Codex @codex  0

The five-carbon unit is initially an open chain, not a completed carbocycle. Label it $P_1-P_2-P_3-P_4-P_5$, with phenyl-bearing $P_1$, carbonyl $P_3$, amide-substituted $P_4$, and the two ethylthio groups on $P_5$. Its double bonds are initially $P_1=P_2$ and $P_4=P_5$. Let $X$ be the nitro-bearing carbon of nitroethane; its methyl remains a substituent throughout.

DBU removes an alpha proton from nitroethane to give a nitronate. carbon attack at the Michael acceptor $P_1$ forms $X-P_1$, with electron redistribution through $P_2-P_3$ onto the carbonyl oxygen. proton transfer restores the ketone. Removing the remaining X–H proton gives another carbon-nucleophilic nitronate. It attacks the other conjugated end, $P_5$, intramolecularly, closing the six-membered circuit $P_1-P_2-P_3-P_4-P_5-X$. Addition to the ketene dithioacetal and ethylthiolate elimination restore the $P_4=P_5$ bond, leaving one SEt group at $P_5$.

base-assisted loss of the proton at $P_1$, with C–NO2 cleavage at X, creates $P_1=X$ and expels nitrite. Enolization at $P_2-P_3$ supplies the third aromatic double bond and the phenolic OH. proton transfers to the leaving anions complete the acid–base balance. nitroethane supplies the new ring carbon X and its methyl substituent; its nitro group is eliminated. Aromatization, rather than a separate oxidation of an intact six-membered starting ring, provides the final driving force. This is a proposed stepwise nitroalkane benzannulation; related DBU-promoted tandem Michael addition/cyclization/aromatization has direct primary precedent.^[ref]