Out of the two alpha-positions adjacent to the carbonyl group in the compound (XII) or its salt, the hydrogen at the alpha-position, which is not substituted with R', R or --Z--B--COOR3, is drawn in accordance with a conventional method to cause formation of carbanion, and an ester residue (e.g. carboxyl group which is esterified by a C1-6 alkyl such as methyl, ethyl, n-propyl, iso-propyl, n-buty