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Palladium Nanoparticle Catalyzed Hiyama Coupling Reaction of Benzyl Halides Dipankar Srimani, Ansuman Bej, and Amitabha Sarkar* Department of Organic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032
[email protected] Received February 24, 2010
An efficient Hiyama coupling reaction between benzylic halide and aryltrialkoxysilane using Pd nanoparticles has been developed. This procedure accommodates various functional groups to yield a diverse range of diarylmethanes which are ubiquitous units of natural products and pharmaceuticals.
as macrocycles, catenanes, and rotaxanes.2 The common methods for making diarylmethanes are reduction of diaryl ketones or diarylcarbinols, Friedel-Crafts alkylation,3 and transition metal-catalyzed coupling reactions (either a benzyl halide with an arylmetal species or an aryl halide with a benzylmetal species).4-11 Transition-metal-catalyzed cross-coupling reactions have emerged as a powerful method for the construction of carbon-carbon bonds.4 However, benzylic halides have been studied to a lesser extent compared to aryl halides in coupling reaction. The most promising methods for diarylmethane synthesis have been the Suzuki-Miyaura-type coupling of benzyl halides,5 benzyl carbonates,6a,b benzyl phosphates,6c or benzyl acetate.6d Organozinc,7 organotin,8 and organoindium9 have also been used in such reactions. Cu(I)-catalyzed cross-coupling reactions between Grignard reagents and benzylic halides10a or phosphates10b and nickel-,11a iron-,11b or cobalt-catalyzed7 cross-coupling with zinc organometallics have also been reported.
(1) (a) Leeson, P. D.; Emmett, J. C.; Shah, V. P.; Showell, G. A.; Novelli, R.; Prain, H. D.; Benson, M. G.; Ellis, D.; Pearce, N. J.; Underwood, A. H. J. Med. Chem. 1989, 32, 320. (b) Ku, Y.-Y.; Patel, R. R.; Sawick, D. P. Tetrahedron Lett. 1996, 37, 1949. (c) Gangjee, A.; Devraj, R.; Queener, S. F. J. Med. Chem. 1997, 40, 470. (d) Gangjee, A.; Vasudevan, A.; Queener, S. F. J. Med. Chem. 1997, 40, 3032. (e) McPhail, K. L.; Rivett, D. E. A.; Lack, D. E.; Davies-Coleman, M. T. Tetrahedron 2000, 56, 9391. (f) Wai, J. S.; Egbertson, M. S.; Payne, L. S. T.; Fisher, E.; Embrey, M. W.; Tran, L. O.; Melamed, J. Y.; Langford, H. M.; Guare, J. P.; Zhuang, L.; Grey, V. E.; Vacca, J. P.; Holloway, M. K.; Naylor-Olsen, A. M.; Hazuda, D. J.; Felock, P. J.; Wolfe, A. L.; Stillmock, K. A.; Schleif, W. A.; Gabryelski, L. J.; Young, S. D. J. Med. Chem. 2000, 43, 4923. (g) Boyd, R. E.; Rasmussen, C. R.; Press, J. B.; Raffa, R. B.; Codd, E. E.; Connelly, C. D.; Li, Q. S.; Martinez, R. P.; Lewis, M. A.; Almond, H. R.; Reitz, A. B. J. Med. Chem. 2001, 44, 863. (h) Hsin, L. W.; Dersch, C. M.; Baumann, M. H.; Stafford, D.; Glowa, G. R.; Rothman, R. B.; Jacobon, A. E.; Rice, K. C. J. Med. Chem. 2002, 45, 1321. (i) Rosowsky, A.; Chen, H.; Fu, H.; Queener, S. F. Bioorg. Med. Chem. 2003, 11, 59. (j) Long, Y.-Q.; Jiang, X.-H.; Dayam, R.; Sachez, T.; Shoemaker, R.; Sei, S.; Neamati, N. J. Med. Chem. 2004, 47, 2561. (k) Forsch, R. A.; Queener, S. F.; Rosowsky, A. Bioorg. Med. Chem. Lett. 2004, 14, 1811. (l) Chappie, T. A.; Humphrey, J. M.; Allen, M. P.; Estep, K. G.; Fox, C. B.; Lebel, L. A.; Liras, S.; Marr, E. S.; Menniti, F. S.; Pandit, J.; Schmidt, C. J.; Tu, M.; Williams, R. D.; Yang, F. V. J. Med. Chem. 2007, 50, 182. (m) Mertins, K.; Iovel, I.; Kischel, J.; Zapf, A.; Beller, M. Adv. Synth. Catal. 2006, 348, 691. (2) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154. (b) Conn, M. M.; Rebek, J. Chem. Rev. 1997, 97, 1647. (c) Jasat, A.; Sherman, J. C. Chem. Rev. 1999, 99, 931. (d) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303. (3) (a) Olah, G. A. Friedel-Crafts and Related Reactions; Interscience Publishers: New York, 1965. (b) Olah, G. A. Friedel-Crafts Chemistry; Wiley: New York, 1973. (c) De la Cruz, M. H. C.; Da Silva, J. F. C.; Lachter, E. R. Appl. Catal. A: Gen. 2003, 245, 377. (d) Ivoel, I.; Mertins, K.; Kischel, J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 3913.
(4) (a) Metal Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998. (b) Tsuji, J. Palladium Reagents and Catalysts, Innovations in Organic Synthesis; Wiley: Chichester, U.K., 1995. (c) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic Press: New York, 1985. (d) Metal Catalysed Cross Coupling Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004. (e) Liegault, B.; Renaud, J,-L.; Bruneau, C. Chem. Soc. Rev. 2008, 37, 290. (5) (a) Burns, M. J.; Fairlamb, I. J. S.; Kapdi, A. R.; Sehnal, P.; Taylor, R. J. K. Org. Lett. 2007, 9, 5397. (b) Chahen, L.; Doucet, H.; Santelli, M. Synlett 2003, 1668. (c) Nobre, S. M.; Monteiro, A. L. Tetrahedron Lett. 2004, 45, 8225. (d) Langle, S.; Abarbri, M.; Duchene, A. Tetrahedron Lett. 2003, 44, 9255. (e) Chowdhury, S.; Georghiou, P. E. Tetrahedron Lett. 1999, 40, 7599. (f) Ines, B.; Moreno, I.; SanMartin, R.; Domı´ nguez, E. J. Org. Chem. 2008, 73, 8448. (g) Molander, G. A.; Elia., M. D. J. Org. Chem. 2006, 71, 9198. (h) Singh, R.; Viciu, M. S.; Kramareva, N.; Navarro, O.; Nolan, S. P. Org. Lett. 2005, 7, 1829. (i) Bandgar, B. P.; Bettigeri, S. V.; Phopase, J. Tetrahedron Lett. 2004, 45, 6959. (j) Srimani, D.; Sarkar, A. Tetrahedron Lett. 2008, 49, 6304. (6) (a) Yu, J.-Y.; Kuwano, R. Org. Lett. 2008, 10, 973. (b) Kuwano, R.; Kondo, Y.; Shirahama, T. Org. Lett. 2005, 7, 2973. (c) McLaughlin, M. Org. Lett. 2005, 7, 4875. (d) Kuwano, R.; Yokogi., M. Chem. Commun. 2005, 5899. (7) (a) Amator, M.; Gosmini, C. Chem. Commun. 2008, 5019. (b) Rottl€ ander, M.; Knochel, P. Tetrahedron Lett. 1997, 38, 1749. (c) Piber, M.; Jensen, A. E.; Rottl€ ander, M.; Knochel, P. Org. Lett. 1999, 9, 1323. (d) Manolikakes, G.; Dong, M.Sc. Z.; Mayr, H.; Li, J.; Knochel, P. Chem.;Eur. J. 2009, 15, 1324. (e) Rottl€ ander, M.; Knochel, P. J. Org. Chem. 1998, 63, 203. (f) Metzger, A.; Melzig, L.; Despotopoulou, C.; Knochel, P. Org. Lett. 2009, 11, 4228. (g) Piller, F. M.; Metzger, A.; Schade, M. A.; Haag, B. A.; Gavryushin, A.; Knochel, P. Chem.;Eur. J. 2009, 15, 7192. (h) Metzger, A.; Schade, M. A.; Knochel, P. Org. Lett. 2008, 10, 1107. (i) Duplais, C.; Krasovskiy, A.; Wattenberg, A.; Lipshutz, B. H. Chem. Commun. 2010, 46, 562. (8) (a) Milstein, D.; Stille, J. K. J. Am. Chem. Soc. 1979, 101, 4992. (b) Kamlage, S.; Sefkow, M.; Peter, M. G. J. Org. Chem. 1999, 64, 2938. (c) Crawforth, C. M.; Fairlamb, I. J. S.; Taylor, R. J. K. Tetrahedron Lett. 2004, 45, 461. (d) Crawforth, C. M.; Burling, S.; Fairlamb, I. J. S.; Kapdi, A. R.; Taylor, R. J. K.; Whitwood, A. C. Tetrahedron 2005, 61, 9736. (e) Crawforth, C. M.; Fairlamb, I. J. S.; Kapdi, A. R.; Serrano, J. L.; Taylor, R. J. K.; Sanchez, G. Adv. Synth. Catal. 2006, 348, 405. (f) Appukkuttan, P.; Husain, M.; Gupta, R. K.; Parmar, V. S.; Van der Eycken, E. Synlett 2006, 1491. (g) Nichele, T. Z.; Monteiro, A. L. Tetrahedron Lett. 2007, 48, 7472. (9) (a) Perez, I.; Sestelo, J. P.; Sarandeses, L. A. J. Am. Chem. Soc. 2001, 123, 4155. (b) Mosquera, A.; Riveiros, R.; Perez Sestelo, J.; Sarandeses, L. A. Org. Lett. 2008, 10, 3745. (c) Chupak, L. S.; Wolkowski, J. P.; Chantigny, Y. A. J. Org. Chem. 2009, 74, 1388. (10) (a) Dohle, W.; Lindsay, D. M.; Knochel, P. Org. Lett. 2001, 3, 2871. (b) Kofink, C. C.; Knochel, P. Org. Lett. 2006, 8, 4121. (11) (a) Schade, M. A.; Metzger, A.; Hug, S.; Knochel, P. Chem. Commun. 2008, 3046. (b) Bedford, R. B.; Huwea., M.; Wilkinson, M. C. Chem. Commun. 2009, 600.
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Diarylmethanes constitute an important class of compounds with pharmacological activity.1 They are also frequently used as subunits in supramolecular structures such
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DOI: 10.1021/jo1003373 r 2010 American Chemical Society
JOC Note
Srimani et al. TABLE 1.
Hiyama Coupling of Benzyl Halide with Trialkoxysilane Catalyzed by Pd Nanoparticlea
a
Reaction conditions: 1 mmol of benzyl halide, 2 mmol of siloxane, 4 mol % of K2PdCl4, 1.16 mmol of PEG-600, 1.5 mL of 1 M TBAF in THF at 70 °C under argon. bYield of the isolated product.
It was originally believed that organosilicon compounds are too stable to undergo cross-coupling reactions as the Si-C bond has a significantly low polarity.12 Hiyama demonstrated (12) Negishi, E. Acc. Chem. Res. 1982, 15, 340. (13) Hatanaka, Y.; Hiyama, T. J. Org. Chem. 1988, 53, 918. (14) (a) Hiyama, T. In Metal Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998; Chapter 10. (b) Denmark, S. E.; Sweis, R. F. Acc. Chem. Res. 2002, 35, 835. (c) Hiyama, T. J. Organomet. Chem. 2002, 653, 58. (d) Hatanaka, Y.; Fukushima, S.; Hiyama, T. Chem. Lett. 1989, 1711–1714. (e) Hatanaka, Y.; Goda, K.; Okahara, Y.; Hiyama, T. Tetrahedron 1994, 50, 8301–8316. (d) Hirabayashi, K.; Mori, A.; Kawashima, J.; Suguro, M.; Nishihara, Y.; Hiyama, T. J. Org. Chem. 2000, 65, 5342–5349. (f) Nakao, Y.; Oda, T.; Sahoo, A. K.; Hiyama, T. J. Organomet. Chem. 2003, 687, 570–573.
that arylfluorosilanes undergo palladium-catalyzed crosscoupling reactions with aryl iodides.13 Use of organosilicon compounds as transmetalation reagents has attracted attention since then because of their low cost, ready availability, low toxicity, and chemical stability.14 Lee and Wolf successfully applied organotrimethoxysilanes as efficient coupling reagents in the Hiyama coupling reaction with aryl chlorides and bromides.15 DeShong reported efficient coupling reactions of vinyl and aryl halides, aryl triflates, and allylic benzoates with hypervalent silanes.16 Denmark and others have developed (15) (a) Wolf, C.; Lerebours, R. Org. Lett. 2004, 6, 1147. (b) Lee, H. M.; Nolan, S. P. Org. Lett. 2000, 2, 2053.
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JOC Note Lewis base-promoted reactions using hypervalent silicates17 and demonstrated the use of alkenyl- and arylsilanols18 as efficient coupling partners in Hiyama cross-coupling reactions. However, the Hiyama coupling reaction using benzyl halide is rare in the literature.19 Recently, Hiyama et al.20 reported the cross-coupling reaction of benzylic carbonates with organo[2-hydroxymethyl)phenyl]dimethylsilanes using a combination of (η5-cyclopentadienyl)(η3-allyl)palladium [Cp(allyl)Pd] and 1,10 -bis(diphenylphosphino)ferrocene (dppf). In transition-metal-catalyzed reactions, ligands play a significant role. Numerous phosphine and nonphosphine ligands for palladium are described in the literature for cross-coupling reactions. Several of these ligands are air and moisture sensitive, difficult to prepare, and expensive. Thus, catalysis by a ligand-free metal center is an area of high importance.21 The catalytic system based on metal nanoparticles appears to be a promising area because of the high surface to volume ratio of such particles. Recently, we successfully synthesized Pd nanoparticle of different sizes and reported their catalytic properties in different C-C cross-coupling reactions.22 Herein, we report the first example of Hiyama coupling reaction of benzyl halides and aryl trialkoxysilane with nanosized Pd particles. PEG-600 was used as a stabilizing agent as well as the reducing agent for the preparation of the nanoparticle.23 The nanoparticles were prepared by stirring 14 mg of K2PdCl4 with 700 mg of PEG-600 for 15 min at 70 °C. They were characterized by transmission electron microscopy (TEM). SAED pattern reveals that the Pd nanoparticles are polycrystalline in nature (see the Supporting Information). C(sp3)-chlorides suffer from poor activity in cross-coupling reactions due to the low reactivity of the strong C-Cl bond and serious competition from β-hydride elimination when (16) (a) Brescia, M.-R.; DeShong, P. J. Org. Chem. 1998, 63, 3156. (b) Mowery, M. E.; DeShong, P. J. Org. Chem. 1999, 64, 1684. (c) Mowery, M. E.; DeShong, P. Org. Lett. 1999, 1, 2137. (d) DeShong, P.; Handy, C. J.; Mowery, M. E. Pure Appl. Chem. 2000, 72, 1655. (e) Riggleman, S.; DeShong, P. J. Org. Chem. 2003, 68, 8106. (f) Correia, R.; DeShong, P. J. Org. Chem. 2001, 66, 7159. (g) Hoke, M., E.; Brescia, M.-R.; Bogaczyk, S.; DeShong, P.; King, B. W.; Crimmins, M. T. J. Org. Chem. 2002, 67, 327. (h) McElroy, W. T.; Deshong, P. Org. Lett. 2003, 5, 4779. (i) Seganish, W. M.; Handy, C. J.; DeShong, P. J. Org. Chem. 2005, 70, 8948. (17) (a) Denmark, S. E.; Stavenger, R. A. J. Am. Chem. Soc. 2000, 122, 8837. (b) Denmark, S. E.; Fujimori, S. Org. Lett. 2002, 4, 3473. (c) Denmark, S. E.; Fujimori, S. Org. Lett. 2002, 4, 3477. (d) Denmark, S. E.; Sweis, R. F. Acc. Chem. Res. 2002, 35, 835. (18) (a) Denmark, S. E.; Sweis, R. F. J. Am. Chem. Soc. 2001, 123, 6439. (b) Denmark, S. E.; Ober, M. H. Org. Lett. 2003, 5, 1357. (c) Denmark, S. E.; Regens, C. S. Acc. Chem. Res. 2008, 41, 1486. (19) Hirabayashi, K.; Mori, A.; Kawashima, J.; Suguro, M.; Nishira, Y.; Hiyama, T J. Org. Chem. 2000, 65, 5342. (20) Nakao, Y.; Ebata, S.; Chen, J.; Imanaka, H.; Hiyama, T. Chem. Lett. 2007, 36, 606. (21) (a) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (b) Roucoux, A.; Schulz, J.; Patin, H. Chem. Rev. 2002, 102, 3757. (c) Moreno-Ma~ nas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638. (d) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (e) Astruc, D. Inorg. Chem. 2007, 46, 1884. (22) Srimani, D.; Sawoo, S.; Sarkar, A Org. Lett. 2007, 9, 3639. (b) Sawoo, S.; Srimani, D.; Dutta, P.; Lahiri, R.; Sarkar, A. . Tetrahedron 2009, 65, 4367. (23) The amount of PEG is high, Pd/PEG 1:29, as precedented ( Luo, C.; Zhang, Y.; Wang, Y. J. Mol. Catal. A: Chem. 2005, 229, 7–12. ). (24) (a) Su, C.-R.; Shen, Y.-C.; Kuo, P.-C.; Leu, Y.-L.; Damu, A. G.; Wang, Y.-H.; Wu, T.-S. Bioorg. Med. Chem. Lett. 2006, 16, 6155. (b) Belley, M.; Chan, C. C.; Gareau, Y.; Gallant, M.; Juteau, H.; Houde, K.; Lachance, N.; Labelle, M.; Sawyer, N.; Tremblay, N.; Limontage, S.; Carriere, M.-C.; Denis, D.; Greig, G. M.; Slipez, D.; Gordon, R.; Chauret, N.; Lo, C.; Zamboni, R. J.; Metters, K. M. Bioorg. Med. Chem. Lett. 2006, 16, 5639.
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β-hydrogen is present. Encouraged by recent reports24 where aliphatic chloro compounds were found to participate in coupling reactions, we decided to examine reactive benzyl chlorides that have no β-hydrogen. The cross-coupling of benzyl chloride with phenyltrimethoxysilane with a palladium nanoparticle in THF was investigated using a range of different base activators. Use of 1.5 equiv of TBAF as base yielded optimum results. CsF or KF was not useful. Other bases such as K2CO3, Cs2CO3, K3PO4, NaOAc, and CsOAc also failed to promote the cross-coupling reaction. It was observed that 2 equiv of phenyltrimethoxysilane and 4 mol % of Pd were required for 1 equiv of benzyl chloride to obtain 90% of the coupled product consistently at 70 °C in THF for 1.5 h (Table 1, entry 1). This condition was used for all of the examples in Table 1. On decreasing the Pd concentration from 4 to 2 mol %, the yield of diarylmethane dropped to 30% after 2 h, but it was improved to 80% after stirring for 8 h at 70 °C. No product was observed in the absence of any palladium catalyst. The electronic influence of the aromatic substituents in the benzyl chloride moiety was found to be minimal. No significant steric effect was observed (Table 1, entries 5, 26, and 27). The results summarized in the Table 1 show that a wide variety of functional groups are compatible with this procedure (Table 1, entries 9, 20-25). The reaction is completely chemoselective: benzyl chlorides reacts in preference to aryl chlorides (Table 1, entries 3, 4, 17-19). The arylalkyl chlorides derived from naphthalene or heterocycles can also be used as substrates (Table 1, entries 12-14, 19). The scope of these coupling reactions could be extended to benzylic tosylates, which gave coupled products in excellent yield (Table 2). However, benzylic acetates and carbonate are not suitable for this reaction. Palladium-catalyzed allyl cross-coupling reaction of cinnamyl chlorides with aryl siloxanes provides a powerful tool for the synthesis of 1,3-diarylpropenes, which constitutes an important structural assembly in many molecules of TABLE 2. Hiyama Coupling of Benzyl Electrophile with Trialkoxysilane Catalyzed by Pd Nanoparticlea
a Reaction conditions: 1 mmol of benzyl electrophile, 2 mmol of siloxane, 4 mol % of K2PdCl4, 1.16 mmol of PEG-600, 1.5 mL of 1 M TBAF in THF at 70 °C under argon. bYield of the isolated product.
JOC Note
Srimani et al. TABLE 3. Hiyama Coupling of Allyl Halide with Trialkoxysilane Catalyzed by Pd Nanoparticlea
SCHEME 2. phenol
a Reaction conditions: 1 mmol of allyl halide, 2 mmol of siloxane, 4 mol % of K2PdCl4, 1.16 mmol of PEG-600, 1.5 mL of 1 M TBAF in THF at 70 °C under argon. bYield of the isolated product.
positions, and then its reaction with triethoxy(4-methoxyphenyl)silane afforded the desired coupled product.27 Demethylation28 of this product has been reported to yield the natural product. In summary, we have developed a new attractive method for preparing diarylmethanes from functionalized benzyl and cinnamyl halides via the Hiyama coupling reaction using Pd nanoparticles as catalyst. This operationally simple and relatively inexpensive procedure shows good functional group tolerance and affords the desired diarylmethane in high yield.
SCHEME 1.
Hiyama Coupling with Allyl Chloride
Synthetic Route to 2, 4-Bis(4-hydroxybenzyl)-
Experimental Section
biological importance.25 The coupling reactions with allylic halide proceeded to give excellent yield of the coupling product despite the presence of the β-hydrogen in the allylic substrate26 (Table 3). Although mechanistically viable (Scheme 1), no allenic byproduct was isolated from this reaction. We successfully applied our methodology to synthesize naturally occurring 2,4-bis(4-hydroxybenzyl)phenol from anisole, a simple and inexpensive starting material (Scheme 2). First, anisole was chloromethylated at the ortho and para (25) (a) Terao, J.; Watanabe, H.; Ikumi, A.; Kuniyasu, H.; Kambe, N. J. Am. Chem. Soc. 2002, 124, 4222. (b) Frisch, A. C.; Shaikh, N.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2002, 41, 4056. (c) Frisch, A. C.; Rataboul, F.; Zapf, A.; Beller, M. J. Organomet. Chem. 2003, 687, 403. (26) When R-methylbenzyl bromide was used as substrate, the desired coupled product was not obtained; styrene was obtained instead as the major product: Srimani, D. Unpublished work.
General Procedure for the Preparation of Pd Nanoparticle and Catalysis of Hiyama Coupling Reaction. A weighed amount of K2PdCl4 (13 mg, 0.04 mmol) and PEG 600 (700 mg, 1.16 mmol) was heated at 70 °C for 15 min to complete the formation of Pd nanoparticles. To this suspension of colloidal Pd at ambient temperature were added aryltrialkoxysilane (2 mmol), benzyl halide (1 mmol), and tetrabutylammonium fluoride The reaction mixture was allowed to cool at room temperature, and THF was removed. The residue was extracted with ether (15 mL 3), and the combined organic extract was dried over anhydrous Na2SO4 and concentrated in vacuum. The residue thus obtained was subjected to flash column chromatography on silica gel (230-400 mesh) with either petroleum ether or ethyl acetate (2-10%) in petroleum ether as eluent.
Acknowledgment. We thank the reviewers for their extremely valuable input. We thank CSIR, India, and UKIERI for financial support. D.S. and A.B. are thankful to CSIR, India, for Research Fellowships. Supporting Information Available: Detailed experimental procedures and characterization data (1H, 13C) of the coupling products. This material is available free of charge via the Internet at http://pubs.acs.org. (27) Noda, N.; Kobayashi, Y.; Miyahara, K.; Fukahori, S. Phytochemistry 1995, 39, 1247. (28) Flaherty, A.; Trunkfield, A.; Barton, W. Org. Lett. 2005, 7, 4975.
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