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Silver(I)-mediated reaction of trimethylsilylated arylacetylenes with sulfonyl chlorides: Unexpected formation of vinyl sulfones
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Chinese Chemical Letters 23 (2012) 1115–1118 www.elsevier.com/locate/cclet
Silver(I)-mediated reaction of trimethylsilylated arylacetylenes with sulfonyl chlorides: Unexpected formation of vinyl sulfones Gui Sheng Deng a,b,*, Teng Fei Sun a a
b
College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Hunan Normal University), Ministry of Education, Changsha 410081, China Received 7 May 2012 Available online 23 September 2012
Abstract A novel reaction of trimethylsilylated arylacetylenes with sulfonyl chlorides was performed in the presence of silver nitrate or triflate. Conjugated vinyl sulfones as dramatic products were obtained in moderate yields and with Z-selectivity. A free radical mechanism has been proposed to account for the formation of the products. # 2012 Gui Sheng Deng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Alkynyl trimethylsilane; Sulfonyl chlorides; Vinyl sulfones; Silver nitrate
Since silyl derivatives are widely used as protecting groups for terminal alkynes, many attentions have been paid to the direct transformation of silyl-protected alkynes into useful precursors. For example, the carbon–carbon bondforming reactions with a variety of carbon electrophiles such as acid chloride [1], cross-coupling reactions with vinylic iodides [2], aryl halides [3] and desilylative halogenation for the conversion to alkynyl halides with NBS [4], NIS [5], and iodine [6] have been achieved recently. CuCl, InBr3 and Ag(I) compounds have been shown to be efficient promoters in the above corresponding transformations. Although remarkable achievements have been obtained, the development of efficient and practical methodology for the direct transformation of silyl-protected alkynes to other useful precursors for organic synthesis is still an important issue. To the best of our knowledge, there are no examples of the coupling reaction of alkynylsilanes with sulfonyl chlorides to date. To accomplish this transformation, it is crucial to choose an appropriate transition-metal reagent, which can deprotect alkynylsilanes 1 and immediately convert the resulting alkynes into the corresponding metal acetylides 2. In theory, the reaction of the resulting metal acetylides 2 with sulfonyl chlorides 3 seems to give the corresponding alkynyl sulfones 4 [7]. This protocol has been outlined in Scheme 1. Many transition-metal reagents may encounter difficulties due to lack of ability to deprotect alkynylsilanes. On the basis of this consideration, silver salts, which succeeded in deprotection of alkynylsilanes [2,3,8], were examined for this purpose.
* Corresponding author at: College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China. E-mail address: [email protected] (G.S. Deng). 1001-8417/$ – see front matter # 2012 Gui Sheng Deng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.07.017
1116
G.S. Deng, T.F. Sun / Chinese Chemical Letters 23 (2012) 1115–1118 R 1
SiMe3
[M]
R
2
M
R'SO2Cl 3
?
R 4
SO2R'
Scheme 1. A proposed reaction between alkynylsilanes and sulfonyl chlorides.
Trimethylsilyl phenylacetylene 1a and p-tosyl chloride 3a were first used as a model to screen the commonly available silver(I) compounds in THF at 20 8C under nitrogen. Surprisingly, the use of 1 equiv. silver nitrate gave vinyl sulfone 5a in 53% yield (entry 1 in Table 1), and no expected product 4a (R = Ph, R0 = p-MeC6H4) was observed by TLC. The reaction was carried out without any additives except silver nitrate. Silver triflate also proved to be efficient for the conversion into 5a, but less active than silver nitrate (35% yield, entry 2 in Table 1). It is worth noting that stoichiometric amount of silver nitrate relative to trimethylsilyl phenylacetylene 1a was required to efficiently carry out this reaction. Utilization of catalytic amount of silver nitrate only gave 5a in very low yield (entry 3 in Table 1). Other silver salts such as silver halides (X = F, Cl, Br, I), silver trifloroacetate, silver acetate and silver carbonate, were not effective for this transformation (entries 4–10 in Table 1). No desired product 5a was formed in the presence of 1 equiv. silver oxide (entry 11 in Table 1). Solvent effect on the reaction is complicated. The reaction efficiency is lower in CH3CN than in THF. No reactions took place in dichloromethane and 1,4-dioxane because of insolubility of silver nitrate (entries 13 and 14 in Table 1). In protic solvent such as EtOH, only ethyl 4-methylbenzenesulfonate was formed without 5a (entry 15 in Table 1). Wet THF proved to be the best alternative in comparison to the solvents investigated. In order to study the scope of this novel transformation and acquire information regarding the influence of the substitution pattern of the substrate on the product distribution, representative alkynyl trimethylsilanes and sulfonyl chlorides were submitted to the above optimal conditions. The results are summarized in Table 2. As shown in Table 2, the reaction of trimethylsilylated aromatic alkynes 1 with arylsulfonyl chlorides 3 mainly generated vinyl sulfones 5 in 50–61% yields (entries 1–10 in Table 2). Obviously, changing the aryl substituent of
Table 1 Effect of silver compounds and solvents on the reaction of trimethylsilyl phenylacetylene 1a with p-tosyl chloride 3a.a Ag (I), wet solvent Ph Ts Ph SiMe3 + TsCl o 20 C, N2 H H 1a 3a 5a Entry
Ag(I) (equiv.)b
Solvent
Reaction time (h)
Yieldc of 5a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
AgNO3 (1 equiv.) AgOTf (1 equiv.) AgNO3 (0.1 equiv.) AgF (1 equiv.) AgCl (1 equiv.) AgBr (1 equiv.) AgI (1 equiv.) CF3COOAg (1 equiv.) CH3COOAg (1 equiv.) Ag2CO3 (1 equiv.) Ag2O (1 equiv.) AgNO3 (1 equiv.) AgNO3 (1 equiv.) AgNO3 (1 equiv.) AgNO3 (1 equiv.)
THF THF THF THF THF THF THF THF THF THF THF CH3CN CH2Cl2 1,4-Dioxane EtOH
24 38 32 48 48 48 48 48 48 48 48 30 40 40 24
53 35 8 NRd NRd NRd NRd NRd NRd NRd NDe 32 NRd NRd NDe
a b c d e
Molar ratio of trimethylsilyl phenylacetylene and p-tosyl chloride is 1:0.5. Equivalent quantity relative to trimethylsilyl phenylacetylene. Isolated yield. No reaction. Some competing reactions were carried out, however, no product 3a was obtained.
G.S. Deng, T.F. Sun / Chinese Chemical Letters 23 (2012) 1115–1118 Table 2 Silver
nitrate-promoted
R
SiMe3 + R'SO2Cl 1
3
reaction
of
various
AgNO3 (1 eq), N2 o
wet THF, 20 C
R H
5
trimethylsilylated alkynes O O O SO2R' S R R' H 6
0
Entry
Product
R
R
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
a b c d e f g h i j k l m n o
Ph p-CH3C6H4 p-nPrC6H4 p-FC6H4 p-CH3OC6H4 Ph p-CH3C6H4 p-nPrC6H4 p-FC6H4 p-CH3OC6H4 Ph p-CH3C6H4 n-C4H9 n-C4H9 Me3Si
p-CH3C6H4 p-CH3C6H4 p-CH3C6H4 p-CH3C6H4 p-CH3C6H4 Ph Ph Ph Ph Ph CH3 CH3 p-CH3C6H4 CH3 p-CH3C6H4
a b c
1
1117
with
sulfonyl
chlorides
3.a
Reaction time (h)
Yield (%)b of vinyl sulfone 5 (Z/E)c
24 30 30 28 32 26 24 28 32 28 36 36 40 40 40
53 56 51 58 60 50 57 57 59 61 42 45 0 0 0
(2:1) (1.5:1) (2:1) (1.7:1) (1.5:1) (3.3:1) (1.5:1) (2.5:1) (2:1) (1.5:1) (1.5:1) (1.2:1)
Molar ratio of alkynyl trimethylsilane, sulfonyl chloride and silver nitrate is 1:0.5:1. Isolated yield. Determined by 1H NMR (500 MHz) of the crude product.
arylacetylenes and arylsulfonyl chlorides did not significantly affect the course of the reaction in terms of efficiency and selectivity (entries 1–10 in Table 2). The methylsulfonyl chloride was less active than arylsulfonyl chloride, so yields were lower (entries 11 and 12 in Table 2). In the above cases, the reactions favored Z-selectivity. Besides the product of vinyl sulfones 5, the corresponding b-keto sulfones 6 have been yielded as side products in 10–15% isolated yields (entries 1–12 in Table 2). No reactions of trimethylsilylated aliphatic alkynes with sulfonyl chlorides 3 were carried out under the same conditions probably due to less activity of the resulting aliphatic alkynyl silver than aromatic alkynyl silver (entries 13–15 in Table 2). In conclusion, we have described a novel reaction [9] of trimethylsilylated alkynes with sulfonyl chlorides in the presence of silver salt without additives. Unexpected vinyl sulfones were obtained in mild yields. Some control experimental has been accomplished to explain the reaction mechanism. The data is available free of charge via the Internet at http://www.chinchemlett.com.cn. Further works in this area and a detailed mechanistic investigation are now in progress. References [1] [2] [3] [4]
[5] [6] [7] [8] [9]
J.S. Yadav, B.V.S. Reddy, M.S. Reddy, Synlett (2003) 1722. J.A. Marshall, H.R. Chobanian, M.M. Yanik, Org. Lett. 3 (2001) 4107. S.E. Denmark, S.A. Tymonko, J. Org. Chem. 68 (2003) 9151. (a) S.N. Georgiades, J. Clardy, Org. Lett. 7 (2005) 4091; (b) X. Nie, G. Wang, J. Org. Chem. 71 (2006) 4734; (c) D.M. Bowles, J.E. Anthony, Org. Lett. 2 (2000) 85. K. Gao, N.S. Goroff, J. Am. Chem. Soc. 122 (2000) 9320. S. Garrais, J. Turkington, W.P.D. Goldring, Tetrahedron 65 (2009) 8418. J.B. Hendricksan, K.W. Bair, J. Org. Chem. 42 (1977) 3875. (a) A. Orsini, A. Vite´risi, A. Bodlenner, et al. Tetrahedron Lett. 46 (2005) 2259; (b) A. Vite´risi, A. Orsini, J.M. Weibel, et al. Tetrahedron Lett. 47 (2006) 2779. Typical procedure: To a suspension of AgNO3 (0.187 g, 1 mmol) and p-tosylchloride 3a (95 mg, 0.5 mmol) in THF (10 mL) under nitrogen atmosphere, was added trimethylsilyl phenylacetylene 1a (1 mmol, 0.174 g, 196 mL). The resulting mixture was stirred at 20 8C for 24 h. The reaction progress was monitored by analytical TLC. Following filtration, the filtrate was concentrated under reduced pressure to afford a crude residue. Purification of the crude residue by column chromatography using 25% (v/v) ethyl acetate–petroleum ether as the eluant afforded a
1118
G.S. Deng, T.F. Sun / Chinese Chemical Letters 23 (2012) 1115–1118
colorless solid 5a, a mixture of Z-5a and E-5a (68 mg, 53% yield, 2:1 molar ratio of Z-5a to E-5a). Data for Z-5a: 1H NMR (500 MHz, CDCl3): d 7.68 (d, 2H, J = 8.5 Hz), 7.54–7.56 (m, 2H), 7.38–7.39 (m, 3H), 7.23 (d, 2H, J = 8.0 Hz), 7.05 (d, 1H, J = 12.0 Hz), 6.50 (d, 1H, J = 12.5 Hz), 2.38 (s, 3H); 13C NMR (125.8 MHz, CDCl3): d 144.34, 141.09, 138.12, 132.39, 131.27, 130.17, 129.62, 129.09, 128.06, 127.57, 21.60. Data for E-5a [10]: 1H NMR (500 MHz, CDCl3): d 7.82 (d, 2H, J = 8.0 Hz), 7.69 (d, 1H, J = 15.0 Hz), 7.45–7.47 (m, 2H), 7.33–7.36 (m, 3H), 7.23 (d, 2H, J = 8.0 Hz), 6.85 (d, 1H, J = 15.5 Hz), 2.43 (s, 3H); 13C NMR (125.8 MHz, CDCl3): d 144.45, 141.96, 137.74, 132.44, 131.15, 130.01, 129.71, 128.55, 127.71, 127.64, 21.64. Data for a mixture of Z-5a and E-5a: IR (KBr): nmax 3057, 2924, 1680, 1597, 1575, 1493, 1448, 1401, 1301, 1212, 1144, 1085, 1030, 1018, 974, 922, 857, 812, 747, 692, 664, 634, 615, 604, 558, 538 cm1; MS (EI) 258 (M+), 209, 193, 178, 165, 139, 119, 103, 91 (100), 77, 65, 51, 39. [10] G. Deng, J. Zou, ARKIVOC (ii) (2010) 186.
Chinese Chemical Letters 23 (2012) 1115–1118 www.elsevier.com/locate/cclet
Silver(I)-mediated reaction of trimethylsilylated arylacetylenes with sulfonyl chlorides: Unexpected formation of vinyl sulfones Gui Sheng Deng a,b,*, Teng Fei Sun a a
b
College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Hunan Normal University), Ministry of Education, Changsha 410081, China Received 7 May 2012 Available online 23 September 2012
Abstract A novel reaction of trimethylsilylated arylacetylenes with sulfonyl chlorides was performed in the presence of silver nitrate or triflate. Conjugated vinyl sulfones as dramatic products were obtained in moderate yields and with Z-selectivity. A free radical mechanism has been proposed to account for the formation of the products. # 2012 Gui Sheng Deng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Alkynyl trimethylsilane; Sulfonyl chlorides; Vinyl sulfones; Silver nitrate
Since silyl derivatives are widely used as protecting groups for terminal alkynes, many attentions have been paid to the direct transformation of silyl-protected alkynes into useful precursors. For example, the carbon–carbon bondforming reactions with a variety of carbon electrophiles such as acid chloride [1], cross-coupling reactions with vinylic iodides [2], aryl halides [3] and desilylative halogenation for the conversion to alkynyl halides with NBS [4], NIS [5], and iodine [6] have been achieved recently. CuCl, InBr3 and Ag(I) compounds have been shown to be efficient promoters in the above corresponding transformations. Although remarkable achievements have been obtained, the development of efficient and practical methodology for the direct transformation of silyl-protected alkynes to other useful precursors for organic synthesis is still an important issue. To the best of our knowledge, there are no examples of the coupling reaction of alkynylsilanes with sulfonyl chlorides to date. To accomplish this transformation, it is crucial to choose an appropriate transition-metal reagent, which can deprotect alkynylsilanes 1 and immediately convert the resulting alkynes into the corresponding metal acetylides 2. In theory, the reaction of the resulting metal acetylides 2 with sulfonyl chlorides 3 seems to give the corresponding alkynyl sulfones 4 [7]. This protocol has been outlined in Scheme 1. Many transition-metal reagents may encounter difficulties due to lack of ability to deprotect alkynylsilanes. On the basis of this consideration, silver salts, which succeeded in deprotection of alkynylsilanes [2,3,8], were examined for this purpose.
* Corresponding author at: College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China. E-mail address: [email protected] (G.S. Deng). 1001-8417/$ – see front matter # 2012 Gui Sheng Deng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.07.017
1116
G.S. Deng, T.F. Sun / Chinese Chemical Letters 23 (2012) 1115–1118 R 1
SiMe3
[M]
R
2
M
R'SO2Cl 3
?
R 4
SO2R'
Scheme 1. A proposed reaction between alkynylsilanes and sulfonyl chlorides.
Trimethylsilyl phenylacetylene 1a and p-tosyl chloride 3a were first used as a model to screen the commonly available silver(I) compounds in THF at 20 8C under nitrogen. Surprisingly, the use of 1 equiv. silver nitrate gave vinyl sulfone 5a in 53% yield (entry 1 in Table 1), and no expected product 4a (R = Ph, R0 = p-MeC6H4) was observed by TLC. The reaction was carried out without any additives except silver nitrate. Silver triflate also proved to be efficient for the conversion into 5a, but less active than silver nitrate (35% yield, entry 2 in Table 1). It is worth noting that stoichiometric amount of silver nitrate relative to trimethylsilyl phenylacetylene 1a was required to efficiently carry out this reaction. Utilization of catalytic amount of silver nitrate only gave 5a in very low yield (entry 3 in Table 1). Other silver salts such as silver halides (X = F, Cl, Br, I), silver trifloroacetate, silver acetate and silver carbonate, were not effective for this transformation (entries 4–10 in Table 1). No desired product 5a was formed in the presence of 1 equiv. silver oxide (entry 11 in Table 1). Solvent effect on the reaction is complicated. The reaction efficiency is lower in CH3CN than in THF. No reactions took place in dichloromethane and 1,4-dioxane because of insolubility of silver nitrate (entries 13 and 14 in Table 1). In protic solvent such as EtOH, only ethyl 4-methylbenzenesulfonate was formed without 5a (entry 15 in Table 1). Wet THF proved to be the best alternative in comparison to the solvents investigated. In order to study the scope of this novel transformation and acquire information regarding the influence of the substitution pattern of the substrate on the product distribution, representative alkynyl trimethylsilanes and sulfonyl chlorides were submitted to the above optimal conditions. The results are summarized in Table 2. As shown in Table 2, the reaction of trimethylsilylated aromatic alkynes 1 with arylsulfonyl chlorides 3 mainly generated vinyl sulfones 5 in 50–61% yields (entries 1–10 in Table 2). Obviously, changing the aryl substituent of
Table 1 Effect of silver compounds and solvents on the reaction of trimethylsilyl phenylacetylene 1a with p-tosyl chloride 3a.a Ag (I), wet solvent Ph Ts Ph SiMe3 + TsCl o 20 C, N2 H H 1a 3a 5a Entry
Ag(I) (equiv.)b
Solvent
Reaction time (h)
Yieldc of 5a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
AgNO3 (1 equiv.) AgOTf (1 equiv.) AgNO3 (0.1 equiv.) AgF (1 equiv.) AgCl (1 equiv.) AgBr (1 equiv.) AgI (1 equiv.) CF3COOAg (1 equiv.) CH3COOAg (1 equiv.) Ag2CO3 (1 equiv.) Ag2O (1 equiv.) AgNO3 (1 equiv.) AgNO3 (1 equiv.) AgNO3 (1 equiv.) AgNO3 (1 equiv.)
THF THF THF THF THF THF THF THF THF THF THF CH3CN CH2Cl2 1,4-Dioxane EtOH
24 38 32 48 48 48 48 48 48 48 48 30 40 40 24
53 35 8 NRd NRd NRd NRd NRd NRd NRd NDe 32 NRd NRd NDe
a b c d e
Molar ratio of trimethylsilyl phenylacetylene and p-tosyl chloride is 1:0.5. Equivalent quantity relative to trimethylsilyl phenylacetylene. Isolated yield. No reaction. Some competing reactions were carried out, however, no product 3a was obtained.
G.S. Deng, T.F. Sun / Chinese Chemical Letters 23 (2012) 1115–1118 Table 2 Silver
nitrate-promoted
R
SiMe3 + R'SO2Cl 1
3
reaction
of
various
AgNO3 (1 eq), N2 o
wet THF, 20 C
R H
5
trimethylsilylated alkynes O O O SO2R' S R R' H 6
0
Entry
Product
R
R
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
a b c d e f g h i j k l m n o
Ph p-CH3C6H4 p-nPrC6H4 p-FC6H4 p-CH3OC6H4 Ph p-CH3C6H4 p-nPrC6H4 p-FC6H4 p-CH3OC6H4 Ph p-CH3C6H4 n-C4H9 n-C4H9 Me3Si
p-CH3C6H4 p-CH3C6H4 p-CH3C6H4 p-CH3C6H4 p-CH3C6H4 Ph Ph Ph Ph Ph CH3 CH3 p-CH3C6H4 CH3 p-CH3C6H4
a b c
1
1117
with
sulfonyl
chlorides
3.a
Reaction time (h)
Yield (%)b of vinyl sulfone 5 (Z/E)c
24 30 30 28 32 26 24 28 32 28 36 36 40 40 40
53 56 51 58 60 50 57 57 59 61 42 45 0 0 0
(2:1) (1.5:1) (2:1) (1.7:1) (1.5:1) (3.3:1) (1.5:1) (2.5:1) (2:1) (1.5:1) (1.5:1) (1.2:1)
Molar ratio of alkynyl trimethylsilane, sulfonyl chloride and silver nitrate is 1:0.5:1. Isolated yield. Determined by 1H NMR (500 MHz) of the crude product.
arylacetylenes and arylsulfonyl chlorides did not significantly affect the course of the reaction in terms of efficiency and selectivity (entries 1–10 in Table 2). The methylsulfonyl chloride was less active than arylsulfonyl chloride, so yields were lower (entries 11 and 12 in Table 2). In the above cases, the reactions favored Z-selectivity. Besides the product of vinyl sulfones 5, the corresponding b-keto sulfones 6 have been yielded as side products in 10–15% isolated yields (entries 1–12 in Table 2). No reactions of trimethylsilylated aliphatic alkynes with sulfonyl chlorides 3 were carried out under the same conditions probably due to less activity of the resulting aliphatic alkynyl silver than aromatic alkynyl silver (entries 13–15 in Table 2). In conclusion, we have described a novel reaction [9] of trimethylsilylated alkynes with sulfonyl chlorides in the presence of silver salt without additives. Unexpected vinyl sulfones were obtained in mild yields. Some control experimental has been accomplished to explain the reaction mechanism. The data is available free of charge via the Internet at http://www.chinchemlett.com.cn. Further works in this area and a detailed mechanistic investigation are now in progress. References [1] [2] [3] [4]
[5] [6] [7] [8] [9]
J.S. Yadav, B.V.S. Reddy, M.S. Reddy, Synlett (2003) 1722. J.A. Marshall, H.R. Chobanian, M.M. Yanik, Org. Lett. 3 (2001) 4107. S.E. Denmark, S.A. Tymonko, J. Org. Chem. 68 (2003) 9151. (a) S.N. Georgiades, J. Clardy, Org. Lett. 7 (2005) 4091; (b) X. Nie, G. Wang, J. Org. Chem. 71 (2006) 4734; (c) D.M. Bowles, J.E. Anthony, Org. Lett. 2 (2000) 85. K. Gao, N.S. Goroff, J. Am. Chem. Soc. 122 (2000) 9320. S. Garrais, J. Turkington, W.P.D. Goldring, Tetrahedron 65 (2009) 8418. J.B. Hendricksan, K.W. Bair, J. Org. Chem. 42 (1977) 3875. (a) A. Orsini, A. Vite´risi, A. Bodlenner, et al. Tetrahedron Lett. 46 (2005) 2259; (b) A. Vite´risi, A. Orsini, J.M. Weibel, et al. Tetrahedron Lett. 47 (2006) 2779. Typical procedure: To a suspension of AgNO3 (0.187 g, 1 mmol) and p-tosylchloride 3a (95 mg, 0.5 mmol) in THF (10 mL) under nitrogen atmosphere, was added trimethylsilyl phenylacetylene 1a (1 mmol, 0.174 g, 196 mL). The resulting mixture was stirred at 20 8C for 24 h. The reaction progress was monitored by analytical TLC. Following filtration, the filtrate was concentrated under reduced pressure to afford a crude residue. Purification of the crude residue by column chromatography using 25% (v/v) ethyl acetate–petroleum ether as the eluant afforded a
1118
G.S. Deng, T.F. Sun / Chinese Chemical Letters 23 (2012) 1115–1118
colorless solid 5a, a mixture of Z-5a and E-5a (68 mg, 53% yield, 2:1 molar ratio of Z-5a to E-5a). Data for Z-5a: 1H NMR (500 MHz, CDCl3): d 7.68 (d, 2H, J = 8.5 Hz), 7.54–7.56 (m, 2H), 7.38–7.39 (m, 3H), 7.23 (d, 2H, J = 8.0 Hz), 7.05 (d, 1H, J = 12.0 Hz), 6.50 (d, 1H, J = 12.5 Hz), 2.38 (s, 3H); 13C NMR (125.8 MHz, CDCl3): d 144.34, 141.09, 138.12, 132.39, 131.27, 130.17, 129.62, 129.09, 128.06, 127.57, 21.60. Data for E-5a [10]: 1H NMR (500 MHz, CDCl3): d 7.82 (d, 2H, J = 8.0 Hz), 7.69 (d, 1H, J = 15.0 Hz), 7.45–7.47 (m, 2H), 7.33–7.36 (m, 3H), 7.23 (d, 2H, J = 8.0 Hz), 6.85 (d, 1H, J = 15.5 Hz), 2.43 (s, 3H); 13C NMR (125.8 MHz, CDCl3): d 144.45, 141.96, 137.74, 132.44, 131.15, 130.01, 129.71, 128.55, 127.71, 127.64, 21.64. Data for a mixture of Z-5a and E-5a: IR (KBr): nmax 3057, 2924, 1680, 1597, 1575, 1493, 1448, 1401, 1301, 1212, 1144, 1085, 1030, 1018, 974, 922, 857, 812, 747, 692, 664, 634, 615, 604, 558, 538 cm1; MS (EI) 258 (M+), 209, 193, 178, 165, 139, 119, 103, 91 (100), 77, 65, 51, 39. [10] G. Deng, J. Zou, ARKIVOC (ii) (2010) 186.