Structures and conformations of the fourteen-membered ring diterpene ovatodiolide and its acid cyclization product: nuclear Overhauser effect studies in solution and X-ray crystal structure analyses of ovatodiolide and ovatodiolic acid

Abstract

The conformation and relative stereochemistry of ovatodiolide (1), a fourteen-membered ring diterpene dilactone isolated from Anisomeles ovata, have been elucidated by ¹H n.m.r. spectroscopy including intramolecular nuclear Overhauser effect measurements. The structure of ovatodiolic acid (3), an acid cyclization product of (1) has also been investigated. Single-crystal X-ray analyses have confirmed the structural and stereochemical assignments of (1) and (3) both of which crystallize in the orthorhombic system, space group P2₁2₁2₁, with a= 12.798(8), b= 13.255(8), c= 10.950(8)Å, Z= 4, for (1), and a= 12.994(8), b= 14.435(9), c= 9.432(8)Å, Z= 4, for (3). The crystal structures were solved by direct non-centrosymmetric phase-determining procedures and atomic parameters refined from diffractometer data by full-matrix least-squares calculations to R 0.071 [1 280 reflections (1)] and 0.074 [1 399 reflections, (3)]. The trisubstituted trans double bonds in (3) are distinctly distorted from planarity.