The invention provides antitumour compounds of formula (I), wherein R
1
is an aromatic substituent; R
2
is hydrogen or a substituent when the dotted line is absent, or R
2
is absent when the dotted line represents a bond to give a double bond between the nitrogen which bears R
2
and the carbon which bears R
3
; R
3
is an oxo group =0 when the dotted line is absent or is a substituent when the dotted line represents a bond to give a double bond between the nitrogen bearing R
2
and the carbon bearing R
3
; R
4
is hydrogen or a substituent; and pharmaceutically acceptable salts thereof.