Dimerization of (+)-Lysergic Acid Esters

Dimer isomer mixtures, characterized by a bridgehead C8-C8' bond, (6a-7a; 6b-7b) were obtained from (+)-lysergic acid methyl or ethyl ester (1b; 1c) in a solution of methanol or ethanol. The isomers were separated, and their structures were determined by detailed NMR measurements and X-ray analysis. Density functional theory was applied to provide insight into the reaction mechanism. Based on an extended examination and the theoretical calculations, a plausible reaction sequence leading to dimers is also presented. The proposed mechanism has been verified by detecting the formation of the superoxide radical anion (O 2 * -).