= Solution
Silver carbonate promotes deprotonation of the methylene group in methyl isocyanoacetate, giving the nucleophilic carbon $\mathrm{MeO_2C-CH^- -N\equiv C}$. 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. \b[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.
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