Reinvestigation by carbon-13 NMR spectroscopy of the aluminium chloride catalysed cyclisation of methyl-substituted β-chloropropionanilides to 3,4-dihydroquinolin-2(1H)-ones

Abstract

Aluminium chloride-catalysed cyclisation of methyl-substituted β-chloropropionanilides occurs via kinetically controlled intramolecular Friedel–Crafts cyclisation to give 3,4-dihydroquinolin-2(1H)-ones. In the case of the ortho-methylanilide, ipso attack is not much slower than attack at the free ortho-position, resulting in a mixture of both the expected 8-methyl isomer and the 5-isomer, the latter resulting from a 1,2-methyl shift during cyclisation. The anomalous results reported by earlier workers for the meta- and para-methylanilides are due to partial equilibration of the primary cyclisation products, brought about by excessively harsh reaction conditions and inadvertent selective crystallisation of one pure product or of mixed products from mixtures containing all four possible products.