(2aS,3R,4aR,5S,6R,7R,7aS,7bR)-7-Acetoxy-4a-hydroxy-6-isopropenyl-7b-methyl-2-oxo-2a,3-bis-trimethylsilanyloxy-decahydro-indeno[7,1-bc]furan-5-carboxylic acid methyl ester