Secondary Metabolites from the Leaves and Defoliated Twigs of Alstonia scholaris
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SECONDARY METABOLITES FROM THE LEAVES AND DEFOLIATED TWIGS OF Alstonia scholaris
M. S. Ali,1* E. A. Ali,1 H. Kausar,1 S. Zikr-Ur-Rehman,1 Z. S. Saify,1 and M. Latif2
Apocynaceae is one of the largest families with 415 genera and 4555 species [1]. The family is very rich in alkaloids [2]. The main genera of this family are Rauwolfia and Alstonia. The genus Alstonia itself comprises 60 species. One of the species is Alstonia scholaris which is one of the traditional medicinal plant well known for its therapeutic effect against various diseases [3]. It is a beautiful tree popularly known as the “Saptaparni” or “Devil′s tree” [4], growing usually in tropical regions of Africa and Asia [5]. It is also cultivated ornamentally in the temperate areas of California, Texas, and Florida [6]. A variety of secondary metabolites has been reported from different parts of A. scholaris such as alkaloids, terpenoids, steroids, flavonoids, coumarins, phenolics, iridoids, saponins, tannins, and phlobatanins [3]. Literature survey of the plant further showed its strong ethnomedicinal importance in the ayurvedic, homeopathic, and folklore systems of medicine [5]. The ethnopharmacological and chemotaxonomic significance of the genus Alstonia prompted us to reinvestigate the secondary metabolites from the leaves and defoliated twigs of A. scholaris with the particular goal of correlating the structure–activity relationship. As a result of the present phytochemical study, 17 known constituents (1–17) were isolated along with their biological activities. The isolates include three indole alkaloids, namely echitamine (1), scholaricine (2), and picrinine (3) [7] along with nine triterpenes: 3β-hydroxy-11α,12α-epoxy-friedoolean-14-enyl palmitate (4), wallichianic acid (5), cycloeucalenol (6) [8], lupeol acetate (7) [9], betulin (8) [10], betulinic acid (9) [11], oleanolic acid (10) [12], ursolic acid (11) [13], and β -amyrin acetate (12) [9]. In addition, kaempferol-3-O-β-D-galactoside (13), β-sitosterol-3-O-β-D-glucopyranoside (14) [14], 1-palmitoylglycerol (15), dotriacontanyl-3-(4-hydroxyphenyl) propionate (16), and vanillic acid (17) [14] have also been isolated. Out of these compounds 4, 5, and 15–17 have been reported for the first time from this species. The leaves and twigs of A. scholaris (L.) R.Br. were collected from Karachi, Pakistan in May, 2017 and identified by the taxonomist of the Department of Botany, University of Karachi, where a voucher specimen (G.H-94482) has been deposited. Fresh leaves (40 kg) and twigs of A. scholaris (35 kg) were separately shade dried and extracted thrice with ethanol for 5 days. The resulting crude extracts were concentrated on a thin film rotary evaporator to afford 655 g residue from the leaves and 641 g from the twigs. Each of these was divided into fractions soluble in EtOAc, n-BuOH, and water. The EtOAc soluble fraction (214 g) of leaves was chromatographed over silica gel eluting with mixtures of n-hexane–EtOAc and EtOAc–MeOH in increasing order of polarity by 10% and 5%, respectively. The
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