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Abstract: A Lewis acid BF3•Et2O-mediated intramolecular cycloaddition reaction of mahanimbine (1) is described. Three cycloadducts, bicyclomahanimbine (2), cyclomahanimbine (3) and murrayazolinine (4) were obtained. The structural characterization of these compounds was determined by 1D and 2D NMR experiments. These compounds showed no cytotoxic activity against human MRC-5 cells (IC50 > 60 μg/mL). Compound 3 showed the highest inhibition cytotoxic effects against HeLa and HL-60 cancer cells with IC50 values of 10.0 and 28.7 μg/mL, respectively. This strategy opens a new approach for the synthesis of biologically significant cyclic monoterpenoid pyrano[3,2-a]carbazole alkaloids.

Study on chemical constituents from Kopsia singapurensis (Apocynaceae) yielded four new aspidofractinines: Singaporentine A (1), N(1)-formylkopsininic acid (2), N(1)-formylkopsininic acid-N(4)-oxide (3), 15-hydroxykopsamine (4); a new aspidospermatan: 14α-hydroxy-N(4)-methylcondylocarpine (5) and a new akuamiline: singaporentinidine (6) together with 18 known indole alkaloids from different parts of the plant. The structures of these compounds were elucidated by combination of various spectroscopic methods.

Quorum sensing enables bacterial to communicate and at the same time control the gene expression in response to the cell density [1]. It is widely used by both gram-positive and gram-negative bacteria to regulate variety of bacterial physiological functions such as biofilm formation [2], bioluminescence [3], virulence factors [4] and swarming [5] which has been shown contribute to bacterial pathogenesis. With this knowledge, the use of QS inhibitor would be a particular value in treating bacterial pathogenicity and infections. In this work we have tested caffeine as quorum sensing inhibitor by using C.violaceum CV026 as a bioassay. This C.violaceum CV026 mutant strain is incapable of producing the purple pigment called violacein unless there are exogenous supply of N-hexanoylhomoserine lactone (C6-HSL) or other short chain AHL .The Inhibitory activity was measured by quantifying violacein production using a spectrophotometer. The results have revealed that Caffeine significantly reduced violacein production in a concentration dependent manner, indicating inhibition of quorum sensing. The presence of caffeine that exhibit anti-quorum sensing activity may be useful as the lead of anti-infective drugs.