Two new simple iridoids from the ant-plant Myrmecodia tuberosa and their antimicrobial effects

Abstract Six iridoid derivatives (1–6), including two new compounds myrmecodoides A and B (1 and 2), were isolated from the ant-plant Myrmecodia tuberosa. Their structures were determined on the basis of spectroscopic data (1H and 13C NMR, HSQC, HMBC, 1H-1H COSY, NOESY and HR-ESI-MS) and by comparison with the literature values. Among isolates, 3 and 4 exhibit weak antibacterial effect against Staphylococcus aureus subsp. aureus with MIC value of 100.0 μg/mL.


Introduction
The ant-plant Myrmecodia tuberosa Jack. (synonym: M. armata DC.) is an epiphyte shrub; the young stem swells to become a chambered tuber, succulent, ovoid or oval, 20-40 cm long and 5-15 cm diametre; the tuber is spiny, has many cavities and allows entry of ants to form a symbiotic relationship (so called ant-plant); the inflorescences are sessile and occur at cavities on the tuber. In Vietnam, this species is distributed in Hoa Binh, Vinh Phuc, Kon Tum, Gia Lai, Lam Dong and Dak Lak (Chu Yang Sin) provinces and usually found at altitudes above 800 m. The tubers were used as folk medicine to treat hepatitis and stomachache (Chi 2012). The M. tuberosa hypocotyl extract was found to exhibit potent immunomodulatory effects (Sumardi & Sasmito 2013).
As a part of our recent investigations on medicinal plants growing in Lam Dong province -Vietnam (Phan et al. 2014), this paper deals with the isolation and structure elucidation of six iridoid derivatives (1-6) from the M. tuberosa tubers. Their antimicrobial effects were also evaluated.

Results and discussion
Two new iridoid derivatives (1 and 2) and four known compounds (3-6, Figure 1) were isolated from the water and dichloromethane subfractions of crude extract using combined chromatographic methods. Detailed analysis of the spectroscopic data (1D, 2D NMR and MS) and comparison with previously reported values led to the elucidation of the known iridoid glucosides 3-6 as asperulosidic acid, deacetylasperulosidic acid (Tzakou et al. 2007), premnosidic acid (Negi et al. 2004) and asperuloside (Otsuka et al. 1991), respectively. This is the first report of these compounds from Myrmecodia species.

Plant material
The tubers of M. tuberosa Jack. (Rubiaceae) were collected in Dam Rong, Lamdong, Vietnam during August 2013 and identified by Dr Nguyen Quoc Binh (Vietnam National Museum of Nature, VAST). A voucher specimen (TN3/326) was deposited at the Herbarium of Institute of Ecology and Biological Resources, VAST, Vietnam.

Supplementary material
Antimicrobial assays; 1D and 2D NMR spectra and data for the new compounds 1 and 2.