The invention relates to a new process for preparing active esters or carboyxlic acids, which consists in reacting a carboxylic acid, in the presence of an agent for binding hydrohalic acid, with a carbonate of formula: ##STR1## in which R.sup.1 denotes either a radical of formula ##STR2## in which R.sup.3 and R.sup.4, which may be identical or different, are not hydrogen atoms and denote organic radicals which may be substituted or unsubstituted and saturated or unsaturated, and may or may not be bound to a polymer, and which can be joined together to form a hetero-cyclic system with the nitrogen, atom, or a substituted or unsubstituted aryl radical which may or may not be bound to a polymer, R.sup.2 denotes a hydrogen atom, an aliphatic or cycloaliphatic radical which may be substituted or unsubstituted and saturated or unsaturated, or a substituted or unsubstituted aromatic radical, and X denotes a halogen atom. This process is especially useful for the synthesis of active esters of N-protected amino acids. The invention also relates to the new carbonates described above and the method of producing them, which consists in reacting an alpha-halogenated chloroformate of formula: with an alcohol of formula R.sup.1 OH in an inert solvent medium in the presence of an organic or inorganic base.