Unlocking Access to Enantiopure Fused Uracils by Chemodivergent [4+2] Cross‐Cycloadditions: DFT‐Supported Homo‐Synergistic Organocatalytic Approach

The discovery of chemical methods enabling the construction of carbocycle‐fused uracils which embody a three‐dimensional and functional‐group‐rich architecture is a useful tool in medicinal chemistry oriented synthesis. In this work, an unprecedented amine‐catalyzed [4+2] cross‐cycloaddition is documented; it involves remotely enolizable 6‐methyluracil‐5‐carbaldehydes and β‐aryl enals, and chemoselectively produces two novel bicyclic and tricyclic fused uracil chemotypes in good yields with a maximum level of enantiocontrol. In‐depth mechanistic investigations and control experiments support an intriguing homo‐synergistic organocatalytic approach, where the same amine organocatalyst concomitantly engages both aldehyde partners in a stepwise eliminative [4+2] cycloaddition, whose vinylogous iminium ion intermediate product may diverge—depending upon conditions—to either bicyclic targets by hydrolysis or tricyclic products by a second homo‐synergistic trienamine‐mediated stepwise [4+2] cycloaddition. [4+2] Cross‐cycloadditions between uracil‐based aldehydes and β‐aryl enals were carried out under mild reaction conditions in the presence of the popular diphenylsilyl‐protected prolinol organocatalyst, which provided homo‐synergistic activation of both substrates. Bicyclic and tricyclic fused uracils were chemodivergently forged in good yields with maximum levels of enantioselectivity.