The present invention concerns compounds of general formula (1), where in:—-a- is a single or double bond, Ar is an aromatic group, substituted or unsubstituted, R1 and R2 each independently or together are: a hydrogen atom or C1-C6 alkyl group, branched or unbranched, saturated or unsaturated, substituted or unsubstituted; the groups R1 and R2 may also form a heterocycle, R3 and R3′ each independently or together are a hydrogen atom or C1-C6 alkyl group, X is an oxygen atom or a sulphur atom, and the addition salts of the compounds of general formula (1) with pharmaceutically acceptable mineral acids or organic acids.