Method of preparation of a compound of formula in which: - R1 represents a methyl or an ethyl; - R2 represents independently a hydrogen or a C1-C5 alkyl or C2-C5 alkenyl group; - R3 represents an alkyl or alkenyl group, optionally substituted by an aryl, or R3 represents a cyclic alkyl or cyclic alkenyl group, optionally substituted by one or more C1-C6 alkyl groups, it being understood that R3 includes in total 3 to 10 carbon atoms; - Z represents C or CR4, with - R4 representing a hydrogen or a C1-C8 alkyl or C2-C8 alkenyl group; - R5 representing a hydrogen or a C1-C8 alkyl or alkanoyl or C2-C8 alkenyl or alkenoyl group; knowing that a double bond is present or absent in the ring and that, when it is present, it is - either in position 2-3 and R2 is absent in position 2, - or in position 3-4 and R2 is present in position 2 and is as defined above, characterised in that said method comprises the following steps: i) reaction of an α-methylene aldehyde, in the presence of a base, with a symmetrical ketone to obtain a compound of formula (Ia), in which R1 and R3 are as defined above, R2 is a hydrogen and a double bond is present at 2-3 or 3-4 in the ring and this reaction being optionally followed by steps ii), and/or iii), and/or iv), ii) mono- or bis-alkylation reaction in order to obtain a compound of formula (Ia) in which R2 is a C1-C5 alkyl or C2-C5 alkenyl group; iii) conversion of the Z = C function of the compound obtained in the preceding step into a Z=CR4 function, R4 and R5 being as defined above; iv) reduction of the double bond at 2-3 or 3-4 present in the ring of the compound obtained in the preceding step, step iv) being able to be performed after any one of steps i), ii), or iii). - R1 represents a methyl or an ethyl; - R2 represents independently a hydrogen or a C1-C5 alkyl or C2-C5 alkenyl group; - R3 represents an alkyl or alkenyl group, optionally substituted by an aryl, or R3 represents a cyclic alkyl or cyclic alkenyl group, optionally substituted by one or more C1-C6 alkyl groups, it being understood that R3 includes in total 3 to 10 carbon atoms; - Z represents C or CR4, with - R4 representing a hydrogen or a C1-C8 alkyl or C2-C8 alkenyl group; - R5 representing a hydrogen or a C1-C8 alkyl or alkanoyl or C2-C8 alkenyl or alkenoyl group; - R4 representing a hydrogen or a C1-C8 alkyl or C2-C8 alkenyl group; - R5 representing a hydrogen or a C1-C8 alkyl or alkanoyl or C2-C8 alkenyl or alkenoyl group; - either in position 2-3 and R2 is absent in position 2, - or in position 3-4 and R2 is present in position 2 and is as defined above, characterised in that said method comprises the following steps: i) reaction of an α-methylene aldehyde, in the presence of a base, with a symmetrical ketone to obtain a compound of formula (Ia), in which R1 and R3 are as defined above, R2 is a hydrogen and a double bond is present at 2-3 or 3-4 in the ring and this reaction being optionally followed by steps ii), and/or iii), and/or iv), ii) mono- or bis-alkylation reaction in order to obtain a compound of formula (Ia) in which R2 is a C1-C5 alkyl or C2-C5 alkenyl group; iii) conversion of the Z = C function of the compound obtained in the preceding step into a Z=CR4 function, R4 and R5 being as defined above; iv) reduction of the double bond at 2-3 or 3-4 present in the ring of the compound obtained in the preceding step, step iv) being able to be performed after any one of steps i), ii), or iii). i) reaction of an α-methylene aldehyde, in the presence of a base, with a symmetrical ketone to obtain a compound of formula (Ia), in which R1 and R3 are as defined above, R2 is a hydrogen and a double bond is present at 2-3 or 3-4 in the ring and this reaction being optionally followed by steps ii), and/or iii), and/or iv), ii) mono- or bis-alkylation reaction in order to obtain a compound of formula (Ia) in which R2 is a C1-C5 alkyl or C2-C5 alkenyl group; iii) conversion of the Z = C function of the compound obtained in the preceding step into a Z=CR4 function, R4 and R5 being as defined above; iv) reduction of the double bond at 2-3 or 3-4 present in the ring of the compound obtained in the preceding step, step iv) being able to be performed after any one of steps i), ii), or iii).