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Efficient and mild room temperature reduction of benzophenones under ultrasound irradiation
Ultrasonics Sonochemistry 12 (2005) 169–172 www.elsevier.com/locate/ultsonch
Efficient and mild room temperature reduction of benzophenones under ultrasound irradiation Yanqing Peng, Wujun Zhong, Gonghua Song
*
Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides and Pharmaceuticals, East China University of Science and Technology, 130 Meilong Road, P.O. Box 544, Shanghai 200237, China Received 11 August 2003; accepted 11 December 2003 Available online 4 February 2004
Abstract Benzophenones without strong electron-releasing substituents were reduced into the corresponding benzhydrols at room temperature in moderate to good yields with Zn–NaOH-95% ethanol under ultrasound irradiation. This method is also propitious to the synthesis of 3-phenylphthalide and its derivates. 2004 Elsevier B.V. All rights reserved. Keywords: Ultrasound; Reduction; Zinc; Benzophenones; Benzhydrols
1. Introduction Benzhydrol and its analogues are important intermediates in pharmaceutical industry [1]. Routinely, benzhydrols were prepared by reduction from corresponding benzophenones. A variety of reagents, such as sodium amalgam [2], aluminium amalgam [3], sodium borohydride [4], LiAlH4 [5] and Al (OiPr)3 /iPrOH [6] have been employed to reduce benzophenones to benzhydrols. In addition, catalytic hydrogenation [7] and electrolysis [8] processes have also been investigated for this transformation. Zinc is one of the most abundantly used metal reductant. It has been reported that benzophenones were reduced into benzhydrols with zinc dust in the presence of sodium hydroxide [9]. Unfortunately, about 2 h was needed to perform this transformation at reflux temperature. In view of the importance of benzhydrols, efficient and mild method for these transformations is still in great demand.
*
Corresponding author. Tel.: +86-21-642-52945; fax: +86-21-64251386. E-mail address: [email protected] (G. Song). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2003.12.002
The application of ultrasonic irradiation in a wide variety of chemical transformations has gained significant potential in the last two decades [10]. A large number of organic reactions can be carried out in a higher yield, shorter reaction time or milder conditions under ultrasonic irradiation. Sato et al. [11] have reported that benzophenones could be reduced with metallic Al/liquid ammonia/halide under ultrasonic wave irradiation to the corresponding monohydric alcohols in good yields. However, this method should be carried out under pressurized conditions because the liquid ammonia was used. To the best of our knowledge, up to now, zinc-mediated reduction of carbonyl compounds under ultrasound irradiation has not been described. We have recently reported the ultrasound-promoted N-hydroxymethylation at ambient conditions [12], hydrazolysis of esters [13] and Williamson ether synthesis [14] under simultaneous ultrasound and microwave irradiation. In continuation of our ongoing research program on sonochemistry, we report for the first time the use of Zn/NaOH/95% EtOH as reductant system for the reduction of various benzophenones into the corresponding benzhydrols under ultrasound irradiation. In addition, the presented room temperature approach also meets the requirements of green chemistry.
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Y. Peng et al. / Ultrasonics Sonochemistry 12 (2005) 169–172
2. Method 2.1. Apparatus and analysis All reagents were available commercially. The products are all known compounds and identified by melting point and spectrum data. FT-IR spectra were obtained on a Nicolet Nexus 470 infrared spectrometer in KBr discs and 1 H NMR spectra were recorded on a 500 MHz Bruker AM 500 spectrometer with TMS as internal standard. Melting points were taken in open capillary on WRS-1 digital point instrument and uncorrected. The ultrasound apparatus (Shanghai Jump Ultrasonic Co., Ltd.) consists of energy source (stepless regulation, 0–100 W), transducer, and probe (0–100 W, 20 ± 1 kHz, type JHN-M-1) (Fig. 1). 2.2. General procedure To a suspension of zinc dust (1.6 g, 25 mmol) in 20 ml 95% ethanol in a cylindrical flask was added benzophenones (10 mmol) and NaOH (2.0 g, 50 mmol). The tip of detachable horn (20 ± 1 kHz) should be immersed just under the liquid surface in order to obtain optimal
sonication. The mixture was sonicated (50 W) for a specified period at room temperature. After completion of the reaction (monitored by TLC), the mixture was cooled in ice bath and then acidified with aqueous HCl. The precipitate was collected by filtration and the filtrate was extracted with diethyl ether (2 · 20 ml). The extract was dried over anhydrous MgSO4 and the evaporation of the solvent afforded another portion of crude product. The combined crude product was then purified by recrystallization (heptane) to give the pure product. The authenticity of the products was established by comparing their melting points with the literature and by the spectra data of IR and 1 H NMR. Selected characteristic data of as-synthesized compounds: 2b: white needles, mp ¼ 59–60 C. IR (cm1 , KBr): 3580, 3410, 3330, 3200, 1085. 1 H NMR (ppm, CDCl3 ): 3.05 (brs, 1H, OH), 5.60 (s, 1H, CH), 7.16–7.40 (m, 9H, ArH). 2h: white needles, mp ¼ 66–67 C. IR (cm1 , KBr): 3600, 3450, 3310, 3200, 1110. 1 H NMR (ppm, CDCl3 ): 2.20 (brs, 1H, OH), 3.67 (s, 3H, OMe), 5.78 (s, 1H, CH), 6.86–7.38 (m, 9H, ArH). 5: white needles, mp ¼ 119–120 C (Lit [15]: 117 C). IR (cm1 , KBr): 3000, 1700, 1605, 1455, 970. 1 H NMR (ppm, CDCl3 ): 6.38 (s, 1H, Ar–CH–O), 7.27–7.40 (m, 6H, 4-H and PhH), 7.55 (t, 1H, 6-H), 7.66 (t, 1H, 5H), 7.96 (d, 1H, 7-H). 3. Results and discussion
Fig. 1. Apparatus for ultrasound-promoted reduction of benzophenones. (1) Reaction vessel; (2) horn; (3) reaction mixture; (4) rubber stopper; (5) transducer housing.
As shown in Scheme 1 and Table 1, the effect of ultrasound irradiation in reduction was noticeable. These reactions performed at room temperature and afforded moderate to good yields of desired products under ultrasound irradiation within 10–40 min in most cases. The dramatic improvement was observed with regard to reaction time and temperature. It should be pointed out that in the absence of sonication, the reaction was sluggish. For example, in the absence of ultrasound, the mixture of 4-methylbenzophenones (1d), Zn and NaOH was stirred in an alcoholic solution at room temperature for 9 h exhibited only 26% conversions. Even after refluxing for 2 h, 4methylbenzhydrol (2d) was produced in 72% yield, while higher yield of 78% could be achieved after 20 min under ultrasound irradiation at room temperature.
O
OH Zn, NaOH, 95% EtOH
R1
1
R2
r.t., ))))
Scheme 1.
R1
2
R2
Y. Peng et al. / Ultrasonics Sonochemistry 12 (2005) 169–172
171
Table 1 Ultrasound-promoted conversion of benzophenones into benzhydrols Entry
Product R1
R2
a b c d e f g h i
H H H H F Cl Me2 N H H
H Cl Br Me F Cl Me2 N MeO OH
a b
Timea (min)
Yieldb (%)
Mp (C) Obs.
Lit.
40 10 10 20 10 10 60 60 30
97 88 74 78 81 86 0 21 Trace
66 59–60 65–66 60–61 47–48 91 – 66–67 –
67 [16] 61–62 [17] 63–65 [6] 60 [6] 47–48 [18] 90–92 [19] – 66–67 [20] –
All of these reactions were carried out at room temperature. Isolated yields.
O
COOH
OH
COONa
Zn, NaOH, 95% EtOH r.t., )))) 5 min
3
4
O O
HCl / H2O
( 94% )
r.t.
5 Scheme 2.
It was found that the electronic effects of phenyl ring substituents affected the reaction to a major degree. The reduction of benzophenones bearing electron withdrawing substituents proceeded smoothly in all of the cases, yielded the desired products in a range of 74–97%. Benzophenones with strong electron-releasing groups appended, however, afforded the products in much lower yields because of the sluggish reactions. In the case of 4,40 -di(dimethylamino)benzophenones 1g, no desired product was detected even after 60 min irradiation. In the case of 2-benzoylbenzoic acid 3, the starting substrate was consumed within 5 min at room temperature. Unexpectedly, a lactone, 3-phenylphthalide 5, was precipitated (94% yield) after acidification (Scheme 2). According to prevenient literatures, 3-phenylphthalide was prepared by treating 2-benzoylbenzoic acid with NaBH4 for 12 h [21], or heating a mixture of 2-benzoylbenzoic acid, zinc, and acetic anhydride for 3 h at elevated temperature (160 C) [15]. Hence, our procedure also provides a rapid and mild method for the syntheses of 3-phenylphthalide and its derivatives. It has been reported that benzophenone reducted by metallic reagents such as Zn/HOAc [22] or Mg/MgI2 [23] would yield corresponding pinacol (1,1,2,2-tetraphenylethane-1,2-diol). It is also worth mentioning that only a small quantity of pinacol was detected by using our procedure.
4. Conclusion In conclusion, the ultrasound-promoted benzhydrols synthesis is an attractive method, which leads to shorter reaction times and allows the investigator to work at ambient temperature and pressure. The rapid conversation as well as mild condition makes the described protocol interesting from both an economic and environmental point of view. Further investigations to apply this process to other synthetic organic reactions are in progress.
Acknowledgement Financial support of this work from the Shanghai Education Committee is greatly acknowledged.
References [1] D.K. Yung, M.L. Gilroy, D.E. Mahony, J. Pharm. Sci. 67 (1978) 900; D.J. Abraham, P.E. Kennedy, A.S. Mehanna, D.C. Patwa, F.L. Williams, J. Med. Chem. 27 (1984) 967; P.C. Meltzer, A.Y. Liang, B.K. Madras, J. Med. Chem. 39 (1996) 371. [2] E. Fischer, O. Fischer, Justus Liebigs Ann. Chem. 194 (1878) 290.
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Y. Peng et al. / Ultrasonics Sonochemistry 12 (2005) 169–172
[3] R. Cohen, Recl. Trav. Chim. Pays-Bas. 38 (1919) 121. [4] G. Gopalakrishnan, M.E.P. Babu, B.R. Pai, Indian J. Chem. Sect. A 20 (1981) 998; S. Gunasekaran, N. Venkatasubramanian, J. Chem. Soc., Perkin Trans. 2 (1983) 949; S. Nishida, J. Org. Chem. 32 (1967) 2692; J. Mindl, M. Vecera, Collect. Czech. Chem. Commun. 37 (1972) 1143; M.R. Pavia, S.J. Lobbestael, D. Nugiel, D.R. Mayhugh, V.E. Gregor, C.P. Taylor, R.D. Schwarz, L. Brahce, M.G. Vartanian, J. Med. Chem. 35 (1992) 4238. [5] H. Yamataka, N. Miyano, T. Hanafusa, J. Org. Chem. 56 (1991) 2573; D.G. Lee, M. Raptis, Tetrahedron 29 (1973) 1481; T. Zeiss, J. Am. Chem. Soc. 75 (1953) 897; H. Yamataka, N. Miyano, T. Hanafusa, J. Org. Chem. 56 (1991) 2573. [6] M. Muraoka, T. Itoh, T. Mizuma, S. Toyoshima, Chem. Pharm. Bull. 8 (1960) 860; C. Bachmann, E. Carlson Jr., J.C. Moran, J. Org. Chem. 13 (1948) 916. [7] K. Hattori, H. Sajiki, K. Hirota, Tetrahedron 57 (2001) 4817; H. Sajiki, K. Hattori, K. Hirota, J. Chem. Soc., Perkin Trans. 1 (1998) 4043; T. Ohkuma, M. Koizumi, H. Ikehira, T. Yokozawa, R. Noyori, Org. Lett. 2 (2000) 659. [8] D.W. Sopher, J.H.P. Utley, J. Chem. Soc., Perkin Trans. 2 (1984) 1361; A. Sivakumar, S. Jayarama Reddy, V.R. Krishnan, Indian J. Chem. Sect. A 22 (1983) 800.
[9] J.F. Norris, J.T. Blake, J. Am. Chem. Soc. 50 (1928) 1808; E.D. Hughes, C.K. Ingold, N.A. Taher, J. Chem. Soc. (1940) 949. [10] J.P. Lorimer, T.J. Mason, Chem. Soc. Rev. 16 (1987) 239; J. Lindley, T.J. Mason, Chem. Soc. Rev. 16 (1987) 275; K.S. Suslick, Sci. Am. 260 (1989) 80; C. Einhorn, J. Einhorn, J. Luche, Synthesis (1989) 787; R.L. Hunicke, Ultrasonics 28 (1990) 291; J. Berlan, T.J. Mason, Ultrasonics 30 (1992) 203; T.J. Mason, Chem. Soc. Rev. 26 (1997) 443; P. Cintas, J. Luche, Green Chem. 1 (1999) 115. [11] R. Sato, T. Nagaoka, T. Goto, M. Saito, Bull. Chem. Soc. Jpn. 63 (1990) 290. [12] W. Zhong, G. Song, Y. Peng, X. Qian, Synth. Commun. 30 (2000) 3801. [13] Y. Peng, G. Song, Green Chem. 3 (2001) 302. [14] Y. Peng, G. Song, Green Chem. 4 (2002) 349. [15] I. Yokoe, K. Higuchi, Y. Shirataki, M. Komatsu, Chem. Pharm. Bull. 29 (1981) 894. [16] W.L. Truett, J. Am. Chem. Soc. 73 (1951) 5913. [17] M. Isola, E. Ciuffarin, L. Sagramora, Synthesis (1976) 326. [18] A. Streitwieser Jr., E.R. Vorpagel, C.-C. Chen, J. Am. Chem. Soc. 107 (1985) 6970. [19] W.S. Trahanovsky, J.A. Lawson, D.E. Zabel, J. Org. Chem. 32 (1967) 2287. [20] M.M. Bokadia, B.R. Brown, W. Cummings, J. Chem. Soc. (1960) 3308. [21] D. Tobia, B. Rickborn, J. Org. Chem. 51 (1986) 3849. [22] A. Zagumenny, Chem. Ber. 14 (1881) 1402. [23] M. Gomberg, W.E. Bachmann, J. Am. Chem. Soc. 49 (1927) 236.
Efficient and mild room temperature reduction of benzophenones under ultrasound irradiation Yanqing Peng, Wujun Zhong, Gonghua Song
*
Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides and Pharmaceuticals, East China University of Science and Technology, 130 Meilong Road, P.O. Box 544, Shanghai 200237, China Received 11 August 2003; accepted 11 December 2003 Available online 4 February 2004
Abstract Benzophenones without strong electron-releasing substituents were reduced into the corresponding benzhydrols at room temperature in moderate to good yields with Zn–NaOH-95% ethanol under ultrasound irradiation. This method is also propitious to the synthesis of 3-phenylphthalide and its derivates. 2004 Elsevier B.V. All rights reserved. Keywords: Ultrasound; Reduction; Zinc; Benzophenones; Benzhydrols
1. Introduction Benzhydrol and its analogues are important intermediates in pharmaceutical industry [1]. Routinely, benzhydrols were prepared by reduction from corresponding benzophenones. A variety of reagents, such as sodium amalgam [2], aluminium amalgam [3], sodium borohydride [4], LiAlH4 [5] and Al (OiPr)3 /iPrOH [6] have been employed to reduce benzophenones to benzhydrols. In addition, catalytic hydrogenation [7] and electrolysis [8] processes have also been investigated for this transformation. Zinc is one of the most abundantly used metal reductant. It has been reported that benzophenones were reduced into benzhydrols with zinc dust in the presence of sodium hydroxide [9]. Unfortunately, about 2 h was needed to perform this transformation at reflux temperature. In view of the importance of benzhydrols, efficient and mild method for these transformations is still in great demand.
*
Corresponding author. Tel.: +86-21-642-52945; fax: +86-21-64251386. E-mail address: [email protected] (G. Song). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2003.12.002
The application of ultrasonic irradiation in a wide variety of chemical transformations has gained significant potential in the last two decades [10]. A large number of organic reactions can be carried out in a higher yield, shorter reaction time or milder conditions under ultrasonic irradiation. Sato et al. [11] have reported that benzophenones could be reduced with metallic Al/liquid ammonia/halide under ultrasonic wave irradiation to the corresponding monohydric alcohols in good yields. However, this method should be carried out under pressurized conditions because the liquid ammonia was used. To the best of our knowledge, up to now, zinc-mediated reduction of carbonyl compounds under ultrasound irradiation has not been described. We have recently reported the ultrasound-promoted N-hydroxymethylation at ambient conditions [12], hydrazolysis of esters [13] and Williamson ether synthesis [14] under simultaneous ultrasound and microwave irradiation. In continuation of our ongoing research program on sonochemistry, we report for the first time the use of Zn/NaOH/95% EtOH as reductant system for the reduction of various benzophenones into the corresponding benzhydrols under ultrasound irradiation. In addition, the presented room temperature approach also meets the requirements of green chemistry.
170
Y. Peng et al. / Ultrasonics Sonochemistry 12 (2005) 169–172
2. Method 2.1. Apparatus and analysis All reagents were available commercially. The products are all known compounds and identified by melting point and spectrum data. FT-IR spectra were obtained on a Nicolet Nexus 470 infrared spectrometer in KBr discs and 1 H NMR spectra were recorded on a 500 MHz Bruker AM 500 spectrometer with TMS as internal standard. Melting points were taken in open capillary on WRS-1 digital point instrument and uncorrected. The ultrasound apparatus (Shanghai Jump Ultrasonic Co., Ltd.) consists of energy source (stepless regulation, 0–100 W), transducer, and probe (0–100 W, 20 ± 1 kHz, type JHN-M-1) (Fig. 1). 2.2. General procedure To a suspension of zinc dust (1.6 g, 25 mmol) in 20 ml 95% ethanol in a cylindrical flask was added benzophenones (10 mmol) and NaOH (2.0 g, 50 mmol). The tip of detachable horn (20 ± 1 kHz) should be immersed just under the liquid surface in order to obtain optimal
sonication. The mixture was sonicated (50 W) for a specified period at room temperature. After completion of the reaction (monitored by TLC), the mixture was cooled in ice bath and then acidified with aqueous HCl. The precipitate was collected by filtration and the filtrate was extracted with diethyl ether (2 · 20 ml). The extract was dried over anhydrous MgSO4 and the evaporation of the solvent afforded another portion of crude product. The combined crude product was then purified by recrystallization (heptane) to give the pure product. The authenticity of the products was established by comparing their melting points with the literature and by the spectra data of IR and 1 H NMR. Selected characteristic data of as-synthesized compounds: 2b: white needles, mp ¼ 59–60 C. IR (cm1 , KBr): 3580, 3410, 3330, 3200, 1085. 1 H NMR (ppm, CDCl3 ): 3.05 (brs, 1H, OH), 5.60 (s, 1H, CH), 7.16–7.40 (m, 9H, ArH). 2h: white needles, mp ¼ 66–67 C. IR (cm1 , KBr): 3600, 3450, 3310, 3200, 1110. 1 H NMR (ppm, CDCl3 ): 2.20 (brs, 1H, OH), 3.67 (s, 3H, OMe), 5.78 (s, 1H, CH), 6.86–7.38 (m, 9H, ArH). 5: white needles, mp ¼ 119–120 C (Lit [15]: 117 C). IR (cm1 , KBr): 3000, 1700, 1605, 1455, 970. 1 H NMR (ppm, CDCl3 ): 6.38 (s, 1H, Ar–CH–O), 7.27–7.40 (m, 6H, 4-H and PhH), 7.55 (t, 1H, 6-H), 7.66 (t, 1H, 5H), 7.96 (d, 1H, 7-H). 3. Results and discussion
Fig. 1. Apparatus for ultrasound-promoted reduction of benzophenones. (1) Reaction vessel; (2) horn; (3) reaction mixture; (4) rubber stopper; (5) transducer housing.
As shown in Scheme 1 and Table 1, the effect of ultrasound irradiation in reduction was noticeable. These reactions performed at room temperature and afforded moderate to good yields of desired products under ultrasound irradiation within 10–40 min in most cases. The dramatic improvement was observed with regard to reaction time and temperature. It should be pointed out that in the absence of sonication, the reaction was sluggish. For example, in the absence of ultrasound, the mixture of 4-methylbenzophenones (1d), Zn and NaOH was stirred in an alcoholic solution at room temperature for 9 h exhibited only 26% conversions. Even after refluxing for 2 h, 4methylbenzhydrol (2d) was produced in 72% yield, while higher yield of 78% could be achieved after 20 min under ultrasound irradiation at room temperature.
O
OH Zn, NaOH, 95% EtOH
R1
1
R2
r.t., ))))
Scheme 1.
R1
2
R2
Y. Peng et al. / Ultrasonics Sonochemistry 12 (2005) 169–172
171
Table 1 Ultrasound-promoted conversion of benzophenones into benzhydrols Entry
Product R1
R2
a b c d e f g h i
H H H H F Cl Me2 N H H
H Cl Br Me F Cl Me2 N MeO OH
a b
Timea (min)
Yieldb (%)
Mp (C) Obs.
Lit.
40 10 10 20 10 10 60 60 30
97 88 74 78 81 86 0 21 Trace
66 59–60 65–66 60–61 47–48 91 – 66–67 –
67 [16] 61–62 [17] 63–65 [6] 60 [6] 47–48 [18] 90–92 [19] – 66–67 [20] –
All of these reactions were carried out at room temperature. Isolated yields.
O
COOH
OH
COONa
Zn, NaOH, 95% EtOH r.t., )))) 5 min
3
4
O O
HCl / H2O
( 94% )
r.t.
5 Scheme 2.
It was found that the electronic effects of phenyl ring substituents affected the reaction to a major degree. The reduction of benzophenones bearing electron withdrawing substituents proceeded smoothly in all of the cases, yielded the desired products in a range of 74–97%. Benzophenones with strong electron-releasing groups appended, however, afforded the products in much lower yields because of the sluggish reactions. In the case of 4,40 -di(dimethylamino)benzophenones 1g, no desired product was detected even after 60 min irradiation. In the case of 2-benzoylbenzoic acid 3, the starting substrate was consumed within 5 min at room temperature. Unexpectedly, a lactone, 3-phenylphthalide 5, was precipitated (94% yield) after acidification (Scheme 2). According to prevenient literatures, 3-phenylphthalide was prepared by treating 2-benzoylbenzoic acid with NaBH4 for 12 h [21], or heating a mixture of 2-benzoylbenzoic acid, zinc, and acetic anhydride for 3 h at elevated temperature (160 C) [15]. Hence, our procedure also provides a rapid and mild method for the syntheses of 3-phenylphthalide and its derivatives. It has been reported that benzophenone reducted by metallic reagents such as Zn/HOAc [22] or Mg/MgI2 [23] would yield corresponding pinacol (1,1,2,2-tetraphenylethane-1,2-diol). It is also worth mentioning that only a small quantity of pinacol was detected by using our procedure.
4. Conclusion In conclusion, the ultrasound-promoted benzhydrols synthesis is an attractive method, which leads to shorter reaction times and allows the investigator to work at ambient temperature and pressure. The rapid conversation as well as mild condition makes the described protocol interesting from both an economic and environmental point of view. Further investigations to apply this process to other synthetic organic reactions are in progress.
Acknowledgement Financial support of this work from the Shanghai Education Committee is greatly acknowledged.
References [1] D.K. Yung, M.L. Gilroy, D.E. Mahony, J. Pharm. Sci. 67 (1978) 900; D.J. Abraham, P.E. Kennedy, A.S. Mehanna, D.C. Patwa, F.L. Williams, J. Med. Chem. 27 (1984) 967; P.C. Meltzer, A.Y. Liang, B.K. Madras, J. Med. Chem. 39 (1996) 371. [2] E. Fischer, O. Fischer, Justus Liebigs Ann. Chem. 194 (1878) 290.
172
Y. Peng et al. / Ultrasonics Sonochemistry 12 (2005) 169–172
[3] R. Cohen, Recl. Trav. Chim. Pays-Bas. 38 (1919) 121. [4] G. Gopalakrishnan, M.E.P. Babu, B.R. Pai, Indian J. Chem. Sect. A 20 (1981) 998; S. Gunasekaran, N. Venkatasubramanian, J. Chem. Soc., Perkin Trans. 2 (1983) 949; S. Nishida, J. Org. Chem. 32 (1967) 2692; J. Mindl, M. Vecera, Collect. Czech. Chem. Commun. 37 (1972) 1143; M.R. Pavia, S.J. Lobbestael, D. Nugiel, D.R. Mayhugh, V.E. Gregor, C.P. Taylor, R.D. Schwarz, L. Brahce, M.G. Vartanian, J. Med. Chem. 35 (1992) 4238. [5] H. Yamataka, N. Miyano, T. Hanafusa, J. Org. Chem. 56 (1991) 2573; D.G. Lee, M. Raptis, Tetrahedron 29 (1973) 1481; T. Zeiss, J. Am. Chem. Soc. 75 (1953) 897; H. Yamataka, N. Miyano, T. Hanafusa, J. Org. Chem. 56 (1991) 2573. [6] M. Muraoka, T. Itoh, T. Mizuma, S. Toyoshima, Chem. Pharm. Bull. 8 (1960) 860; C. Bachmann, E. Carlson Jr., J.C. Moran, J. Org. Chem. 13 (1948) 916. [7] K. Hattori, H. Sajiki, K. Hirota, Tetrahedron 57 (2001) 4817; H. Sajiki, K. Hattori, K. Hirota, J. Chem. Soc., Perkin Trans. 1 (1998) 4043; T. Ohkuma, M. Koizumi, H. Ikehira, T. Yokozawa, R. Noyori, Org. Lett. 2 (2000) 659. [8] D.W. Sopher, J.H.P. Utley, J. Chem. Soc., Perkin Trans. 2 (1984) 1361; A. Sivakumar, S. Jayarama Reddy, V.R. Krishnan, Indian J. Chem. Sect. A 22 (1983) 800.
[9] J.F. Norris, J.T. Blake, J. Am. Chem. Soc. 50 (1928) 1808; E.D. Hughes, C.K. Ingold, N.A. Taher, J. Chem. Soc. (1940) 949. [10] J.P. Lorimer, T.J. Mason, Chem. Soc. Rev. 16 (1987) 239; J. Lindley, T.J. Mason, Chem. Soc. Rev. 16 (1987) 275; K.S. Suslick, Sci. Am. 260 (1989) 80; C. Einhorn, J. Einhorn, J. Luche, Synthesis (1989) 787; R.L. Hunicke, Ultrasonics 28 (1990) 291; J. Berlan, T.J. Mason, Ultrasonics 30 (1992) 203; T.J. Mason, Chem. Soc. Rev. 26 (1997) 443; P. Cintas, J. Luche, Green Chem. 1 (1999) 115. [11] R. Sato, T. Nagaoka, T. Goto, M. Saito, Bull. Chem. Soc. Jpn. 63 (1990) 290. [12] W. Zhong, G. Song, Y. Peng, X. Qian, Synth. Commun. 30 (2000) 3801. [13] Y. Peng, G. Song, Green Chem. 3 (2001) 302. [14] Y. Peng, G. Song, Green Chem. 4 (2002) 349. [15] I. Yokoe, K. Higuchi, Y. Shirataki, M. Komatsu, Chem. Pharm. Bull. 29 (1981) 894. [16] W.L. Truett, J. Am. Chem. Soc. 73 (1951) 5913. [17] M. Isola, E. Ciuffarin, L. Sagramora, Synthesis (1976) 326. [18] A. Streitwieser Jr., E.R. Vorpagel, C.-C. Chen, J. Am. Chem. Soc. 107 (1985) 6970. [19] W.S. Trahanovsky, J.A. Lawson, D.E. Zabel, J. Org. Chem. 32 (1967) 2287. [20] M.M. Bokadia, B.R. Brown, W. Cummings, J. Chem. Soc. (1960) 3308. [21] D. Tobia, B. Rickborn, J. Org. Chem. 51 (1986) 3849. [22] A. Zagumenny, Chem. Ber. 14 (1881) 1402. [23] M. Gomberg, W.E. Bachmann, J. Am. Chem. Soc. 49 (1927) 236.