The configuration at C-13 in labdanolic and eperuic acids

Abstract

The common C-13 configuration in labdanolic and eperuic acids is inferred from the behaviour of the bicyclic 8,14-diketones derived from the two acids when subjected to alkaline and acidic conditions: the diketone derived from labdanolic acid forms a tricyclic hydroxy-ketone very readily with weak alkali whereas the diketone derived from eperuic acid is totally unaffected under the most vigorous alkaline conditions. Both diketones furnish the corresponding βγ-unsaturated ketones with acid. An interpretation is offered which points to the 13(R)-configuration in both acids. This is supported by the circular dichroism of the diketones and by an X-ray analysis of p-bromophenacyl labdanolate.