Process for the preparation of 4-oxo-hexahydro-pyrazinoisoquinoline derivatives

In a process for preparing a 4-oxo-hexahydro-4H-pyrazino-isoquinoline compound of the formula ##STR5## wherein R.sup.2, R.sup.3 and R.sup.4 are each H or methyl and R.sup.5 and R.sup.6 are each H, methyl or methoxy, as well as the physiologically compatible acid addition salts thereof, the improvement which comprises:

• hydrolyzing a 2-acyl-4-oxo-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinoline of the formula ##STR6## wherein R.sup.1 is H, alkyl of 1-5 carbon atoms, phenyl or phenyl substituted by one or more alkyl or alkoxy of up to 4 carbon atoms or by halogen, with an oxygen-containing inorganic acid of pentavalent phosphorus or of hexavalent sulfur or with an acid salt thereof to form said 4H-pyrazino-isoquinoline derivative.