Vincristine Impurity I (Methyl Vincristine)

(5S,7S,9S)-9-((3aR,3a1R,4R,5S,5aR,10bR)-4-acetoxy-3a-ethyl-6-formyl-5-hydroxy-8-methoxy-5-(methoxycarbonyl)-3a,3a1,4,5,5a,6,11,12-octahydro-1H-indolizino[8,1-cd]carbazol-9-yl)-5-ethyl-5-hydroxy-9-(methoxycarbonyl)-3-methyl-1,2,3,4,5,6,7,8,9,10-decahydro-3,7-methano[1]azacycloundecino[5,4-b]indol-3-ium iodide