Synthesis of stilbene and distyrylbenzene derivatives through rhodium-catalyzed ortho -olefination and decarboxylation of benzoic acids

Article abstract of DOI:10.1021/ol1027392

Ortho-substituted benzoic acids efficiently undergo precisely ordered ortho-olefination/decarboxylation upon treatment with styrenes in the presence of a rhodium catalyst and silver salt oxidant to afford the corresponding meta-substituted stilbene deriva

Full text of DOI:10.1021/ol1027392

ORGANIC
LETTERS
2010
Vol. 12, No. 24
5776-5779
Synthesis of Stilbene and
Distyrylbenzene Derivatives through
Rhodium-Catalyzed Ortho-Olefination
and Decarboxylation of Benzoic Acids
Satoshi Mochida, Koji Hirano, Tetsuya Satoh,* and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity,
Suita, Osaka 565-0871, Japan
satoh@chem.eng.osaka-u.ac.jp; miura@chem.eng.osaka-u.ac.jp
Received November 12, 2010
ABSTRACT
Ortho-substituted benzoic acids efficiently undergo precisely ordered ortho-olefination/decarboxylation upon treatment with styrenes in the
presence of a rhodium catalyst and silver salt oxidant to afford the corresponding meta-substituted stilbene derivatives. The selective syntheses
of 1,3- and 1,4-distyrylbenzenes have also been realized through the reactions of simple benzoic acid and phthalic acid, respectively, with
styrene under similar conditions.
Since stilbene and distyrylbenzene structures are found in
various natural products1 and organic materials,2 the devel-
opment of methods for their effective construction has
attracted considerable attention in organic synthesis. The
conventional syntheses of such derivatives through transition-
metal-catalyzed cross-coupling and Mizoroki-Heck reactions
or Wittig-type reactions usually need complicated multisteps
forming halogen- and phosphorus-containing byproducts.
On the other hand, direct olefination reactions on aromatic
rings involving regioselective C-H bond cleavage have been
developed to provide more simple synthetic routes for
vinylarene derivatives.3 As early examples, we reported that
2-phenylphenols (Scheme 1, YH ) 2-HOC6H4),4 N-(aryl-
Scheme 1
(1) For recent examples, see: (a) Hua, X.; Fu, Y.-J.; Zu, Y.-G.; Wu, N.;
Kong, Y.; Li, J.; Peng, X.; Efferth, T. J. Pharm. Biomed. Anal. 2010, 52,
273. (b) Kong, Y.; Fu, Y.-J.; Zu, Y.-G.; Liu, W.; Wang, W.; Hua, X.; Yang,
M. Food Chem. 2010, 117, 152. (c) Vitrac, X.; Bornet, A.; Vanderlinde,
R.; Valls, J.; Richard, T.; Delaunay, J.-C.; Me´rillon, J.-M.; Teisse´dre, P.-L.
J. Agric. Food Chem. 2005, 53, 5664. (d) Syah, Y. M.; Aminah, N. S.;
Hakim, E. H.; Aimi, N.; Kitajima, M.; Takayama, H.; Achmad, S. A.
Phytochemistry 2003, 63, 913.
sulfonyl)-2-phenylanilines (YH ) 2-(ArSO2NH)C6H4),5 and
benzoic acids (YH ) CO2H)5 undergo regioselective olefi-
nation at the ortho-position of YH upon treatment with an
alkene such as acrylate ester and an appropriate oxidant under
(2) For selected reviews, see: (a) Grimsdale, A. C.; Chan, K. L.; Martin,
R. E.; Jokisz, P. G.; Holmes, A. B. Chem. ReV. 2009, 109, 897. (b) Kraft,
A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37,
402. (c) Marder, S. R.; Kippelen, B.; Jen, A. K.-Y.; Peyghambarian, N.
Nature 1997, 388, 845.
10.1021/ol1027392  2010 American Chemical Society
Published on Web 11/18/2010

palladium/copper catalysis. In each case, subsequent nucleo-
philic cyclization takes place to afford the corresponding
heterocyclic product. Later, acetanilides,6 benzylamines,7
phenylacetic acids,8 1-phenylprazoles,9 and 2-phenyletha-
nols10 have also been shown to undergo direct ortho-
olefination under palladium or rhodium catalysis.
In the course of our further study of the direct olefination
of benzoic acids,12 we unexpectedly observed that meta-
methoxystilbenes were exclusively produced upon treatment
of 2-methoxybenzoic acids with styrene under rhodium
catalysis, accompanied by decarboxylation13-15 (Scheme 3).
Recently, Yu and co-workers reported the novel palladium-
catalyzed meta-olefination of arenes possessing an electron-
withdrawing group (Scheme 2).11 In this case, however,
Scheme 3
Scheme 2
Note that methoxy-substituted stilbenes are of considerable
interest due to their unique biological16 and photophysical
and -chemical properties.17 The procedure was also found
to be applicable to the syntheses of 1,3- and 1,4-distyryl-
benzenes as well as a range of meta-substituted stilbenes.
Expectedly, some distyrylbenzenes obtained have been found
to show solid-state fluorescence. These new findings are
described herein.
considerable amounts of para-olefinated byproduct are also
formed.
(3) For selected reviews concerning C-H bond functionalization, see:
(a) Satoh, T.; Miura, M. Chem.sEur. J. 2010, 16, 11212. (b) Satoh, T.;
Miura, M. Synthesis 2010, 3395. (c) Karimi, B.; Behzadnia, H.; Elhamifar,
D.; Akhavan, P. F.; Esfahani, F. K. Synthesis 2010, 1399. (d) Colby, D. A.;
Bergman, R. G.; Ellman, J. A. Chem. ReV. 2010, 110, 624. (e) Sun, C.-L.;
Li, B.-J.; Shi, Z.-J. Chem. Commun. 2010, 46, 677. (f) Ackermann, L.;
Vicente, R.; Kapdi, A. R. Angew. Chem., Int. Ed. 2009, 48, 9792. (g) Chen,
X.; Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009,
48, 5094. (h) Daugulis, O.; Do, H.-Q.; Shabashov, D. Acc. Chem. Res. 2009,
42, 1074. (i) McGlacken, G. P.; Bateman, L. M. Chem. Soc. ReV. 2009,
38, 2447. (j) Kakiuchi, F.; Kochi, T. Synthesis 2008, 3013. (k) Lewis, J. C.;
Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2008, 41, 1013. (l) Ferreira,
E. M.; Zhang, H.; Stoltz, B. M. Tetrahedron 2008, 64, 5987. (m) Park,
Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008, 41, 222. (n) Beccalli,
E. M.; Broggini, G.; Martinelli, M.; Sottocornola, S. Chem. ReV. 2007, 107,
5318. (o) Herrerias, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. ReV. 2007, 107,
2546. (p) Alberico, D.; Scott, M. E.; Lautens, M. Chem. ReV. 2007, 107,
174. (q) Godula, K.; Sames, D. Science 2006, 312, 67. (r) Conley, B. L.;
Tenn, W. J., III; Young, K. J. H.; Ganesh, S. K.; Meier, S. K.; Ziatdinov,
V. R.; Mironov, O.; Oxgaard, J.; Gonzales, J.; Goddard, W. A., III; Periana,
R. A. J. Mol. Catal. A: Chem. 2006, 251, 8. (s) Kakiuchi, F.; Chatani, N.
AdV. Synth. Catal. 2003, 345, 1077. (t) Ritleng, V.; Sirlin, C.; Pfeffer, M.
Chem. ReV. 2002, 102, 1731. (u) Kakiuchi, F.; Murai, S. Acc. Chem. Res.
2002, 35, 826. (v) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698. (w)
Kakiuchi, F.; Murai, S. Top. Organomet. Chem. 1999, 3, 47. (x) Shilov,
A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879.
(12) Compared with benzoic acids, heteroarene carboxylic acids tend
to readily lose their carboxyl group. For our recent studies on the
decarboxylative arylation and vinylation of heteroarene carboxylic acids,
see: (a) Miyasaka, M.; Fukushima, A.; Satoh, T.; Hirano, K.; Miura, M.
Chem.sEur. J. 2009, 15, 3674. (b) Nakano, M.; Tsurugi, H.; Satoh, T.;
Miura, M. Org. Lett. 2008, 10, 1851. (c) Maehara, A.; Tsurugi, H.; Satoh,
T.; Miura, M. Org. Lett. 2008, 10, 1159. (d) Okazawa, T.; Satoh, T.; Miura,
M.; Nomura, M. J. Am. Chem. Soc. 2002, 124, 5286.
(13) For Pd-catalyzed decarboxylative ipso-vinylation of ortho-substi-
tuted benzoic acids, see: (a) Hu, P.; Kan, J.; Su, W.; Hong, M. Org. Lett.
2009, 11, 2341. (b) Tanaka, D.; Romeril, S. P.; Myers, A. G. J. Am. Chem.
Soc. 2005, 127, 10323. (c) Tanaka, D.; Myers, A. G. Org. Lett. 2004, 6,
433. (d) Myers, A. G.; Tanaka, D.; Mannion, M. R. J. Am. Chem. Soc.
2002, 124, 11250
.
(14) For Pd-catalyzed decarboxylative ipso-arylation of activated benzoic
acids, see: (a) Goossen, L. J.; Lange, P. P.; Rodriguez, N.; Linder, C.
Chem.sEur. J. 2010, 16, 3906. (b) Goossen, L. J.; Rodriguez, N.; Lange,
P. P.; Linder, C. Angew. Chem., Int. Ed. 2010, 49, 1111. (c) Goossen, L. J.;
Linder, C.; Rodriguez, N.; Lange, P. P. Chem.sEur. J. 2009, 15, 9336. (d)
Shang, R.; Fu, Y.; Wang, Y.; Xu, Q.; Yu, H.-Z.; Liu, L. Angew. Chem.,
Int. Ed. 2009, 48, 9350. (e) Goossen, L. J.; Rodriguez, N.; Linder, C. J. Am.
Chem. Soc. 2008, 130, 15248. (f) Goossen, L. J.; Zimmermann, B.; Knauber,
T. Angew. Chem., Int. Ed. 2008, 47, 7103. (g) Voutchkova, A.; Coplin, A.;
Leadbeater, N. E.; Crabtree, R. H. Chem. Commun. 2008, 6312. (h) Becht,
J.-M.; Le Drian, C. Org. Lett. 2008, 10, 3161. (i) Becht, J.-M.; Catala, C.;
Le Drian, C.; Wagner, A. Org. Lett. 2007, 9, 1781. (j) Goossen, L. J.;
Rodr´ıguez, N.; Melzer, B.; Linder, C.; Deng, G.; Levy, L. M. J. Am. Chem.
Soc. 2007, 129, 4824. (k) Goossen, L. J.; Deng, G.; Levy, L. M. Science
(4) Miura, M.; Tsuda, T.; Satoh, T.; Nomura, M. Chem. Lett. 1997, 1103.
(5) (a) Miura, M.; Tsuda, T.; Satoh, T.; Nomura, M. J. Org. Chem. 1998,
63, 5211. For the Rh-catalyzed version, see: (b) Ueura, K.; Satoh, T.; Miura,
M. J. Org. Chem. 2007, 72, 5362.
(6) (a) Patureau, F. W.; Glorius, F. J. Am. Chem. Soc. 2010, 132, 9982.
(b) Nishikata, T.; Lipshutz, B. H. Org. Lett. 2010, 12, 1972. (c) Rauf, W.;
Thompson, A. L.; Brown, J. M. Chem. Commun. 2009, 3874. (d) Houlden,
C. E.; Bailey, C. D.; Ford, J. G.; Gagne´, M. R.; Lloyd-Jones, G. C.; Booker-
Milburn, K. I. J. Am. Chem. Soc. 2008, 130, 10066. (e) Wang, J.-R.; Yang,
C.-T.; Liu, L.; Guo, Q.-X. Tetrahedron Lett. 2007, 48, 5449. (f) Amatore,
C.; Cammoun, C.; Jutand, A. AdV. Synth. Catal. 2007, 349, 292. (g) Lee,
G. T.; Jiang, X.; Prasad, K.; Repicˇ, O.; Blacklock, T. J. AdV. Synth. Catal.
2005, 347, 1921. (h) Boele, M. D. K.; van Strijdonck, G. P. F.; de Vries,
A. H. M.; Kamer, P. C. J.; de Vries, J. G.; van Leeuwen, P. W. N. M.
J. Am. Chem. Soc. 2002, 124, 1586.
2006, 313, 662
.
(15) For protodecarboxylation of benzoic acids, see: (a) Cornella, J.;
Sanchez, C.; Banawa, D.; Larrosa, I. Chem. Commun. 2009, 7176. (b)
Gooꢀen, L. J.; Linder, C.; Rodr´ıguez, N.; Lange, P. P.; Fromm, A. Chem.
Commun. 2009, 7173. (c) Goossen, L. J.; Thiel, W. R.; Rodr´ıguez, N.;
Linder, C.; Melzer, B. AdV. Synth. Catal. 2007, 349, 2241. (d) Dickstein,
J. S.; Mulrooney, C. A.; O’Brien, E. M.; Morgan, B. J.; Kozlowski, M. C.
(7) Cai, G.; Fu, Y.; Li, Y.; Wan, X.; Shi, Z. J. Am. Chem. Soc. 2007,
129, 7666.
Org. Lett. 2007, 9, 2441.
(16) For recent example, see: (a) Mazue´, F.; Colin, D.; Gobbo, J.;
Wegner, M.; Rescifina, A.; Spatafora, C.; Fasseur, D.; Delmas, D.; Meunier,
P.; Tringali, C.; Latruffe, N. Eur. J. Med. Chem. 2010, 45, 2972. (b) Wang,
T. T. Y.; Schoene, N. W.; Kim, Y. S.; Mizuno, C. S.; Rimando, A. M.
Mol. Nutr. Food Res. 2010, 54, 335. (c) Weng, C.-J.; Wu, C.-F.; Huang,
H.-W.; Wu, C.-H.; Ho, C.-T.; Yen, G.-C. J. Agric. Food Chem. 2010, 58,
2886. (d) Sarpietro, M. G.; Ottimo, S.; Giuffrida, M. C.; Spatafora, C.;
Tringali, C.; Castelli, F. Int. J. Pharm. 2010, 388, 144.
(8) (a) Engle, K. M.; Wang, D.-H.; Yu, J.-Q. J. Am. Chem. Soc. 2010,
132, 14137. (b) Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Angew. Chem., Int.
Ed. 2010, 49, 6169. (c) Shi, B.-F.; Zhang, Y.-H.; Lam, J. K.; Wang, D.-H.;
Yu, J.-Q. J. Am. Chem. Soc. 2010, 132, 460. (d) Wang, D.-H.; Engle, K. M.;
Shi, B.-F.; Yu, J.-Q. Science 2010, 327, 315.
(9) Umeda, N.; Hirano, K.; Satoh, T.; Miura, M. J. Org. Chem. 2009,
74, 7094.
(10) Lu, Y.; Wang, D.-H.; Engle, K. M.; Yu, J.-Q. J. Am. Chem. Soc.
2010, 132, 5916.
(17) (a) Chen, G.; Cao, C. J. Phys. Org. Chem. 2010, 23, 776. (b)
Roberts, J. C.; Pincock, J. A. Can. J. Chem. 2003, 81, 709. (c) Papper, V.;
Pines, D.; Likhtenshtein, G.; Pines, E. J. Photochem. Photobiol., A 1997,
111, 87.
(11) Zhang, Y.-H.; Shi, B.-F.; Yu, J.-Q. J. Am. Chem. Soc. 2009, 131,
5072.
Org. Lett., Vol. 12, No. 24, 2010
5777

In an initial attempt, the reaction of 2,4-dimethoxybenzoic
acid (1a) with styrene (2a) (2 equiv) was conducted under
different conditions (Table S1 in the Supporting Information).
Although [Cp*RhCl2]2 (1 mol %) and AgOAc (2-3 equiv)
were found to be suitable choices for the catalyst and oxidant,
respectively, the yield of 3,5-dimethoxystilbene (3a) was
moderate (∼50%).
Table 1. Reaction of Benzoic and Naphthoic Acids 1 with
Styrenes 2a
The reaction appears to proceed via (i) the regioselective
olefination at the 6-position of 2-methoxybenzoic acids under
rhodium catalysis (Scheme 4, step 1) and (ii) the subsequent
Scheme 4
decarboxylation of resulting 2-methoxy-6-styrylbenzoic acids
(step 2).
It was revealed that a relatively high temperature (160 °C)
was needed for not step 1 but step 2. Thus, the olefination
of 1a with 2a proceeded smoothly even at 100 °C to produce
the corresponding 2-stilbenecarboxylic acid, which was
subsequently methyl-esterified for quantification (Scheme 5).
Scheme 5
To our delight, the stepwise heating, at 120 °C for 8 h
(olefination) and then at 160 °C for 4 h (decarboxylation),
significantly increased the yield of 3a. It was further
improved by the addition of K2CO3 and AgOAc15 at the
second step up to 85% (Table 1, entry 1). Under similar
conditions, 2-methoxy- (1b), 2-acetylamino- (1c), 2-fluoro-
(1d), and 2-phenyl- (1e) benzoic acids also underwent
decarboxylative olefination to afford the corresponding meta-
substituted stilbenes 3b-e (entries 2-5). 2-Styrylnaphthalene
(3f) could be synthesized effectively from 1-naphthoic acid
(1f) and 2a (entry 6). Substituted styrenes 2b-d also reacted
with 1a to give substituted stilbenes 3g-i (entries 7-9). It
should be noted that the thus obtained 3,5,4′-trimethoxystil-
bene (3g), an analog of naturally occurring resveratrol,18 is
of particular interest for its biological activities including
anticancer properties.16
a Reaction conditions: (1) 1 (0.5 mmol), 2 (1 mmol), [(Cp*RhCl2)2]
(0.005 mmol), AgOAc (1-1.5 mmol), DMAc (2.5 mL) at 120 °C under
N2 for 8 h; (2) With the addition of AgOAc (1 mmol) and K2CO3 (1 mmol)
at 160 °C under N2 for 4 h. b GC yield based on the amount of 1 used.
Value in parentheses indicates yield after isolation.
Unsubstituted benzoic acid (1g) also underwent the present
decarboxylative olefination upon treatment with 2a under the
stepwise heating conditions (Scheme 6). In this case, double
olefination took place at the 2- and 6-positions to form 1,3-
distyrylbenzene (5)19 selectively. As in the reaction of 1a
(Scheme 5), the intermediary formation of 2,6-distyrylben-
5778
Org. Lett., Vol. 12, No. 24, 2010

Scheme 6
Scheme 7
by a factor of 2.2 (λemis ) 413 nm, Figure S1 in the
Supporting Information).
zoic acid was confirmed and quantitated after esterifiaction
to the methyl ester (6) (Scheme 6).
In summary, we have succeeded in preparing a series of
meta-substituted stilbenes as well as 1,3- and 1,4-distyryl-
benzene derivatives from readily available benzoic acids and
styrenes through precisely ordered ortho-olefination/decar-
boxylaion under rhodium catalysis. In these reactions, the
carboxyl function acts as a unique, removable directing
group. Work is underway toward further development of
relevant reactions around the key functional group.
Interestingly, a 1,4-distyrylbenzene derivative, which is
of importance in organic materials field,20 could be obtained
through the reaction of phthalic acid (1h) with 2a (Scheme
7). Thus, treatment of 1h with 2a in the presence of
[Cp*RhCl2]2 and Cu(OAc)2·H2O, as a catalyst and an oxidant,
respectively, in DMF at 100 °C for 10 h followed by methyl
esterification resulted in the selective formation of methyl
2,5-distyrylbenzoate (7).
Acknowledgment. This work was partly supported by
Grants-in-Aid from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Some obtained distyrylbenzenes showed solid-state fluo-
rescence as described above. Notably, 6 exhibited a relatively
strong emission compared to a typical emitter, anthracene,
Supporting Information Available: Additional results,
standard experimental procedure, and characterization data
of products. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) (a) Iriti, M.; Faoro, F. Med. Hypotheses 2006, 67, 833. (b)
Heynekamp, J. J.; Weber, W. M.; Hunsaker, L. A.; Gonzales, A. M.; Orland,
R. A.; Deck, L. M.; Vander Jagt, D. L. J. Med. Chem. 2006, 49, 7182. (c)
Vessie`res, A.; Top, S.; Pigeon, P.; Hillard, E.; Boubeker, L.; Spera, D.;
Jaouen, G. J. Med. Chem. 2005, 48, 3937. (d) Jang, M.; Cai, L.; Udeani,
G. O.; Slowing, K. V.; Thomas, C. F.; Beecher, C. W. W.; Fong, H. H. S.;
Farnsworth, N. R.; Kinghorn, A. D.; Mehta, R. G.; Moon, R. C.; Pezzuto,
J. M. Science 1997, 275, 218.
OL1027392
(19) For example, see: (a) Ginocchietti, G.; Mazzucato, U.; Spalletti,
A. J. Photochem. Photobiol., A 2008, 196, 233. (b) Tang, Y.; Wang, Y.;
Wang, X.; Xun, S.; Mei, C.; Wang, L.; Yan, D. J. Phys. Chem. B 2005,
109, 8813. (c) Marri, E.; Galiazzo, G.; Mazzucato, U.; Spalletti, A. Chem.
Phys. 2008, 312, 205. (d) Tang, Y.; Wang, Y.; Wang, G.; Wang, H.; Wang,
L.; Yan, D. J. Phys. Chem. B 2004, 108, 12921. (e) Konstandakopoulou,
F. D.; Iconomopoulou, S. M.; Gravalos, K. G.; Kallitsis, J. K. Chem. Mater.
2000, 12, 2957. (f) Lane, P. A.; Mellor, H.; Martin, S. J.; Hagler, T. W.;
Bleyer, A.; Bradley, D. D. C. Chem. Phys. 2000, 257, 41. (g) Hidaka, T.;
Nakatani, H.; Yamanaka, K. Jpn. Kokai Tokkyo Koho 1991; 03056449. (h)
Pavia, M. R.; Moos, W. H.; Hershenson, F. M. J. Org. Chem. 1990, 55,
560. (i) Noller, K.; Kosteyn, F.; Meier, H. Chem. Ber. 1988, 121, 1609. (j)
Colle, R.; Gozzo, F.; Camaggi, G.; Siddi, G. Jpn. Kokai Tokkyo Koho 1978,
53127425.
(20) For example, see: (a) Wang, Y.; Liu, D.; Ikeda, S.; Kumashiro,
R.; Nouch, R.; Xu, Y.; Shang, H.; Ma, Y.; Tanigaki, K. Appl. Phys. Lett.
2010, 97, 033305. (b) Shimizu, M.; Hiyama, T. Chem.sAsian J. 2010, 5,
1516. (c) Nielsen, C. B.; Arnbjerg, J.; Johnsen, M.; Jørgensen, M.; Ogilby,
P. R. J. Org. Chem. 2009, 74, 9094. (d) Xie, Z.; Xie, W.; Li, F.; Liu, L.;
Wang, H.; Ma, Y. J. Phys. Chem. C 2008, 112, 9066. (e) Xie, W.; Li, F.;
Wang, H.; Xie, Z.; Shen, F.; Ma, Y.; Lu, W.; Zhang, D.; Ma, D. Appl. Opt.
2007, 46, 4431. (f) Xie, Z.; Yang, B.; Li, F.; Cheng, G.; Liu, L.; Yang, G.;
Xu, H.; Ye, L.; Hanif, M.; Liu, S.; Ma, D.; Ma, Y. J. Am. Chem. Soc.
2005, 127, 14152. (g) Koch, F.; Heitz, W. Macromol. Chem. Phys. 1997,
198, 1531. (h) Oberski, J.; Bolle, B.; Schaper, A.; Greiner, A. Polym. AdV.
Technol. 1993, 5, 105. (i) Vestweber, H.; Oberski, J.; Greiner, A.; Heitz,
W.; Mahrt, R. F.; Ba¨ssler, H. AdV. Mater. Opt. Electron. 1993, 2, 197.
Org. Lett., Vol. 12, No. 24, 2010
5779