The Total Synthesis of Diquinane-Containing Natural Products

Total Synthesis of the Hexacyclic Sesterterpenoid Niduterpenoid B via Structural Reorganization Strategy

To date, it remains challenging to precisely and efficiently construct structurally intriguing polycarbocycles with densely packed stereocenters in organic synthesis. Niduterpenoid B, a naturally occurring ERα inhibitor, exemplifies this complexity with its intricate polycyclic network comprising 5 cyclopentane and 1 cyclopropane rings, featuring 13 contiguous stereocenters, including 4 all-carbon quaternary centers. In this work, we describe the first total synthesis of niduterpenoid B using a structural reorganization strategy. Key features include the following: (1) an efficient methoxy-controlled cascade reaction that precisely forges a highly functionalized tetraquinane (A-D rings) bearing sterically hindered contiguous quaternary stereocenters; (2) a rhodium-catalyzed [1 + 2] cycloaddition that facilitates the construction of a strained 3/5 bicycle (E-F rings) angularly fused with ring D.

Development of Controllable Hetero-Pauson-Khand Polymerization to Functional Stimuli-Responsive Poly(γ-lactam)s

Polymers containing lactam structures play a crucial role in both natural biological systems and human life, and their synthesis, functions and applications are of utmost importance for biomimetics and the creation of new materials. In this study, we developed an efficient heterogeneous Pauson-Khand polymerization (h-PKP) method for the controlled synthesis of main-chain poly(γ-lactam)s containing α, β-unsaturated γ-lactam functionalities using readily available internal alkynes and imines. The molecular weights of the resulting poly(N-Ts/γ-lactam)s can be precisely controlled by adjusting the ratio of phenyl formate and nickel. These polymers exhibit high solid-state luminescence and demonstrate rapid and sensitive dual responsiveness to light and acid stimuli. They further demonstrate strong reactive oxygen species (ROS) generation capability. The unique dual-emission peaks observed in poly(N-H/γ-lactam)s obtained through post-treatment under acidic conditions demonstrate a mechanism of aggregation-induced intermolecular excited-state proton transfer specific to lactam structures. The efficient one-pot synthetic method for poly(γ-lactam) provides a novel strategy for constructing polymers with γ-lactam structures in the main chain and the simple and efficient post-modification method offer a versatile toolbox for functionalizing poly(γ-lactam)s to expand their potential applications.

Phosphine-promoted intramolecular Rauhut-Currier/Wittig reaction cascade to access (hetero)arene-fused diquinanes

We describe the first phosphine-promoted intramolecular Rauhut-Currier reaction that triggers an intramolecular Wittig process assembling new classes of diquinanes. The one-pot strategy provides ready access to simple diquinanes and various (hetero)arene-fused diquinanes incorporated with up to two contiguous all-carbon quaternary centers under metal-free and neutral conditions. We showcased the generality of the method on a broad range of substrates and demonstrated its synthetic utility in accessing various advanced intermediates relevant to natural product synthesis and material science.

Decarboxylative Intramolecular [3 + 2] Cycloaddition of Cyclic Enol Carbonates: Construction of a Bicyclo[3.3.0]octanone Skeleton

Stereoselective synthesis of bicyclic cyclopentanones was achieved by sequential Tf2O-catalyzed decarboxylation and intramolecular [3 + 2] cycloaddition reactions of cyclic enol carbonates bearing an alkene unit. Four stereogenic centers in the obtained cyclopentanone were stereoselectively constructed. This method could be applied to the synthesis of various fused bicyclic products in moderate-to-good yields.

Total Syntheses of Polycyclic Diterpenes Phomopsene, Methyl Phomopsenonate, and iso-Phomopsene via Reorganization of C-C Single Bonds

The first total syntheses of polycyclic diterpenes phomopsene (1), methyl phomopsenonate (2), and iso-phomopsene (3) have been accomplished through the unusual cascade reorganization of C-C single bonds. This approach features: (i) a synergistic Nazarov cyclization/double ring expansions in one-step, developed by authors, to rapid and stereospecific construction of the 5/5/5/5 tetraquinane scaffold bearing contiguous quaternary centers and (ii) a one-pot strategic ring expansion through Beckmann fragmentation/recombination to efficiently assemble the requisite 5/5/6/5 tetracyclic skeleton of the target molecules 1-3. This work enables us to determine that the correct structure of iso-phomopsene is, in fact, the C7 epimer of the originally assigned structure. Finally, the absolute configurations of three target molecules were confirmed through enantioselective synthesis.

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.

Phosphine-Catalyzed Sequential [3 + 2]/[3 + 2] Annulation between Allenoates and Arylidenemalononitriles for the Enantioselective Construction of Bicyclo[3,3,0]octenes and Cyclopenta[c]quinolinones

A highly diastereo- and enantioselective phosphine-catalyzed sequential [3 + 2]/[3 + 2] annulation of allenoates with arylidenemalononitriles has been developed. This reaction allows for the facile construction of multifunctionalized cis-fused bicyclic[3,3,0]octene scaffolds, encompassing three consecutive stereogenic centers with one quaternary carbon center, in a one-step operation from readily available materials. The reported protocol is scalable, operates under mild reaction conditions, and creates the core structural motif of a number of natural products.

Divergent Total Syntheses of (-)-Crinipellins Facilitated by a HAT-Initiated Dowd-Beckwith Rearrangement

A hydrogen atom transfer (HAT)-initiated Dowd-Beckwith rearrangement reaction was developed, which enables the efficient assembly of diversely functionalized polyquinane frameworks. By incorporation of an iridium-catalyzed regio- and enantioselective hydrogenation and a diastereocontrolled ODI-[5+2] cycloaddition/pinacol rearrangement cascade reaction, the asymmetric total syntheses of eight tetraquinane natural products, including (-)-crinipellins A-F and (-)-dihydrocrinipellins A and B, have been achieved in a concise and divergent manner.

Five-membered carbocycle construction in the synthesis of Daphniphyllum alkaloids: recent strategic and methodological advances

In this review article, we summarize novel or non-standard strategies and methods for the five-membered carbocycle construction in recent Daphniphyllum alkaloid synthesis.