Ethyl (1R,1′S,2′S,7a'R)-2-oxo-1′-[(3aR,5R,5aS,8aS,8bR)-2,2,7,7-tetramethyltetrahydro-3aH-bis[1,3]dioxolo[4,5-b:4′,5′-d]pyran-5-yl]-1′,2′,5′,6′,7′,7a'-hexahydro-2H-spiro[acenaphthylene-1,3′-pyrrolizine]-2′-carboxylate