Reaction Of Lithiated Propargyl Ethers With Carbonyl Compounds - A Regioselective Route To Furan Derivatives

The deprotonation of 3-aryl-substituted alkyl propargyl ethers with n-butyllithium provides an ambident anion that reacts with carbonyl compounds to provide mixtures of gamma-substituted products with alkoxyallene substructure and of alpha-substituted propargyl ethers. The ratio of the two product types strongly depends on the solvent: in diethyl ether the gamma-substituted products predominate whereas the more polar tetrahydrofuran favors the alpha-adducts. The primary addition products undergo 5-endo-trig or 5-endo-dig cyclizations under various reaction conditions to afford isomeric furan derivatives. The highest selectivity in favor of alpha-substituted products was achieved by employing a MOM-protected propargyl ether. During the protonation step no evidence for a proton shift leading to an isomeric allenyl anion was found. A brief mechanistic discussion tries to rationalize the observed regio-selectivities.