A New Configurational Analysis of 1,6,7-triacetoxy-8,13-epoxy-14-labden- 11-one Isolated from Plectranthus ornatus Based on NMR and Theoretical Calculations

This work describes the configurational and conformational analyses of 1,6,7-triacetoxy-8,13-epoxy-14-labden11-one, a labdane diterpenoid isolated from Plectranthus ornatus. Its relative stereochemistry was previously proposed by comparison with H and C NMR data of forskolin (2); however, the configurations of C-8 and C-13 have not been confirmed. Correlations between C NMR experimental data and HF/6-31G* calculated values of carbon chemical shifts were performed for configurational and conformational analyses. This procedure was formerly applied to the configurational analysis of 2. Among the seven different forskolin-type structures investigated, the C NMR data of 2 correlated best with those with the same stereochemistry of 2 described in the literature. Thus, the same procedure was considered valid for configurational analysis of the labdane diterpenoid isolated from P. ornatus. The relative stereochemistry of this compound based on theoretical calculations was similar to the structure previously proposed, but the best results were obtained considering configurational inversion at C-13.