e) reacting a compound of the formula (II) with a C1-5 alkyl isocyanate or phenyl isocyanate, to obtain compounds of the formula (I), wherein R2 and the dotted lines are as defined above, R3 is hydrogen, R4 is a C1-5 alkylcarbamoyl or phenylcarbamoyl group, R and R1 are absent and a double bond is present between the N(3) and C(4) atoms; or