(1R,2R,3S,8R,10R,11R,15S,16S)-3-(acetyloxy)-15-[(4R)-4-[(2S)-3,3-dimethyloxiran-2-yl]-1,4-dihydroxybutan-2-yl]-2,7,7,11,16-pentamethyl-5-oxo-6-oxatetracyclo[9.7.0.0(2,8).0(12,16)]octadec-12-en-10-yl acetate + reduced [NADPH--hemoprotein reductase] + O2 = (1R,2R,3S,8R,10R,11R,15S,16S)-3-(acetyloxy)-15-(1-hydroxy-4-oxobutan-2-yl)-2,7,7,11,16-pentamethyl-5-oxo-6-oxatetracyclo[9.7.0.0(2,8).0(12,16)]octadec-12-en-10-yl acetate + 2-methylpropanoate + oxidized [NADPH--hemoprotein reductase] + H2O + 2 H(+) — reaction