Isolation and structure characterization of cytotoxic alkaloids from Micromelum integerrimum

Highlights

• Ten undescribed alkaloids were isolated from Micromelum integerrimum.

• A racemic heterodimer of carbazole and indole was firstly obtained.

• Two rare natural aldoximes were isolated.

• A racemic pyrrolone was isolated.

• Four compounds showed cytotoxicities against four tumor cell lines.

Abstract

Ten undescribed alkaloids, named integerrines A–J, including one racemic heterodimer of carbazole and indole, two racemic, two scalemic, and one enantiomerically enriched biscarbazoles, two aldoximes, and one racemic pyrrolone, were isolated from the dried leaves and stems of Micromelum integerrimum. The racemic or scalemic compounds were resolved using chiral-phase HPLC and their configurations were determined by comparison of experimental and calculated ECD data. Four compounds exhibited moderate to weak cytotoxicities against HepG2, HTC-116, HeLa, and PANC-1 cell lines, with IC₅₀ values of 14.1–67.5 μM.

Introduction

Micromelum is a genus of the Rutaceae family, including about 10 species. Many of Micromelum species have medicinal values and are usually used to treat various diseases in traditional folk medicine (Editorial Committee of flora of China, 1997; Suthiwong et al., 2014). Micromelum integerrimum (Buch.-Ham.) Roem. (Rutaceae) is a small tree distributed in several countries including the south of China, Vietnam, Thailand, and India. The whole plant of M. integerrimum has been used as a herbal medicine to treat influenza, cough, ostalgia, gastralgia, swellings, and pains (Susidarti et al., 2009). Previous chemical investigations have confirmed that M. integerrimum contains coumarins, flavonoids, and alkaloids, which possess potent anti-inflammatory, antibacterial, and cancer cell cytotoxicity (Huang et al., 2011; Lekphrom et al., 2016; Susidarti et al., 2009).

As part of an ongoing search for bioactive alkaloids from the Micromelum plants, the 95% aqueous EtOH extract of the leaves and stems of M. integerrimum was phytochemically investigated to yield ten previously undescribed alkaloids (1a/1b–5a/5b, 6–9, and 10a/10b) (Fig. 1), including one racemic indole−carbazole, two racemic, two scalemic, and one enantiomerically enriched biscarbazoles, two aldoximes, and one racemic pyrrolone. Compound 1a/1b is the first heterodimeric racemate of carbazole and indole linked via a C–C bond, and compounds 8 and 9 are two rare natural aldoximes. Compounds 2a/2b–5a/5b are four biscarbazole racemates or scalemic mixtures with a rare C-5–C-5′ linkage, and 10a/10b is a racemic pyrrolone. These enantiomers were separated by chiral-phase HPLC to obtain enantiomerically pure compounds. Herein, the isolation, structure elucidation, and biological evaluation on cytotoxic activities of ten previously undescribed alkaloids are reported.