Aldeídos podem ser convertidos a acilas por tratamento com anidrido acético na presença de zircônia sulfatada a 0 o C, com excelentes rendimentos. Cetonas e aldeídos alifáticos não são afetados nessas condições de reação. A desproteção das acilas resultantes é obtida usando-se o mesmo catalisador a 60 o C. O catalisador pode ser reutilizado por duas vezes sem perder sua atividade.Aldehydes can be converted to acylals by treatment with acetic anhydride in the presence of sulfated zirconia in excellent yield at 0 o C. Ketones and aliphatic aldehydes are found to be unaffected under the reaction conditions. The deprotection of the resulting acylals is achieved using the same catalyst at 60 o C. The catalyst can be reused in two cycles without losing its activity.
Keywords: acylals, sulfated zirconia, protection, deprotection
IntroductionThe use of protecting groups is very important in organic synthesis, being often the key for the success of many synthetic enterprises. Geminal or 1,1-diacetates are synthetically useful as aldehyde protecting groups, 1 as alternatives to acetals. These diacetates are stable towards aqueous acids and mild bases.2 In addition, they can be used as building blocks for the synthesis of dienes for Diels-Alder cycloaddition reactions.3 Chiral allylic esters have been obtained using palladium catalysts by an asymmetric allylic alkylation of gem-diesters 4 and also preparation of homoallyl acetates by allylation of 1,1-diacetates have been described.
Results and DiscussionSulfated zirconia characterization, Figure 1 shows XRD plot corresponding to the crystalline zirconia. The sample analyzed presented tetragonal as unique phase (ICSD collection code: 066787) given by reflections in 2 = 30.18° (relative intensity in 100) as well as peaks 34. 616°, 35.283°, 43.002°, 50.214°, 50.770°, 59.291°, 60.187°, 62.724°, 72.894°, 74.617° and 81.768°. Adsorption-desorption of nitrogen gave an isotherm plot type IV of BET classification (Brunauer Emmett and Teller theory), no microporous area was observed by t-plot equation. BET specific surface-area, pore volume and pore size values are in , desorption curve is showed in Figure 2.To test the reciclability, the sulfated zirconia catalyst was filtered after reaction and activated at 500 °C for 1 h in air flow. There is no significant change in the activity and selectivity after two cycles.A variety of aldehydes and ketones react with acetic anhydride at 110 °C in the presence of commercial sulfated zirconia 36 (10% m/m of carbonyl compound) with surface area of 10 m 2 g -1 to afford the corresponding 1,1-diacetates. Sulfated metal oxides have gained attention due to their unusual ability to initiate acid catalyzed reactions at low temperatures. 37,38 We now wish to describe an more mild and efficient method for the chemoselective preparation and deprotection of acylals from aromatic and heteroaromatic aldehydes using prepared sulfated zirconia as catalyst (Scheme 1). Table 2 different aromatic and heteroaromatic aldehydes are converted to the corresp...