Å molecular sieves catalyzed ring-opening of epoxides to 1 , 2-diacetates with acetic anhydride

Article history: Received February21, 2015 Received in revised form April 29, 2015 Accepted 2 June 2015 Available online 2 June2015 This study was aimed at highly stereoselective ring-opening of epoxides with acetic anhydride in the presence of molecular sieves 4Å as an efficient reagent. Yields of the 1,2-diacetates are uniformly good to excellent and the reagent may be recycled without the requirement for purification. High yields, short reaction times, ease of preparation and simple work-up procedure (filtration) are advantages of this method. © 2015Growing Science Ltd. All rights reserved.


Introduction
The ring strain and polarization of carbon-oxygen bonds make epoxides versatile synthetic intermediates in organic synthesis. 1Nucleophilic ring-opening of epoxides with water to give the corresponding 1,2-diols is one of the most common reactions and the subsequent acetylation of these diols provides an effective means for the protection of hydroxyl groups and preparation of 1,2diacetoxy ester products. 2Therefore, direct transformation of epoxides to the 1,2-diacetates is a very attractive reaction.A review of the literature shows that this transformation has been achieved with the reaction of epoxides and acetic anhydride in the presence of catalysts or reagents such as HCl, ZnCl2, 3 (NH4)3(PMo12O40)6, 4 (TBA)4PFeW11O39•3H2O, 5 Cp2TiCl2, 6 (n-Bu)4NOAc, 7 DBU-LiCl, 8 BF3•Et2O, 9 cerium(III) chloride, 10 NaBH4, 11 HY zeolite, 12 Bu3P, 13 LiClO4 14 and Er(OTf)3. 15Among these, only four reports detailed general values (i.e.refs.11-13 and 15), and the others were limited in scope.Some of the previously-reported studies required expensive, exotic or hazardous reagents, extended reaction times, and/or limitation to a narrow range of epoxides.For these reasons, the development of convenient methods which use cheap and readily-available reagents is necessary.
In conjunction with ongoing work in our laboratory on the catalysis of epoxide ring-opening, we observed that molecular sieves (4Å) can be utilized as an efficient reagent to the ring-opening of epoxides with acetic anhydride under reflux to form the 1,2-diacetates in excellent yields (Scheme 1).

Results and Discussion
The cleavage of epoxides with acetic anhydride is unique and highly useful in organic synthesis due to production of the 1,2-diacetates as an effective protecting group.A molecular sieve is a material with very small pores in its structure, increasing contact area and activating the ring-opening of epoxides in order to proceed to the diacetate products.They appear to function as a Lewis acid upon the epoxide oxygen lone pairs to polarize the ring for heterolysis.We utilized 4Å molecular sieves in this study; these are composed of alkali metal silicic acid salts and can absorb water and other molecules with a critical diameter less than 4Å.The reagent can be easily recovered and reused several times.To the best of our knowledge, the above reagent has not been employed in the ring-opening of epoxides up to now.
As illustrated in Table 1, all reactions proceeded in high to excellent yields within 2 hours or less.The results obtained from the ring-opening of different classes of epoxides containing electronreleasing or -withdrawing groups with molecular sieves in acetic anhydride as both reagent and solvent under reflux shows that the reactions were completed by using a ratio of 0.150.55g of molecular sieves per 1 mmol of substrate.The desired 1,2-diacetate products were obtained in 8298% yields.Product analysis revealed that only one diastereomer with anti 1,2-diacetoxy groups was prepared by the reaction of cyclohexene oxide with acetic anhydride, which gives exclusively the corresponding trans-1,2-diacetate.Stereochemical assignment was achieved by: i) comparison of the obtained 1 H NMR spectrum with one of authentic sample reported in the literature, 16 and ii) hydrolysis of rac-trans-1,2-diacetoxycyclohexane to white crystalline rac-trans-1,2-cyclohexanediol (mp 101103 °C, found: 101104 °C, lit. 17

Conclusions
In this study, 4Å molecular sieves have been used as an efficient reagent in order to form the 1,2diacetates via the ring-opening of epoxides with acetic anhydride under reflux.The amount of molecular sieves is dependent on the structure of epoxides.Some epoxides can convert to products with small amount of reagent and some of them need higher amount of reagent for fully conversion.All reactions were carried out by using 1 mmol of epoxide with different amount of molecular sieves 4Å.In addition, the high yields, ease of preparation, easy catalyst recovery and short reaction times are the advantages of our method.

Materials and methods
All reagents and substrates were purchased in high-purity grade from Merck and Fluka, and were used without further purification.FT-IR and 1 H/ 13 C NMR spectra were recorded on Thermo Nicolet Nexus 670 and Bruker Avance 300 MHz spectrometers, respectively.The products were characterized by their FT-IR and 1 H/ 13 C NMR spectra and compared with data reported in literature.All yields refer to isolated pure products.Column chromatography was carried out on short columns of silica gel 60 (230400 mesh) in glass columns (1-2 cm diameter) using of silica gel (10-20 g per 1 g of isolated crude 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 4Å molecular sieves in acetic anhydride
In a round-bottomed flask (15 mL) equipped with a condenser and a magnetic stirrer bar, molecular sieves 4Å (150550 mg) were added to a solution of epoxide (1 mmol) in acetic anhydride (1 mL).The reaction mixture was stirred under reflux for 2 h.After completion of the reaction, an aqueous solution of NaHCO3 (5%, 5 mL) was added and the mixture was stirred for additional 5 min.The mixture was extracted with CH2Cl2 (2 × 5 mL) and then dried over anhydrous sodium sulfate.The eluted product was concentrated under reduced pressure and then subjected to chromatography on a normal-length silica gel column to give the pure product in excellent yield (8298%, Table 1).

Physical and spectral data
carried out with 1 mmol of epoxide in refluxing Ac a isolated pure products.