(1S,3bR,4R,5aR,9aR,9bR,11aS)-1-[(4R)-5-[(2S)-3,3-dimethyloxiran-2-yl]-1,4-dihydroxybutan-2-yl]-3b,6,6,9a,11a-pentamethyl-7-oxo-1H,2H,3bH,4H,5H,5aH,6H,7H,9aH,9bH,10H,11H,11aH-cyclopenta[a]phenanthren-4-yl acetate + reduced [NADPH--hemoprotein reductase] + O2 = (1S,3bR,4R,5aR,9aR,9bR,11aS)-1-(1-hydroxy-4-oxobutan-2-yl)-3b,6,6,9a,11a-pentamethyl-7-oxo-1H,2H,3bH,4H,5H,5aH,6H,7H,9aH,9bH,10H,11H,11aH-cyclopenta[a]phenanthren-4-yl acetate + 2-methylpropanoate + oxidized [NADPH--hemoprotein reductase] + H2O + 2 H(+) — reaction