Asymmetric Total Synthesis of Indole Alkaloids Containing an Indoline Spiroaminal Framework

The total synthesis of the indole alkaloids, neoxaline, oxaline and meleagrinA, all containing a unique indoline spiroaminal framework, was accomplished through the stereoselective introduction of a reverse prenyl group to the congested benzylic carbon of furoindoline, a two-pot transformation of indoline (containing three nitrogen atoms at appropriate positions) to the featured indoline spiroaminal framework, and elimination of carbonate assisted by the adjacent imidazole moiety to construct the (E)-dehydrohistidine. The absolute stereochemistry of neoxaline was elucidated through our total synthesis. In addition, we evaluated the bioactivity, especially the anti-infectious properties, of neoxaline and oxaline, and of some synthetic intermediates. Synthetic spiroaminals: The total synthesis of neoxaline, oxaline, and meleagrinA (see scheme), all containing a unique indoline spiroaminal framework, was accomplished through the stereoselective introduction of a reverse prenyl group to the congested benzylic carbon of furoindoline, a two-pot transformation of indoline to the featured indoline spiroaminal framework, and elimination of carbonate assisted by the adjacent imidazole moiety to construct the (E)-dehydrohistidine.