in ethanol; toluene according to: M. Boualam et al., Appl. Organomet. Chem. 4 (1990) 335; addn. of diorganotin oxide to soln. (toluene/ethanol 4:1) of 4-aminosalicylic acid (molar ratio 2:1), refluxing (4 h), azeotrop distn.; evapn. (red. pressure), recrystn. (petroleum ether);