R1 and R2 each represent a hydrogen atom, a hydrocarbon radical with 1 to 20 carbon atoms that is aliphatic, linear or branched and may or may not be saturated, cycloaliphatic or aromatic, or an alkyl-ether chain, whereby R1 and R2 together can form an oxygenated heterocyclic radical (for example furanic or tetrahydrofuranic);R3 is a radical that is defined as R1 or R2 except for the hydrogen atom