The benzyl ether function is one of the mqst common protecting groups for alcohols. It is usually removed by catalytic hydrogenation or less often by sodium in liquid ammonia [21. If, however, the substrate contains additional reducible functions, this procedure is unsatisfactory.We now report a simple and mild method for the removal of the p-methoxybenzyl ether protecting group by homogeneous electron transfer under oxidative conditions. The stable cation radical of tris(p-bromopheny1)amine is employed as electron transfer agent (mediator). Bond cleavage takes place in moist acetonitrile according to the following reaction scheme: and regenerated from the amine during the course of the reaction (method 2). In the latter case only catalytic amounts of the electron transfer agent are necessary. Both methods usually give good to excellent yields and the alcohol is obtained in high purity. In contrast to the reductive deprotection procedures, double bonds are not attacked. Numerous oxidation-labile functions should tolerate the new procedure because the redox potential of the mediator is very low. Direct anodic deprotection, however, needs an anode potential of at least f1.6V us. SCE and fouling of the electrode is often encounteredC6!
General procedureMethod 1 : To a solution of 2 mmol of the respective p-methoxybenzyl ether (3) (2 mmol) and 2,6-dimethylpyridine (0.212g, 2mmol) in CH3CN[71/CH2C12 (5: 1) a solution of tris-(p-bromopheny1)ammoniumyl hexachloroantimonate (4.08 g, 5mmol) in CH3CN/CH2CI2 (5: 1) is added dropwise until the blue colour of the cation radical (I) persists. For product workup, water is added to the reaction solution which is subsequently reduced, saturated with KC1, and extracted with ether. After extraction with NaHS03 solution the ether phase is dried over MgS04. The alcohols are identified by gas chromatography and mass spectrometry on comparison with authentic samples and isolated by bulb-to-bulb distillation.Method 2: In a divided beaker-type glass cell (20°C, Pt anode, Pt cathode) a solution of tris(p-bromopheny1)amine (0.964 g, 2 mmol) in CH3CN/CH2C12 (5 : 1 ; 0.2 M LiC104) is used as anolyte. The catholyte consists of CH,CN/I.OM LiC104. After application of an anode potential of + 1.2 V (us. Ag/AgCl) the blue color of ( 1 ) appears. When the respective p-methoxybenzyl ether (3) ( 5 mmol) and 2,6-dimethylpyridine (0.535 g, 5 mmol) are added the solution is decolorized. After reappearance of the blue color of the electron transfer agent (consumption of about 0.013 F) the electrolysis is stopped. The workup procedure is the same as in method 1.