To obtain the subject compound without causing drain containing an ammonium salt, other salts and formaldehyde, by reacting a specific C-H-acid compound with formaldehyde.

A C-H-acid compound of formula I (X1 and X2 are each an electron attractive group) (e.g. dimethyl malonate) is reacted with formaldehyde in the form of p-formaldehyde, preferably using a polar solvent such as a 1-4C alkanol in the presence of a basic catalyst such as an alkali metal hydroxide, an alkali metal carbonate, an alkali metal hydrogencarbonate or an alkali metal alcoholate preferably at -30 to 120°C, more preferably 15-30°C in a water- free solvent to give the objective compound of formula II.