Base-Promoted [4 + 3] Cycloadditions Enabling the Construction of Polycycle-Fused Uracils from <i>N</i> -Aminoisoquinolinium Derivatives

As the sole nucleobase characteristic of RNA, uracil possesses a unique heterocyclic architecture that endows it with exceptional value as a privileged scaffold in medicinal chemistry. Herein, we report a versatile and efficient strategy for constructing polycyclic uracil derivatives through rare [4 + 3] cycloaddition reactions of <i>N</i>-aminoisoquinolinium, <i>N</i>-aminoquinolinium, or <i>N</i>-aminopyridinium salts with uracil-derived aldehydes. This protocol exhibits a broad substrate scope (36 examples), excellent functional-group tolerance, consistently high yields, and good scalability. The resulting products display diverse synthetic utility, as demonstrated by downstream derivatizations.