Method for the preparation of a formula I product: in which A represents a radical selected from: - CO-R 2 - CO-O-R 2 a - CN - C -N Ra R'a - CH- (CO 2 Rb) 2 R 1 represents a hydrogen atom or a linear or branched alkyl or alkylene radical having at most 12 carbon atoms or R 1 represents a saturated or unsaturated cycloalkyl radical having from 3 to 7 carbon atoms and possibly containing one or more heteroatoms chosen from nitrogen, sulphur or oxygen atoms, or R 1 represents a carbocyclic or heterocyclic aryl radical, each of these alkyl, alkylene or heterocyclic radicals being possibly substituted, R 2 represents a linear or branched alkyl radical having from 1 to 12 carbon atoms, possibly substituted, or R 2 represents a saturated or unsaturated cycloalkyl radical having from 3 to 7 carbon atoms and possibly containing one or more heteroatoms chosen from nitrogen, sulphur or oxygen atoms, or R 2 represents a carbocyclic or heterocyclic aryl radical, each of these alkyl, aryl or heterocyclic radicals being possibly substituted, R 2 a represents a linear or branched alkyl radical having from 1 to 12 carbon atoms. Ra and R'a, which may be identical or different, are chosen from linear or branched alkyl or alkoxy radicals having from 1 to 12 carbon atoms, it being understood that Ra and R'a cannot simultaneously represent a linear or branched alkoxy radical having from 1 to 12 carbon atoms. Rb is selected from linear or branched alkyl radicals having from 1 to 12 carbon atoms. R 3 represents a hydrogen atom, linear or branched alkyl or alkylene having at most 6 carbon atoms or R 3 represents a carbocyclic or heterocyclic aryl radical, each of these alkyl, alkylene or aryl radicals being possibly substituted, or the radicals R 2 and R 3 are linked together to form a ring having from 5 to 7 carbon atoms, said chain possibly including one or more heteroatoms selected from nitrogen, sulphur or oxygen atoms, R 4 is hydrogen or is selected from possibly substituted alkyl, acylamido with 2 to 12 carbon atoms, carboxylester, preferably R 4 is hydrogen, characterised in that a product of the formula II: with a formula III product in the presence on the one hand of an organic compound comprising at least one ring chosen from a 1,3-imidazolium, a 1,2,4-triazolium or a thiazolium and on the other hand ascorbic acid in free or salified form of a compound comprising a in which R 5 and R 6, independently of each other, represent a hydrogen atom, a linear or branched alkyl radical having from 1 to 12 carbon atoms or R 5 and R 6 are linked together to form a 3- to 7-membered ring, the said alkyl or the said ring being possibly substituted by one or more heteroatoms, in particular chosen from O, N and S, in particular a reductone, preferably 2-hydroxypropanedial or 2,3- dihydroxycyclopentadial -2-en-1-one or croconic acid in an organic solvent. - CO-R 2 - CO-O-R 2 a - CN - C -N Ra R'a - CH- (CO 2 Rb) 2 R 1 represents a hydrogen atom or a linear or branched alkyl or alkylene radical having at most 12 carbon atoms or R 1 represents a saturated or unsaturated cycloalkyl radical having from 3 to 7 carbon atoms and possibly containing one or more heteroatoms chosen from nitrogen, sulphur or oxygen atoms, or R 1 represents a carbocyclic or heterocyclic aryl radical, each of these alkyl, alkylene or heterocyclic radicals being possibly substituted, R 2 represents a linear or branched alkyl radical having from 1 to 12 carbon atoms, possibly substituted, or R 2 represents a saturated or unsaturated cycloalkyl radical having from 3 to 7 carbon atoms and possibly containing one or more heteroatoms chosen from nitrogen, sulphur or oxygen atoms, or R 2 represents a carbocyclic or heterocyclic aryl radical, each of these alkyl, aryl or heterocyclic radicals being possibly substituted, R 2 a represents a linear or branched alkyl radical having from 1 to 12 carbon atoms. Ra and R'a, which may be identical or different, are chosen from linear or branched alkyl or alkoxy radicals having from 1 to 12 carbon atoms, it being understood that Ra and R'a cannot simultaneously represent a linear or branched alkoxy radical having from 1 to 12 carbon atoms. Rb is selected from linear or branched alkyl radicals having from 1 to 12 carbon atoms. R 3 represents a hydrogen atom, linear or branched alkyl or alkylene having at most 6 carbon atoms or R 3 represents a carbocyclic or heterocyclic aryl radical, each of these alkyl, alkylene or aryl radicals being possibly substituted, or the radicals R 2 and R 3 are linked together to form a ring having from 5 to 7 carbon atoms, said chain possibly including one or more heteroatoms selected from nitrogen, sulphur or oxygen atoms, R 4 is hydrogen or is selected from possibly substituted alkyl, acylamido with 2 to 12 carbon atoms, carboxylester, preferably R 4 is hydrogen, characterised in that a product of the formula II: with a formula III product in the presence on the one hand of an organic compound comprising at least one ring chosen from a 1,3-imidazolium, a 1,2,4-triazolium or a thiazolium and on the other hand ascorbic acid in free or salified form of a compound comprising a in which R 5 and R 6, independently of each other, represent a hydrogen atom, a linear or branched alkyl radical having from 1 to 12 carbon atoms or R 5 and R 6 are linked together to form a 3- to 7-membered ring, the said alkyl or the said ring being possibly substituted by one or more heteroatoms, in particular chosen from O, N and S, in particular a reductone, preferably 2-hydroxypropanedial or 2,3- dihydroxycyclopentadial -2-en-1-one or croconic acid in an organic solvent.