(3a'S,4S,6'R,7a'S,10a'S,10b'R,11'R)-11'-(2-chloroethyl)-2,2,9',9'-tetramethyldihydro-6'H-spiro[[1,3]dioxolane-4,7'-[6,10b]methano[1,3]dioxolo[4',5':5,6]oxocino[4,3-d]oxazole]-2',4'(1'H,3a'H)-dione