Catalytic Enantioselective [5+2] Cycloaddition between Oxidopyrylium Ylides and Enals under Dienamine Activation

Abstract

Abstract 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.