Catalyzes the C6-oxidation step in brassinosteroids biosynthesis (PubMed:15710611). Converts 6-deoxocastasterone (6-deoxoCS) to castasterone (CS) (PubMed:15710611, PubMed:9990098). May also convert 6-deoxoteasterone (6-deoxoTE) to teasterone (TE), 3-dehydro-6-deoxoteasterone (6-deoxo3DT, 6-deoxo3DHT) to 3-dehydroteasterone (3DT, 3-DHT), and 6-deoxotyphasterol (6-deoxoTY) to typhasterol (TY), but not castasterone (CS) to brassinolide (BL) (PubMed:15710611). 6-deoxocastasterone + O2 + reduced [NADPH--hemoprotein reductase] = 6alpha-hydroxycastasterone + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 6alpha-hydroxycastasterone + O2 + reduced [NADPH--hemoprotein reductase] = castasterone + H(+) + 2 H2O + oxidized [NADPH--hemoprotein reductase] 6-deoxocastasterone + 2 O2 + 2 reduced [NADPH--hemoprotein reductase] = castasterone + 2 H(+) + 3 H2O + 2 oxidized [NADPH--hemoprotein reductase] Plant hormone biosynthesis; brassinosteroid biosynthesis. Expressed in sub-meristematic regions of shoot and root apexes, in zones undergoing lateral root formation, in fruits, and in all flower parts, with a high expression in young flower buds and at the joint in the pedicel. Strong expression in the locular jelly during seed development. Belongs to the cytochrome P450 family.