The present invention relates to novel derivatives of 6,7-dihydro-3H-oxazolo[3,4-a]pyrazine-5,8-dione represented by formula (I): and a mixture of these derivatives. The present invention also encompasses the pharmaceutical compositions comprising an effective amount of a said compounds, object of the present invention, as well as to the use of the compounds and/or derivatives as a phosphodiesterase enzyme inhibitor, and the use of the compounds and/or derivatives in the treatment of erectile dysfunction, disorders and/or conditions treatable with relaxation of tissues and disorders treatable with phosphodiesterase inhibitors, more particularly PDE-5 inhibitor. A further objective of the present invention is to provide a medication comprising a therapeutically effective amount of said compound and a method of treating using the said novel compounds.