Methyl dodonates, a new type of diterpenes with a modified clerodane skeleton from Dodonaea viscosa

Three new modified clerodanes (10, 12, 17) containing a tricyclo[5.4.0.01,3]undecane ring system were isolated from Dodonaea viscosa. These types of compounds have not been found in nature until now, but they have been proposed as hypothetical intermediates in the biogenetic pathway of diterpenes possessing bicyclo[5.4.0]undecane or bicyclo[5.3.0]decane ring systems. In support of this hypothesis, 17 was transformed into the bicyclo[5.4.0]undecane derivative 20. Additionally, we isolated the new 5,10-secoclerodane 21 and several known diterpenes. The structures were elucidated by spectroscopic methods, chemical reactions and X-ray diffraction analysis.