SELECTIVE DEMETHYLATION OF SOME ACONITINE-TYPE NORDITERPENOID ALKALOIDS

Demethylation of some aconitine-type norditerpenoid alkaloids was carr ied out with trimethylsilyl iodide and with HBr in glacial AcOH. Aconi tine (10), cammaconine (23), delphinine (3), falconerine (18), lappaco nitine (22), and talatizamine (24) afforded partially demethylated pro ducts. When methoxyl groups are present at the C-16 and C-18 positions , these are demethylated, and the methoxyl group at the C-1 position u nderwent demethylation in none the alkaloids studied except falconerin e (18). With HBr-AcOH, in the case of alkaloids possessing a C-3 hydro xyl group, the methoxymethyl at C-18 formed a tetrahydrofuran, cyclizi ng at the C-6 position. Detailed NMR spectral studies (H-1, C-13, H-1 homonuclear COSY, HETCOR, and selective INEPT) carried out on the deme thylation products have enabled accurate chemical shift assignments to be made for the demethylated alkaloids.