings with petroleum ether (3 × 2
mL), 3 (72 mg, 0.165 mmol, 98%) was obtained as a purple solid
d
λab (nm)b
λem (nm)c
ΦF
1
(mp 212-214 °C): H NMR (200 MHz, CDCl3) δ 7.90 (s, 2H),
compd no. CH THF
CH
THF
CH
THF
7.49-7.31 (m, 6H), 7.24-7.05 (m, 10H), 6.55 (dd, J1 ) 4.0 Hz,
J2 ) 1.8 Hz, 2H); 13C NMR (50 MHz, CDCl3) δ 151.1, 147.7,
146.6, 142.9, 134.8, 132.5, 131.3, 129.9, 126.4, 126.1, 124.9, 120.2,
118.2; IR (KBr, cm-1) 3105 (w), 3059 (w), 3036 (w), 1587 (s),
1558 (s), 1533 (s), 1488 (s), 1412 (s), 1388 (s), 1332 (m), 1294
(m), 1261 (s), 1224 (m), 1192 (m), 1118 (s), 1076 (s), 980 (s), 911
(m), 757 (m), 742 (m), 696 (m). HRMS FABS (M + H+) calcd
for C27H21BF2N3 436.1796, found 436.1791.
meta
5
16
17
20
21
18
19
3
4
6
13
14
15
22
501 501 517
502 501 519
501 501 518
503 501 521
502 501 517
526
528
530
525
527
0.0360 0.0066
0.0447 0.0051
0.0489 0.0058
0.0548 0.0066
0.0707 0.0070
0.0061 0.0041
0.0034 0.0048
0.1233 0.0074
0.1394 0.0804
0.1498 0.0074
0.0283 0.0027
0.1609 0.0323
ortho
para
503 504 528, 663 531
502 500 530
501 498 542
512 511 531
497 493 519
502 501 521
514 513 531
501 501 541
514 512 604
534
530, 686
544, 646
530, 662
527, 754
535, 686
General Procedure (GP) for Suzuki Cross-Coupling: BO-
DIPY 13. An oven-dry Schlenk tube, equipped with a stir bar, was
charged with 8-(p-bromophenyl)bodipy 7 (40 mg, 0.115 mmol, 1
equiv), 4-(diphenylamino)phenylboronic acid (100 mg, 0.346 mmol,
3 equiv), Na2CO3 (73 mg, 0.692 mmol, 6 equiv), and DME/H2O
(4/1 v/v, 5 mL) under N2. The stirred solution was sparged with
N2 for 10 min, whereupon Pd(PPh3)4 (8.0 mg, 6.92 µmol, 6 mol
%) was added followed by 5 min of N2-purge. The reaction mixture
was heated at 80 °C. The reaction was monitored by TLC (Rf 0.38,
SiO2-gel, 20% EtOAc/hexanes), which showed that the reaction
was finished after 30 min. The reaction mixture was allowed to
reach rt and was adsorbed on SiO2-gel. After flash-chromatography
(SiO2-gel, EtOAc/hexanes gradient) purification, followed by re-
crystallization from petroleum ether/toluene, 13 (26 mg, 0.0508
mmol, 44%) was obtained as a purple solid: mp 108-109 °C; IR
(KBr, cm-1) 3033 (w), 1590 (s), 1558 (s), 1537 (s), 1489 (s), 1412
528 (broad) 0.0690 0.0067
545, 753 0.1704 0.0734
a In cyclohexane (CH) and THF solutions with a concentration of
∼10 µM. b Absorption maximum. c Emission maximum. d Quantum
yield estimated with fluorescein as standard (ΦF ) 79% in 0.1 N
NaOH). Abbreviation: meta, ortho, and para ) meta-, ortho-, and
para-substituted BODIPYs.
emission. The optical behavior of compound 18 was rationalized
based upon its crystal structure. Bromo-containing BODIPY
derivatives 7-12 were prepared in an efficient fashion. These
compounds will allow the construction of other architectures
via the ever growing transition metal-based cross-coupling
protocols. The methods developed herein are amenable for the
synthesis of a second generation of the polyarylamine-BODIPY
hybrids, especially because many of the compounds prepared
in this work showed potentially useful properties. These new
avenues are currently under investigation in our laboratories and
will be disclosed in due course.
1
(s), 1387 (s), 1261 (s), 1115 (s), 1077 (s); H NMR (200 MHz,
CDCl3) δ 7.95 (s, 2H), 7.73 (d, J ) 8.6 Hz, 2H), 7.63 (d, J ) 8.6
Hz, 2H), 7.57-7.51 (m, 2H), 7.34-7.30 (m, 2H), 7.27-7.26 (m,
2H), 7.19-7.01 (m, 10H), 6.56 (dd, J1 ) 4.2 Hz, J2 ) 1.6 Hz,
2H); 13C NMR (50 MHz, CDCl3) δ 148.4, 147.6, 147.5, 144.0,
143.5, 135.0, 133.1, 132.3, 131.7, 131.4, 129.6, 128.0, 126.6, 125.0,
123.6, 123.5, 118.7; HRMS FABS (M + H+) calcd for C33H25BF2N3
512.2110, found 512.2114.
Acknowledgment. We are grateful to CIAM program grant
2005-C02-51833/A-1, SEP- 2004-C01-45970/A1, and FOMIX-
GTO (grant number GTO-2007-C02-69094). B.Z.T. wishes to
thank the Hong Kong Research Grants Council (Project No.
603008). We thank Gary Allred of Synthonix for kind donations
of boronic acids and CuTC. A.A.-A. wishes to thank CONACyT
for a graduate scholarship.
Experimental Section
General Procedure (GP) for the Liebeskind-Srogl Cross-
Coupling: BODIPY 3. An oven-dry round-bottom flask, equipped
with a stir bar, was charged with thiomethylbodipy 1 (40 mg, 0.168
mmol, 1 equiv), 4-(diphenylamino)phenylboronic acid (146 mg,
0.504 mmol, 3 equiv), and anhydrous THF (20 mL) under N2. The
stirred solution was sparged with N2 for 10 mi., whereupon Pd2dba3
(3.8 mg, 4.2 µmol, 2.5 mol %), tri(2-furyl)phosphine (2.9 mg, 12.6
µmol, 7.5 mol %), and copper(I) thiophene-2-carboxylate (96 mg,
0.504 mmol, 3 equiv) were added followed by 5 min of N2-purge.
The reaction mixture was heated at 55 °C. The reaction was
monitored by TLC (Rf 0.44, SiO2-gel, 20% EtOAc/hexanes), which
showed that the reaction was finished after 1.5 h. The reaction
mixture was allowed to reach rt and was adsorbed on SiO2-gel.
Supporting Information Available: A complete description
of the synthesis of all of the compounds described in the text,
1
copies of their spectroscopic data (IR, H, 13C), and a CIF file
and Ortep diagram of 18. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO802519B
2058 J. Org. Chem. Vol. 74, No. 5, 2009