A process for the preparation of a compound of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 1 and R 2 , independently from each other, represent a hydrogen atom or a C 1-2 alkyl group; R 9 represents a hydrogen atom or a methyl group; R 3 , R 4 , R 5 , R 6 , R 7 and R 8 , independently from each other, represent a hydrogen atom or a C 1-4 alkyl group; or two groups among R 3 , R 4 , R 5 , R 6 , R 7 and R 8 are taken together and form a C 3-8 cycloalkyl or C 5-8 cycloalkenyl group and the others groups have the same meaning as defined above; comprising the steps of a) converting a compound of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 have the same meaning as defined above; into an acetal of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 have the same meaning as defined above; R a and R b , independently from each other, represent a C 1-4 alkyl group or R a and R b , when taken together, represent a C 2-5 alkanediyl group; b) treating the acetal obtained in step a) with an acid and a compound of formula CHR 1 =CH-OR c ; wherein R c represents a C 1-4 alkyl group and R 1 has the same meaning as defined above; to obtain a compound of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R a , R b and R c have the same meaning as defined above; and c) treating the compound of formula (IV) with an acid to obtain a compound of formula (I). a) converting a compound of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 have the same meaning as defined above; into an acetal of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 have the same meaning as defined above; R a and R b , independently from each other, represent a C 1-4 alkyl group or R a and R b , when taken together, represent a C 2-5 alkanediyl group; in the form of any one of their stereoisomers or a mixture thereof, and wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 have the same meaning as defined above; into an acetal of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 have the same meaning as defined above; R a and R b , independently from each other, represent a C 1-4 alkyl group or R a and R b , when taken together, represent a C 2-5 alkanediyl group; b) treating the acetal obtained in step a) with an acid and a compound of formula CHR 1 =CH-OR c ; wherein R c represents a C 1-4 alkyl group and R 1 has the same meaning as defined above; to obtain a compound of formula in the form of any one of their stereoisomers or a mixture thereof, and wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R a , R b and R c have the same meaning as defined above; and c) treating the compound of formula (IV) with an acid to obtain a compound of formula (I). <EOS>