(III) wherein R1 has the same meaning as above and R3 is a halogen, to give a 2-(2-imino-6-oxocyclohexylidene)-propionic acid derivative of the formula: ##STR11## wherein R1, R2 and R3 have the same meanings as above; forming a fused 5-membered ring from said propionic acid derivative (V) to give a compound of the formula: ##STR12## wherein R1 and R2 have the same meanings as above; and