Catalytic enantioselective [5+2] cycloaddition between oxidopyrylium ylides and enals under dienamine activation.

Benzopyrylium ylides generated in situ from 1-acetoxyisochroman-4-ones reacted with α,β-unsaturated aldehydes in the presence of a bifunctional secondary-amine/squaramide catalyst to furnish [5+2] cycloaddition products in good yield with high diastereo- and enantioselectivity. The reaction proceeds by dienamine activation and involves β,γ-functionalization of the enal. The dienamine intermediates showed exclusive β,γ-reactivity and provided direct access to compounds with the 8-oxabicyclo[3.2.1]octane framework. The ability of the bifunctional secondary-amine/squaramide catalyst to engage in hydrogen-bonding interactions with the ylide made it particularly effective in terms of both the yield and the stereoselectivity of the transformation.