Conformation of ring A in 3-oxotriterpenoids. sofa conformation in 3-oxolupane-28-nitrile crystal

Abstract

Crystal structure of 3-oxolupane-28-nitrile (I) has been determined. As usual, the B, C and D rings adopt a chair, and the E ring an envelope conformation. Surprisingly enough, the A ring exists in a sofa arrangement with coplanar C(1) to C(5) and O atoms. The existence of this uncommon and energetically unfavourable conformation is explained by an intermolecular interaction of carbonyl dipoles facilitated by the particular mode of crystal packing. The comparison with crystal structure of 3-oxolupanes II-IV demonstrates that these compounds differ in conformation of the A and E rings whereas the rest of the skeleton remains essentially the same and is remarkably bent about B and C rings. Differences in the effect of 8β-methyl group on the energy of the chair and boat forms of the A ring are tentatively explained on the basis of the B ring deformation. Conformation of the A ring in solution is generally unlike that in the crystal since energy differences between the conformers tend to be effectively overbalanced by crystal packing forces.