in which R1 is a hydrogen atom or a lower alkyl radical and R3 is a reactive residue, to give a compound of the general formula: ##STR7## in which R1, R2 and R3 have the same meanings as above, which is cyclised to give a compound of the general formula: ##STR8## in which R1 and R2 have the same meanings as above, which is subsequently dehydrogenated in known manner to give the desired compound of