The present invention relates to the field of organic synthesis and more particularly to a new process for the preparation of a compound of formula (I), in the form of any one of its isomers or a mixture thereof, ##STR00001## wherein, more preferably, G represents a C.dbd.O group, R.sup.1 represents a butyl group and R.sup.2 represents a methyl group.The process of the invention involves an 2-(1-hydroxyalkyl)-cyclopent-2-en-1-one derivative, as starting material, which can be then converted into a compound of formula (I) by a process comprising a thermal rearrangement. The 2-alkylidene-3-oxo-cyclopentylacetate derivative and the 2-(1-hydroxyalkyl)-cyclopent-2-en-1-one derivative are also an object of the invention.