One-step synthesis of 2,9-disubstituted phenanthrenes via Diels–Alder reactions using 1,4-disubstituted naphthalenes as dienophiles

The normal electron-demand Diels–Alder reaction between 1,4-disubstituted naphthalenes, nitro being one of these two groups, and 1-methoxy-3-trimethylsililoxy-1,3-butadiene gives hydroxyphenanthrene derivatives, the yields being enhanced by placement of more electron-withdrawing groups on the dienophilic system. The nitro group as substituent directs the cycloaddition, and its cis-extrusion as nitrous acid along with the loss of methanol from primary adducts leads to the expected aromatized products.