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)A, Z= 4, for (1), and a= 12.994(8), b= 14.435(9), c= 9.432(8)A, 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.