A Modular Strategy for the Synthesis of Macrocycles and Medium-Sized Rings via Cyclization/Ring Expansion Cascade Reactions

Nine-Membered Ketolactams by Oxidative Cyclization Expansion

Considerable progress has been made over decades in synthetic organic chemistry in order to build up molecular complexity, often through the design of advanced catalytic systems. Yet, simply exposing organic molecules to benchmark oxidants in catalyst free conditions can sometimes lead to surprising and highly valuable products. Thus, a synthetic method for accessing rare 9-membered ketolactams is herein described, under mild oxidative conditions. Key 18O and 17O label experiments revealed an unexpected oxygen atom migration event in the ring expansion process. The scope, mechanism, synthetic applications, and 9 to 7 membered ring contraction opportunities are discussed.

Constructions of Biaryl-Bridged Macrocycles Through Phosphine-Ligand-Controlled Switch of Selectivity Between Intra- and Intermolecular Cyclizations

The rigid biaryl-bridged macrocycles serve as core scaffolds in numerous bioactive molecules. However, constructing biaryl-bridged macrocycles smaller than 14-membered ring remains challenging, partially due to the strain present in the cyclic intermediates formed during their synthesis. Here, we report that a Pd catalyst supported by P(tBu)3 effectively promotes the intramolecular ortho C─H arylation of phenols with aryl bromides, leading to the efficient synthesis of both strained 2-hydroxybiaryl-bridged macrocycles and strain-free bimolecular counterparts. The success of this Pd-catalyzed intramolecular cyclization in generating strained macrocycles originates from the dissociation of the P(tBu)3 ligand from the Pd center, which facilitates dual chelation of functional groups of substrates to the Pd center of palladacycle intermediate for alleviating its strain. Additionally, Pd catalysts supported by PPh3 promote intermolecular cyclization of the same substrates, enabling formation of larger macrocycles bridged by two 2-hydroxybiaryl motifs. These two distinct cyclization modes illustrate how phosphine ligands control the switch of chemoselectivity in Pd-catalyzed cyclizations.

Synthesis and Elaboration of Medium-Sized Ring Building Blocks Prepared via Cascade Ring Expansion Reactions

A general approach is described for the synthesis and elaboration of medium-sized ring mono- and difunctionalized 8- or 9-membered ring lactone building blocks. The lactones are prepared via cascade ring expansion reactions and elaborated via Suzuki-Miyaura cross coupling and various N-functionalization reactions. This enables efficient access to diverse, medium-sized ring building blocks in a synthetically challenging and under-represented area of the pharmaceutical chemical space.

Synthesis of All Ring Sizes of Medium-Sized Heterocycles Bridged Biaryls via VQM-Enabled Diversity-Oriented Synthetic Strategy

Herein, for the first time, the controllable, accurate, and diverse synthesis of all ring sizes of medium-sized (8- to 11-membered) indole-derived bridged biaryls has been realized by using ingeniously designed o-alkynylnaphthols that feature cyclic amines with adjustable ring sizes. The transformation may proceed through a DBN-mediated in-situ generation of vinylidene ortho-quinone methides/indole-ring formation/ring expansion cascade sequence, which is characterized by acceptable to excellent yields and good functional group tolerance.

C-C Cleavage/Cross-Coupling Approach for the Modular Synthesis of Medium-to-Large Sized Rings: Total Synthesis of Resorcylic Acid Lactone Natural Products

The chemical synthesis of medium (8-11 membered) and large sized (≥12 membered) cyclic systems is often challenging. The introduction of transannular strain and loss of degrees of freedom in forming macrocycles often result in poor reaction kinetics and thermodynamics (i.e., thermodynamically disfavored at equilibrium). To address these challenges, we herein report a strategy for the synthesis of medium-to-large sized rings, which leverages strain-release and metal templating through a palladium-mediated C-C cleavage/cross-coupling. By means of DOSY NMR techniques, we identified an undesired competing β-hydrogen elimination pathway, which was substrate dependent. Using a streamlined synthesis of the requisite precursors, our method enables the rapid generation of complex medium-to-large sized rings in a modular fashion through a C(sp2)-C(sp3) macrocyclization. The transformation enabled the short total synthesis of various resorcylic acid lactone (RAL) natural products and unnatural analogues of late-stage intermediates. A mechanistic proposal for the macrocyclization is supported by computational studies of the reaction using density functional theory.

Stereoselective synthesis of atropisomeric amides enabled by intramolecular acyl transfer

C-N atropisomeric amides are important compounds in medicinal chemistry and agrochemistry. Atropselective methods for their synthesis are therefore important. In this study, a novel strategy to make C-N atropisomeric amides based on intramolecular acyl transfer via a tethered Lewis basic pyridine or tertiary amine group is reported. The reactions operate under kinetic control and in most cases are highly atropselective, with the products isolable as pure, single diastereoisomers following chromatography. The kinetically favored atropisomer can also be isomerised into the alternative thermodynamically favored atropisomer upon heating. The kinetic and thermodynamic outcomes are supported by computational studies, while additional mechanistic studies support operation via initial fast acylation of the Lewis basic group, followed by rate-determining acyl transfer, which also enables control over the atropisomer formed.

Enzymatic ester bond formation strategies in fungal macrolide skeletons

Covering: up to August 2024Macrolides, the core skeletons of numerous marketed drugs and bioactive natural products, have garnered considerable scientific interest owing to their structural diversity and broad spectrum of pharmaceutical activities. The formation of intramolecular ester bonds is a critical biocatalytic step in constructing macrolide skeletons. Here, we summarised enzymatic ester bond formation strategies in fungal polyketide (PK)-type, nonribosomal peptide (NRP)-type, and PK-NRP hybrid-type macrolides. In PK-type macrolides, ester bond formation is commonly catalysed by a trans-acting thioesterase (TE) or a cis-acting TE domain during the product release process. In NRP-type and PK-NRP hybrid-type macrolides, the ester bond is usually introduced through condensation (C) domain-catalysed esterification during the elongation or product release step. Although the TE and C domains share similarities in their catalytic mechanism, using hydroxyl groups as nucleophiles in an intramolecular nucleophilic attack, they differ in terms of the hydroxyl origin, the timing of ester bond formation, and domain location. Furthermore, some TE domains are utilized as chemoenzymatic catalysts to construct macrolides with different ring sizes. A comparison of ester bond formation between fungi and bacteria is also discussed. Exploring the biosynthetic pathways of fungal macrolides, elucidating the diverse strategies employed in the formation of ester bonds, and understanding the application of enzymes/domains in chemoenzymatic synthesis hold promise for the discovery of new bioactive macrolides in the future.

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Expanding the scope of the successive ring expansion strategy for macrocycle and medium-sized ring synthesis: unreactive and reactive lactams

New methods are described that expand the scope of the Successive Ring Expansion (SuRE) with respect to synthetically challenging lactams. A protocol has been developed for use with 'unreactive' lactams, enabling SuRE reactions to be performed on subsrates that fail under previously established conditions. Ring expansion is also demonstarted on 'reactive' lactams derived from iminosugars for the first time. The new SuRE methods were used to prepare a diverse array of medium-sized and macrocyclic lactams and lactones, which were evaluted in an anti-bacterial assay against E. coli BW25113WT.