A Brønsted Acid Mediated Cascade Enone Synthesis from Aldehydes Containing a Tethered Propargylsilane

MeSO3H effects the intramolecular allenylation of a series
of aldehydes 1 to provide allenyl alcohol product 3 as a single
diastereoisomer. Cyclization proceeds rapidly at −78 °C.
However, when the reaction is performed at room temperature, aldehyde 1 provides enone product 7 instead. A
mechanism for the formation of this product is proposed in
which the initially formed allenyl alcohol 3 undergoes
dehydration to provide an allyl carbocation, which is trapped
with water, thereby installing the enone.