2,2-Dimethyl-propionic acid (2R,3R)-3-(tert-butyl-dimethyl-silanyloxy)-3-{(4S,5S)-5-[(E)-(S)-3-(1,4-dioxa-spiro[4.4]non-2-yl)-buta-1,3-dienyl]-2,2-dimethyl-[1,3]dioxolan-4-yl}-2-methyl-propyl ester