1,1-Dichloro-1a, 10b-dihydrodibenzo(b,f)cycloprop(d)azepine-6(1H)-carboxaldehyde is produced in good yield through a reaction between a compound of the general formula ##SPC1## Wherein R stands for hydrogen or formyl group and chloroform. Thus obtained product can be used as a valuable intermediate for synthesizing medicines and is further convertible to known drugs, e.g. 6-(3-dimethylamino)propyl)-1, 1a, 6, 10b-tetrahydrodibenzo(b,f)cycloprop(d)azepine.