((4aR,5aS,6aR,8S,9R,10aS,11aR,13aS)-2,2,9,10a-Tetramethyl-9-trimethylsilanyloxy-dodecahydro-1,3,5,7,11-pentaoxa-benzo[4,5]cyclohepta[1,2-b]naphthalen-8-yl)-acetic acid ethyl ester