A Mild and Efficient Method for the Synthesis of Acylals from Aromatic Aldehydes and Their Deprotections Catalyzed by Synthetic Phosphates under Solvent-Free Conditions

An efficient and clean preparation of acylals from aromatic aldehydes in the presence of synthetic phosphates (flourapatite and hydroxyapatite doped with ZnCl2 and ZnBr2) and acetic anhydride was achieved easily in high yields (86% - 97%) at room temperature under solvent-free conditions. Deprotection of the resulting acylals has also been attained by using the same catalysts under microwave irradiation. This method consistently has advantage of excellent yields (82% - 96%) and a short reaction time (3 - 4 min).


Introduction
There has been an increasing emphasis among researchers from both academia and industry to design synthetic strategies keeping in view the principles of "Green Chemistry" [1]. Adopting the principles of green chemistry means to reduce or eliminate the generation and use of hazardous substances. In recent years, replacement of hazardous solvents with environmentally benign solvents [2] [3] or development of solvent-free syntheses [4] [5] [6] is one of the major focus areas of Green Chemistry. In other cases, the use of heterogeneous cata-lysts under solvent-free conditions is becoming very popular as it has many advantages: reduced pollution, reusability, high selectivity, low cost, and simplicity in process and in handling. These factors are especially important in industry.
The 1,1-diacetates (acylals) have been introduced as a suitable protection group for aldehyde because of their remarkable stability to neutral and basic conditions [7]. In addition, they can be converted into other useful functional groups by reaction with appropriate nucleophiles [8] and used as carbonyl surrogates for asymmetric synthesis [9]. 1,1-Diacetates, on the other hand, are ambident substrates containing two types of reactive carbon centres, the carbon atom of the protected aldehyde function and the carbonyl group in the ester moieties [10]. The relative acid stability of 1,1-diacetates is another interesting feature of such 1,1-diacetates in the field of protection-deprotection chemistry [11].
The synthesis of acylals is usually achieved via the reaction of an aldehyde with acetic anhydride (AC 2 O) in the presence of a protic acid [12] [13] [14], Lewis acids [15]- [23], or heterogeneous catalyst [24]- [30]. Many of reported methods, however, involve strongly acidic or oxidizing conditions, corrosive reagents, high temperature, high catalyst loading, longer reaction time and cumbersome procedures. In view of these, the search for finding a cost-effective, mild and simple protocol for synthesis of acylals from aldehydes is still relevant.

Results and Discussion
The catalytic activity of the materials prepared has been evaluated in the protection of aromatic aldehydes. First of all, we have tested the apatite (FAP or HAP) alone in the catalysis of the synthesis of acylals ( To explore the scope of this methodology the protection of different substrates was investigated. To appreciate the important enhancement of the activity of the doped materials, we have carried out synthesis of acylals with ZnCl 2 /FAP, ZnBr 2 /FAP, ZnCl 2 /HAP and ZnBr 2 /HAP in the same condition ( Table 2). The positive effect of doped solid catalysts can be observed in all cases. The yields obtained are high (86% -97%) at room temperature except for the product required relatively longer reaction time (entry 2e) at 70˚C, possibly owing to the strong electron-withdrawing nitro substituent (see Table 2). Meanwhile, the presence of electron-donating groups (entry 2b, 2d, 2g, Table 2) increases the yields, demonstrating the participation of both the aldehydes and the acetic anhydride in the rate controlling step of the reaction. Furthermore, α, β-unsatured aldehydes, such as cinnamaldehyde ( Table 2, product 2f) and acid sensitive aldehydes, such as furfural ( Table 2, product 2c) reacted well without any decomposition or polymerization under the selected reaction conditions.
To evaluate the influence of the metal halide in the activity of the doped material, we have compared the yields obtained in the synthesis of product 2a using    Table 3. The reaction were completed within 3 min for  It was noticed that there was no reaction under microwave without catalyst, and to what was observed in traditional heating without solvent. This shows a certain synergy between catalyst and the microwave. It is thus completely reasonable to think that the effect of the temperature is a determining factor to promote this transformation. Unfortunately, domestic microwave was used and therefore it was impossible to measure the exact temperature during the reaction.
In conclusion, we have developed a clean and easy method for the synthesis of geminal-diacetates from aldehydes in solvent free conditions, and regeneration of aromatic aldehydes from the corresponding acylals in microwave irradiation using metal halides doped apatite (HAP or FAP) as heterogeneous catalysts. The positive effect of doping the apatite with ZnCl 2 and ZnBr 2 has been observed, and the comparison of these materials indicates that ZnBr 2 /HAP is the best catalyst for this reaction. The use of metal halides doped apatite offers diverse advantages including simplicity of operation due to the heterogeneous nature of reaction, easy workup and high yields.

Preparation and Characterization of HAP
The synthesis of hydroxyapatite [33] is carried out by reaction between diammonium phosphate and calcium nitrate in presence of ammonia. An amount of 250 ml of aqueous solutions containing 7.92 g of diammonium phosphate, maintained at a pH greater than 12 by addition of ammonium hydroxide (70 ml), were dropped under constant stirring into 150 ml of a solution containing 23.6 g of calcium nitrate (Ca(NO 3 ) 2 H 2 O). The suspension was refluxed for 4 h.
Distilled water (DW) was used to prepare the solutions. The obtained hydroxyapatite was filtered, washed with DW, dried overnight at 80˚C and calcined in air at 800˚C for 1 h before use.
The structure of the obtained hydroxyapatite was confirmed by X-ray diffraction, infrared spectroscopy and chemical analysis. The hydroxyapatite crystallized in the hexagonal system with the space group P63/m. The lattice parameters of the prepared HAP are in excellent agreement with standard data: a = 6.883 A˚ and c = 9.422 A˚. Surface area of the calcined HAP was determined by the BET method from the adsorption-desorption isotherm of nitrogen at its liquid temperature (77 K), and was found to be S = 35 m 2 ·g −1 .

Preparation and Characterization of FAP
FAP was prepared by the co-precipitation method as previously reported [34].
The structure of the obtained FAP was confirmed by X-ray diffraction, infrared and was found to be S = 15 m 2 ·g −1 .

Preparation of ZnX2/Apatite (ZnX2/HAP or ZnX2/FAP) [X = Br, Cl]
The preparation of ZnX 2 /apatite was as follows: 10 mmol of ZnX 2 and 10 g of apatite (FAP or HAP) were mixed in 100 mL of water and then evaporated to dryness and dried for 2 h at 150˚C before use.

Procedure for the Preparation of 1.1-Diacetates
A typical procedure for the preparation of 1,1-diacetates is a follows: the aldehyde (3 mmol) and acetic anhydride (9 mmol) were placed in a two-necked flask with stirring at room temperature. The catalyst (200 mg of apatite (HAP or FAP) or 100 mg of (ZnX 2 /FAP or ZnX 2 /HAP) was added and obtained mixture was maintained at room temperature (only for entry 2e a temperature at 70˚C was needed) for the appropriate time (see Table 2). After the completion of the reaction was monitored by TLC (hexane/ethyl acetate 5% -20%), Et 2 O was added to the mixture and filtered, and then the catalyst was again washed with Et 2 O and collected for reuse. Evaporation of the solvent followed by column chromatography on silica gel afforded the pure compound corresponding to 1,1-diacetates.

Procedure for Deprotection of 1.1-Diacetates
A typical procedure for the regeneration of aldehydes from 1,1-diacetates is a follows: 0.5 g of synthetic phosphates were placed, at 500 w for (HAP or FAP) or 350 w for (ZnX 2 /FAP or ZnX 2 /HAP) in a domestic microwave oven, for the appropriate time (Table 3). After the completion of the reaction was monitored by TLC (hexane/ethyl acetate 5% -20%), Et 2 O was added to the mixture and filtered. Evaporation of the solvent followed by column chromatography on silica gel afforded the pure compound corresponding to aldehydes.