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

Abstract

This work describes the configurational and conformational analyses of 1,6,7-triacetoxy-8,13-epoxy-14-labden-11-one, a labdane diterpenoid isolated from Plectranthus ornatus. Its relative stereochemistry was previously proposed by comparison with 1 H and 13 C NMR data of forskolin (2); however, the configurations of C-8 and C-13 have not been confirmed. Correlations between 13 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 13 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.

Chemistry of South African Lamiaceae: Structures and Biological Activity of Terpenoids

“…The phytochemical studies resulted in the isolation of 11 neoclerodanes (plectrornatins A (160) [75] [79]) in addition to β-sitosterol and stigmasterol, 3β-acetyl-α-amyrin, and friedelin. Inversion at C-13 of 1,6-di-O-acetyl-9-deoxyforskolin (174) was carried out based on correlations between 13 C NMR experimental data and HF/6-31G* calculation [80]. 160, 161 showed moderate antimicrobial.…”

From classical taxonomy to genome and metabolome: Towards comprehensive quality standards for medicinal herb raw materials and extracts

“…A survey of its occurrence in a number of related species (of seven species of Coleus and six of Plectranthus) indicated the presence of forskolin only in C. forskohlii [56]. Perusal of literature reveals the isolation of 'forskolin-like' constituents in species such as Plectranthus ornatus [57] that is used as substitute for P. barbatus for treating digestive disorders in Brazil. Lukhoba et al [58] raised doubts about the lack of distribution of forskolin in other species of Plectranthus indicating the practice of natural product chemists not reporting the occurrence of known compounds and suggesting to re-examine the distribution of diterpenoids within this group.…”