A Direct Synthesis of Trisubstituted Allenes from Propargyl Alcohols via Oxaphosphetane Intermediates

A direct synthetic method for trisubstituted allenes from propargyl alcohol is provided; the synthesis proceeds via an oxaphosphetane intermediate. Functional groups such as formyl and pyridyl exhibited a degree of tolerance during reaction without any protection. The alcohol dimethylated at the propargyl position afforded two structural isomers, allene and 1,3-diene. The product ratio was considerably influenced by the solvent. Allene was predominantly obtained when the reaction was conducted in cyclohexane, and the ratio was inverted by changing the solvent to dichloromethane. The prepared (2-pyridyl)allene served as a substrate for the copper(I) catalyzed cyclization reaction to afford 3,3-dimethylindolizine-2-one.