Six novel quinolinone alkaloids bearing a phenylpropanoid or a coumarin moiety, named melicodenines C-H (1-6), two new cinnamyl alcohol derivatives, named melicodins A (7) and B (8), and a new coumarinolignan, named melicodin C (9), were isolated from the leaves of Melicope denhamii (Seem.) T. G. Hartley. Their structures were established by spectroscopic analyses including extensive 2D-NMR experiments. Compounds 1-5 and 9 possess a cyclobutane ring and are hypothetically produced by a [2+2] cycloaddition, whereas compound 6 is presumed to form through a Diels-Alder cycloaddition followed by the elimination of an acetone molecule. Ten quinolinone alkaloids (1-6, 10-13) and a coumarinolignan (9) were tested for anti-proliferative activity against DLD-1 human colon cancer cells. Melicodenine G (5) showed the most potent inhibitory activity, causing the induction of apoptosis with an IC50 value of 9.4 +/- 0.3 mu M. (C) 2012 Elsevier Ltd. All rights reserved.