2,2-Dimethyl-propionic acid (2R,3R)-3-[(4S,5S)-5-((S)-2-acetoxy-3-1,4-dioxa-spiro[4.4]non-2-yl-but-3-enyl)-2,2-dimethyl-[1,3]dioxolan-4-yl]-3-(tert-butyl-dimethyl-silanyloxy)-2-methyl-propyl ester