159852-70-7 3-Methyl-4-oxo-4H-thieno(2',3':4,5)pyrimido(2,1-b)benzoxazole-2-carboxylic acid ethyl ester 4H-Thieno(2',3':4,5)pyrimido(2,1-b)benzoxazole-2-carboxylic acid, 3-methyl-4-oxo-, ethyl ester