Conversion of 2-Alkylcinnamaldehydes to 2-Alkylindanones via a Catalytic Intramolecular Friedel−Crafts Reaction

The preparation of indanones by the intramolecular acylation of 3-arylpropanoic acids or halides requires the use of noncatalytic acid promoters. In the presence of 5−10 mol % FeCl<sub>3</sub>, in situ generated dimethyl acetals of (<i>E</i>)-2-alkylcinnamaldehydes cyclize to 1-methoxy-2-alkyl-1H-indenes in good-to-high yields. The 1-methoxyindenes were converted in high yield into 2-alkylindanones by treatment with triethylamine, to effect isomerization to the isomeric enol ethers, followed by acid-catalyzed hydrolysis. Thus, a catalytic, intramolecular Friedel−Crafts reaction leading to 2-alkylindanones from 2-alkylcinnamaldehydes was developed.