Methyl cycloheptatrienecarboxylate in which the 3-isomer predominates is obtained from 3-cyanocycloheptatriene and converted to its tricarbonylchromium derivatives of which the 3-, 1- and 7-endo isomers are characterised. Triphenylmethyl tetrafluoroborate converts these to tricarbonyl(carbomethoxytropylium)chromium tetrafluoroborate which undergoes ring contraction to tricarbonyl(methyl benzoate)chromium in contact with alumina or silica.