Solvent-free Cleavage of tert-Butyl Esters under Microwave Condition

摘要

Protection and deprotection of carboxylic acid with steri-cally bulky alkyl groups are essential steps in synthetic organic chemistry. Among them, tert-butyl is the most commonly used to protect the carboxylic acid group because the resulting tert-butyl esters are highly stable under neutral and basic conditions, and can be easily prepared from a variety of free carboxylic acids. The tert-butyl group could be removed usually by using strong acids such as CF3COOH, HNO3, and H2SO4. However, major drawbacks of these strong acid conditions are formation of unwanted side products and decomposition of acid labile substrates. On the other hand, the use of Lewis acids such as MgI2, and ZnBr2 have a merit of mild reaction condition but need to long reaction time, higher reaction temperature, and involve toxic metals which are detrimental to the environment. In 2001, Jackson reported an interesting method using silica gel as an efficient reagent for the cleavage of tert-butyl esters. Unfortunately this method also required long reaction time at high reaction temperature as exemplified by cleavage of tert-butyl from the tert-butyl 4-methoxybenzoate took 7 h at reflux.