A process for the synthesis of a monounsaturated nitrile-ester (acid) by cross metathesis starting from an unsaturated compound I comprising at least one trivalent nitrile, acid or ester functional group, wherein it comprises a stage in which two types of cross metathesis are alternated consecutively:
- a first type of cross metathesis (cm1) with an acrylic compound II chosen from acrylonitrile, acrylic acid or an acrylic ester, one of the compounds I or II comprising a nitrile functional group and the other of these compounds II or I comprising an acid or ester functional group, and
- a second type of cross metathesis (cm2) with a light C2 to C4 alkene compound III, preferably with ethylene.