Impurities of tazarotene: Isolation and structural characterisation

Two impurities showing structures 5 and 6 were isolated and characterised by means of NMR analysis, during the optimisation of a synthetic procedure to tazarotene. Impurity 5, i.e. ethyl 6-((4,4-dimethyl-4H-thiochromen-6-yl)ethynyl)nicotinate, was a by-product of the reduction of the intermediate sulfoxide 7 with PCl3. Impurity 6, i.e. 1,4-bis(4,4-dimethylthiochroman-6-yl)buta-1,3-diyne, was due to a side reaction of the Sonogashira coupling.