An alkyne-isocyanide [3 + 2] cycloaddition followed by a Boulton-Katritzky rearrangement and a ring expansion is demonstrated. Different from the typical Boulton-Katritzky rearrangement, which forms five-membered ring products, the rearrangement-ring expansion method provides a mild, efficient, and atom-economical access to fused 9-deazaguanine structures in high yields.