Syntheses of taiwaniaquinoid and icetexane natural products based on biogenetic hypotheses

The syntheses of natural products based on biogenetic hypotheses have the potential not only for giving access to short, elegant and efficient synthetic routes, but also to shine light on how nature constructs these breathtaking structures. This work, divided in three experimentally interconnected chapters, will highlight the importance of this approach with the syntheses of taiwaniaquinoid and icetexane natural products.
A protecting-group-free route to (−)-taiwaniaquinone F based on a Wolff-rearrangement ring contraction and subsequent aromatic oxidation of a sugiol derivative was deviced following a proposed biogenetic pathway. In addition, the first synthesis of (+)-taiwaniaquinol A is reported via short time exposure of (−)-taiwaniaquinone F to sunlight triggering a remote C−H functionalization. The hypothesis that the biogenesis of several methylenedioxy bridged natural products could proceed via similar nonenzymatic mechanisms is presented.
A divergent synthesis of thirteen members of icetexane natural products based on a proposed biogenetic cationic ring expansion of a reduced carnosic acid derivative is described. Among these members, (+)-salvicanol, (−)-cyclocoulterone, (−)-coulterone, (−)-obtusinone D, (−)-obtusinone E, and (−)-euolutchuol E are synthesized for the first time. Following this approach, an additional support to our methylenedioxy biogenetic hypothesis is reported via the photolysis of (+)-komaroviquinone to (−)-cyclocoulterone and (+)-komarovispirone.
Preliminary results towards the mechanistic elucidation of the discovered remote C−H activation are outlined. Photolysis of a model substrate shows the formation of the methylenedioxy moiety for the first time on a non-natural quinone. Preliminary evidence that account for the formation of biradical species are given.