{(2S,4S,6R,8S,10S)-4-acetoxy-8-[(3R)-4-(tert-butyldimethylsilanyloxy)-3-methyl-2-methylenebutyl]-10-methyl-10-(triethylsilanyloxy)-1,7-dioxaspiro[5.5]undec-2-yl}acetic acid allyl ester