1. Process for the preparation of cycloaliphatic ketones of formula 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 ₁ to C ₃ alkyl radical, said process being characterized in that there is added to the cyclopentanone, in the presence of a basic agent, an aldehyde of formula wherein the double bond is of (E) configuration and wherein R has the meaning indicated above, to provide a diene cyclopentanone of formula and, optionally, in that the cyclopentanone thus formed is subjected to a thermal treatment to provide a cyclopentenone of formula

2. Process according to claim 1, characterized in that the diene cyclopentanone of formula (Ia)E,E is subjected to a thermal treatment at a temperature of from about 200° to about 450° C. to provide a cyclopentenone of formula I(b)Z.

3. Compounds of formula (Ia) and (Ib) such as defined in claim 1, selected from the following group:

4. Carboxylic diesters of formula wherein R represents a C ₁ to C ₃ alkyl radical and R ¹ defines a lower alkyl.

5. A carboxylic diester according to claim 4, specifically as

6. A method for making the carboxylic diesters of formula wherein R represents a C ₁ to C ₃ alkyl radical and R ¹ defines a lower alkyl, which comprises subjecting the cyclopentenone of formula (Ib)Z according to claim 2 to an addition reaction with a dialkyl malonate.

7. A method for making the cis and trans dihydrojasmonates of formula wherein R represents a C ₁ to C ₃ alkyl radical and R ¹ defines a lower alkyl, which comprises decarboxylating the carboxylic diesters of formula III according to claim 6, and hydrogenating the decarboxylation product to produce the cis and trans dihydrojasmonates wherein the ratio of cis to trans isomers is 1:1.