(3Z,9E)-(1R,7S,11S,15S,17R)-11-(tert-Butyl-dimethyl-silanyloxy)-7-(tert-butyl-diphenyl-silanyloxymethyl)-15-methyl-13-methylene-6,21-dioxa-bicyclo[15.3.1]henicosa-3,9,19-trien-5-one