4,5α-epoxy-3-methoxy-17-methylmorphinan-6-one; (4R,4aR,7aR,12bS)-9-methoxy-3-methyl-1,2,4,4a,5,6,7a,13-octahydro-4,12-methanobenzofuro[3,2-e]isoquinolin-7-one; Dihydrocodeine EP Impurity C