Alkaloid studies LXV. Confirmation of the heptacyclic structure of the $\text{Aspidosperma obscurinervium}$ alkaloids through x-ray analysis of obscurinervine hydrobromide☆

Thus, a range of synthetic tactics toward these heterocyclic scaffolds have been developed over the past decade.7 For example, the [5 + 1] type of annulation using 2-halo-1,3-dicarbonyl compounds and imines,8 the palladium-catalyzed intramolecular etherification,9 the stepwise reactions of epoxides with o-haloanilines,10 and the reduction of 2H-benzo[b][1,4]oxazines11 are typical methods of the category. Regardless these important advances, however, the most frequently used methods in 3,4-dihydro-1,4-benzoxazine ring construction are the annulation using o-aminophenol or its derivatives to incorporate different electrophilic partners such as vicinal dihaloalkanes,12 imines,13 vinyl selenones,14 α-haloketones,15 α-haloesters16 and vinyl bromides.17