Phytochemical investigation and chemotaxonomic significance of Melodinus axillaris (Apocynaceae)

In this study, twenty known alkaloids, including nineteen monoterpenoid indole alkaloids (MIAs) and one rare β-carboline alkaloid, were isolated and identified from the stems and leaves of Melodinus axillaris . Their structures were unambiguously confirmed by comprehensive spectroscopic analyses (HRESIMS, 1D and 2D NMR), as well as by comparison with previously reported literature data. Among these isolates, compounds 3 , 8 , 16 , and 18 – 19 were obtained from this species for the first time, whereas compounds 2 , 9 – 10 , 17 , and 20 are reported from the genus Melodinus for the first time. Notably, the vindolinine-type alkaloids ( 9 – 10 ) and the β-carboline alkaloid ( 20 ) represent chemotypes that have been rarely documented in M. axillaris . The vindolinine subtype, together with compounds 2 and 17 , may act as potential chemotaxonomic markers for M. axillaris . Furthermore, the chemotaxonomic significance and distribution patterns of these compounds within the genus Melodinus are systematically discussed.