2-Methylthio-4, 6, 7-triphenylpteridine (1) is slowly converted by potassium amide in liquid ammonia at -33°into 2-amino-4, 6, 7-triphenylpteridine (2) as the main product and into a ring contraction product which is proved to be 6, 8-diphenyl-2-methylthiopurine (3). The latter is found to be the major product when 4, 6-diphenyl-2-methylthiopteridine (4) and 4, 7-diphenyl-2-methylthiopteridine (5) are treated likewise. The mechanism of the ring contraction is discussed.
