Hetero Diels-Alder Cycloaddition of Siloxy Vinylallenes-Synthesis of the Indolizidine Skeleton: Experimental and Computational Studies.

Vinylallenes have been used in Diels-Alder reactions with a variety of dienophiles. However, vinylallenes activated at the allenic part of the molecule have been reacted only with carbon-carbon double bonds. We prepared siloxy vinylallenes by the base-induced equilibrium of silyl-protected allyl-propargyl alcohols. We found that these systems react with imines to form cycloadducts with total regio and facial selectivity, but only moderate endo:exo selectivity. The cycloadducts obtained were transformed into indolizidine derivatives. The reaction was studied computationally using DFT and compared to the reaction of siloxydienes. It was found that the main difference between those systems is the higher nucleophilicity of the siloxydienes compared to the siloxy vinylallenes.