Salicifoxazines A and B, new cytotoxic tetrahydro-1,2-oxazine-containing tryptamine-derived alkaloids from the leaves of Chimonanthus salicifolius

Two new [named salicifoxazines A (1) and B (2)] and six known dimeric tryptamine-derived alkaloids (3–8) were isolated from the leaves of Chimonanthus salicifolius. The new structures were established on the basis of extensive spectroscopic methods and chemical transformations. The absolute configuration of 1 was determined by the observed Cotton effects in its circular dichroism (CD) spectrum, whereas 2 is a pseudo-mesomer of 1. In fact, the first two naturally occurring asymmetric tetrahydro-1,2-oxazine-containing tryptamine-derived alkaloids 1 and 2 are diastereoisomers at C-3a/C-8a or at C-3a′/C-8a′. Compounds 1, 2, (−)-chimonanthine (3), and meso-chimonanthine (4) were found to show moderate cytotoxic effects against human colorectal carcinoma cells (SW-1116 and HCT-116) and/or gastric carcinoma cells (SGC-7901).