HETEROCYCLES

Short Paper | Regular issue | Vol. 83, No. 1, 2011, pp. 117-123
Received, 29th September, 2010, Accepted, 16th November, 2010, Published online, 24th November, 2010.
DOI: 10.3987/COM-10-12074

■ Heck Reactions in 2,6-Diaryl-3,5-dibromo-4-pyrones in the Presence of N,N’-Dibutylbenzimidazolium Bromide

Zarrin Ghasemi, Vahideh Nazari-Belvirdi, Maryam Allahvirdinasab, and Aziz Shahrisa*

Department of Organic and Bioorganic Chemistry, Faculty of Chemistry, University of Tabriz, 51664, Tabriz, Iran

Abstract

Synthesis of two 2,6-diaryl-3,5-dibromo-4-pyrone derivatives is described, by using of NBS/DMF bromination of related 2,6-diaryl-4-pyrones. PdCl2 catalyzed Heck coupling reactions of these dibromides, with emphasis on the use of N,N′-dibutylbenzimidazol-2-ylidene as ligand, resulted in the formation of two class of mono- and di-vinylated products.

Palladium-catalyzed cross-coupling Heck reaction of aryl and heteroaryl halides with various olefins is one of the most intensively studied reactions and an versatile C-C bond forming tool in synthesis of important functionalized compounds.1 Phosphine-palladium complexes in homogeneous solutions or heterogeneous systems, palladacycles and N-heterocyclic carbene (NHC)-palladium complexes are the well established palladium species and generally employed as efficient catalysts for the Heck reactions. The use of N-heterocyclic carbens as potential candidates for many palladium catalyzed coupling reactions is rapidly increased.2 The electron richness of NHC ligands and strong palladium-carbon bond in such complexes provide the ancillary supporting and stabilization of metal center, and prevent the possible dissociation of the carbon-metal bond at different stages of these catalytic cycles, thereby making them thermally and oxidatively stable.3
4-Pyrone derivatives also, are of considerable pharmacological relevance and found in a whole spectrum of bioactive systems.4 Through our recently attempts for synthesis of cyclic and polyfunctional frameworks of 4-pyrones,5 we were interested in the coupling reactions in 4-pyrone backbone which have rarely been reported.6 We report here the 3,5-dibromination reactions of 2,6-diaryl-4-pyrones and the Heck vinylation reactions of these dibromides, catalyzed by N,N΄-dibutylbenzimidazolium bromides (DBBIB)-PdCl2 system.
The 2,6-diaryl-4-pyrones 1a and 1b used for bromination reactions, were prepared through cyclization of related 1,3,5-triketone derivatives under acidic conditions, which is a known route for the synthesis of a variety of 2,6-disubstituted-4-pyrone structures.7 We examined the bromination of compounds 1a and 1b by using of NBS in dry DMF at room temperature and achieved the brominated products 2a and 2b in good yields, respectively (Scheme 1). Although by the using of excess amount of brominating reagent (4 eq), the aryl rings of 2- and 6- positions were not affected. It should be noticed that treatment of 1b with 2 eq. of NBS, gave a mixture of mono- and di- brominated products. The reagent NBS/DMF has been applied for electrophilic bromination at electron rich positions of various aryl and heterocyclic rings.8

For evalution the reactivity of dibromides 2a-b toward Pd(II) catalyzed Heck coupling reactions, we originally treated the compound 2a with methyl acrylate (4 eq), in the presence of PdCl2 (1 eq%) as catalyst, triphenylphosphine (2 eq%) as ligand, and triethylamine as base at 130 οC for 24 h. Under these conditions, products 3a and 4a were formed along with the large amounts of unreacted starting materials (Scheme 2). Formation of product 4a can be explained by partial reduction of the in situ formed 3-bromo-5-alkenyl-4-pyrone intermediate. It is noteworthy that changes in the amounts of catalyst, ligand, alkene and base did not influence the situation of the results. Subsequently, we attempted these reactions under the same stoichiometric conditions by using N,N,′-dibutylbenzimidazolium bromide insted of PPh3, consequently complete conversion of starting matarials as well as formation of two class products 3 and 4 were observed after 18 h. Yields of each of these products in the reactions of 2a,b with alkyl acrylates have been monitored in Table 1. As the table shows, the reaction of dibromides 2a,b with n-butyl acrylate, have exclusively resulted in the formation of only divinylated products (Entries 3, 6).

In conclusion We developed efficient electrophilic bromination of 2,6-diaryl-4-pyrone derivatives 1a,b by using of NBS/DMF reagent, which in result, 2,6-diaryl-3,5-dibromo-4-pyrones were obtained by simple workup and good yields. Furthermore the Heck coupling reactions of these dibromides with methyl, ethyl and butyl acrylates in the presence of PdCl2-DBBIB system gave the di- and mono- vinylated products. Photocyclization reactions of compounds 3a-f, 4a,b and 4d,e are under investigation.

EXPERIMENTAL
Melting points were measured on a Electrothermal MEL-TEMP apparatus (model 1202D) and are uncorrected. FT-IR spectra were recorded on a Bruker Tensor 27 spectrometer. 1H, and 13C NMR spectra were recorded at 400 and 100 MHz respectively on a Bruker Spectrospin Avance 400 spectrometer with CDCl3 as solvent and TMS as internal standard. Mass spectra were recorded on a Shimadzo spectrometer operating at an ionization potential of 70 eV. Elemental analyses for C and H were obtained using a Vario EL ΙΙΙ apparatus (Elementar Co.). Preparative layer chromatography (PLC) was done using silica gel (Merk Kieselgel 60 PF254+366) .
General procedure for the electrophilic bromination of 2,6-diaryl-4-pyrones (1a-b)
To a 100 mL flask charged with 2,6-diaryl-4-pyrone (2 mmol) in dry DMF (10 mL) was added a solution of NBS (1.42 g, 8 mmol) in dry DMF (4 mL). The mixture was stirred at room temperature for 24 h and then poured into water (100 mL). Resulted white suspension was extracted with Et2O (3 × 50 mL). The ether extracts were dried with Na2SO4, filtered, and concentrated in vacuo to dryness. Obtained solid was recrystallized with EtOH to give 2,6-diaryl-3,5-dibromo-4-pyrones (2a-b) as white crystals.
3,5-Dibromo-2,6-diphenyl-4H-pyran-4-one (2a)
White crystals, yield 61%, mp 169-170 οC. υmax (KBr) 3060, 2924, 1645 (pyrone C=O), 1612, 1342, 1004, 755, 694 cm-1. 1H NMR (CDCl3), δ 7.48-7.56 (6H, m, phenyl-H), 7.79-7.82 (4H, m, phenyl-H). 13C NMR (CDCl3), δ 109.9, 127.4, 128.1, 130.3, 130.4, 160.5, 168.8. MS, m/z (%)= 408 (M+4, 4), 406 (M+2, 12), 404 (M, 5), 377 (15), 113 (30), 105 (94), 77 (100). Anal. Calcd for C17H10O2Br2 (406.08): C, 50.28; H, 2.48. Found: C, 50.22; H, 2.55.
2,6-Bis(4-methylphenyl)-3,5-dibromo-4H-pyran-4-one (2b)
White crystals, yield 60%, mp 186-187 οC. υmax (KBr) 3031, 2950, 1646 (pyrone C=O), 1611, 1505, 1340, 1006, 820, 756 cm-1. 1H NMR (CDCl3): δ 2.43 (6H, s, CH3), 7.30 (4H, d, J=8.1 Hz, aryl-H), 7.71 (4H, d, J=8.1 Hz, aryl-H). 13C NMR (CDCl3): δ 20.5, 109.4, 127.6, 128.0, 128.1, 140.9, 160.5, 168.9. MS, m/z (%)= 436 (M+4, 42), 434 (M+2, 58), 432 (M, 24), 405 (100), 403 (50), 353 (16), 194 (22). Anal. Calcd for C19H14O2Br2 (434.13): C, 52.57; H, 3.25. Found: C, 52.22; H, 3.55.
General procedure for the Heck coupling reactions of dibromides (2a-b) with alkyl acrylates
A mixture of 2,6-diaryl-3,5-dibromo-4-pyrone (1.0 mmol), alkyl acrylate (4.0 mmol), PdCl2 ( 0.002 g, 1 mol%), N,N′-dibutylbenzimidazolium bromide (0.006 g, 2 mol%), triethylamine (0.4 mL, 4 mmol) in dry DMF (4 mL) was stirred at 130 οC for 18 h. Then, water (150 mL) was added, and after stirring (2 min), the mixture was extracted with EtOAc (3 × 50 mL). The organic layers were combined, dried with Na2SO4 and concentrated in vacuo. The residue was purified by PLC on silicagel (hexane/CH2Cl2 1:1) to give the mono- and di-vinylated products (3, 4).
3,5-Bis[trans-2-(methoxycarbonyl)ethenyl]-2,6-diphenyl-4H-pyran-4-one (3a)
Yellow solid, yield 65%, mp 188-190 οC. υmax (KBr) 3060, 2996, 1714 (ester C=O), 1648 (pyrone C=O), 1602, 1284, 1161 cm-1. 1H NMR (CDCl3): δ 3.77 (6H, s, CO2CH3), 7.43 (2H, d, J=15.9 Hz, CH=CH-CO2Me), 7.49-7.61 [8H, m, (6H, phenyl-H), (2H, CH=CH-CO2Me)], 7.62-7.64 (4H, m, phenyl-H). 13C NMR (CDCl3): δ 51.6, 118.5, 124.5, 128.8, 129.7, 131.2, 131.5, 135.2, 164.9, 168.0, 178.7. Anal. Calcd for C25H20O6 (416.43): C, 72.11; H, 4.84. Found: C, 72.03; H, 4.93.
3,5-Bis[trans-2-(ethoxycarbonyl)ethenyl]-2,6-diphenyl-4H-pyran-4-one (3b)
Yellow solid, yield 57%, mp 158-160 οC. υmax (KBr) 3080, 2963, 1710 (ester C=O), 1647 (pyrone C=O), 1623, 1261, 1030 cm-1. 1H NMR (CDCl3): δ 1.36 (6H, t, J=7.1 Hz, CH2CH3), 4.29 (4H, q, J=7.1 Hz, CH2CH3), 7.49 (2H, d, J=15.8 Hz, CH=CH-CO2Et), 7.57-7.64 [8H, m, (6H, phenyl-H), (2H, CH=CH-CO2Et)], 7.68-7.70 (4H, m, phenyl-H). 13C NMR (CDCl3): δ 13.2, 59.4, 117.5, 123.9, 127.8, 128.7, 130.2, 130.5, 133.9, 163.8, 166.6, 175.9. Anal. Calcd for C27H24O6 (444.21): C, 72.99; H, 5.40. Found: C, 72.62; H, 5.16.
3,5-Bis[trans-2-(butoxycarbonyl)ethenyl]-2,6-diphenyl-4H-pyran-4-one (3c)
Yellow solid, yield 73%, mp 85-87 οC. υmax (KBr) 3070, 2929, 1712 (ester C=O), 1646 (pyrone C=O), 1625, 1274, 1025 cm-1. 1H NMR (CDCl3): δ 0.93 (6H, t, J=7.3 Hz, (CH2)3-CH3), 1.25-1.45 (4H, m, -(CH2)2-CH2-CH3), 1.59-1.67 (4H, m, CH2CH2CH2CH3), 4.15 (4H, t, J=6.5 Hz, CO2CH2), 7.42 (2H, d, J=15.8 Hz, CH=CH-CO2Bu), 7.50-7.58 [8H, m, (6H, phenyl-H), (2H, CH=CH-CO2Bu)], 7.60-7.62 (4H, m, phenyl-H). 13C NMR (CDCl3): δ 12.7, 18.2, 29.6, 63.3, 117.5, 123.9, 127.8, 128.7, 130.2, 130.5, 133.9, 163.8, 166.7 (ester C=O), 175.9 (pyrone C=O). Anal. Calcd for C31H32O6 (500.25): C, 74.42; H, 6.39. Found: C, 74.46; H, 6.66.
3,5-Bis[trans-2-(methoxycarbonyl)ethenyl]-2,6-bis (4-methylphenyl)-4H-pyran-4-one (3d)
Yellow solid, yield 64%, mp 201-202 οC, υmax (KBr) 3069, 2940, 1714 (ester C=O), 1648 (pyrone C=O), 1613, 1415, 1282, 1221 cm-1. 1H NMR (CDCl3): δ 2.45 (6H, s, benzylic CH3), 3.79 (6H, s, CO2CH3), 7.34 (4H, d, J=8.1 Hz, aryl-H), 7.43 (2H, d, J=15.9 Hz, CH=CH-CO2Me), 7.50 (2H, d, J=15.9 Hz, CH=CH-CO2Me), 7.52 (4H, d, J=8.1 Hz, aryl-H). 13C NMR (CDCl3): δ 20.5, 50.6, 117.0, 122.9, 127.3, 128.5, 128.7, 134.6, 141.2, 164.1, 167.2, 175.9. Anal. Calcd for C27H24O6 (444.21): C, 72.99; H, 5.40. Found: C, 72.67; H, 5.45.
3,5-Bis[trans-2-(ethoxycarbonyl)ethenyl]-2,6-bis (4-methylphenyl)-4H-pyran-4-one (3e)
Yellow solid, yield 60%, mp 159-160 οC. υmax (KBr) 3085, 2950, 1714 (ester C=O), 1646 (pyrone C=O), 1625, 1420, 1270 cm-1. 1H NMR (CDCl3): δ 1.25 (6H, t, J=7.0 Hz, CH2CH3), 2.46 (6H, s, benzylic CH3), 4.20 (4H, q, J=7.0 Hz, CH2CH3), 7.28 (4H, d, J=8.2 Hz, aryl-H), 7.35 (2H, d, J=15.9 Hz, CH=CH-CO2Et), 7.51 (2H, d, J=15.9 Hz, CH=CH-CO2Et), 7.69 (4H, d, J=8.2 Hz, aryl-H), 13C NMR (CDCl3): δ 13.2, 20.5, 59.3, 109.3, 123.1, 124.7, 128.2, 128.7, 128.8, 134.5, 163.7, 166.5, 175.8. Anal. Calcd for C29H28O6 (472.23): C, 73.75; H, 5.93. Found: C, 73.48; H, 5.88.
3,5-Bis[trans-2-(buthoxycarbonyl)ethenyl]-2,6-bis (4-methylphenyl)-4H-pyran-4-one (3f)
Yellow solid, yield 74%, mp 87-89 οC. υmax (KBr) 3084, 2950, 1714 (ester C=O), 1648 (pyrone C=O), 1613, 1415, 1282, 1221 cm-1. 1H NMR (CDCl3): δ 0.93 (6H, t, J=7.3 Hz, -(CH2)3-CH3), 1.37-1.43 (4H, m, -(CH2)2CH2CH3), 1.62-1.65 (4H, m, -CH2CH2CH2CH3), 2.43 (6H, s, benzylic CH3), 4.15 (4H, t, J=6.6 Hz, CO2CH2-), 7.31 (4H, d, J=8.0 Hz, aryl-H), 7.42 (2H, d, J=15.8 Hz, CH=CH-CO2Bu), 7.49 (4H, d, J=8.0 Hz, aryl-H), 7.51 (2H, d, J=15.8 Hz, CH=CH-CO2Bu). 13C NMR (CDCl3): δ 12.7, 18.2, 20.5, 29.7, 63.2, 117.0, 123.4, 127.3, 128.5, 128.7, 134.4, 141.1, 164.1, 166.9, 176.1. Anal. Calcd for C33H36O6 (528.27): C, 75.02; H, 6.81. Found: C, 74.64; H, 6.69.
2,6-Diphenyl-3-[trans-2-(methoxycarbonyl)ethenyl]-4H-pyran-4-one (4a)
Light yellow solid, yield 19%, mp 162-164 οC. υmax (KBr) 3070, 2993, 1712, 1648, 1607, 1403, 1286 cm-1. 1H NMR (CDCl3): δ 3.76 (3H, s, CO2CH3), 6.90 (1H, s, pyrone-H), 7.42 (1H, d, J= 15.5 Hz, CH=CH-CO2Me), 7.50-7.60 [7H, m, (1H, CH=CH-CO2Me), (6H, phenyl-H)], 7.65-7.67 (2H, m, phenyl-H), 7.80-7.84 (2H, m, phenyl-H). 13C NMR (CDCl3): δ 51.6, 111.1, 118.9, 124.2, 125.8, 128.9, 129.1, 129.7, 130.5, 130.7, 131.4, 131.6, 135.4, 162.0, 166.4, 168.2, 178.5. Anal. Calcd for C21H16O4 (332.17): C, 75.93; H, 4.82. Found: C, 76.06; H, 4.63.
2,6-Diphenyl-3-[trans-2-(ethoxycarbonyl)ethenyl]-4H-pyran-4-one (4b)
Light yellow solid, yield 21%, mp 131-133 οC. υmax (KBr) 3066, 2928, 1707, 1646, 1611, 1261, 1096 cm-1. 1H NMR (CDCl3): δ 1.36 (3H, t, J=7.1 Hz, CH2CH3), 4.29 (2H, q, J=7.1 Hz, CH2CH3), 6.97 (1H, s, pyrone-H), 7.48 (1H, d, J=15.8 Hz, CH=CH-CO2Et), 7.55-7.67 [7H, m, (1H, CH=CH-CO2Et), (6H, phenyl-H)], 7.72-7.74 (2H, m, phenyl-H), 7.87-7.89 (2H, m, phenyl-H). 13C NMR (CDCl3): δ 13.2, 59.3, 110.1, 117.9, 123.6, 124.8, 127.8, 128.1, 128.7, 129.7, 130.3, 130.5, 130.6, 134.1, 160.9, 165.0, 166.7, 177.6. Anal. Calcd for C22H18O4 (346.18): C, 76.32; H, 5.19. Found: C, 76.09; H, 5.11.
2,6-Bis(4-methylphenyl)-3-[trans-2-(methoxycarbonyl)ethenyl]-4H-pyran-4-one (4d)
Light yellow solid, yield 22%, mp 167-168 οC. υmax (KBr) 3080, 2994, 1713, 1645, 1607, 1405, 1290, 1034 cm-1. 1H NMR (CDCl3): δ 2.43 (3H, s, benzylic CH3), 2.45 (3H, s, benzylic CH3), 3.77 (3H, s, CO2CH3), 6.92 (1H, s, pyrone-H), 7.27 (2H, d, J=8.1 Hz, aryl-H), 7.36 (2H, d, J=8.0 Hz, aryl-H), 7.43 (1H, d, J=15.8 Hz, CH=CH-CO2Me), 7.50 (1H, d, J=15.8 Hz, CH=CH-CO2Me), 7.54 (2H, d, J=8.0 Hz, aryl-H), 7.68 (2H, d, J=8.1 Hz, aryl-H). 13C NMR (CDCl3): δ 20.7, 20.9, 51.5, 111.3, 115.3, 123.6, 123.9, 127.2, 127.8, 128.1, 128.3, 128.5, 128.7, 134.2, 139.9, 158.9, 164.8, 166.6, 173.3. Anal. Calcd for C23H20O4 (360.19): C, 76.69; H, 5.55. Found: C, 76.38; H, 5.24.
2,6-Bis(4-methylphenyl)-3-[trans-2-(ethoxycarbonyl)ethenyl]-4H-pyran-4-one (4e)
Light yellow solid, yield 15%, mp 161-162 οC. υmax (KBr) 3060, 2965, 1709, 1645, 1611, 1507, 1453, 1414, 1283, 1033 cm-1. 1H NMR (CDCl3): δ 1.32 (3H, t, J=7.0 Hz, CH2CH3), 4.20 (2H, q, J=7.0 Hz, CH2CH3), 2.42 (3H, s, benzylic CH3), 2.46 (3H, s, benzylic CH3), 6.84 (1H, s, pyrone-H), 7.28 (2H, d, J=8.2 Hz, aryl-H), 7.36 (2H, d, J=8.0 Hz, aryl-H), 7.41 (1H, d, J=15.8 Hz, CH=CH-CO2Et), 7.50 (1H, d, J=15.8 Hz, CH=CH-CO2Et), 7.53 (2H, d, J=8.0 Hz, aryl-H), 7.69 (2H, d, J=8.2 Hz, aryl-H). 13C NMR (CDCl3): δ 13.3, 20.5, 20.6, 59.4, 112.3, 115.4, 123.8, 124.9, 127.1, 127.9, 128.1, 128.2, 128.5, 128.7, 134.1, 140.8, 159.6, 164.9, 166.8, 172.1. Anal. Calcd for C24H22O4 (374.20): C, 77.03; H, 5.88. Found: C, 76.86; H, 5.50.

ACKNOWLEDGEMENTS
We thank research affair of University of Tabriz for financial support.

References

1. W. Pei, J. Mo, and J. Xiao, J. Organometal. Chem., 2005, 690, 3546; M. Hussain, N. T. Hung, and P. Langer, Tetrahedron Lett., 2009, 50, 3929; CrossRef S. M. T. Toguem, M. Hussain, I. Malik, A. Villinger, and P. Langer, Tetrahedron Lett., 2009, 50, 4962; CrossRef N. Robert, C. Hoarau, S. Celanire, P. Ribereau, and F. Marsais, Tetrahedron, 2005, 61, 4569; CrossRef F. Berthiol, M. Feuerstein, H. Doucet, and M. Santelli, Tetrahedron Lett., 2002, 43, 5625; CrossRef I. Malik, M. Hussain, A. Ali, S. M. T. Toguem, and P. Langer, Tetrahedron, 2010, 66, 1637. CrossRef
2. S. Gulcemal, S. Kahraman, J. C. Daran, and E. Cetinkaya, J. Organometal. Chem., 2009, 694, 3580; S. Demir, I. Ozdemir, and B. Cetinkaya, Appl. Organometal. Chem., 2009, 23, 520; CrossRef G. Zou, W. Huang, Y. Xiao, and J. Tang, New J. Chem., 2006, 30, 803; CrossRef B. Altava, M. I. Burguete, N. Karbass, and S. V. Luis, Tetrahedron Lett., 2006, 47, 2311; CrossRef B. Yiğit, M. Yiğit, İ. Özdemir, and E. Çetinkaya, Heterocycles, 2010, 81, 943; CrossRef Ö. Doğan, N. Gürbüz, İ. Özdemir, B. Çetinkaya, and O. Büyükgüngör, Dalton Ttrans., 2009, 35, 7087.
3. J. Ye, W. Chen, and D. Wang, Dalton Trans., 2008, 4015; CrossRef S. Diez-Gonzalez, N. Marion, and S. P. Nolan, Chem. Rev., 2009, 109, 3612. CrossRef
4. W. Wilk, H. Waldmann, and M. Kaiser, Bioorg. Med. Chem., 2009, 17, 2304; CrossRef Y. L. Yan, and S. M. Cohen, Org. Lett., 2007, 9, 2517; CrossRef M. S. South, C. C. Ma, and K. J. Koeller, United States Patents, 2005, 6,916,847 B2 (Chem. Abstr., 2001, 135, 303776e); T. Sengoku, T. Takemura, E. Fukasawa, and H. Kigoshi, Tetrahedron Lett., 2009, 50, 325. CrossRef
5. A. Shahrisa, Z. Ghasemi, and M. Saraei, J. Heterocycl. Chem., 2009, 46, 273; CrossRef A. Shahrisa and Z. Ghasemi, Chem. Hetrocycl. Compounds, 2010, 46, 30. CrossRef
6. J. Farard, C. Loge, B. Pfeiffer, and B. Lesur, Tetrahedron Lett., 2009, 50, 5729; CrossRef O. D. Lopez, J. T. Goodrich, F. Yang, and L. B. Snyder, Tetrahedron Lett., 2007, 48, 2063; CrossRef T. Kamino, K. Kuramochi, and S. Kobayashi, Tetrahedron Lett., 2003, 44, 7349. CrossRef
7. A. Sakakura, H. Watanabe, and K. Ishihara, Org. Lett., 2008, 10, 2569; CrossRef V. I. Tyvorskii, D. N. Bobrov, O. G. Kulinkovich, Tetrahedron, 1998, 54, 2819; CrossRef H. Arimoto, S. Nishiyama, and S. Yamamura, Tetrahedron Lett., 1990, 31, 5619; CrossRef A. Shahrisa and S. Hemmati, Indian J. Chem., 2000, 39B, 190.
8. R. H. Mitchell, Y. H. Lai, and R. V. Williams, J. Org. Chem., 1979, 44, 4733; CrossRef M. J. Marsella, K. Yoon, and F. S. Tham, Org. Lett., 2001, 3, 2129; CrossRef C. Xia, X. Fan, J. Locklin, and R. C. Advincula, Org. Lett., 2002, 4, 2067; CrossRef M. A. Ismail, R. Brun, T. Wenzler, and F. A. Tanious, J. Med. Chem., 2004, 47, 3658; CrossRef D. C. M. Chan and A. Rosowsky, J. Org. Chem., 2005, 70, 1364. CrossRef