A process for synthesizing a long-chain saturated a,omega-amino ester (acid) comprising from 6 to 17 carbon atoms, said process comprising a first step of cross metathesis between a first acrylic compound chosen from acrylonitrile, acrylic acid or an acrylic ester and a second monounsaturated compound comprising at least one nitrile, acid or ester trivalent function, one of the compounds comprising a nitrile function and the other an acid or ester function, in the presence of a ruthenium carbene metathesis catalyst, and a second step of hydrogenation of the monounsaturated nitrile-ester (acid) obtained, characterized in that said monounsaturated compound comprising at least one nitrile, acid or ester trivalent function has previously been subjected to a purification by thermal and/or chemical treatment.