Diverse prieurianin-type limonoids with oxygen-bridged caged skeletons from two Aphanamixis species: discovery and biomimetic conversion

The spatially close and highly reactive ester appendages through the opening of A and B rings of prieurianin-type limonoids would further rearrange into diverse ring system via oxygen-bridges or new C–C bonds. In our current research, two limonoids with an unprecedented 7/6/5 tricyclic skeleton (1) and 2,6-dioxabicyclo[3.2.2]nonan-3-one caged ring A system (2), along with eight other new ones (3–10) were obtained from Aphanamixis polystachya and Aphanamixis sinensis, and the structure of 1 was confirmed by X-ray crystallographic diffraction. Meanwhile, a reliable solution based on biomimetic alkaline hydrolysis to build new oxygen-bridges via OH-1 was established to resolve the difficulties in the structural elucidation of prieurianin limonoids with broad or missing NMR signals and applied for the determination of 9 and 10. Moreover, the potential of Dieckmann reaction as a key biosynthetic step in the formation of C-3/C-6 bonds in the aphanamolide-type backbone was verified by chemical conversions of 13–16. These findings provided new ideas and perspectives for structural elucidation and chemical communication of prieurianin-type limonoids.