Described herein is a process including alpha alkylation of an aldehyde with a polycyclic olefin followed by a ring opening step in order to provide a compound of formula in a form of any one of its stereoisomers or a mixture thereof and where n represents 1 to 4 and R2 to R5 represent independently of each other, a hydrogen atom or a C1-6 linear alkyl group or a C3-6 branched alkyl group; and R3 and R4 or R5 or, alternatively, R2 and R3 when taken together form a saturated or unsaturated optionally substituted 5 or 6 membered carbocyclic ring.