4-[5-(3-benzyloxy-1-methylpropyl)-4-methyl-tetrahydro-furan-2-yl]-3-triisopropylsilanyloxy-butyric acid 2-but-3-enyl-10a,11a-dimethyl-dodecahydro-1,5,10-trioxa-cyclohepta[b]naphthalen-3-yl ester