Cyclopentannulation of conjugated enones using a vinyldiazomethane-based reagent

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Deconstructive and Divergent Synthesis of Bioactive Natural Products

Natural products play a key role in innovative drug discovery. To explore the potential application of natural products and their analogues in pharmacology, total synthesis is a key tool that provides natural product candidates and synthetic analogues for drug development and potential clinical trials. Deconstructive synthesis, namely building new, challenging structures through bond cleavage of easily accessible moieties, has emerged as a useful design principle in synthesizing bioactive natural products. Divergent synthesis, namely synthesizing many natural products from a common intermediate, can improve the efficiency of chemical synthesis and generate libraries of molecules with unprecedented structural diversity. In this review, we will firstly introduce five recent and excellent examples of deconstructive and divergent syntheses of natural products (2021-2023). Then, we will summarize our previous work on the deconstructive and divergent synthesis of natural products to demonstrate the high efficiency and simplicity of these two strategies in the field of total synthesis.

Stereoconvergent Direct Ring Expansion of Cyclopropyl Ketones to Cyclopentanones

Recyclization of the ring-opening species of alkyl cyclopropyl ketones to cyclopentanones, which proceeds through an unfavored 5-endo-trig cyclization predicted by Baldwin's rules, is elusive. Herein, as assisted by a strong aryl donor and the Thorpe-Ingold strain on a quaternary cyclopropyl center, stereoconvergent direct ring expansion of cyclopropyl ketones to cyclopentanones promoted by TfOH or BF₃·Et₂O is described, providing a modular construction of polysubstituted cyclopentanones from aldehydes, alkyl methyl ketones, and α-keto esters within three steps.

Acid-catalyzed oxidative cross-coupling of acridans with silyl diazoenolates and a Rh-catalyzed rearrangement: two-step synthesis of γ-(9-acridanylidene)-β-keto esters

A MsOH-catalyzed oxidative cross-coupling of acridans and silyl diazoenolates and a Rh2(OAc)4-catalyzed rearrangement of the resultant diazo products are described. The reactions provide various γ-(9-acridanylidene)-β-keto esters in good yields, which bear an active α-methylene unit for further functionalization.

Synthesis of cis-hydrindan-2,4-diones bearing an all-carbon quaternary center by a Danheiser annulation

A straightforward synthetic entry to functionalized hydrindane compounds based on a rapid assembly of the core nucleus by a Danheiser cycloaddition is reported. Valuable bicyclic building blocks containing the fused five and six-membered carbocyclic ring system can be achieved in only four steps from a simple acyclic β-keto ester.

Cyclopentene Annulations of Alkene Radical Cations with Vinyl Diazo Species Using Photocatalysis

A direct (3+2) cycloaddition between alkenes and vinyl diazo reagents using either Cr or Ru photocatalysis is described. The intermediacy of a radical cation species enables a nucleophilic interception by vinyl diazo compounds, a departure from their traditional electrophilic behavior. A variety of cyclopentenes are synthesized using this method, and experimental insights implicate a direct cycloaddition instead of a cyclopropanation/rearrangement process.

A Stereoselective [3+1] Ring Expansion for the Synthesis of Highly Substituted Methylene Azetidines

The reaction of rhodium-bound carbenes with strained bicyclic methylene aziridines results in a formal [3+1] ring expansion to yield highly substituted methylene azetidines with excellent regio- and stereoselectivity. The reaction appears to proceed through an ylide-type mechanism, where the unique strain and structure of the methylene aziridine promotes a ring-opening/ring-closing cascade that efficiently transfers chirality from substrate to product. The resultant products can be elaborated into new azetidine scaffolds containing vicinal tertiary-quaternary and even quaternary-quaternary stereocenters.

Cycloaddition reactions of enoldiazo compounds

Enoldiazo esters and amides have proven to be versatile reagents for cycloaddition reactions that allow highly efficient construction of various carbocycles and heterocycles. Their versatility is exemplified by (1) [2+n]-cycloadditions (n = 3, 4) by the enol silyl ether units of enoldiazo compounds with retention of the diazo functionality to furnish α-cyclic-α-diazo compounds that are themselves subject to further transformations of the diazo functional group; (2) [3+n]-cycloadditions (n = 1-5) by metallo-enolcarbenes formed by catalytic dinitrogen extrusion from enoldiazo compounds; (3) [2+n]-cycloadditions (n = 3, 4) by donor-acceptor cyclopropenes generated in situ from enoldiazo compounds that produce cyclopropane-fused ring systems. The role of dirhodium(ii) and the emergence of copper(i) catalysts are described, as are the different outcomes of reactions initiated with these catalysts. This comprehensive review on cycloaddition reactions of enoldiazo compounds, with emphasis on methodology development, mechanistic insight, and catalyst-controlled chemodivergence, aims to provide inspiration for future discoveries in the field and to catalyze the application of enoldiazo reagents by the wider synthetic community.