A new, convenient reductive procedure for the deprotection of 4-methoxybenzyl (MPM) ethers to alcohols

Selective introduction and removal of protecting groups is of great significance in organic synthesis. The benzyl ether function is one of the most common protecting groups for alcohols. Selective oxidative removal of the 4-methoxybenzyl (MPM) ethers in the presence of benzyl ethers made the MPM moiety an alternative protecting group, and its utility in carbohydrate chemistry is well established. Several procedures have been developed for the cleavage of the 4-methoxybenzyl moiety, e.g. DDQ oxidation (eq 1), electrochemical oxidation, homogeneous electron transfer, photoinduced single electron transfer, boron trichloride-dimethyl sulfide, etc. However, in all these methods isolation of the alcohol from the inevitable byproduct, 4-methoxybenzaldehyde [also dichlorodicyanohydroquinone (DDHQ) in the most commonly used method employing DDQI can be troublesome. Recently Wallace and Hedgetts discovered that acetic acid at 90°C cleaves the aromatic MPM ethers into the corresponding phenols and 4-methoxybenzyl acetate (eq 2), whereas the aliphatic MPM ethers generated, instead of alcohols, the corresponding acetates (eq 3). Complimentary to this methodology, herein we report that sodium cyanoborohydride and boron trifluoride etherate reductively cleaves, cleanly and efficiently, the aliphatic MPM ethers to an easily separable mixture of the corresponding alcohols and 4-methylanisole (eq 4)