(S)-2-tert-Butoxycarbonylamino-3-[(3aR,5aR,7R,8aS,8bR)-7-((S)-1,2-dihydroxy-ethyl)-2,2-dimethyl-hexahydro-[1,3]dioxolo[4,5]furo[2,3-b]pyran-7-yl]-propionic acid methyl ester