Radical photocyclization route for macrocyclic lactone ring expansion and conversion to macrocyclic lactams and ketones

Organic Photoredox Reactions in Two-Molecule Photoredox System

Using our recent relevant results, this account shows the featured reactivities of two-molecule photoredox systems compared to one-molecule photoredox systems. The low efficiency of electron transfer processes, such as photoinduced and back-electron transfer, in the two-molecule photoredox system, furnishes unique products through different pathways. The facile replacement of photoredox catalysts with appropriate oxidation/reduction potentials in this system provides valuable insights into photoredox reactions.

Palladium-Catalyzed Remote Hydro-Oxygenation of Internal Alkenes: An Efficient Access to Primary Alcohols

As a general method for the synthesis of alcohols, the direct oxygenation of alkenes is difficult to afford linear alcohols. Herein, we communicate the remote hydro-oxygenation of alkenes under palladium catalysis, in which both terminal and internal alkenes are suitable to yield the corresponding linear alcohols efficiently. A compatible SelectFluor/silane redox system plays an essential role for the excellent chemo- and regioselectivities. The reaction features a broad substrate scope and excellent functional group compatibility.

Aza-Oxa-Triazole Based Macrocycles with Tunable Properties: Design, Synthesis, and Bioactivity

A modular platform for the synthesis of tunable aza-oxa-based macrocycles was established. Modulations in the backbone and the side-chain functional groups have been rendered to achieve the tunable property. These aza-oxa-based macrocycles can also differ in the number of heteroatoms in the backbone and the ring size of the macrocycles. For the proof of concept, a library of macrocycles was synthesized with various hanging functional groups, different combinations of heteroatoms, and ring sizes in the range of 17–27 atoms and was characterized by NMR and mass spectrometry. In light of the importance of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction and the significance of triazole groups for various applications, we employed the click-reaction-based macrocyclization. The competence of the synthesized macrocycles in various biomedical applications was proven by studying the interactions with the serum albumin proteins; bovine serum albumin and human serum albumin. It was observed that some candidates, based on their hanging functional groups and specific backbone atoms, could interact well with the protein, thus improving the bioactive properties. On the whole, this work is a proof-of-concept to explore the backbone- and side-chain-tunable macrocycle for different properties and applications.

Direct decarboxylative Giese reactions

The quest to find milder and more sustainable methods to generate highly reactive, carbon-centred intermediates has led to a resurgence of interest in radical chemistry. In particular, carboxylic acids are seen as attractive radical precursors due their availability, low cost, diversity, and sustainability. Moreover, the corresponding nucleophilic carbon-radical can be easily accessed through a favourable radical decarboxylation process, extruding CO₂ as a traceless by-product. This review summarizes the recent progress on using carboxylic acids directly as convenient radical precursors for the formation of carbon-carbon bonds via the 1,4-radical conjugate addition (Giese) reaction.

Flow Chemistry under Extreme Conditions: Synthesis of Macrocycles with Musklike Olfactoric Properties

Starting from small cyclic ketones, continuous flow synthesis is used to produce medium-sized rings and macrocycles that are relevant for the fragrance industry. Triperoxides are important intermediates in this process and are pyrolyzed at temperatures above 250 °C. The synthesis is carried out in two continuously operated flow reactors connected by a membrane-operated separator. The practicality of flow chemistry is impressively demonstrated in this work by the use of hazardous reagent mixtures (30% H₂O₂, 65% HNO₃) and the pyrolysis of no less problematic peroxides. All new macrocycles were tested for their olfactory properties in relation to musk.

Phenylene-bridged bis(benzimidazolium) (BBIm2+): a dicationic organic photoredox catalyst

A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm2+) was designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation and dimerisation of carboxylic acids. The catalytic activity of BBIm2+ was higher than that for a monocation analogue, suggesting that the dicationic nature of BBIm2+ plays a key role in these decarboxylative reactions. The rate constant for the decay of the triplet-triplet absorption of the excited BBIm2+ increased with increasing concentration of the carboxylate anion with a saturated dependence, suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the triplet excited state of BBIm2+ and a carboxylate anion.

Biomimetic Macrocyclic Inhibitors of Human Cathepsin D: Structure-Activity Relationship and Binding Mode Analysis

Human cathepsin D (CatD), a pepsin-family aspartic protease, plays an important role in tumor progression and metastasis. Here, we report the development of biomimetic inhibitors of CatD as novel tools for regulation of this therapeutic target. We designed a macrocyclic scaffold to mimic the spatial conformation of the minimal pseudo-dipeptide binding motif of pepstatin A, a microbial oligopeptide inhibitor, in the CatD active site. A library of more than 30 macrocyclic peptidomimetic inhibitors was employed for scaffold optimization, mapping of subsite interactions, and profiling of inhibitor selectivity. Furthermore, we solved high-resolution crystal structures of three macrocyclic inhibitors with low nanomolar or subnanomolar potency in complex with CatD and determined their binding mode using quantum chemical calculations. The study provides a new structural template and functional profile that can be exploited for design of potential chemotherapeutics that specifically inhibit CatD and related aspartic proteases.

Photoinduced Electron Transfer-Promoted Reactions Using Exciplex-Type Organic Photoredox Catalyst Directly Linking Donor and Acceptor Arenes

Directly linked donor and acceptor arenes, such as phenanthrene/naphthalene/biphenyl and 1,3-dicyanobenzene were found to work as photoredox catalysts in the photoreactions of indene, 2,3-dimethyl-2-butene, and 4-methoxyphenylacetic acid. The new stable organic photocatalyst forms an intramolecular exciplex (excited complex) when irradiated in a polar solvent and shows redox catalyst activity, even at low concentrations. To the best of our knowledge, this is the first example of an intramolecular exciplex working as a redox catalyst.

Synthesis of 23-, 25-, 27-, and 29-Membered ( Z)-Selective Unsaturated and Saturated Macrocyclic Lactones from 16- and 17-Membered Macrocyclic Lactones and Bromoalcohols by Wittig Reaction, Yamaguchi Macrolactonization, and Photoinduced Decarboxylative Radical Macrolactonization

A new strategy for the synthesis of 23-, 25-, 27-, and 29-membered ( Z)-selective unsaturated and saturated macrocyclic lactones from commercially available 16- and 17-membered macrocyclic lactones and bromoalcohols by Wittig reaction, Yamaguchi macrolactonization, and photoinduced decarboxylative radical macrolactonization is described. The position of the unsaturated part in the macrocyclic lactones can be controlled by changing the number of carbons in the starting materials. This protocol can provide facile access to the desired large-ring ( Z)-selective unsaturated and saturated macrocyclic lactones from simple starting materials.

Highly selective macrocyclic ring-closing metathesis of terminal olefins in non-chlorinated solvents at low dilution

New ruthenium complexes featuring two unsymmetrical NHCs proved to be highly selective in macrocyclic RCM performed in green solvents at low dilution.

Two Types of Cross-Coupling Reactions between Electron-Rich and Electron-Deficient Alkenes Assisted by Nucleophilic Addition Using an Organic Photoredox Catalyst

Two types of photoreactions between electronically differentiated donor and acceptor alkenes assisted by nucleophilic addition using an organic photoredox catalyst efficiently afforded 1:1 or 2:1 cross-coupling adducts. A variety of alkenes and alcohols were employed in the photoreaction. Control of the reaction pathway (i.e., the formation of the 1:1 or 2:1 adduct) was achieved by varying the concentration of the alcohol used. Detailed mechanistic studies suggested that the organic photoredox catalyst acts as an effective electron mediator to promote the formation of the cross-coupling adducts.

A strategy for generating aryl radicals from arylborates through organic photoredox catalysis: photo-Meerwein type arylation of electron-deficient alkenes

Generation of a variety of aryl radicals from arylboronic acids through metal-free photoredox catalysis under mild conditions.

Metal-Free Photoinduced Decarboxylative Radical Polymerization Using Carboxylic Acids as Benign Radical Initiators: Introduction of Complex Molecules into Polymer Chain Ends

Metal-free photoinduced decarboxylative radical polymerization of aliphatic carboxylic acids with a variety of monomers was found to proceed smoothly to give the corresponding polymers under mild conditions. Complex carboxylic acids such as those of sugars, steroids, and peptides can function as benign radical initiators via decarboxylation and can be incorporated at the polymer chain ends. This synthetic methodology represents a facile introduction of molecules and functionalities to polymers by using commercially available carboxylic acids.

Transition metal-free decarboxylative alkylation reactions

This review summarizes recent advances in the transition metal-free decarboxylative alkylation of carboxylic acids and their derivatives.

Access to bicyclic hydroxamate macrocycles via intramolecular aza-(4 + 3) cyloaddition reactions of aza-oxyallylic cation intermediates

The intramolecular aza-(4 + 3) cycloaddition reactions of in situ generated aza-oxyallylic cations and furans have been reported for the construction of medium sized hydroxamate macrocycles.

Decarboxylative allylation of amino alkanoic acids and esters via dual catalysis

A combination of photoredox and palladium catalysis has been employed to facilitate the room temperature decarboxylative allylation of recalcitrant α-amino and phenylacetic allyl esters. This operationally simple process produces CO₂ as the only byproduct and provides direct access to allylated alkanes. After photochemical oxidation, the carboxylate undergoes radical decarboxylation to site-specifically generate radical intermediates which undergo allylation. A radical dual catalysis mechanism is proposed. Free phenylacetic acids were also allylated utilizing similar reactions conditions.

Photoinduced electron transfer promoted radical ring expansion and cyclization reactions of α-(ω-carboxyalkyl) β-keto esters

Photoinduced electron transfer (PET) promoted decarboxylation of α-(ω-carboxyalkyl) β-keto esters undergoes radical ring expansion and cyclization reactions. This mild and environmentally friendly method can provide one-carbon expanded γ-keto esters and bicyclic alcohols, and the product distribution is strongly dependent on the length of the alkyl chain containing the terminal carboxylate group.