Direct Transformation of Epoxides to 1,2-Diacetates with Ac2O/B(OH)3 System.
Summary: Direct transformation of different kinds of epoxides to 1,2-diacetates was carried out easily and efficiently with Ac2O/B(OH)3 system. All reactions were carried out under reflux conditions within 2 h to afford 1,2-diacetates in high yields.
Keywords: Acetic anhydride, B(OH)3, 1,2-Diacetate, Epoxide, Ring-opening.
The ring strain makes epoxides multipurpose intermediates in the synthesis of organic compounds . Epoxides ring-opening in the presence of water leads to 1,2-diols and so acetylation of these products prepares an effective tool to protect hydroxyl groups as 1,2-diacetoxy esters . In a simple way, however, epoxides could be directly converted to 1,2-diacetates. The literature review shows that direct transformation of epoxides to 1,2-diacetates has been obtained in the presence of various catalysts/reagents such as HCl/ZnCl2 , BF3 Et2O , DBU/LiCl , n-Bu4NCl , n- Bu4NOAc , Bu3P , LiClO4 , Er(OTf)3 , HY zeolite , (NH4)3PMo12O40 , (TBA)4P- FeW11O39 3H2O , ZrO(OTf)2 , NaBH4 , phosphomolybdic acid or its supported on silica gel  and molecular sieves 4A .
Among these, some reports detailed general values and the others were limited in scope. Moreover, they generally suffer from using expensive reagents, strongly acidic/basic reaction conditions, long reaction times and limitation to use both aromatic and aliphatic epoxides as starting materials. Therefore, the development of simple procedures which utilize inexpensive and convenient reagents is the subject of interest and still is demanded.
In line of the outlined strategies and continuation of our research program directed to the straightforward preparation of 1,2-diacetates from epoxides [15-17], herein, we wish to introduce boric acid, B(OH)3, in acetic anhydride as an easily available and efficient promoter system for the titled transformation (Scheme 1).
Scheme-1: Ring opening of epoxides with AC2O/B (OH)3 system.
Materials and Methods
All reagents and substrates were purchased from commercial sources with the best quality and were used without further purification. FT-IR and 1H, 13C NMR spectra were recorded on Thermo Nicolet Nexus 670 and Bruker Avance 300 MHz spectro- meters, respectively. The products were characterized by their FT-IR and 1H, 13C NMR spectra and compared with the reported data in literature. All yields refer to isolated pure products. TLC was applied for the purity determination of the substrates, products and for monitoring of the reactions over silica gel 60 F254 aluminum sheet.
A General Procedure for Conversion of Epoxides to 1,2-Diacetates with Ac2O/B(OH)3 System
In a round-bottomed flask (15 mL) equipped with a magnetic stirrer and condenser, B(OH)3 (1.5-2 mmol) were added to a solution of epoxide (1 mmol) in acetic anhydride (1 mL). The reaction mixture was stirred for 2 h under reflux conditions. After completion of the reaction, the mixture was cooled to the room temperature. Then, an aqueous solution of NaHCO3 (5%, 5 mL) was added and the reaction mixture was stirred for additional 5 min. The mixture was extracted with EtOAc (2 A- 5 mL) and then dried over anhydrous sodium sulfate. The eluted solvent was passed through a short column of silica gel and then concentrated under reduced pressure to afford the pure product in high yield (8096%, Table 1). Spectral data for compounds (1-8)b:
1,2-Diacetoxy-1-phenylethane (1b): 1H NMR (CDCl3, 300 MHz) d 7.40-7.31 (m, 5H), 6.02 (dd, J 4.2, 7.8 Hz, 1H), 4.39-4.25 (m, 2H), 2.10 (s, 3H), 2.06 (s, 3H); 13C NMR (CDCl3, 75.5 MHz) d 170.64, 170.05, 136.50, 128.64, 128.63, 126.70, 73.32, 66.09, 21.10, 20.78; FT-IR (max/cm-1, neat) 3034, 2954, 1743, 1455, 1435, 1371, 1223, 1047.
1,2-Diacetoxycyclohexane (2b): 1H NMR (CDCl3, 300 MHz) d 4.79-4.5 (m, 2H), 1.97-1.93 (m, 2H), 1.94 (s, 3H), 1.93 (s, 3H), 1.69-1.62 (m, 2H), 1.36-1.16 (m, 4H); 13C NMR (CDCl3, 75.5 MHz) d 170.37, 170.22, 73.49, 30.16, 29.94, 23.25, 23.11, 21.17, 21.14; FT-IR (max/cm-1, neat) 2942, 2866, 1739, 1452, 1368, 1251, 1042.
(2,3-Diacetoxypropyl) methacrylate (3b): 1H NMR (CDCl3, 300 MHz) d 6.10-6.04 (m, 1H), 5.55-5.54 (m, 1H), 5.30-5.16 (m, 1H), 4.33-4.05 (m, 4H), 2.02 (s, 3H), 2.00 (s, 3H), 1.87 (s, 3H); 13C NMR (CDCl3, 75.5 MHz) d 170.41, 170.03, 166.66, 135.65, 126.37, 69.28, 62.39, 62.23, 20.76, 20.57, 18.13; FT-IR (max/cm-1, neat) 2988, 2880, 1720, 1638, 1453, 1372, 1320, 1296, 1166, 1077, 1055.
1,2-Diacetoxy-3-phenoxypropane (4b): 1H NMR (CDCl3, 300 MHz) d 7.32-7.25 (m, 2H), 6.99-6.89 (m, 3H), 5.41-5.34 (m, 1H), 4.43 (dd, J 3.9, 12 Hz, 1H), 4.29 (dd, J 6, 12 Hz, 1H), 4.11 (d, J 5.1 Hz, 2H), 2.09 (s, 3H), 2.06 (s, 3H); 13C NMR (CDCl3, 75.5 MHz) d 170.57, 170.26, 158.27, 129.53, 121.36, 114.59, 69.76, 65.96, 62.54, 20.92, 20.70; FT-IR (max/cm-1, neat) 3041, 2957, 1746, 1600, 1588, 1497, 1371, 1228, 1050.
1,2-Diacetoxy-3-isopropoxypropane (5b): 1H NMR (CDCl3, 300 MHz) d 5.15-4.96 (m, 1H), 4.28 (dd, J 3.6, 12 Hz, 1H), 4.12 (dd, J 3, 12 Hz, 1H), 3.58-3.40 (m, 3H), 2.05 (s, 3H), 2.04 (s, 3H), 1.09 (d, J 6 Hz, 6H); 13C NMR (CDCl3, 75.5 MHz) d 170.68, 170.34, 72.32, 70.59, 66.12, 63.00, 21.91, 21.84, 21.00, 20.74; FT-IR (max/cm-1, neat) 2918, 2849, 1743, 1463, 1371, 1229, 1118, 1047.
1,2-Diacetoxy-3-allyloxypropane (6b): 1H NMR (CDCl3, 300 MHz) d 5.92-5.76 (m, 1H), 5.27 (dd, J 1.5, 17.4 Hz, 2H), 5.21 (dd, J 3, 17.4, 1H), 5.20-5.12 (m, 1H), 4.32 (dd, J 3.9, 12 Hz, 1H), 4.15 (dd, J 6.3, 12 Hz, 1H), 4.05-3.91 (m, 2H), 3.55 (d, J 5.1 Hz, 2H), 2.07 (s, 3H), 2.03 (s, 3H); 13C NMR (CDCl3, 75.5 MHz) d 170.58, 170.26, 134.13, 117.33, 72.19, 70.21, 68.07, 62.80, 20.94, 20.67; FT- IR (max/cm-1, neat) 2957, 2868, 1743, 1433, 1372, 1225, 1092, 1048.
1,2-Diacetoxy-3-chloropropane (7b): 1H NMR (CDCl3, 300 MHz) d 5.25-5.13 (m, 1H), 4.37- 4.1 (m, 2H), 3.75-3.55 (m, 2H), 2.09 (s, 3H), 2.07 (s, 3H); 13C NMR (CDCl3, 75.5 MHz) d 170.49, 170.41, 70.39, 62.36, 42.10, 20.86, 20.79; FT-IR (max/cm-1, neat) 2963, 1744, 1436, 1371, 1221, 1046.
1,2-Diacetoxy-3-butoxypropane (8b): 1H NMR (CDCl3, 300 MHz) d 5.20-5.12 (m, 1H), 4.31 (dd, J 3.7, 11.9 Hz, 1H), 4.13 (dd, J 6.3, 12 Hz, 1H), 3.52 (d, J 5.4 Hz, 2H), 3.47-3.35 (m, 2H), 2.06 (s, 3H), 2.04 (s, 3H), 1.58-1.45 (m, 2H), 1.40-1.26 (m, 2H), 0.88 (t, J 7.3 Hz, 3H); 13C NMR (CDCl3, 75.5 MHz) d 170.73, 170.39, 71.43, 70.32, 68.83, 62.98 31.56, 21.05, 20.78, 19.18, 13.85; FT-IR ( max cm-1, neat) 2959, 2935, 2871, 1743, 1465, 1378, 1238, 1109.
Results and Discussion
Boric acid, also called hydrogen borate, is a weak, monobasic Lewis acid of boron often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other chemical compounds . In recent years, boric acid has gained special attention in organic synthesis for a number of synthetic transformations, such as Biginelli reaction , transesterification of ethyl acetoacetate , aza Michael  and thia Michael addition , oxidation of sulphides , preparation of a- hydroxyamides , silylation of alcohols , synthesis of 1,5-benzodiazepine , benzimidazoles , N,N'-alkylidene bisamides , 1-amidoalkyl-2-naphthols  and 4H-isoxazol-5-ones . This reagent is commercially available, environmentally benign, cheap, easy to handle, and stable. A literature review shows that the application of boric acid for direct transformation of epoxides to 1,2-diacetates has not been investigated yet.
So, this subject and our interests to develop new synthetic methods for the promoted ring-opening of epoxides with the easily available and more efficient reagents, encouraged us to investigate the capability of boric acid for the titled reaction.
Transformation of epoxides to 1,2-diacetates was first examined by the reaction of styrene oxide as a model compound with acetic anhydride in the absence of B(OH)3. The result showed that under different conditions the reaction did not any take place and styrene oxide was recovered from the reaction mixture. However, performing of the reaction in the presence of boric acid, exhibited the more efficient result. Further experiments resulted that using 2 mmol B(OH)3 is the requirement for complete reaction of styrene oxide (1 mmol) in Ac2O (1 mL). The reaction was carried out under reflux conditions within 2 h to give 1,2-diacetoxy-1- phenylethane in 85% yield (Table 1, entry 1).
Encouraged by the result, the capability of this synthetic method was more studied with the reaction of various epoxides containing electron-releasing or withdrawing groups with Ac2O under the optimized conditions. Table 1 shows the general trend and versatility of this synthetic method. As it's seen, all reactions were carried out successfully using 1.5-2 mmol of boric acid within 2 h to give the products in high to excellent yields.
Table-1: Conversion of epoxides to 1,2-diacetates with Ac2O/B(OH)3 systema.
###Entry###Epoxide (a)###1,2-Diacetate (b)###Molar ratiob###Yield (%)c###Ref.
Moreover, in the reaction of cyclohexene oxide with Ac2O/B(OH)3 system, rac-trans-1,2- diacetoxycyclohexane was obtained (Table 1, entry 2). Stereochemistry of the product was assigned by comparison of the obtained 1H NMR spectrum with the reported one of authentic sample [31,32], and hydrolysis of rac-trans-1,2-diacetoxycyclohexane  to white crystalline rac-trans-1,2-cyclohaxane- diol (m.p. 101-103 C, Lit.  101-104 C) (Scheme 2).
Scheme-2: Hydrolysis of rac-trans-1,2-diacetoxycyc- lohexane to rac-trans-1,2-cyclohexanediol.
Although the exact mechanism of this synthetic method is not clear, however, it's seems that boric acid by its weak Bronsted as well as Lewis acidity characters can activate the epoxide ring for heterolysis by Ac2O.
In summary, we have shown that B(OH)3 in acetic anhydride can be used efficiently for direct transformation of different kinds of epoxides to 1,2- diacetates. All reactions were carried out under reflux conditions within 2 h to afford the products in high to excellent yields. The cheapness and availability of the reagents, manipulation to a wide range of epoxides and high efficiency are the advantages which make this protocol a synthetically useful addition to the present methodologies.
The authors gratefully acknowledged the financial support of this work by the research council of Urmia University.
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|Date:||Dec 31, 2015|
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