Sesquiterpenoids. Part X. Stereochemistry of pregeijerene: crystal structure of the pregeijerene–silver nitrate adduct

Abstract

Crystal-structure analysis of C₁₂H₁₈,AgNO₃,H₂O, the silver nitrate adduct of pregeijerene, has shown the trisubstituted double bonds of the 1,5-dimethylcyclodeca-1,5,7-triene to be trans and the disubstituted double bond cis. The silver ion is co-ordinated to the C(5)–C(6) double bond of one molecule and the C(7)–C(8) double bond of a related molecule, with Ag–C distances 2·42–2·47 Å. The olefin adopts a conformation in which the methyl groups are on the same side of the mean plane of the ring. The transannular separation C(2)⋯C(6), 2·91 Å, is notably short. The C(6)–C(7)–C(8)–C(9) torsion angle is 1·5°; the C(10)–C(1)–C(2)–C(3) and C(4)–C(5)–C(6)–C(7) torsion angles are 165·4 and 150·4°. The deviation from planar trigonal bonding at C(1) is small, but the apparent twist of 29·6 in the complexed trans double bond C(5)–C(6) involves a component of 19·5° arising from out-of-plane displacement of the carbon–carbon bonds at C(5). The adduct crystallizes in the monoclinic space group P2₁/c, with Z= 4 in a cell of dimensions a= 10·19, b= 9·17, c= 14·80 Å, β= 100·8°. The structure was determined by Fourier and least-squares methods and refined to R 0·105.