A Synthetic Route to The Core Structure of (-)-Retigeranic Acid A

DFT Study on Retigerane-Type Sesterterpenoid Biosynthesis: Initial Conformation of GFPP Regulates Biosynthetic Pathway, Ring-Construction Order and Stereochemistry

Retigerane-type sesterterpenoids, which feature a unique 5/6/5/5/5 fused pentacyclic structure with an angular-type triquinane moiety, are biosynthesized via successive carbocation-mediated reactions triggered by terpene cyclases. However, the precise biosynthetic pathways/mechanisms, wherein steric inversion of the carbon skeleton occurs at least once, remain elusive. Two plausible reaction pathways have been proposed, which differ in the order of ring cyclization: A → B/C → D/E-ring(s) (Route 1) and A → E → B → C/D-ring(s) (Route 2). Since the reaction intermediates of these complicated domino-type reaction sequences are experimentally inaccessible, we employed comprehensive density functional theory (DFT) calculations to evaluate these routes. The results indicate that retigeranin biosynthesis proceeds via Route 2 involving a multistep carbocation cascade, in which the order of ring cyclization (A → E → B → C/D) is the key to constructing the angular 5/5/5 triquinane structure with the correct stereochemistry at C3. The result also suggests that slight differences in the initial conformation have a significant effect on the order of cyclization and steric inversion. The computed pathway/mechanism also provides a rational basis for the formation of various related terpenes/terpenoids.

Total Synthesis of (-)-Retigeranic Acid A: A Reductive Skeletal Rearrangement Strategy

The asymmetric total synthesis of (-)-retigeranic acid A was described, which relies on a crucial reductive skeletal rearrangement cascade for the controllable assembly of diverse angular triquinane subunits. Taken together with an intramolecular Michael/aldol cyclization, an ODI-[5 + 2] cycloaddition/pinacol rearrangement cascade, a Wolff ring contraction and a stereoselective HAT reduction, our synthetic approach has enabled the access to (-)-retigeranic acid A in a concise and practical manner.

A Modular Approach to Angularly Fused Polyquinanes via Ring-Rearrangement Metathesis: Synthetic Access to Cameroonanol Analogues and the Basic Core of Subergorgic Acid and Crinipellin

We describe a modular approach to angularly fused polyquinanes that are core units of many natural products such as cameroonanol, subergorgic acid, and crinepellin, etc. in excellent yields by employing atom-economic ring-rearrangement metathesis as a key step. Our work highlights, the synthesis of cameroonanol analogues 1-6 and their ester derivatives by using the stereoselective reduction of the carbonyl group by using DIBAL-H- and DCC-mediated coupling as the key reactions. The subergorgic acid core 7 was produced by LDA-mediated kinetically controlled regio- and stereoselective ring-junction allylation as a critical step. Moreover, it is worth mentioning that the present strategy relies on a less explored exo-dicyclopentadiene-1-one (8) and produces highly congested polycyclic frameworks containing up to seven contiguous stereogenic centers including quaternary carbons up to two. All of the new molecules were characterized by NMR data. The structure and relative stereochemistry of some compounds were confirmed by chemical methods and further supported by single-crystal X-ray diffraction studies. The newly reported tri- and tetraquinane skeletons are present in many naturally occurring bioactive polyquinanes. Hence, this strategy is useful for designing various "druglike molecules" and expands the chemical space of cyclopentanoids that are useful in medicinal chemistry.