Diastereoselective Total Synthesis of the Euphorbia Diterpenoid Pepluanol A: A Reductive Annulation Approach

Total Synthesis of the Euphorbia Diterpenoid Pepluacetal

The first total synthesis of the Euphorbia diterpenoid pepluacetal is disclosed in both racemic and chiral fashions. The synthesis strategically relies on a photo-induced Wolff rearrangement/lactonization cascade (WRLC) reaction to access the cyclobutane moiety, a ring-closing metathesis/cyclopropanation sequence to rapidly forge the 7-3 bicyclic system, and a late-stage Rh-catalyzed transannular carbenoid insertion to C(sp3)-H bond followed by a Baeyer-Villiger oxidation and ring-opening manipulations to install the side chain. The synthetic route demonstrates excellent stereochemical control on the non-classical concave-face bond formation, remote traceless stereochemical relay and high scalability to provide 20 mg of (+)-pepluacetal.

Rearrangement of the Tetra- and Tricyclic Skeletons of Pepluanol B to Access the Core Structures of Tigliane- and Myrsinane-Type Euphorbia Diterpenes

Pepluanol B is a new Euphorbia diterpene with an unprecedented tetracyclic backbone. However, its biogenetic relationship with known Euphorbia diterpenes is unclear. We report herein that its β-hydroxyl ketone motif could undergo a base-promoted retro-aldol/aldol process in two pathways and afford the skeletons of tigliane- and myrsinane-type Euphorbia diterpenes through the formation of the C8-C14 and C7-C13 bonds, respectively. The retro-aldol/aldol cascade indicates that pepluanol B is possibly a biosynthetic precursor of lathyranes and other relevant dipterpenes.

The Total Synthesis of Diquinane-Containing Natural Products

Diquinane or bicyclo[3.3.0]octane is a conspicuous structural unit existing in the carbo-frameworks of a wide range of natural products such as alkaloids and terpenoids. These diquinane-containing molecules not merely exhibit intriguing architectures, but also showcase a broad spectrum of significant bioactivities, which draw widespread attention from the global synthetic community. During the past decade, with an aim to accomplish the total syntheses of such specified cornucopias of natural products, a variety of elegant strategies for construction of the diquinane ring system have been disclosed. In this Minireview, the achievements on this subject in the timeline from 2010 to June 2020 are demonstrated and it is discussed how the diquinane unit is strategically forged in the context of the specific target structure. In addition, impacts of the selected works to the field of natural product total synthesis is highlighted and the particular outlook of diquinane-containing natural product synthesis is provided.

Condition Screening for Sustainable Catalysis in Single-Electron Steps by Cyclic Voltammetry: Additives and Solvents

Cyclic voltammetry-based screening method for Cp₂ TiX-catalyzed reactions is extended to the screening of solvents other than tetrahydrofuran for bulk electrolysis of the catalyst and radical arylation. It was found that CH₃ CN can be used as a solvent for both processes without additives. Furthermore, in tetrahydrofuran, squaramide L2 is more efficient than the previously reported supramolecular halide binder, Schreiner's thiourea L1. The results extend the usefulness of the proposed time and resource-efficient screening method for designing catalysis reactions in single-electron steps.

Mechanism-Based Condition Screening for Sustainable Catalysis in Single-Electron Steps by Cyclic Voltammetry

A cyclic-voltammetry-based screening method for Cp₂ TiX-catalyzed reactions is introduced. Our mechanism-based approach enables the study of the influence of various additives on the electrochemically generated active catalyst Cp₂ TiX, which is in equilibrium with catalytically inactive [Cp₂ TiX₂ ]^- . Thioureas and ureas are most efficient in the generation of Cp₂ TiX in THF. Knowing the precise position of the equilibrium between Cp₂ TiX and [Cp₂ TiX₂ ]^- allowed us to identify reaction conditions for the bulk electrolysis of Cp₂ TiX₂ complexes and for Cp₂ TiX-catayzed radical arylations without having to carry out the reactions. Our time- and resource-efficient approach is of general interest for the design of catalytic reactions that proceed in single-electron steps.

Cp2 TiX Complexes for Sustainable Catalysis in Single-Electron Steps

We present a combined electrochemical, kinetic, and synthetic study with a novel and easily accessible class of titanocene catalysts for catalysis in single-electron steps. The tailoring of the electronic properties of our Cp₂ TiX-catalysts that are prepared in situ from readily available Cp₂ TiX₂ is achieved by varying the anionic ligand X. Of the complexes investigated, Cp₂ TiOMs proved to be either equal or substantially superior to the best catalysts developed earlier. The kinetic and thermodynamic properties pertinent to catalysis have been determined. They allow a mechanistic understanding of the subtle interplay of properties required for an efficient oxidative addition and reduction. Therefore, our study highlights that efficient catalysts do not require the elaborate covalent modification of the cyclopentadienyl ligands.