Antibacterial triterpenes from <i>Syzygium guineense</i> (Myrtaceae)

Antibacterial bioassay-guided fractionation of <i>Syzygium guineense</i> leaf extracts afforded 10 triterpenes, namely betulinic acid <b>1</b>, oleanolic acid <b>2</b>, a mixture of 2-hydroxyoleanolic acid <b>3a</b>, 2-hydroxyursolic acid <b>3b</b>, arjunolic acid <b>4a</b>, asiatic acid <b>4b</b>, a mixture of terminolic acid <b>5a</b>, 6-hydroxyasiatic acid <b>5b</b>, and a mixture of arjunolic acid 28-β-glucopyranosyl ester <b>6a</b> and the asiatic acid 28-β-glucopyranosyl ester 6b. Isolated compounds were submitted to an antibacterial assay system against gram-positive and -negative bacteria and human pathogen bacteria. Compounds <b>4a</b> and <b>4b</b> showed the most significant antibacterial activity against <i>Escherichia coli</i>, <i>Bacillus subtilis</i> and <i>Shigella sonnei</i>. The fraction <b>5a–5b</b> was the least active, whereas compounds <b>1, 2</b> and the mixtures of <b>3a–3b</b> and <b>6a–6b</b> were inactive in the assays.