Conformational study of dibenzocyclo-octadiene systems related to the schizandrin-type lignans

X-Ray and ¹H and ¹³C n.m.r. data have been obtained for the dibenzocyclo-octadiene lignan kadsurin and several of its synthetic precursors, in which di-ortho-substitution of the aryl rings leads to high energy barriers for rotation around the biaryl bond. The crystallographic and spectral information shows that for compounds in this series the eight-membered ring adopts either a twist-boat (TB) or twist-boat-chair (TBC) conformation. The relative stability of ring conformers and/or biaryl rotamers is determined by three factors: (i) a preference for the TBC over the TB form, (ii) stabilization of the TB conformation by conjugation of the benzylic carbonyl groups with the aryl rings, and (iii) the presence of benzylic hydroxy (or acetoxy) substituents in endo/exo orientations.