A process for the synthesis of a monounsaturated nitrile-ester (acid) by cross metathesis starting from an unsaturated compound I including at least one trivalent nitrile, acid or ester functional group, wherein the process includes 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 including a nitrile functional group and the other of these compounds II or I including an acid or ester functional group, and—a second type of cross metathesis cm2 with a light C2 to C4 alkene compound III. Also, a process for the synthesis of an α,ω-amino ester (acid) obtained by a process including a stage in which the two types of cross metathesis defined above are alternated consecutively, followed by a hydrogenation stage.