A Green Protocol for Direct Esterification of TMS and THP Ethers with Aldehydes Using KCN and Air as the Simplest Available and Free of Cost Oxidant

Esterification reaction is an important and very useful reaction in organic synthesis. Thechemical structure of many pharmaceutical compounds, fragrances, essences and oils involvesthe ester moiety. In addition, this reaction is applied for protection of hydroxyl groups. Thereaction between a carboxylic acid and alcohol is a known route for ester synthesis. However,these compounds are also produced from other sources such as aldehydes. Various oxidativemethods have been reported for the synthesis of esters from alcohols and aldehydes directlyusing an oxidant such as tBuOOH in the presence of CoII/CF3COOH as a catalyst [1] and ureahydrogen peroxide and p-toluenesulfonyl chloride [2]. However, many of these methods sufferfrom some disadvantages such as restriction to narrow choice of alcohols with relativelylow boiling point such as methanol, ethanol and isopropanol, the use of external oxidant andexpensive and unavailable catalyst, the use of alcohols in excess amounts (as solvent), lowyields and operation in acidic conditions. Herein, in continuation of our previous work [3] andfor first time, a green protocol is reported for direct esterification of trimethylsilyl (TMS) andtetrahydropyranyl (THP) ethers with aldehydes using KCN and air as the simplest availableand free of cost oxidant (Fig. 1). (***) This method is not restricted to only some these ethers, but also different kinds of them are used in nearly equal molar amount to aldehyde. In addition, the present green protocol doesnot need to an external oxidant and also indicates a version of transprotection of hydroxylgroup which can be used in organic synthesis.