(1′S,12′R,13′S,17′S)-15′,15′-Dimethyl-1,2-dihydro-11′,14′,16′,18′-tetraoxa-7′-azaspiro[indole-3,8′-pentacyclo[10.6.0.02,9.03,7.013,17]octadecane]-2,10′-dione