[(1aR,8R,8aS,8bR)-8a-methoxy-5-methyl-4,7-dioxo-6-(pyrrolidin-1-yl)-1,1a,2,4,7,8,8a,8b-octahydroazireno[2',3':3,4]pyrrolo[1,2-a]indol-8-yl]methyl carbamate