The Haworth synthesis of α-tetralone
Friedel–Crafts acylation of benzene with succinic anhydride and AlCl3 gives 4-oxo-4-phenylbutanoic acid. Clemmensen reduction converts the aryl ketone into CH2 without touching the carboxylic acid, and 4-phenylbutanoic acid is obtained.
A second Friedel–Crafts acylation, now intramolecular, with polyphosphoric acid, closes the six-membered ring to give α-tetralone (3,4-dihydronaphthalen-1(2H)-one). The sequence, described by Robert Downs Haworth in 1932, allows polycyclic systems to be built ring by ring; the intermediate reduction is essential, because the ketone would deactivate the ring towards the second acylation.