X-Ray structural and conformational analyses of the Erythrina alkaloids cocculine and coccutrine

Abstract

Crystal structures of the title alkaloids have been established by direct phase-determining methods and atomic parameters refined by full-matrix least-squares calculations to R 0.063 (1) and 0.078 (2) over 960 (1) and 1 342 (2) reflections from diffractometer measurements. The non-isomorphous crystals both belong to the orthorhombic system, space group P2₁2₁2₁, with Z= 4 in unit cells of dimensions: (1), a= 10.51 (1), b= 15.87(1), c= 8.95(1)Å; and (2), a= 10.69(1), b= 16.65(1), c= 8.80(1)Å. Detailed comparisons of the ring conformations are made. Ring A approximates to a half-chair form and ring C to a form intermediate between half-chair and envelope in both compounds. Small variations observed in the ring B conformations are ascribed to different hydrogen-bonded arrangements in the crystals.