Aaptodines A-D, Spiro Naphthyridine-Furooxazoloquinoline Hybrid Alkaloids from the Sponge Aaptos suberitoides

LC-MS-oriented fractionation of the sponge Aaptos suberitoides resulted in the isolation of four heptacyclic alkaloids, aaptodines A-D (1-4), which contain 9,10-dihydrofuro[2,3-f][1,3]oxazolo[5,4-h]-quinolone and 7,8-dihydrocyclopenta[de][1,6]naphthyridine subunits with a spiro carbon atom. The structures were determined on the basis of NMR spectroscopic and single-crystal X-ray diffraction data analysis aided by electronic circular dichroism calculations and Mosher's method. A biosynthetic pathway for the formation of aaptodines A-D is postulated. Aaptodine D exhibits potent inhibition against osteoclast formation.