2,2-Dimethyl-propionic acid (1R,2R,3R,5R,6R,12S)-3-(4-acetylamino-2-oxo-2H-pyrimidin-1-yl)-12-methoxy-7-(2-methyl-propane-2-sulfonyl)-4,10-dioxa-7-aza-tricyclo[4.3.3.01,5]dodec-2-yl ester