Regiospecific Allenylation of Ketones with Propargyltrichlorosilane.

The inherent difficulty in eliciting regio-control over propargyl-allenyl isomerization of the corresponding metal/metalloid species has limited their utility as nucleophiles in organic synthesis. Consequently, synthetically versatile α-allenyl tertiary alcohols are generally synthesized in two steps from ketones (e.g., Crabbé homologation of propargylic alcohols) rather than by the direct addition of allenylation reagents to ketones. We have developed a regiospecific allenylation of ketones with isomerically pure propargyltrichlorosilane as an operationally simple, general method to prepare racemic monosubstituted α-allenyl tertiary alcohols. The method provided 29 α-allenyl tertiary alcohols in 8-99% isolated yields but it proved to be intolerant of ketones containing sterically large groups and enones.