Concise and Stereoselective Total Syntheses of Annotinolides C, D, and E

The annotinolides are one of the most recent additions to the Lycopodium family of alkaloids, with its members possessing challenging, caged structures that include a [3.2.1]-bicyclic core bearing six contiguous stereocenters, including oxa-, aza-, and all-carbon quaternary centers. Herein, we document a concise and stereoselective route that achieves the first total syntheses of three of its members: annotinolides C, D, and E. Key operations include a gold(I)-catalyzed Conia-ene reaction that fashions much of the main core in a single operation, as well as a number of other challenging and chemoselective transformations to generate the remaining elements. Moreover, efforts utilizing the natural products themselves, seeking adjustments in their oxidation states and the rearrangement of individual ring systems, sheds light on their potential biogenesis with some outcomes counter to those originally proposed. Finally, formal enantioenriched syntheses of the target molecules are also presented.