Nitrous acid elimination from 4-alkyl-5-formyl-4-nitrocyclohex-1-enes: synthesis of mono and bicyclic benzene and dihydrobenzene derivatives

Elimination of nitrous acid from 4-alkyl-5-formyl-4-nitrocyclohex-1-enes, by treatment with silica gel or potassium carbonate, is described. Whereas in the former case the product consisted in a mixture of the starting material, 2-alkyl-4,5-dimethylcyclohexa-1,4-diene-1-carbaldehyde and 2-alkyl-4,5-dimethylbenzaldehyde, reaction with potassium carbonate at 60°C yielded exclusively aromatic compounds. Furthermore, when a ketone function was present in the γ-position of the alkyl chain at C-4, the alkaline medium promoted an intramolecular aldol condensation, being obtained bicyclic products. PM3 calculations have been made to justify the result of the cyclization.