. . "f an alkali metal or an alkali metal alcoholate, as a strong base, and at an elevated temperature to give an alkali metal salt of the formula IV ##STR3## in which M is an alkali metal, and subsequently liberating a compound of the formula I by protolysis of a compound of the above formula IV, which comprises treating, in stages, a compound of the above formula IV in at least two portions, as a pro" . .