Involved in the transformation of a taxadienyl acetate by hydroxylation at C13 to yield taxadien-5-alpha-acetoxy-13-alpha-ol. O2 + reduced [NADPH--hemoprotein reductase] + taxa-4(20),11-dien-5alpha-ol = H(+) + H2O + oxidized [NADPH--hemoprotein reductase] + taxa-4(20),11-dien-5alpha,13alpha-diol Alkaloid biosynthesis; taxol biosynthesis. Belongs to the cytochrome P450 family.