A happy medium: the synthesis of medicinally important medium-sized rings via ring expansion

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

Hydantoin-bridged medium ring scaffolds by migratory insertion of urea-tethered nitrile anions into aromatic C-N bonds

Bicyclic or tricyclic nitrogen-containing heterocyclic scaffolds were constructed rapidly by intramolecular nucleophilic aromatic substitution of metallated nitriles tethered by a urea linkage to a series of electronically unactivated heterocyclic precursors. The substitution reaction constitutes a ring expansion, enabled by the conformationally constrained tether between the nitrile and the heterocycle. Attack of the metallated urea leaving group on the nitrile generates a hydantoin that bridges the polycyclic products. X-ray crystallography reveals ring-dependant strain within the hydantoin.

Visible-light-driven palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade

A visible-light-induced palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade is described. A range of six to nine-membered β-alkenylated cyclic ketones possessing a quaternary carbon center were accessed under mild conditions. Besides styrenes, the electron-rich alkenes such as silyl enol ethers and enamides were also compatible, providing the desired β-alkylated cyclic ketones in moderate to good yields.

Enantioselective Iron-Catalyzed Cross-[4+4]-Cycloaddition of 1,3-Dienes Provides Chiral Cyclooctadienes

Chiral cyclooctadienes are a frequently occurring scaffold in natural products and specialty chemicals, and are used as ligands in asymmetric catalysis. Accessing substituted cyclooctadienes in an efficient asymmetric fashion has been notoriously challenging. We report an iron-catalyzed enantioselective cross-[4+4]-cycloaddition of 1,3-dienes to form substituted cyclooctadienes under very mild conditions. A highly tailored chiral α-diimine iron complex is key for the success of the transformation providing a balanced performance between reactivity, excellent cross-selectivity and very high enantioselectivity. Steric maps of the complexes help accounting for the observed selectivity. The developed method allows rapid and atom-economic access to novel differently functionalized cyclooctadienes in very high yields and enantioselectivities.

Evaluating the Viability of Successive Ring-Expansions Based on Amino Acid and Hydroxyacid Side-Chain Insertion

The outcome of ring-expansion reactions based on amino/hydroxyacid side-chain insertion is strongly dependent on ring size. This manuscript, which builds upon our previous work on Successive Ring Expansion (SuRE) methods, details efforts to better define the scope and limitations of these reactions on lactam and β-ketoester ring systems with respect to ring size and additional functionality. The synthetic results provide clear guidelines as to which substrate classes are more likely to be successful and are supported by computational results, using a density functional theory (DFT) approach. Calculating the relative Gibbs free energies of the three isomeric species that are formed reversibly during ring expansion enables the viability of new synthetic reactions to be correctly predicted in most cases. The new synthetic and computational results are expected to support the design of new lactam- and β-ketoester-based ring-expansion reactions.

Divergent Strategy for Diastereocontrolled Synthesis of Small- and Medium-Ring Architectures

Nitrogen and oxygen medium rings, in particular nine-membered rings, epitomize a unique area of chemical space that occurs in many natural products and biologically appealing compounds. The scarcity of 8- to 12-membered rings among clinically approved drugs is indicative of the difficulties associated with their synthesis, principally owing to the unfavorable entropy and transannular strain. We report here a scandium triflate-catalyzed reaction that allows for a modular access to a diverse collection of nine-membered ring heterocycles in a one-pot cascade and with complete diastereocontrol. This cascade features an intramolecular addition of an acyl group-derived enol to a α,β-unsaturated carbonyl moiety, leading to N- and O-derived medium-ring systems. Computational studies using the density functional theory support the proposed mechanism. Additionally, a one-pot cascade leading to hexacyclic chromeno[3',4':2,3]indolizino[8,7-b]indole architectures, with six fused rings and four contiguous chiral centers, is reported. This novel cascade features many concerted events, including the formation of two azomethine ylides, [3 + 2]-cycloaddition, 1,3-sigmatropic rearrangement, Michael addition, and Pictet-Spengler reaction among others. Phenotypic screening of the resulting oxazonine collection identified chemical probes that regulate mitochondrial membrane potential, adenosine 5'-triphosphate contents, and reactive oxygen species levels in hepatoma cells (Hepa1-6), a promising approach for targeting cancer and metabolic disorders.

Phosphoranyl Radical Fragmentation Reactions Driven by Photoredox Catalysis

Photocatalytic generation of phosphoranyl radicals is fast emerging as an essential method for the generation of diverse and valuable radicals, typically via deoxygenation or desulfurization processes. This Perspective is a comprehensive evaluation of all studies using phosphoranyl radicals as tunable mediators in photoredox catalysis, highlighting how two distinct methods for phosphoranyl radical formation (radical addition and nucleophilic addition) can be used to generate versatile radical intermediates with diverse reactivity profiles.