Crystal chemistry of some synthetic 2-oxa-steroids:: conformation, packing motifs and isostructurality

The crystal structures of six synthetic 2-oxa-steroids (A-ring lactone steroids) have been determined by single-crystal X-ray diffraction. The conformation and hydrogen bonding in these oxa-steroids is compared with packing motifs in the natural steroids and the anabolic agent, Anavar(R). O-H ... O hydrogen bonding with lactone carbonyl O is the preferred arrangement in molecules with a C-OH group. The donor H atoms of A, B and D rings participate in C-H ... O interactions with lactone carbonyl O and D-ring hydroxyl/ketone O acceptor atoms. The conformation of the lactone ring in these analogues is different from the natural androgens because replacement of the C2-methylene group by an O atom changes the geometry of the A ring. Two structurally related lactone steroids provide the first example of O-H ... O/C-H ... O interaction mimicry and furthermore the two components form a binary solid solution. The O-H ... O and C-H ... O hydrogen bonds in 2-oxa-steroid crystal structures are analysed and the observed preferences discussed in terms of geometric and chemical factors.