The cycloaliphatic ketones of formula ##STR1## having two conjugated double bonds in positions 1 and 3, or 2 and 4, such as indicated by the dotted lines, and wherein R defines a C.sub.1 to C.sub.3 alkyl radical, usefuil intermediate products for the preparation of jasmonic derivatives, are prepared by an addition reaction btween the cyclopentanone and an aldehyde of formula ##STR2## wherein the double bond is of (E) configuration and wherein R has the meaning indicated above, to provide a diene cyclopentanone of formula ##STR3## and, optionally, in that the cyclopentanone thus formed is subjected to a thermal treatment to provide a cyclopentenone of formula ##STR4## The 2-(E,2'Z)-but-2'-enylidene-cyclopentanone, 2-(1'Z)-but-1'-enyl-cyclopent-2-enone, 2-(E,2'E)-pent-2'-enylidene-cyclopentanone, 2-(Z,2'E)-pent-2'-enylidene-cyclopentanone and 2-(1'Z)-pent-l'-enyl-cyclopent-2-enone, as well as the malonic esters dimethyl 2-(pent-(1'Z)-enyl)-3-oxo-1-cyclopentyl!-malonate and dimethyl 2-(but-(1'Z)-enyl)-3-oxo-1-cyclopentyl!-malonate, are novel chemical entities.