Three new sesquiterpene alkaloids of the polyveoline type containing eight-membered lactam rings named greenwaylactam A (1), B (2) and C (3) were isolated from the leaves of a Cameroonian medicinal plant Greenwayodendron oliveri (Engl.) Verdc along with three known compounds diacetylpolyveoline (4), N-acetyl-8 alpha-polyveolinone (5) and methyl pheophorbide-A (6). These comprise the first members of a new class of xi-lactam sesquiterpenes. Compounds were isolated by means of successive column chromatographies and their structures were elucidated by extensive NMR spectroscopic analyses and HRESIMS. The configuration of 1 was deduced from its single-crystal X-ray diffraction. The crude extract and some isolated compounds were assessed for their antibacterial activity against four enteric bacterial strains Escherichia coli, Salmonelles typhi, Shigella dysenterie and Klebseilla pneumonie using the micro-dilution method. Compounds 1, 2, 5 and 6 were the most active with MIC values varying between 31.25-250 mu g/mL.