Dehydrogenation reaction

A process for the preparation of a compound of formula (I)

• in a form of any one of its stereoisomers or a mixture thereof and wherein R1, R2 and R3, simultaneously or independently, represent a linear or branched C1-6 alkyl or alkenyl group or a linear C2-5 alkynyl group, optionally comprising one or two functional groups selected amongst ether and ester;
• and R4 represents a hydrogen atom, a linear or branched C1-6 alkyl or alkenyl group or a linear C2-5 alkynyl group, optionally comprising one or two functional groups selected amongst ether, and ester;
• R1 and R3, taken together, represent a C2-9 alkanediyl or alkenediyl group optionally substituted by one to two linear or branched C1-5 alkyl or C2-5 alkenyl groups;
• or/and R2 and R4, taken together, represent a C2-9 alkanediyl or alkenediyl group optionally substituted by one to five linear or branched C1-5 alkyl or C2-5 alkenyl groups;
• or/and R3 and R4, taken together, represent a C2-9 alkanediyl or alkenediyl group optionally substituted by one to two linear or branched C1-5 alkyl or C2-5 alkenyl groups;
• comprising the step of dehydrogenate compound of the formula (II)
• in a form of any one of its stereoisomers and wherein R1, R2, R3, and R4 have the same meaning as in formula (I);
• said dehydrogenation being performed under an inert atmosphere in the presence of Palladium) (Pd0) catalyst in elemental metallic form, wherein the reaction is carried out in the liquid phase.