Massularosides A-D, new triterpenoid saponins from the stem bark of Massularia acuminata with an N-acetylglucosamine residue and their antifungal properties

Four triterpenoid saponins named massularosides A-D (1-4), sharing a common α-L-rhamnopyranosyl-(1 → 3)-2-acetamido-2-deoxy-β-d-glucopyranosyl moiety, were isolated from the stem bark of Massularia acuminata. All four compounds are newly identified natural products, although compound 1 was previously chemically synthesized. The structures of the isolates were established by high-resolution ESI mass spectrometry (HRESIMS), 1D (¹H and ¹³C-DEPTq) and 2D (COSY, ROESY, HSQC and HMBC) NMR spectroscopy and by comparison with literature data to be 3-O-[α-L-rhamnopyranosyl-(1 → 3)-2-acetamido-2-deoxy-β-d-glucopyranosyl] oleanolic acid (1); 3-O-[α-L-rhamnopyranosyl-(1 → 3)-2-acetamido-2-deoxy-β-d-glucopyranosyl]-28-O-[β-d-glucopyranosyl] oleanolic acid (2); 3-O-[α-L-rhamnopyranosyl-(1 → 3)-[β-d-glucopyranosyl-(1 → 6)]-2-acetamido-2-deoxy-β-d-glucopyranosyl]-28-O-[β-d-glucopyranosyl] oleanolic acid (3) and 3-O-[β-d-glucopyranosyl (1 → 4)-α-L-rhamnopyranosyl-(1 → 3)-2-acetamido-2-deoxy-β-d-glucopyranosyl]-28-O-[β-d-glucopyranosyl] oleanolic acid (4), respectively. The antifungal activity of the butanol-soluble fraction, saponins-enriched fractions and compounds 1 and 3 was evaluated against a series of plant pathogenic fungi and oomycetes. The butanol-soluble fraction (>60 % inhibition at 125 μg/mL) and two saponins-enriched fractions (>65 % inhibition at 62.5 μg/mL) showed inhibitory activity against the oomycetes Pythium myriotylum and Phytophtora megakarya. Compound 1 inhibited the growth of P. myriotylum with a minimum inhibitory concentration (MIC) of 125 μg/mL.