 Solution

Silver carbonate promotes deprotonation of the methylene group in methyl isocyanoacetate, giving the nucleophilic carbon $\mathrm{Me}{\mathrm{O}}_2\mathrm{C}-\mathrm{C}{\mathrm{H}}^{-}-\mathrm{N}\equiv \mathrm{C}$. It adds to the alkyne carbon farther from the electron-withdrawing sulfone. This creates the new bond between the future pyrrole C2 and methyl-bearing C3 and leaves the vinyl anion at the sulfone-bearing carbon, future C4.

The vinyl anion then attacks the terminal carbon of the isocyanide intramolecularly, closing a five-membered ring with atom sequence C2–C3–C4–C5–N. Proton transfers place one of the starting methylene hydrogens on nitrogen and the other on C5, giving an aromatic pyrrole. The product has CO${}_2$Me at C2, methyl at C3 and the retained arylsulfonyl group at C4. This is an addition–cyclization–aromatization sequence: the isocyanide contributes both ring nitrogen and the otherwise unsubstituted ring carbon, rather than acting as a cyanide leaving group.