A Simple and Efficient Esterification Method

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1 Journal of the Chinese Chemical Society, 2003, 50, A Simple and Efficient Esterification Method Ming-Yi Chen a ( ) and Adam Shih-Yuan Lee b *( ) a Department of General Education, Taipei Nursing College, Taipei, Taiwan 112, R.O.C. b Department of Chemistry, Tamkang University, Tamsui, Taiwan 251, R.O.C. A convenient and practical esterification was realized and this reaction proceeded without a dehydrating reagent or water removal equipment. The synthesis of esters by reaction of carboxylic acids with various alcohols such as methyl, ethyl, isopropyl, isobutyl, allyl, benzyl, propargyl and decanyl alcohols were achieved with a catalytic amount of CBr 4 under refluxing reaction conditions. Keywords: Esterification; Carboxylic acid; Carbon tetrabromide. INTRODUCTION Transformation of a carboxylic acid into an ester is one of the most important reactions in organic synthesis. 1-6 Although many reliable reactions for the preparation of esters have been reported, there is still a need for simple and efficient methods for the condensation reaction of carboxylic acids and alcohols. 7 Recently, our laboratory reported a chemoselective esterification method for preparation of methyl esters. 8 Methyl esters were produced in high yield but low yields (or no yield) were obtained when the reaction alcohol was more sterically hinderd than methanol. Therefore, we further investigated and improved this esterification technigue which now enables carboxylic acids to react with hindered alcohols such as ethanol and isopropanol. Herewith, we wish to report a simple and highly efficient esterification method for carboxylic acids with a range of alcohols in the presence of a catalytic amount of CBr 4 (Scheme I). Scheme I R- R'OH RESULTS AND DISCUSSION CBr 4 (5-20 mol%) reflux R'OH = MeOH, EtOH, iproh R-CO 2 R' A series of carboxylic acids were esterified under the reaction conditions and the results are shown in Table 1. Primary and secondary alkyl carboxylic acids were converted into their esters in high yields (Entries 1-5). It should be noted that the esterification becomes much slower when the more sterically hindered alcohol, isopropanol, was used. The esterification of benzoic acid was quite slow, and significant quantities of benzoic acid were recovered in the reaction with ethanol and isopropanol even after a relatively prolonged reaction time (Entry 6). The sp 2 -C and sp-c tethered carboxylic acids undergo esterification in high yields after a prolonged reaction time (Entries 7, 8). The N-acetyl glutamic acid was esterified to the corresponding N-acetyl diester in high yield under the standard reaction conditions (Entry 9). The specific optical rotations of dimethyl N-acetylglutamate and diethyl N-acetylglutamate revealed [ ] D = 11.3 (CHCl 3, c = 1.1) and [ ] D = (EtOH, c = 2.0), respectively. These data are consistent with the reported values. 9,10 The absolute configuration of the -amino acid was retained under these reaction conditions. It is worth noting that the esterification becomes much slower when sterically hindered carboxylic acids were used. When a mixture of phenylacetic and benzoic acids was investigated for selective esterification, phenylacetic acid was selectively transformed to the methyl ester, whereas benzoic acid was retained unchanged (Scheme II). This result led us to investigate further competetive esterification between sp 3 -C and sp 2 -C tethered carboxylic acids. A mixture of 1-decanoic acid and 2,4-hexadienoic acid in CBr 4 /MeOH (10%/10 ml) was heated under reflux for three hours (Scheme III). The sp 3 -C tethered primary acid was esterificated to its corresponding methyl ester in 95% yield, whereas sp 2 -C tethered carboxylic acid remained intact and was recovered in 98% after chromatography. Dedicated to Professor Fa-Ching Chen on the occasion of his ninetieth birthday. * Corresponding author. adamlee@mail.tku.edu.tw

2 104 J. Chin. Chem. Soc., Vol. 50, No. 1, 2003 Chen and Lee Table 1. Esterification of Carboxylic Acids with Alcohols Entry R R'-OH CBr 4 (mol%) Time (h) Yield a a The yields were determined after chromatographic purification. b The yield is lower because the product is highly volatile. c The recovery yield of carboxylic acid. d The yield of diester. MeOH % EtOH % iproh % MeOH % EtOH % iproh % MeOH % b EtOH % iproh % MeOH % EtOH % iproh % MeOH % EtOH % iproh %(12%) c MeOH %(3%) c EtOH %(27%) c iproh %(54%) c MeOH % EtOH % iproh %(18%) c MeOH % EtOH % iproh % MeOH % d EtOH % d iproh % d Scheme II CO 2 H CBr 4 /MeOH (10%/10 ml), CO 2 Me reflux, 2h 96% 94% Scheme III ( ) 6 CBr 4 / MeOH (10%/10mL), reflux, 3h ( ) 6 CO 2 Me 95% 98% An interesting result was observed that the esterification of a secondary alkyl acid is much slower than the corresponding primary species (Table 1, Entries 4 and 5). Thus, we attempted competitive esterification between phenylacetic acid and 2-methylphenylacetic acid (Scheme IV). The primary carboxylic acid was esterified to its corresponding methyl ester in 94% yield, whereas secondary carboxylic acid was recovered almost quantitatively by column chromatography. The direct condensation reaction of a carboxylic acid with an alcohol is the most straightforward and efficient way to afford an ester Our reported esterification was carried out by a simple condensation reaction of carboxylic acids with alcohols in the presence of catalytic amounts of CBr 4

3 A Simple and Efficient Esterification Method J. Chin. Chem. Soc., Vol. 50, No. 1, Scheme IV CO 2 Me 94% CBr 4 / MeOH (10%/10mL), reflux, 2h CH 3 CH 3 92% CO 2 Me and with no requirement for water removal apparatus. The success of this esterification relies on the in situ generation of HBr from CBr 4 with ROH which provides a mild and an anhydrous acidic reaction environment We envisaged that other functionalized alcohols could also be introduced quantitatively to achieve the goal of a direct condensation reaction between a carboxylic acid and an alcohol. Therefore, we investigated the reaction of 1-decanoic acid (1.0 eq.), allyl alcohol (40 eq.) and CBr 4 (0.2 eq.). After stirring at 65 C for 45 hours, allyl decanoate was obtained in 87% yield by chromatographic purification. It should be noted that the esterification was performed under neat reaction conditions. Esterification of 1-decanoic acid CH 3 5% with various alcohols was then investigated and the results are shown in Table 2. When esterification of 1-decanoic acid with liquid alcohols such as allyl, benzyl, propargyl, isobutyl and 1-decanyl alcohols were investigated, the corresponding esters were obtained in high yields. Thus, these reaction conditions also provide an alternative protection method for converting carboxylic acids to the corresponding allyl or benzyl esters. 1,2 The results showed that the alcohol behaves as a reactant, a reagent and a solvent in these esterifications. Our previous studies showed that numerous functionalities (e.g.; -Cl, -OR, -CHO, -COR, -NR 2, CONR 2 ) were tolerated in the CBr 4 /ROH protocols under thermal, photochemical or ultrasonic reaction conditions In conclusion, we have demonstrated a simple and an efficient esterification method and this reaction is applicable to a variety of carboxylic acids and alcohols. This method enables esterification of sp 3 -C tethered carboxylic acids, whereas sp 2 -C tethered carboxylic acids were more resistent with this protocol. This method also allows esterification of primary carboxylic acids, whereas a secondary carboxylic Table 2. Esterification of 1-Decanoic Acid with Various Alcohols a Entry R-OH Equivalent Time (h) Product Yield b % % c % % % a A mixture of 1-decanoic acid, alcohol and CBr 4 (20 mol%) was stirred at 65 C for h and then the excess alcohol was removed directly under reduced pressure. b The yields were determined after chromatographic purification. c The 20% yield of PhCH 2 OCH 2 OCH 2 Ph was also obtained.

4 106 J. Chin. Chem. Soc., Vol. 50, No. 1, 2003 Chen and Lee acid was more stable under the reaction conditions. This esterification method demands no dehydrating reagents or water removal equipment. This protocol provides a practical method for the simple condensation reaction between a carboxylic acid and an alcohol. EXPERIMENTAL SECTION General The 1 H-NMR (proton nuclear magnetic resonance) spectra were recorded at 300 MHz (Bruker-AC300P) with deuteriochloroform (CDCl 3, Aldrich 99.8 atom% D) as the solvent and the internal standard. The 13 C-NMR (carbon nuclear magnetic resonance) spectra were recorded at 75.5 MHz (Bruker-AC300P) with CDCl 3 as the solvent and the internal standard. Chemical shifts are reported in parts per million and resonance patterns are reported with the notations of either s (singlet), d (doublet), t (triplet), q (quartet), or m (multiplet). Coupling constants (J) are reported in hertz (Hz). All experiments were carried out under a nitrogen atmosphere which was dried primarily by passing through a column of potassium hydroxide (KOH) layered with calcium sulfate (CaSO 4 ). MeOH, EtOH and iproh were distilled from sodium and recirculated prior to use. Hexane and ethyl acetate were distilled from calcium hydride. Thin-layer chromatography (TLC) analysis was performed on a plastic plate (or aluminum sheet) precoated with silica gel (Merck, 5554 Silica gel 60F 254 ). Visualization was accomplished by UV light or developed by spraying with a 10% phosphomolybdic acid ethanol solution. Column chromatography was performed using silica gel (Merck mesh) and ethyl acetate/hexane mixture as the eluent. All carboxylic acids (Table 1 and 2) were purchased from Aldrich, Merck and Riedel-deHaen and all were used directly without further purification. Esterifications were investigated under the typical procedure shown below and the yields are the isolated yields after chromatography. All these esters were characterized by spectral analysis and by comparsion with authentic compounds. Preparation of Methyl, Ethyl and Isopropyl esters A solution of the carboxylic acid (1.0 mmol), CBr 4 ( mmol) and 10 ml anhydrous MeOH (EtOH or iproh) was refluxed for the specified times indicated in Table 1. After the reaction was complete (analysis by TLC), the excess alcohol was removed directly under reduced pressure. Further purification is achieved by flash chromatography with ethyl acetate/hexane as eluant. Methyl decanoate (Table 1, Entry 1) 1 H-NMR: 0.84 (3H, t, J = 6.8), (12H, m), (2H, m), 2.26 (2H, t, J = 7.6), 3.62 (3H, s). 13 C-NMR: 14.0, 22.6, 24.9, 29.1 (2C), 29.2, 29.3, 31.8, 34.0, 51.3, Ethyl decanoate (Table 1, Entry 1) 1 H-NMR: 0.84 (3H, t, J = 6.8), (12H, m), 1.22 (3H, t, J = 7.2), (2H, m), 2.25 (2H, t, J = 7.5), 4.08 (2H, q, J = 7.2). 13 C-NMR: 14.0, 14.2, 22.6, 24.9, 29.1, 29.2 (2C), 29.3, 31.8, 34.3, 60.0, Isopropyl decanoate (Table 1, Entry 1) 1 H-NMR: 0.84 (3H, t, J = 6.8), (12H, m), 1.19 (6H, d, J = 6.1), (2H, m), 2.22 (2H, d, J = 7.5), 4.97 (1H, m). 13 C-NMR: 14.0, 21.7, 22.6, 25.0, 29.0, 29.1, 29.2, 29.4, 31.8, 34.6, 67.2, Methyl 3-cyclohexylpropanoate (Table 1, Entry 2) 1 H-NMR: (2H, m) (4H, m), (2H, m), (5H, m), 2.28 (2H, t, J = 7.3), 3.63 (3H, s). 13 C-NMR: 26.1, 26.5, 31.6, 32.3, 32.9, 37.2, 51.3, Ethyl 3-cyclohexylpropanoate (Table 1, Entry 2) 1 H-NMR: (2H, m), (4H, m), 1.21 (3H, t, J = 6.9), (2H, m), (5H, m), 2.25 (2H, dd, J = 7.3, 7.3), 4.07 (2H, q, J = 6.9). 13 C-NMR: 14.1, 26.1, 26.4, 31.8, 32.2, 32.8, 37.1, 60.0, Isopropyl 3-cyclohexylpropanoate (Table 1, Entry 2) 1 H-NMR: (2H, m), (4H, m), 1.20 (6H, d, J = 6.3), 1.48 (2H, m), (5H, dm), 2.23 (2H, dd, J = 8.0, 8.0), 4.96 (1H, m). 13 C-NMR: 21.6, 26.1, 26.4, 32.1, 32.2, 32.8, 37.1, 67.1, Methyl cyclohexylmethanoate (Table 1, Entry 3) 1 H-NMR: (3H, m), (2H, m), 1.58 (1H, m), (2H, m), (2H, m), 2.24 (1H, m), 3.59 (3H, s). 13 C-NMR: 25.3, 25.6, 28.9, 43.2, 51.3, Ethyl cyclohexylmethanoate (Table 1, Entry 3) 1 H-NMR: (3H, m), 1.20 (3H, t, J = 7.3), (2H, m), 1.61 (1H, m), (2H, m), (2H, m), 2.24 (1H, m), 4.07 (2H, q, J = 7.3). 13 C-NMR: 14.1, 25.3, 25.7, 28.9, 43.2, 60.0, Isopropyl cyclohexylmethanoate (Table 1, Entry 3) 1 H-NMR: (3H, m), 1.20 (6H, t, J = 6.3),

5 A Simple and Efficient Esterification Method J. Chin. Chem. Soc., Vol. 50, No. 1, (2H, m), 1.60 (1H, m), (2H, m), (2H, m), 2.19 (1H, m), 4.94 (1H, m). 13 C-NMR: 21.7, 25.4, 25.7, 28.9, 43.3, 66.9, Methyl 2-phenylethanoate (Table 1, Entry 4) 1 H-NMR: 3.65 (2H, s), 3.70 (3H, s), (5H, m). 13 C-NMR: 41.2, 52.0, 127.1, 128.6, 129.2, 134.0, Ethyl 2-phenylethanoate (Table 1, Entry 4) 1 H-NMR: 1.24 (3H, t, J = 7.2), 3.61 (2H, s), 4.14 (2H, q, J = 7.2), (5H, m). 13 C-NMR: 14.1, 41.4, 60.8, 127.0, 128.5, 129.2, 134.0, Isopropyl 2-phenylethanoate (Table 1, Entry 4) 1 H-NMR: 1.25 (6H, d, J = 6.4), 3.60 (2H, s), 5.04 (1H, m), (5H, m). 13 C-NMR: 21.6, 41.6, 68.0, 126.9, 128.4, 129.1, 134.3, Methyl 2-phenylpropanoate (Table 1, Entry 5) 1 H-NMR: 1.53 (3H, d, J = 7.3), 3.65 (3H, s), 3.76 (1H, q, J = 7.3), (5H, m). 13 C-NMR: 18.6, 45.4, 51.9, 127.1, 127.4, 128.6, 140.6, Ethyl 2-phenylpropanoate(Table 1, Entry 5) 1 H-NMR: 1.22 (3H, t, J = 7.2), 1.51 (3H, d, J = 7.2), 3.74 (2H, q, J = 7.2), 3.76 (1H, q, J = 7.2), (5H, m). 13 C-NMR: 14.1, 18.6, 45.6, 60.7, 127.0, 127.4, 128.5, 140.7, Isopropyl 2-phenylpropanoate (Table 1, Entry 5) 1 H-NMR: 1.14 (3H, d, J = 6.4), 1.23 (3H, d, J = 6.4), 1.50 (3H, d, J = 7.2), 3.68 (1H, q, J = 7.2), 5.02 (1H, m), (5H, m). 13 C-NMR: 18.4, 21.4, 21.6, 45.6, 67.8, 127.1, 127.4, 128.4, 140.7, Methyl benzonate (Table 1, Entry 6) 1 H-NMR: 3.91 (3H, s), (2H, m), 7.55 (1H, m), 8.03 (2H, dd, J = 7.4, 5.4). 13 C-NMR: 52.0, 128.3, 129.5, 130.1, 132.8, Ethyl benzonate (Table 1, Entry 6) 1 H-NMR: 1.39 (3H, t, J = 7.2), 4.38 (2H, q, J = 7.2), (3H, m), 8.03 (2H, dd, J = 7.3, 5.3). 13 C-NMR: 14.2, 60.9, 128.2, 129.4, 130.1, 132.7, Isopropyl benzonate (Table 1, Entry 6) 1 H-NMR: 1.23 (6H, d, J = 6.0), 4.06 (1H, m), (2H, m), 7.60 (1H, m), 8.12 (2H, dd, J = 7.9, 1.0). 13 C- NMR: 25.1, 64.6, 128.4, 130.1, 132.7, 133.5, Methyl 2,4-hexadienoate (Table 1, Entry 7) 1 H-NMR: 1.86 (3H, dd, J = 5.4, 2.2), 3.74 (3H, s), 5.76 (1H, d, J = 15.5), (2H, m), 7.25 (1H, dd, J = 15.5, 9.7). 13 C-NMR: 18.6, 51.4, 118.5, 129.8, 139.4, 145.2, Ethyl 2,4-hexadienoate (Table 1, Entry 7) 1 H-NMR: 1.27 (3H, t, J = 7.2), 1.83 (3H, dd, J = 5.4, 2.2), 4.19 (2H, q, J = 7.2), 5.75 (1H, d, J = 15.5), (2H, m), 7.20 (1H, dd, J = 15.5, 9.7). 13 C-NMR: 14.2, 18.5, 60.1, 118.9, 129.7, 139.1, 144.8, Isopropyl 2,4-hexadienoate (Table 1, Entry 7) 1 H-NMR: 1.22 (6H, d, J = 6.3), 1.81 (3H, d, J = 5.4), 5.03 (1H, m), 5.71 (1H, d, J = 15.4), (2H, m), 7.19 (1H, dd, J = 15.4, 9.8). 13 C-NMR: 18.5, 21.8, 67.3, 119.5, 129.6, 138.8, 144.6, Methyl 3-phenylprop-2-ynoate (Table 1, Entry 8) 1 H-NMR: 3.83 (3H, s), (3H, m), 7.57 (2H, dd, J = 7.8, 1.5). 13 C-NMR: 52.7, 82.1, 87.2, 119.4, 128.5, 130.6, 132.9, Ethyl 3-phenylprop-2-ynoate (Table 1, Entry 8) 1 H-NMR: 1.34 (3H, t, J = 7.2), 4.28 (2H, q, J = 7.2), (3H, m), 7.56 (2H, dd, J = 8.6, 3.4). 13 C-NMR: 13.9, 62.0, 80.6, 86.0, 119.5, 128.5, 130.5, 132.8, Isopropyl 3-phenylprop-2-ynoate (Table 1, Entry 8) 1 H-NMR: 1.32 (6H, d, J = 6.0), 5.15 (1H, m), (3H, m), 7.58 (2H, dd, J = 7.5, 1.5). 13 C-NMR: 25.1, 64.6, 128.4, 130.1, 132.7, 133.5, Methyl N-acetyl glutamic ester (Table 1, Entry 9) 1 H-NMR: 1.99 (1H, m), 2.00 (3H, s), 2.20 (1H, m), (2H, m), 3.66 (3H, s), 3.73 (3H, s), 4.60 (1H, m), 6.29 (1H, d, J = 6.4). 13 C-NMR: 22.6, 27.3, 29.2, 51.2, 59.0, 60.4, 170.5, 172.0, Ethyl N-acetyl glutamic ester (Table 1, Entry 9) 1 H-NMR: 1.19 (3H, t, J = 7.2), 1.21 (3H, t, J = 7.2), 1.92 (1H, m), 1.95 (3H, s), 2.13 (1H, m), (2H, m), 4.06 (2H, q, J = 7.2), 4.14 (2H, q, J = 7.2), 4.52 (1H, m), 6.52 (1H, d, J = 7.3). 13 C-NMR: 13.9, 14.0, 22.8, 27.2, 30.2, 51.6, 60.5, 61.4, 170.1, 171.9, Isopropyl N-acetyl glutamic ester (Table 1, Entry 9) 1 H-NMR: (12H, m), 1.92 (1H, m), 1.97 (3H, s), 2.09 (1H, m), (3H, m), 4.49 (1H, m),

6 108 J. Chin. Chem. Soc., Vol. 50, No. 1, 2003 Chen and Lee (2H, m), 6.48 (1H, d, J = 7.2). 13 C-NMR: 21.6, 21.7, 23.0, 27.4, 30.7, 51.9, 68.0, 69.3, 170.1, 171.5, Propenyl decanoate (Table 2, Entry 1) 1 H-NMR: 0.86 (3H, t, J = 7.0), (12H, m), (2H, m), 2.30 (2H, d, J = 5.6), 4.54 (2H, d, J = 6.5), (2H, m), 5.88 (1H, m). 13 C-NMR: 14.0, 22.6, 24.9, 29.1 (2C), 29.2, 29.3, 31.8, 34.2, 64.8, 117.9, 132.3, Benzyl decanoate (Table 2, Entry 2) 1 H-NMR: 0.90 (3H, t, J = 6.9), (12H, m), (2H, m), 2.37 (2H, d, J = 7.6), 5.13 (2H, s), (5H, m). 13 C-NMR: 14.1, 22.7, 25.0, 29.1, 29.2, 29.3, 29.4, 31.9, 34.3, 66.0, 128.0, 128.4, 128.5, 136.2, Propynyl decanoate (Table 2, Entry 3) 1 H-NMR: 0.84 (3H, t, J = 6.8), (12H, m), (2H, m), 2.33 (2H, d, J = 7.8), 2.44 (1H, d, J = 2.2), 4.65 (2H, d, J = 2.2). 13 C-NMR: 13.2, 21.8, 24.0, 28.2, 28.4, 28.5, 29.1, 31.0, 33.1, 50.8, 73.8, 77.0, Isobutyl decanoate (Table 2, Entry 4) 1 H-NMR: 0.82 (3H, t, J = 6.6), 0.89 (6H, t, J = 6.6), (13H, m), 1.57 (1H, m), 1.86 (1H, m), 2.24 (2H, t, J = 7.5), 3.80 (2H, d, J = 6.6). 13 C-NMR: d 14.0, 21.7, 22.6, 25.0, 29.0, 29.1, 29.2, 29.4, 31.8, 34.6, 36.6, 60.0, Decanyl decanoate (Table 2, Entry 5) 1 H-NMR: (8H, m), (24H, m), (4H, m), 2.26 (2H, t, J = 7.6), 4.03 (2H, t, J = 6.7). 13 C-NMR: 14.0, 22.5, 22.6, 25.0, 25.8, 25.9, 28.5, 28.6, 29.0, 29.1 (2C), 29.2, 29.3, 29.4, 29.5, 31.5, 31.8, 34.3, 64.3, ACKNOWLEDGEMENT We thank the National Science Council in Taiwan (NSC M ), Tamkang University and National Taipei College of Nursing for financial support. Received August 19, REFERENCES 1. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis; John Wiley & Sons, Inc.: New York, Kocienski, P. J. Protecting Groups; Georg Thieme Verlag: New York, Ladduwahetty, T. Contemp. Org. Synth. 1997, 4, Haslam, E. Tetrahedron 1980, 36, Ogliaruso, M. A.; Wolfe, J. F. Synthesis of Carboxylic Acids, Esters and Their Derivatives; Patai, S.; Rappoport, Z.; Eds.; John Wiley & Sons, Inc.: New York, Sutherland, I. O. Comprehensive Organic Chemistry; Barton, D. H. R., Ollis, W. D., Eds.; Pergamon Press: Oxford, 1997; vol. 6, p Otera, J. Angew. Chem. Int. Ed. 2001, 40, Lee, A. S.-Y.; Yang, H.-C.; Su, F.-Y. Tetrahedron Lett. 2001, 42, Keglevic, D.; Horvat, J.; Plavsic, F. Carbohydr. Res. 1976, 47, 49. L-( )-Dimethyl N-acetylglutamate was reported as [ ] D 12 (CHCl 3,c 1). 10. Cohen, S. G.; Crossley, J. J. Am. Chem. Soc. 1964, 86, L-(-)-Diethyl N-acetylglutamate was reported as [ ] D (EtOH, c 0.98). 11. Ishihara, K.; Ohara, S.; Yamamoto, H. Science 2000, 290, Wakasugi, K.; Misaki, T.; Yamada, K.; Tanabe, Y. Tetrahedron Lett. 2000, 41, Otera, J.; Dan-Oh, N.; Nozaki, H. J. Org. Chem. 1991, 56, Das, B.; Venkataiah, B.; Madhusudhan, P. Synlett 2000, Hughes, D. L. Org. React. 1992, 42, Mitsunobu, O. Synthesis 1981, Eshuis, J. J. W. Tetrahedron Lett. 1994, 35, Lee, A. S.-Y.; Hu, Y.-J.; Chu, S.-F. Tetrahedron 2001, 57, Lee, A. S.-Y.; Su, F.-Y.; Liao, Y.-C. Tetrahedron Lett. 1999, 40, Lee, A. S.-Y.; Yeh, H.-C.; Shie, J.-J. Tetrahedron Lett. 1998, 39, Horspool, W.; Armesto, D. Organic Photochemistry; Ellis Horwood: New York, Luche, J.-L., Ed. Synthetic Organic Sonochemistry; Plenum press: New York, 1998.