Direct, Asymmetric Synthesis of Carbocycle-Fused Uracils via [4+2] Cycloadditions: a Noncovalent Organocatalysis Approach

Abstract

The peculiar versatility of remotely enolizable 6-methyluracil-5-carbaldehydes as useful vinylogous pronucleophiles in direct, asymmetric [4+2] cyclizations with suitable nitroolefins has been demonstrated. Under the strategic exploitation of noncovalent bifunctional organocatalysis, a dearomative remote enolization strategy was implemented, to generate oQDM-type dienolate intermediates that were efficiently and stereoselectively trapped by either aromatic or aliphatic nitroolefins. A series of functionalized, chiral carbocycle-fused uracils embedding three contiguous stereocenters were thus collected in one step in good yields, with generally good levels of enantioselectivity, and complete diastereocontrol. Furthermore, the ability to provide enantiopure products via simple one-cycle recrystallizations and the possibility to further functionalize these scaffolds without losing their chiral integrity were demonstrated. (Figure presented.).