The present invention relates to the field of organic synthesis and more specifically to the alpha alkylation of an aldehyde with a polycyclic olefin followed by a ring opening step in order to provide compound of formula (I) in the form of any one of its stereoisomers or a mixture thereof and wherein R represents a hydrogen atom or C1-8 linear alkyl group； R1, R2, R3 and R4 represent, when taken separately, independently of each other, a hydrogen atom or a C1-2 linear alkyl group or a C3-4 linear or branched alkyl group； or R2 and R3, when taken together, represent a C4-10 linear, branched or cyclic alkanediyl group and n is 1 or 2.
