Transformation of Neoclerodane Diterpenoids into 19-Norneoclerodane Derivatives

Article abstract of DOI:10.1021/jo00023a027

The neoclerodane diterpenoid eriocephalin (1) was transformed into its 19-nor derivatives 3 and 4 by reaction with potassium tert-butoxide in THF, whereas with the same treatment compound 6 yielded only the transacetylation derivative 7.These results indicated that 19-acetoxy-4α,18-epoxy-7α-hydroxy-6-oxoneoclerodane derivatives are transformed into the corresponding 19-nor compounds in a retroaldol reaction by loss of the C-19 carbon as formaldehyde followed by opening of the oxirane ring and intramolecular attack by a C-18 alkoxide on the carbonyl C-6 carbon atom, giving the allylic hemiacetal 3, which is easily dehydrated to the furanic derivative 4.Compound 4 was transformed into the α,β-unsaturated γ,δ-enol γ-lactone 8 by an oxidation reaction with atmospheric oxygen in chloroform solution.Alternatively, it gave the α,β-unsaturated γ-lactone 11 under acid catalysis.These reactions allowed the partial syntheses of the naturally occurring diterpenoids teuscorolide (9) and teucvin (12) and are of interest for chemical transformations of substances of this kind.