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A truly green, solventless and catalyst-free method has been developed for the quantitative protection of various functional groups using 3,4-dihydro-2H-pyran (DHP) under its own pressure at 125 °C. Advantages of this new technique are numerous: (a) no work-up or laborious purification steps are required; (b) no additional chemicals are employed; (c) no waste products are generated; (d) the protected intermediate can be used in the next synthesis step in the same flask in which the protection was carried out; (e) as there are no losses during the protection step, the overall yield is maximized; (f) the robust tetrahydropyranyl protecting group allows substrate derivatization under a variety of reaction conditions, yet can conveniently be removed by mild acidic hydrolysis. The influence of substrate acidity, miscibility of substrate and DHP, amount of DHP employed, nucleophilicity of the atom to be protected, steric hindrance, reaction time and reaction temperature on the efficiency of the protection reaction is discussed.