Crystal Structure and Conformation of Ring A of 2β-Bromo-19β,28-epoxy-18α-oleanan-3-one
The crystal structure of β-bromo-19β,28-epoxy-18α-oleanan-3-one was elucidated. The crystal is orthorhombic, P2 1 2 1 2 1 , a = 9.686(1), b = 14.355(2), c = 19.687(4) A, Z = 4, R = 0.042 for 2 410 observed reflections. Rings B, C, D and E adopt the chair conformation, the five membered ether cycle in ring E occurs in the envelope form. Ring A takes the twist-boat conformation turned towards the classical boat with C2 and C5 in the stem-stern position, in contrast to the conformation in solution, which is turned in the opposite direction towards the classical boat with C3 and C10 in the stem-stern positions.