Process for preparing indene acryladehyde derivatives

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 R1 and R2, independently from each other, represent a hydrogen atom or a C1-2 alkyl group;
• R9 represents a hydrogen atom or a methyl group;
• R3, R4, R5, R6, R7 and R8, independently from each other, represent a hydrogen atom or a C1-4 alkyl group;
• or two groups among R3, R4, R5, R6, R7 and R8 are taken together and form a C3-8 cycloalkyl or C5-8 cycloalkenyl group and the other 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 R2, R3, R4, R5, R6, R7, R8 and R9 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 R2, R3, R4, R5, R6, R7, R8 and R9 have the same meaning as defined above;
• Ra and Rb, independently from each other, represent a C1-4 alkyl group or Ra and Rb, when taken together, represent a C2-5 alkanediyl group;
• b) treating the acetal obtained in step a) with an acid and a compound of formula CHR1═CH—ORc;
• wherein Rc represents a C1-4 alkyl group and R1 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 R1, R2, R3, R4, R5, R6, R7, R8, R9, Ra, Rb and Rc 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).