Alkyl esters and amides of 1,3,3-trisubstituted 1,2,3,4- tetrahydroisoquinoline-1-carboxylic acid as well as 1,2,3,4-tetrahydro-1- (4,5-dihydroimidazol-2-yl)- and 1,2,3,4-tetrahydro-1-(1.4,5,6- tetrahydropyrimidin-2-yl)-1,3,3-trimethylisoquinolines are readily obtained in two steps by the reaction of 3,4-dihydroisoquinolines with dichlorocarbene followed by ring opening of the resulting gem-dichloroaziridine by oxygen and nitrogen nucleophiles.