Synthesis of PS-supported NHC-Pd catalyst derived from theobromine and its applications in Suzuki-Miyaura reaction

Article abstract of DOI:10.1002/jccs.201100565

The synthesis of the PS-supported bis-NHC-palladium catalyst simply prepared from theobromine in four steps was reported. The air-stable PS-supported bis-NHC-Pd catalyst can be used as a good catalyst in running Suzuki-Miyaura cross-coupling reaction.

Full text of DOI:10.1002/jccs.201100565

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Synthesis of PS-supported NHC-Pd Catalyst Derived from Theobromine and its
Applications in Suzuki-Miyaura Reaction
Hung-Kun Lo and Fen-Tair Luo*
Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 11529, R.O.C.
(Received: Oct. 31, 2011; Accepted: Nov. 11, 2011; Published Online: Nov. 18, 2011; DOI: 10.1002/jccs.201100565)
The synthesis of the PS-supported bis-NHC-palladium catalyst simply prepared from theobromine in four
steps was reported. The air-stable PS-supported bis-NHC-Pd catalyst can be used as a good catalyst in
running Suzuki-Miyaura cross-coupling reaction.
Keywords: Suzuki reaction; PS-supported bis-NHC-Pd complex; Theobromine; NHC.
INTRODUCTION
Recently, we have reported the synthesis of a new
ethyl acetate we could obtain 1-allyl-3,7,9-trimethylxan-
thinium iodide as a yellowish white solid in 66% yield.14,15
MS of both FAB+ and ES+ showed one peak at m/z 597
which is corresponding to the cation [2M - I-]+, another
peak at m/z 235 is the corresponding cation of [M - I-]+. MS
of ES- showed one peak at m/z 489 which is the corre-
sponding anion of [M + I-]-. The corresponding proton and
carbon peaks appeared in 1H- and 13C-NMR spectra were
assigned in its chemical structure with the help of the spec-
tral data shown in their 2D-NOESY and HSQC spectra.
The following reaction of 1-allyl-3,7,9-trimethylxanthinium
iodide with sodium t-butoxide in the presence of palladium
acetate in THF at reflux could give bis(1-allyl-3,7,9-tri-
methylxanthine-8-ylidene)palladium diiodide 1 as a yel-
lowish white powder in 51% yield after recrystallization
from benzene and hexane as shown in Scheme I. The bis-
NHC ligands in the Pd complex obtained via the similar re-
action conditions was observed and reported in our previ-
ous studies.6 MS of EI+ of 1 showed its molecular ion at
m/z 828, the radical cation of [M - I]+ showed at m/z 701,
and another radical cation [M – 2 I]+ showed at m/z 574. At-
tempts to do the copolymerization of 1 with six equivalents
of styrene with or without a catalytic amount of free radical
initiator such as azobisisobutyronitrile (AIBN) or benzoyl
peroxide in benzene, toluene, THF or DMF at 70 °C to 150
°C were all failed.
type of soluble polystyrene-supported palladium complex
as an excellent and recyclable palladacycle catalyst and its
applications for carbon-carbon bond formation in Heck-
Mizoroki, Suzuki-Miyaura, and Sonogashira reactions.1,2
In view of the NHC-metal complexes have been proved to
be more effective in many catalytic reactions than phos-
phine-metal complexes,3-5 we developed a simple and
highly efficient synthesis of bis-NHC-Pd catalyst derived
from caffeine and its applications in Suzuki-Miyaura,
Heck-Mizoroki, and Sonogashira cross-coupling reactions
in aqueous solution with or without adding additives.6 In
order to support the development of sustainable chemistry
and to apply the high recyclability and the development of
polystyrene in organic synthesis,7-13 herein, we report an-
other simple synthesis of PS-supported bis-NHC-Pd cata-
lyst derived from theobromine, one appropriate natural
methylated xanthine, in four steps and its applications in
Suzuki-Miyaura reaction in water without adding other ad-
ditives.
RESULTS AND DISCUSSION
Initially, we did the allylation of theobromine at N1-
position via deprotonation by using sodium t-butoxide in
DMF and followed by the allylation with allyl bromide to
give 1-allyl-3,7-dimethylxanthine in 95% yield. 1-Allyl-
3,7-dimethylxanthine was methylated at N9-position by the
reaction with excess of methyl iodide in dry DMF at 100 °C
for 20 h. An excess amount of dry ethyl acetate was added
to the clear solution to obtain a precipitate in yellow color.
After filtration, concentration, and recrystallization from
We then turned our attention to the use of p-styryl-
methylene group at the N1-position in theobromine. Thus,
the benzylation of theobromine was done by deprotonation
with sodium t-butoxide in DMF and followed by the addi-
tion of excess p-vinylbenzyl chloride to form 1-p-vinyl-
benzyltheobromine in 73% yield. 1-p-Vinylbenzyltheo-
Dedicated to the memory of Professor Yung-Son Hon (1955–2011).
* Corresponding author. E-mail: luoft@chem.sinica.edu.tw
394
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
PS-supported NHC-Pd Catalyst Derived from Theobromine
To test the applicabilities of palladium complexes 1-3
in the carbon-carbon bond formation reactions, we exam-
ined the Suzuki-Miyaura cross-coupling reaction of phen-
ylboronic acid with p- or m-bromotoluene and sodium
t-butoxide or potassium carbonate1 as the base in the ab-
sence or presence of a catalytic amount of complex 3 in wa-
ter as shown in Table 1. It was noted that in the absence of
the palladium complexes 1-3, there was no detectable
amount of the desired coupling product as judged by the
GC-MS spectral analysis (entry 1). The reaction was pre-
ferred to run at higher temperature, probably due to the
higher solubility of the catalyst 3 at higher temperature (en-
tries 4-5). When sodium t-butoxide was used as the base,
the cross-coupling reaction using complex 3 as the catalyst
could give a little bit higher yields than that of using potas-
sium carbonate as the base (entries 4, 6-8). Although palla-
dium complexes 1 and 2 could give similar results as that
from complex 3 (entries 2-4), however, in view of the good
solubilities of the PS-supported palladium complex 3 in
tetrahydrofuran, N,N-dimethylacetamide, N,N-dimethyl-
formamide, toluene, chloroform, dichloromethane and not
very soluble in methanol, acetonitrile, hexane, and diethyl
ether, we preferred to use complex 3 as a recyclable catalyst
Scheme I Synthesis of 1 from theobromine
bromine was methylated at N9-position by the reaction with
excess of methyl iodide in dry DMF at 100 °C for 20 h to
give 1-p-vinylbenzyl-3,7,9-trimethylxanthinium iodide in
40% yield. MS EI+ showed a peak at m/z 749 which is cor-
responding to the cation [2 M - I-]+, another peak at m/z 311
is corresponding to the cation [M - I-]+. The reaction of
1-p-vinylbenzyl-3,7,9-trimethylxanthinium iodide with
sodium t-butoxide in the presence of palladium acetate in
THF at reflux could give bis(1-p-vinylbenzyl-3,7,9-tri-
methylxanthine-8-ylidene)palladium diiodide 2 as a yel-
lowish white crystal powder in 45% yield after recrys-
tallization from benzene and hexane. Considering the
proper space around the palladium in the catalyst, the com-
plex 2 was successfully copolymerized with six equiva-
lents of styrene in the presence of a catalytic amount of
AIBN at 150 °C in DMF for 48 h to form the air-stable
polystyrene-supported bis-NHC-Pd complex 3 after re-
peatedly washed by the solvent of THF/n-hexane (v/v =
1/50) in a Soxhlet extraction apparatus as shown in Scheme
II. Gel permeation chromatography analysis (mobile phase:
THF, polystyrene standards) indicated that Mw of PS-sup-
ported bis-NHC-Pd complex 3 was 1.378 ´ 104 g/mol.
Atomic absorption spectrophotometer analysis showed
that palladium was containing as 59.83 mg per gram of the
polymer 3.
Table 1. Palladium complexes 1-3 catalyzed Suzuki-Miyaura
cross-coupling reaction
PhB(OH)2
Br
0.5 mol % Pd cat.
1.5 equiv of base
water, 6 h
Me
Me
Iso. Yield
(%) of the
Producta
Temp
Entry Bromotoluene Pd catalyst Base
(°C)
1
2
3
4
5
6
7
8
p-
p-
p-
p-
p-
m-
p-
m-
--
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
80
80
80
80
25
80
--
1
75
71
73
18
66
75
71
2
3
3
3
3
NaO-t-Bu 80
NaO-t-Bu 80
Synthesis of palladium complexes 2 and 3
Scheme II
3
1st
from theobromine
9
p-
p-
p-
K2CO3
K2CO3
K2CO3
80
80
80
67
65
61
recycled 3
2nd
10
11
recycled 3
3rd
recycled 3
a The products were identified by comparison their 1H-NMR and
MS with that of authentic samples.
J. Chin. Chem. Soc. 2012, 59, 394-398
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395

Article
Lo and Luo
597.1435, found, 597.1431.
for our further studies. The recyclability test of complex 3
showed that the yields of the Suzuki-Miyaura cross-cou-
pling products could be isolated more than 60% after recy-
cling one to three times of the complex 3 with diethyl ether
as the solvent to precipitate 3 (entries 9-11).
Preparation of bis(1-allyl-3,7,9-trimethylxanthine-8-
ylidene)palladium diiodide 1
In one 100 mL round-bottom flask was added 1-allyl-
3,7,9-trimethylxanthinium iodide (1.3 g, 3.6 mmol), palla-
dium acetate (0.4 g, 1.8 mmol), and sodium t-butoxide
(0.35 g, 3.6 mmol). The air in the flask was removed under
vacuum and the flask was flushed with nitrogen. The flask
was added 20 mL of dry THF and the reaction mixture was
stirred at room temperature for 1.5 h. Then the reaction
mixture was heated and refluxed for another 3 h. After
cooling, 60 mL of hexane was added slowly into the flask
to obtain precipitate as the crude product with yellowish
white color. The crude product was recrystallized three
times with benzene (10 mL) and hexane (30 mL) to give the
title compound (0.77 g, 0.92 mmol) as powder crystal with
yellowish white color in 51% yield. mp. 175-176 °C. 1H-
NMR (400 MHz, CDCl3): d 3.79 (s, 3H), 4.24 (s, 3H), 4.35
(s, 3H), 4.57 (d, J = 6 Hz, 2H), 5.21 (d, J = 11 Hz, 1H), 5.28
(d, J = 17 Hz, 1H), 5.80-5.89 (m, 1H) ppm. 13C-NMR (125
MHz, CDCl3): d 32.14, 37.70, 39.42, 44.26, 110.87,
118.92, 131.48, 140.54, 150.40, 152.87, 175.94 ppm. EI+:
828 (M+), 701, 574, 466, 360, 339, 234. HRMS EI+ calcd
for C22H28N8O4I2Pd (M+) 827.9358, found, 827.9368.
HRMS EI+ calcd for C22H28N8O4IPd (M+ - I) 701.0313,
found, 701.0310; calcd for C22H28N8O4Pd (M+ - 2 I)
574.1268, found, 574.1262.
EXPERIMENTAL
Preparation of 1-allyltheobromine16
Theobromine (9.0 g, 50 mmol) and sodium t-but-
oxide (5.28 g, 55 mmol) were dissolved in dry DMF (250
mL). The solution was added allyl bromide (4.32 mL, 50
mmol) at 150 °C and stirred for another 1 h. The solution
was added another allyl bromide (4.32 mL, 50 mmol) and
stirred for another 12 h at 100 °C. The reaction was then
cooled to room temperature. Volatiles were removed by ro-
tary evaporator and the crude material was extracted three
times by water (50 mL´ 3) and ethyl acetate (200 mL´ 3) to
give the crude title compound as pale yellow crystals.
Recrystallization from methanol could give 10.5 g (47.5
mmol) of the title compound as white crystal in 95% yield.
mp. 142-143 °C. 1H-NMR (500 MHz, CDCl3): d 3.56 (s,
3H, CH3 at N3), 3.96 (s, 3H, CH3 at N7), 4.61 (d, J = 6 Hz,
2H), 5.17 (d, J = 11 Hz, 1H), 5.25 (d, J = 17 Hz, 1H),
5.85-5.94 (m, 1H), 7.49 (s, 1H) ppm. 13C-NMR (125 MHz,
CDCl3): d 29.92, 33.79, 43.53, 107.86, 117.78, 132.51,
141.70, 149.13, 151.54, 155.23 ppm. FAB+: m/z 221 (M+ +
1), 205, 193, 181. HRMS FAB+ calcd for C10H13N4O2 ([M
+ 1]+) 221.1039, found 221.1034.
Preparation of 1-p-vinylbenzyltheobromine
Theobromine (3.3 g, 18 mmol) and sodium t-but-
oxide (1.8 g, 19 mmol) was added into one 250 mL round-
bottom flask. The air in the flask was removed under vac-
uum and then the flask was flushed with nitrogen. Dry
DMF (90 mL) was added into the flask and the reaction
mixture was heated to 80 °C, p-vinylbenzyl chloride (2.8
mL, 54 mmol) was added slowly into the mixture and re-
acted for another 1 h, then another portion of p-vinylbenzyl
chloride (2.8 mL, 54 mmol) was added into the mixture and
reacted for another 24 h. After cooling, DMF was removed
under vacuum and the reaction mixture was added 100 mL
of deionized water and the desired product was extracted
with chloroform (300 mL´ 2). After the organic layer was
concentrated to about 50 mL, 300 mL of hexane was slowly
added into the organic layer to obtain the title compound
(3.9 g, 13.2 mmol) with white color in 73% yield. mp.
141-143 °C. 1H-NMR (400 MHz, CDCl3): d 3.57 (s, 3H,
CH3 at N3), 3.99 (s, 3H, CH3 at N7), 5.18 (s, 2H), 5.20 (d, J
= 11 Hz, 1H), 5.69 (d, J = 17 Hz, 1H), 6.68 (dd, J = 11, 17
Preparation of 1-allyl-3,7,9-trimethylxanthinium io-
dide
1-Allyltheobromine (2.2 g, 10 mmol) was dissolved
in 20 mL of dry DMF at 100 °C for 1 h until the solution ap-
peared clear, then the solution was added iodomethane (3.1
mL, 50 mmol) dropwise and stirred for another 20 h at 100
°C. The solution was poured slowly into 100 mL of ethyl
acetate to obtain precipitate with cream yellow color. The
crude product was washed with ethyl acetate (60 mL ´ 3).
After drying in the air, the title compound was obtained as
yellowish powder (2.4 g, 6.6 mmol) in 66% yield. mp.
152-154 °C. 1H-NMR (500 MHz, CDCl3): d 3.83 (s, 3H),
4.23 (t, 3H), 4.40 (s, 3H), 4.58 (d, J = 6 Hz, 2H), 5.24 (d, J =
11 Hz, 1H), 5.31 (d, J = 17 Hz, 1H), 5.78-5.88 (m, 1H),
10.45 (s, 1H) ppm. 13C-NMR (125 MHz, CDCl3): d 32.44,
36.59, 38.91, 44.65, 108.58, 119.41, 130.68, 139.68,
139.88, 149.94, 153.16 ppm. FAB+: m/z 597 (2 M - I-), 235
(M - I-). ES+: m/z 597 (2 M - I-), 235 (M - I-). ES-: 489 (M +
I-). HRMS FAB+ calcd for C22H30N8O4I (2 M - I-)
396
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PS-supported NHC-Pd Catalyst Derived from Theobromine
Hz, 1H), 7.34 (d, J = 8 Hz, 2H), 7.46 (d, J = 8 Hz, 2H), 7.49
(s, 1H) ppm. 13C-NMR (125 MHz, CDCl3): d 29.99, 33.82,
44.44, 107.92, 114.01, 126.46, 129.31, 136.76, 137.13,
137.16, 141.76, 149.13, 151.84, 155.48 ppm. MS EI+ m/z
296 (M+), 117. HRMS EI+: calcd. for C16H16O2N4
296.1273, found: 296.1267.
135.78, 135.87, 136.21, 140.42, 149.77, 152.18, 174.11
ppm. FAB+: m/z 980 (M+), 853, 726. HRMS FAB+: calcd.
for C34H36O4N8I2Pd (M+) 979.9984, found: 979.9993;
calcd. for C34H36O4N8IPd (M+ - I) 853.0939, found:
853.0936; calcd. for C34H36O4N8Pd (M+ - 2I) 726.1894,
found: 726.1885.
Preparation of 1-p-vinylbenzyl-3,7,9-trimethylxan-
thinium iodide
Synthesis of polystyrene-supported bis-NHC-Pd cata-
lyst 3
1-p-Vinylbenzyltheobromine (2.96 g, 10 mmol) was
dissolved in 20 mL of dry DMF within 1 h at 100 °C under
nitrogen atmosphere. Methyl iodide (3.1 mL, 50 mmol)
was added to the above solution to react for 20 h at 100 °C.
After cooling, DMF was removed under vacuum and the
desired product was recrystallized from ethyl acetate to
give yellowish crystal (1.75 g, 4 mmol) in 40% yield. mp.
162-164 °C. 1H-NMR (400 MHz, CDCl3): d 3.82 (s, 3H,
CH3 at N3), 4.23 (s, 3H, CH3 at N7), 4.37 (s, 3H, CH3 at N9),
5.14 (s, 2H), 5.22 (d, J = 11 Hz, 1H), 5.71 (d, J = 17 Hz,
1H), 6.65 (dd, J = 17, 11 Hz, 1H), 7.34 (d, J = 8 Hz, 2H),
7.40 (d, J = 8 Hz, 2H), 10.40 (s, 1H) ppm. 13C-NMR (125
MHz, DMSO-D6): d 31.41, 35.62, 36.81, 44.32, 107.77,
114.37, 126.04, 128.02, 135.86, 136.11, 136.41, 139.61,
139.78, 150.01, 153.12 ppm. TOF-MS-ES+: 749 (2 M - I-),
639, 343, 311 (M - I-). HRMS ES+: calcd. for C34H38O4N8I
(2 M - I-) 749.2061, found: 749.2064; calcd. for C17H19O2N4
(M - I-) 311.1508, found: 311.1510.
Bis(1-p-vinylbenzyl-3,7,9-trimethylxanthine-8-yli-
dene)palladium diiodide 2 (980 mg, 1 mmol) was added
into a test tube and flushed with nitrogen. A mixture of sty-
rene (620 mg, 6 mmol) and dry DMF (5 mL) was injected
into the test tube. Then, a mixture of AIBN (50 mg, 0.3
mmol) and DMF (2 mL) was injected into the stirring test
tube. The reaction mixture was then heated at 150 °C for 48
h. After cooling, the mixture was concentrated to dryness.
The dry solid was repeatedly washed by the solvent (THF/
hexane = 1:50) in Soxhlet extractor. The isolated polymer
1
was 0.90 g (56% yield). H NMR (500 MHz, CDCl3) d
1.3-2.2 (m, Ar-CHCH2), 3.7-3.8 (m, CH3 at N3),4.2-4.4 (m,
CH3 at N7), 4.4-4.6 (m, CH3 at N9), 5.0-5.1 (m, ArCH2N),
6.3-6.8 (m, Ar-H), 6.8-7.4 (m, Ar-H) ppm; Gel permeation
chromatography analysis (mobile phase: THF, polystyrene
standards) indicated that Mw of catalyst 3 was 1.378 ´ 104
g/mol. Atomic absorption spectrophotometer analysis
showed that palladium was containing at 59.83 mg of Pd
per gram of polymer 3.
Preparation of bis(1-p-vinylbenzyl-3,7,9-trimethylxan-
thine-8-ylidene)palladium diiodide 2
General Procedure for Suzuki-Miyaura Cross Cou-
pling Reaction
1-p-Vinylbenzyl-3,7,9-trimethylxanthinium iodide
(0.79 g, 1.8 mmol), palladium acetate (0.1 g, 0.9 mmol),
and sodium t-butoxide (0.173 g, 1.8 mmol) were added into
one 100 mL round-bottom flask. The air in the flask was re-
moved by vacuum and the flask was then flushed with ni-
trogen. Dry THF (10 mL) was added into the above mixture
and the reaction mixture was stirred at room temperature
for 1.5 h and at reflux temperature for another 3 h. After
cooling, hexane (60 mL) was slowly added into the flask to
obtain yellowish white precipitate as the crude product.
Recrystallization with benzene (10 mL) and hexane (30
mL) could give the title compound (0.4 g, 0.4 mmol) as yel-
lowish white crystal powder in 45% yield. mp. 280-282 °C.
1H-NMR (500 MHz, DMSO-D6): d 3.74 (s, 3H, CH3 at N3),
4.12 (s, 3H, CH3 at N7), 4.31 (s, 3H, CH3 at N9), 5.03 (s,
2H), 5.22 (d, J = 11 Hz, 1H), 5.78 (d, J = 17 Hz, 1H), 6.70
(dd, J = 11, 17 Hz, 1H), 7.29 (d, J = 7 Hz, 2H), 7.40 (d, J = 7
Hz, 2H) ppm. 13C-NMR (125 MHz, DMSO-D6): d 31.20,
36.43, 43.71, 108.90, 113.76, 125.64 (2 C’s), 127.50,
A mixture of p-bromotoluene (1 mmol), phenylbo-
ronic acid (1.2 mmol), potassium carbonate (1.5 mmol) and
0.5 mol % of PS-supported bis-NHC-PdI2 catalyst 3 in 1
mL of water was stirred at 80 ºC for 6 h under nitrogen. The
organic layer was extracted with diethyl ether, dried over
magnesium sulfate, and the solvent was removed com-
pletely under high vacuum to give a crude product. The
product was further purified by column chromatography on
silica gel (neutral, 70-230 mesh, n-hexane). The products
were identified by comparison their 1H-NMR and MS spec-
tral data as reported in the literature.
ACKNOWLEDGEMENTS
The authors thank the National Science Council of
the Republic of China and Academia Sinica for financial
supports. The authors also thank Dr. Rajesh Rengasamy for
his preparation of some chemicals in this research.
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Article
Lo and Luo
11. Toy, P. H.; Reger, T. S.; Janda, K. D. Aldrichimica Acta 2000,
33, 87.
REFERENCES
1. Lin, C. A.; Luo, F. T. Tetrahedron Lett. 2003, 44, 7565.
2. Luo, F. T.; Xue, C.; Ko, S. L.; Shao, Y. D.; Wu, C. J.; Kuo, Y.
M. Tetrahedron 2005, 61, 6040.
12. Toy, P. H.; Reger, T. S.; Garibay, P.; Garno, J. C.; Malikayil,
J. A.; Liu, G. Y.; Janda, K. D. J. Comb. Chem. 2001, 3, 117.
13. Shimomura, O.; Lee, B. S.; Meth, S.; Suzuki, H.; Mahajan,
S.; Nomura, R.; Janda, K. D. Tetrahedron 2005, 61, 12160.
14. Panzner, M. J.; Hindi, K. M.; Wright, B. D.; Taylor, J. B.;
Han, D. S.; Youngs, W. J.; Cannon, C. L. Dalton Trans. 2009,
7308.
3. Poater, A.; Ragone, F.; Giudice, S.; Costabile, C.; Dorta, R.;
Nolan, S. P.; Cavallo, P. Organometallics 2008, 27, 2679.
4. Peris, E. Top. Organomet. Chem. 2007, 21, 83.
5. Corberán, R.; Ramírez, J.; Poyatos, M.; Perisa, E.; Fernández,
E. Tetrahedron: Asymmetry 2006, 17, 1759.
15. Kascatan-Nebioglu, A.; Melaiye, A.; Hindi, K.; Durmus, S.;
Panzner, M. J.; Hogue, L. A.; Mallett, R. J.; Hovis, C. E.;
Coughenour, M.; Crosby, S. D.; Milsted, A.; Ely, D. L.;
Tessier, C. A.; Cannon, C. L.; Youngs, W. J. J. Med. Chem.
2006, 49, 6811.
6. Luo, F. T.; Lo, H. K. J. Organomet. Chem. 2011, 696, 1262.
7. Chen, S.; Janda, K. D. Tetrahedron Lett. 1998, 39, 3943.
8. Toy, P. H.; Janda, K. D. Tetrahedron Lett. 1999, 40, 6329.
9. Manzotti, R.; Tang, S. Y.; Janda, K. D. Tetrahedron 2000, 56,
7885.
16. Daly, J. W.; Padgett, W. L.; Shamim, M. T. J. Med. Chem.
1986, 29, 1305.
10. Manzotti, R.; Reger, T. S.; Janda, K. D. I. Tetrahedron Lett.
2000, 41, 8417.
398
www.jccs.wiley-vch.de
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