A method for synthesising a compound of the following formula (I): wherein - X designates an oxygen, sulphur atom or an NH group, preferably an oxygen atom; - R 1 represents a C 1 -C 12 alkyl group or OR', R' being a C 1 -C 12 alkyl group, - n is an integer ranging from 0 to 4; - Sp represents a linear or branched C 1 -C 24 alkylene chain, optionally interrupted by one or more nitrogen or oxygen atoms, the group representing a 5 or 6-atom diazotized or triazotized, preferably diazotized, heterocycle, comprising at least one carbonyl, thiocarbonyl or imine function, comprising two successive steps (b1) and (b2) according to a one-pot synthesis: - (b1) reacting a compound of formula (II) with a compound of formula (III): wherein Z represents a nucleofugal group; in the presence of: - at least one polar solvent S1; - at least one base selected from alkali carbonates; at a temperature T 1 ranging from 70 to 150°C, to form an ether; - (b2) reacting said ether with an aqueous hydroxylamine solution at a temperature T 2 ranging from 30 to 70°C, to obtain an oxime compound of formula (IV): then: - a step (c) of recovering said oxime compound of formula (IV) ; - a step (d) of oxidising the oxime compound of formula (IV) with an oxidising agent, in the presence of at least one organic solvent S 2 . - X designates an oxygen, sulphur atom or an NH group, preferably an oxygen atom; - R 1 represents a C 1 -C 12 alkyl group or OR', R' being a C 1 -C 12 alkyl group, - n is an integer ranging from 0 to 4; - Sp represents a linear or branched C 1 -C 24 alkylene chain, optionally interrupted by one or more nitrogen or oxygen atoms, the group representing a 5 or 6-atom diazotized or triazotized, preferably diazotized, heterocycle, comprising at least one carbonyl, thiocarbonyl or imine function, comprising two successive steps (b1) and (b2) according to a one-pot synthesis: - (b1) reacting a compound of formula (II) with a compound of formula (III): wherein Z represents a nucleofugal group; in the presence of: - at least one polar solvent S1; - at least one base selected from alkali carbonates; at a temperature T 1 ranging from 70 to 150°C, to form an ether; the group representing a 5 or 6-atom diazotized or triazotized, preferably diazotized, heterocycle, comprising at least one carbonyl, thiocarbonyl or imine function, comprising two successive steps (b1) and (b2) according to a one-pot synthesis: - (b1) reacting a compound of formula (II) with a compound of formula (III): - (b1) reacting a compound of formula (II) with a compound of formula (III): wherein Z represents a nucleofugal group; in the presence of: - at least one polar solvent S1; - at least one base selected from alkali carbonates; - at least one polar solvent S1; - at least one base selected from alkali carbonates; at a temperature T 1 ranging from 70 to 150°C, to form an ether; - (b2) reacting said ether with an aqueous hydroxylamine solution at a temperature T 2 ranging from 30 to 70°C, to obtain an oxime compound of formula (IV): then: - a step (c) of recovering said oxime compound of formula (IV) ; - a step (d) of oxidising the oxime compound of formula (IV) with an oxidising agent, in the presence of at least one organic solvent S 2 . <EOS>