Sequential Photocatalytic Reactions for the Diastereoselective Synthesis of Cyclobutane Scaffolds

α-Halocarbonyls as a Valuable Functionalized Tertiary Alkyl Source

This review introduces the synthetic organic chemical value of α-bromocarbonyl compounds with tertiary carbons. This α-bromocarbonyl compound with a tertiary carbon has been used primarily only as a radical initiator in atom transfer radical polymerization (ATRP) reactions. However, with the recent development of photo-radical reactions (around 2010), research on the use of α-bromocarbonyl compounds as tertiary alkyl radical precursors became popular (around 2012). As more examples were reported, α-bromocarbonyl compounds were studied not only as radicals but also for their applications in organometallic and ionic reactions. That is, α-bromocarbonyl compounds act as nucleophiles as well as electrophiles. The carbonyl group of α-bromocarbonyl compounds is also attractive because it allows the skeleton to be converted after the reaction, and it is being applied to total synthesis. In our survey until 2022, α-bromocarbonyl compounds can be used to perform a full range of reactions necessary for organic synthesis, including multi-component reactions, cross-coupling, substitution, cyclization, rearrangement, stereospecific reactions, asymmetric reactions. α-Bromocarbonyl compounds have created a new trend in tertiary alkylation, which until then had limited reaction patterns in organic synthesis. This review focuses on how α-bromocarbonyl compounds can be used in synthetic organic chemistry.

Visible-Light-Mediated [2+2] Photocycloadditions of Alkynes

Visible-light-mediated [2+2] photocycloaddition reaction can be considered an ideal solution due to its green and sustainable properties, and is one of the most efficient methods to synthesize four-membered ring motifs. Although research on the [2+2] photocycloaddition of alkynes is challenging because of the diminished reactivity of alkynes, and the more significant ring strain of the products, remarkable achievements have been made in this field. In this article, we highlight the recent advances in visible-light-mediated [2+2] photocycloaddition reactions of alkynes, with focus on the reaction mechanism and the late-stage synthetic applications. Advances in obtaining cyclobutenes, azetines, and oxetene active intermediates continue to be breakthroughs in this fascinating field of research.

Palladium-Catalyzed Divergent Enantioselective Functionalization of Cyclobutenes

Aliphatic strained rings have been increasingly applied in medicinal chemistry due to their beneficial physicochemical and pharmacokinetic properties. However, the divergent synthesis of enantioenriched cyclobutane derivatives with various structural patterns continues to be a significant challenge. Here, we disclose a palladium-catalyzed enantioselective desymmetrization of cyclobutenes, resulting in a series of hydroarylation and 1,2- and 1,3-diarylation products via the interceptions of a common Heck intermediate. Mechanistic investigations provide valuable insights into understanding the catalytic mode of the palladium catalysts and the observed variations in the deuterium-responsive behavior during reactions. Furthermore, the synthetic utility is demonstrated in the syntheses of deuterated drug candidate belaperidone skeletons and pseudosymmetrical truxinic acid-type derivatives.

Construction of cyclobutane-fused tetracyclic skeletons via substrate-dependent EnT-enabled dearomative [2+2] cycloaddition of benzofurans (benzothiophenes)/maleimides

Herein, a novel and facile organic photosensitizer (thioxanthone)-mediated energy-transfer-enabled (EnT-enabled) dearomative [2+2] cycloaddition of aromatic heterocycles/maleimides for green synthesis of cyclobutane-fused polycyclic skeletons is reported. Mechanistic investigations revealed that different EnT pathways by triplet thioxanthone were initiated when different aromatic heterocycles participated in the reaction, giving the corresponding excited intermediates, which underwent the subsequent intermolecular [2+2] cycloaddition to access the desired highly functionalized cyclobutane-fused polycyclic skeletons.

Visible Light-Induced Diastereoselective Construction of Trifluoromethylated Cyclobutane Scaffolds through [2+2]-Photocycloaddition and Water-Assisted Hydrodebromination

A visible light-induced diastereoselective synthesis of trifluoromethylated cyclobutane derivatives is described, consisting of [2+2]-photocycloaddition and water-assisted hydrodebromination by one pot. Quinolinones, isoquinolinones, and coumarins are able to participate in this one-pot process with 1-bromo-1-trifluoromethylethene. In addition, stereodefined trisubstituted trifluoromethylated cyclobutane alcohols, carboxylic acids, and amines can be obtained in a straightforward manner through the ring opening of lactone or lactam without the loss of original high diastereoselectivity given by the water-tristrimethylsilylsilane coordination. The antineoplastic bioactivities of those compounds are also well studied, which exhibit great antineoplastic potential comparable to cisplatin. In the proposed mechanism, thioxanthone (TX) serves as a dual catalyst and a radical chain pathway may be involved in the hydrodebromination process.

Photochemical [2 + 2] Cycloaddition of Alkenes with Maleimides: Highlighting the Differences between N-Alkyl vs N-Aryl Maleimides

Throughout the last 15 years, there has been increased research interest in the use of light promoting organic transformations. [2 + 2] Cycloadditions are usually performed photochemically; however, literature precedent on the reaction between olefins and maleimides is limited to a handful of literature examples, focusing mainly on N-aliphatic maleimides or using metal catalysts for visible-light driven reactions of N-aromatic maleimides. Herein, we identify the differences in reactivity between N-alkyl and N-aryl maleimides. For our optimized protocols, in the case of N-alkyl maleimides, the reaction with alkenes proceeds under 370 nm irradiation in the absence of an external photocatalyst, leading to products in high yields. In the case of N-aryl maleimides, the reaction with olefins requires thioxanthone as the photosensitizer under 440 nm irradiation.

Boryl Radicals Enabled a Three-Step Sequence to Assemble All-Carbon Quaternary Centers from Activated Trichloromethyl Groups

The construction of diversely substituted all-carbon quaternary centers has been a longstanding challenge in organic synthesis. Methods that add three alkyl substituents to a simple C(sp³) atom rely heavily on lengthy multiple processes, which usually involve several preactivation steps. Here, we describe a straightforward three-step sequence that uses a range of readily accessible activated trichloromethyl groups as the carbon source, the three C-Cl bonds of which are selectively functionalized to introduce three alkyl chains. In each step, only a single C-Cl bond was cleaved with the choice of an appropriate Lewis base-boryl radical as the promoter. A vast range of diversely substituted all-carbon quaternary centers could be accessed directly from these activated CCl₃ trichloromethyl groups or by simple derivatizations. The use of different alkene traps in each of the three steps enabled facile collections of a large library of products. The utility of this strategy was demonstrated by the synthesis of variants of two drug molecules, whose structures could be easily modulated by varying the alkene partner in each step. The results of kinetic and computational studies enabled the design of the three-step reaction and provided insights into the reaction mechanisms.

Recent advances in transition-metal-catalyzed directed C-H alkenylation with maleimides

Transition-metal-catalyzed directed C-H alkenylation with maleimides has attracted much attention in recent years, as maleimide core moieties are present in various natural products and pharmaceuticals. In addition, these derivatives can be readily modified into biologically important compounds including succinimides, pyrrolidines and γ-lactams. The efficient chelation-assisted inert C-H bond activation strategy provides straightforward access to a wide array of structurally diverse molecules containing maleimide units. This review describes the major progress and mechanistic investigations on Heck-type reaction/cyclization of maleimides with organic molecules until early 2022.

Application of the Spin-Center Shift in Organic Synthesis

Spin-center shift (SCS) is a radical process involving 1,2-radical translocation along with a two-electron ionic movement, such as elimination of an adjacent leaving group. Such a process was initially observed in some important biochemical transformations, and the unique property has also attracted considerable interest in synthetic chemistry. Experimental, kinetic, as well as computational studies have been performed, and a series of useful radical transformations have been developed and applied in organic synthesis based on SCS processes in the last 20 years. This Perspective is an overview of radical transformations involving the SCS mechanism.