Deprotection of Homoallyl (~hAllyl) Derivatives ofPhenols, Alcohols, Acids, and Amines

摘要

Chemoselective deprotection is among the most desirablefeatures associated with a protecting group.' The extent ofinertness and the conditions under which unmasking of aprotecting group take place are of prime consideration. Avail-ability and economics can also play important, albeit oftentimessecondary, roles. Surprisingly, the readily available hydrocarbon-based homoallyl residue is rarely seen in synthesis. Indeed, inthe latest edition of Greene's Protective Groups in OrganicSynthesis (2007),2 the homoallyl moiety is not even listed amongthe myriad of hydroxyl protecting groups. The potential for thisrelatively unreactive 4-carbon unit to serve as a protecting groupfor alcohols,3 amines,' and acids5 has been recognized fordecades, but existing conditions for removal may be responsiblefor its lack of utility to date. Thus, Barrett reported on theozonolysis of homoallylic esters as a means of revealing acarboxylic acid following treatment with Et3N (eq 1).6 Morerecently, a single example from the Cossy group described aruthenium-catalyzed isomerization of a homoallyl ether to anenol ether, which was subsequently hydrolyzed under aqueousacidic conditions to the free alcohol in modest overall yield (eq2).