Compounds of general formula (I): ##STR1## in which: R.sub.1 represents a hydrogen atom or a lower alkyl group, and R.sub.2 represents a substituted or unsubstituted alkyl, a phenyl or a substituted or unsubstituted heteroaryl, their enantiomers, diastereoisomers and epimers and their addition salts with a pharmaceutically acceptable base when R.sub.1 represents a hydrogen atom or when R.sub.2 comprises a carboxylic acid group. Medicinal products.