*Article* **Alpha-Glucosidase and Alpha-Amylase Inhibitory Activities of Novel Abietane Diterpenes from** *Salvia africana-lutea*

**Ninon G.E.R. Etsassala 1, Jelili A. Badmus 2, Tesfaye T. Waryo 1, Jeanine L. Marnewick 2, Christopher N. Cupido 3, Ahmed A. Hussein 4,\* and Emmanuel I. Iwuoha 1**


Received: 26 June 2019; Accepted: 17 August 2019; Published: 20 September 2019

**Abstract:** The re-investigation of a methanolic extract of *Salvia africana-lutea* collected from the Cape Floristic Region, South Africa (SA), a fforded four new abietane diterpenes, namely 19-acetoxy-12-methoxycarnosic acid (**1**), 3β-acetoxy-7 α-methoxyrosmanol (**2**), 19-acetoxy-7 α-methoxyrosmanol (**3**), 19-acetoxy-12-methoxy carnosol (**4**), and two known named clinopodiolides A (**5**), and B (**6**), in addition to four known triterpenes, oleanolic, and ursolic acids (**7**, **8**), 11,12-dehydroursolic acid lactone (**9**) and β-amyrin (**10**). The chemical structural elucidation of the isolated compounds was determined on the basis of one and two dimensional nuclear magnetic resonance (1D and 2D NMR), high-resolution mass spectrometry (HRMS), ultra violet (UV), fourier transform infrared (IR), in comparison with literature data. The in vitro bio-evaluation against alpha-glucosidase showed strong inhibitory activities of **8**, **10**, and **7**, with the half inhibitory concentration (IC50) values of 11.3 ± 1.0, 17.1 ± 1.0 and 22.9 ± 2.0 μg/mL, respectively, while **7** demonstrated the strongest in vitro alpha-amylase inhibitory activity among the tested compounds with IC50 of 12.5 ± 0.7 μg/mL. Additionally, some of the compounds showed significant antioxidant capacities. In conclusion, the methanolic extract of *S. africana-lutea* is a rich source of terpenoids, especially abietane diterpenes, with strong antioxidant and anti-diabetic activities that can be helpful to modulate the redox status of the body and could therefore be an excellent candidate for the prevention of the development of diabetes, a disease where oxidase stress plays an important role.

**Keywords:** diabetes mellitus; Lamiaceae; alpha-glucosidase; alpha-amylase; *Salvia africana-lutea*; terpenoids; cape floristic region
