Phytotoxic neo-clerodane diterpenoids from the aerial parts of Scutellaria barbata

Highlights

• The aerial parts of Scutellaria barbata afforded 13 undescribed neo-clerodane diterpenoids.

• The absolute configurations of scutebarbolides A and L and scutebata W were determined by X-ray diffraction.

• Phytotoxic activity of undescribed scutebarbolide K was stronger than that of the positive control.

• The structure-activity relationships of these neo-clerodane diterpenoids were discussed.

Abstract

Bioactivity guided the isolation of extracts from the aerial parts Scutellaria barbata D. Don to discover neo-clerodane diterpenoids with potent phytotoxic activity. Of the 34 isolates, 13 neo-clerodane diterpenoids were described for the first time. The structures of these undescribed compounds were elucidated by extensive analysis of NMR spectroscopic data, and the absolute configurations of scutebarbolides A and L and scutebata W were determined by X-ray diffraction. The phytotoxic activity of all compounds against the growth of the roots and shoots of L. perenne and L. sativa seedlings were first reported, and some compounds showed considerable inhibitory effects, especially scutebarbolide K, whose inhibition rates were higher than those of the positive control at concentrations ranging from 25 to 200 μg/mL. When L. perenne and L. sativa seedlings were treated at a concentration of 200 μg/mL, scutebarbolide K caused wilting symptoms on and finally death of these two tested plant seedlings. In addition, the structure-activity relationships of these neo-clerodane diterpenoids were also discussed.

Introduction

Weeds are one of the important biological factors endangering crop growth and reducing yield by competing with crops for water, nutrients and light (Shaik et al., 2017; Tshewang et al., 2016). At present, weeds can be controlled by a variety of means including manual, mechanical, physical, and chemical weedings (Zhao et al., 2017). Chemical weeding is still the most common choice because it is highly efficient, timeless, labor-saving, and economical. Synthetic herbicides can be of some help for farmers and gardeners−but they also come with drawbacks, such as harm to environment, wildlife, and humans (Macías et al., 2000; Tucci et al., 2019; Owen and Zelaya, 2005). This prompted us to look for herbicides from natural sources due to their advantages of being renewable, environmentally friendly, and less harmful to wildlife and humans.

Scutellaria barbata D. Don is a perennial herb plant of the genus Scutellaria in the Lamiaceae, distributed widely in China, Korea, India, and other Asian countries, and is popularly used in China as a traditional medicine for the treatment of various diseases (Yeon et al., 2015; Yang et al., 2017). The chemical constituents of this plant have been widely studied, having led to the isolation of neo-clerodane diterpenoids, diterpenoid alkaloids, and flavonoids (Dai et al., 2011; Zhu et al., 2011; Li et al., 2014a, 2014b; Wu et al., 2015; Wang et al., 2012, 2018, 2019; Guo et al., 2019). Some neo-clerodane diterpenoids have been reported to possess brilliant phytotoxic activity (Bisio et al., 2011; Li et al., 2014a, 2014b). A bioassay-guided isolation approach to S. barbata was performed, which revealed that the EtOAc fraction showed a better phytotoxic activity than the n-BuOH fraction. Thus, a phytochemical investigation of the EtOAc fraction led to 34 neo-clerodane diterpenoids (13 previously undescribed ones). The phytotoxic activity of these purified compounds was also evaluated. Herein, the isolation, structural characterization, and phytotoxic activity of these neo-clerodane diterpenoids are described. To the best of our knowledge, this is the first report on the phytotoxic activity of neo-clerodane diterpenoids from S. barbata.