Acetic acid (1R,3R,6aS,12aR,12bR)-3-benzyloxy-12a-bromo-3-methyl-7,12-dioxo-1-triisopropylsilanyloxy-1,2,3,4,6,6a,7,12,12a,12b-decahydro-benzo[a]anthracen-8-yl ester