2,2-Dimethyl-propionic acid (2R,3S,4aR,5S,7R,8aR,9aS,10aR)-5-(tert-butyl-dimethyl-silanyloxy)-7-[2-(tert-butyl-diphenyl-silanyloxymethyl)-allyl]-2-(2,2-dimethyl-propionyloxymethyl)-10a-methyl-decahydro-1,8,10-trioxa-anthracen-3-yl ester