Synthesis of polysubstituted cyclobutanes through a photoredox strain-release/[3,3]-rearrangement cascade

Small saturated carbocycles, such as cyclobutanes, with elevated three-dimensionality and rich Csp3 centers are privileged scaffolds in naturally occurring molecules and drug discovery. It remains highly desirable and challenging to develop modular and straightforward strategies to craft densely substituted cyclobutanes. Herein, a photoredox-catalyzed radical strain-release/[3,3]-rearrangement cascade (SRRC) strategy for efficient synthesis of polysubstituted cyclobutanes is disclosed. This protocol operates with readily available α-silylamines as radical precursors, and strained bicyclo[1.1.0]butanes (BCBs) and cyclobutenes as radical acceptors, to access an array of structurally diverse 1,1,3- and 1,1,2-trisubstituted cyclobutanes containing a unique non-natural amino acid scaffold. Mechanistic studies reveal the pivotal reactivity of the silylketene acetal intermediate and the origin of diastereoselectivity. The power and utility of this method are illustrated with diverse transformations and preliminary anticancer assessment.

Photoredox cobalt-catalyzed asymmetric desymmetric reductive coupling of cyclobutenes with alkynes

Catalytic methods to couple alkynes and alkenes are highly valuable in synthetic chemistry. The cobalt-catalyzed intermolecular reductive coupling of alkenes and alkynes is particularly attractive due to the unique reactivity and cost-effectiveness of cobalt catalysts. However, the enantioselective transformations of this kind are less developed. The limited successful enantioselective examples are restricted to the use of electronically biased activated olefins as the coupling partners. Herein, we report an asymmetric desymmetric reductive coupling of electronically unbiased succinimide-containing cyclobutenes with alkynes to synthesize enantioenriched, synthetically important vinyl cyclobutanes via photoredox and cobalt dual catalysis. Excellent enantioselectivities, good diastereoselectivities and regioselectivities are obtained. Preliminary mechanistic studies suggest that Hantzsch ester is a better reducing reagent when used in combination with Et3N. Density functional theory calculations reveal that the reaction proceeds more likely through a Co(III)-H migratory insertion mechanism.

Photoredox/Cobalt-Catalyzed Chemo-, Regio-, Diastereo- and Enantioselective Reductive Coupling of 1,1-Disubstituted Allenes and Cyclobutenes

A dual photoredox/cobalt-catalyzed protocol for chemo-, regio-, diastereo- and enantioselective reductive coupling of 1,1-disubstituted allenes and cyclobutenes through chemo-, regio-, diastereo- and enantioselective oxidative cyclization followed by stereoselective protonation promoted by a chiral phosphine-cobalt complex is presented. Such process represents an unprecedented reaction pathway for cobalt catalysis that enables selective transformation of the less sterically congested alkenes of 1,1-disubstituted allenes with cyclobutenes, incorporating a broad scope of tetrasubstituted alkenes into the cyclobutane scaffolds in up to 86 % yield, >98 : 2 chemo- and regioselectivity, >98 : 2 dr and >99.5:0.5 er. Functionalization delivered a variety of enantioenriched cyclobutanes that are otherwise difficult to access. Preliminary mechanistic studies revealed that the reactions proceeded through oxidative cyclization followed by protonation and protonation might be the rate-determining step.

Access to Alkenyl Cyclobutanols by Ni-Catalyzed Regio- and Enantio-Selective syn-Hydrometalative 4-exo-trig Cyclization of Alkynones

Enantioselective synthesis of (spiro)cyclobutane derivatives poses significant challenges yet holds promising applications for both synthetic and medicinal chemistry. We report here a nickel-catalyzed asymmetric syn-hydrometalative 4-exo-trig cyclization of 1,4-alkynones to synthesize alkenyl cyclobutanols with a tetrasubstituted stereocenter. This strategy features a broad substrate scope, delivering a variety of trifluoromethyl-containing rigid (spiro)carbocycle skeletons in good yields with high enantioselectivities (up to 84 % yield and 98.5 : 1.5 er). The synthetic utility is demonstrated through stereospecific transformations into fused spiro molecules. Experimental and computational mechanistic studies indicate that the reaction is initiated by an active Ni-H species, with carbonyl-directed hydrometalation as the key for regioselective control. This catalytic method provides a general solution for regioselective hydrofunctionalization of alkynes and represents an efficient reaction pattern for assembling highly strained enantioenriched bioisosteres.

Recent advances in catalytic enantioselective carbometallation of cyclopropenes and cyclobutenes

Enantioenriched small carbocycles are key structures in numerous natural products and pharmaceutically important molecules as well as vital intermediates in organic synthesis. Although various catalytic approaches for the construction of such molecules from acyclic precursors have been developed, direct enantioselective functionalization of preformed three-membered and four-membered rings represents the most straightforward and modular strategy, enabling rapid and diversified synthesis of enantioenriched cyclopropanes and cyclobutanes from a single set of starting materials without the need for the incorporation of specific functional groups. In this Feature Article, we have summarized the recent advances in catalytic enantioselective functionalization of cyclopropenes and cyclobutenes through carbometallation. The plausible mechanisms of such reactions and future of this field are also discussed.

A comprehensive review on the enantiomeric separation of chiral drugs using metal-organic frameworks

Chiral drugs play an important role in modern medicine, but obtaining pure enantiomers from racemic mixtures can pose challenges. When a drug is chiral, only one enantiomer (eutomer) typically exhibits the desired pharmacological activity, while the other (distomer) may be biologically inactive or even toxic. Racemic drug formulations introduce additional health risks, as the body must still process the inactive or detrimental enantiomer. Some distomers have also been linked to teratogenic effects and unwanted side effects. Therefore, developing efficient and scalable methods for separating chiral drugs into their pure enantiomers is critically important for improving patient safety and outcomes. Metal-organic frameworks (MOFs) show promise as novel materials for chiral separation due to their highly tunable structures and interactions. This review summarizes recent advancements in using MOFs for chromatographic and spectroscopic resolution of drug enantiomers. Both the opportunities and limitations of MOF-based separation techniques are discussed. A thorough understanding of these methods could aid the continued development of pure enantiomer formulations and help reduce health risks posed by racemic drug mixtures.

Palladium-Catalyzed Regio- and Diastereoselective Hydro(hetero)arylation for Rapid Construction of Quaternary Center Containing Cyclobutanes

Herein we report a Pd-catalyzed regio- and diastereoselective hydro(hetero)arylation of inactivated alkylidenecyclobutanes. This protocol provides a rapid and atom-economical route to access 3-cyclobutyl (hetero)arenes with good functionalities toleration. With the assistance of the directing group, nucleophilic attack happened on the bulkier γ-position to form the quaternary carbon center. Furthermore, the selected products exhibited antitumor bioactivities.

Axis-Unfixed Biphenylphosphine-Oxazoline Ligands: Design and Applications in Asymmetric Catalytic Reactions

The design and synthesis of chiral ligands plays an important role in asymmetric catalytic reactions. Over the past decades, various types of chiral phosphine-oxazolines (PHOX ligands) have been developed and have greatly advanced the field of asymmetric catalysis. Novel chiral PHOX ligand with an axis-unfixed biphenyl backbone, developed by our group, have shown interesting coordination behavior and excellent chiral inducing ability in various transition-metal-catalyzed asymmetric reactions. This personal account focuses on our developed axis-unfixed biphenylphosphine-oxazoline ligand (BiphPHOX), including an overview of its design and applications, which will provide inspiration for the exploration of novel ligands and related reactions.

Palladium-Catalyzed Trans-Selective Synthesis of Spirocyclic Cyclobutanes Using α,α-Dialkylcrotyl- and Allylhydrazones

Herein, we demonstrate the use of E/Z mixtures of α,α-disubstituted crotylhydrazones to obtain spirocyclic vinylcyclobutanes in a diastereoselective fashion. We show 24 examples of a 1,1-insertion/4-exo-trig tandem process to produce these motifs. Additionally, spirocyclic alkylidene cyclobutanes can be obtained by using α,α-disubstituted allylated hydrazones (11 examples). In this study, we show that the aryl migrating group has a dramatic impact on the course of the reaction. Specifically, allylic C-H insertion products can be obtained in good yields using bromoenones as reaction partners. When Pd(0) is used with no aryl or alkenyl bromide, an intramolecular cyclopropanation reaction takes place to afford [2.1.0]-bicycles.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

Chiral Diene Ligands in Asymmetric Catalysis

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.

Catalytic enantioselective synthesis of benzocyclobutenols and cyclobutanols via a sequential reduction/C-H functionalization

We report here a sequential enantioselective reduction/C-H functionalization to install contiguous stereogenic carbon centers of benzocyclobutenols and cyclobutanols. This strategy features a practical enantioselective reduction of a ketone and a diastereospecific iridium-catalyzed C-H silylation. Further transformations have been explored, including controllable regioselective ring-opening reactions. In addition, this strategy has been utilized for the synthesis of three natural products, phyllostoxin (proposed structure), grandisol and fragranol.

A catalytic asymmetric cross-coupling approach to the synthesis of cyclobutanes

Stereodefined four-membered rings are common motifs in bioactive molecules and versatile intermediates in organic synthesis. However, the synthesis of complex, chiral cyclobutanes is a largely unsolved problem and there is a need for general and modular synthetic methods. Here we report a series of asymmetric cross-coupling reactions between cyclobutenes and arylboronic acids which are initiated by Rh-catalysed asymmetric carbometallation. After the initial carborhodation, Rh-cyclobutyl intermediates undergo chain-walking or C-H insertion so that overall a variety of additions such as reductive Heck reactions, 1,5-addition and homoallylic substitution are observed. The synthetic applicability of these highly stereoselective transformations is demonstrated in the concise syntheses of the drug candidates Belaperidone and PF-04862853. We anticipate this approach will be widely adopted by synthetic and medicinal chemists. While the carbometallation approach reported here is exemplified with Rh and arylboronic acids, it is likely to be applicable to other metals and nucleophiles.

Enantioselective Functionalization of Prochiral Cyclobutanones and Cyclobutenones

Enantioselective synthesis of cyclobutane derivatives is still a challenging topic in asymmetric synthesis. [2+2] Cycloaddition and skeleton rearrangement are two primary strategies to this end. Recently, functionalization of cyclobutanones and cyclobutenones, which are readily available via [2+2] cycloadditions as prochiral substrates, has emerged as a powerful tool to access versatile four-membered ring compounds. Herein, we summarize some recent advances in these areas from our and other groups.

1 Introduction

2 Enantioselective Functionalization of Cyclobutanones

2.1 Chiral Lithium Amide Approach

2.2 Enamine Approach

3 Enantioselective Functionalization of Cyclobutenones

4 Conclusion

Chiral 3-Acylglutaric Acid Derivatives from Strain-Induced Nucleophilic Retro-Claisen Ring-Opening Reactions

A nucleophilic retro-Claisen ring-opening of donor-acceptor cyclobutenes, formed with high stereocontrol by [3 + 1]-cycloaddition of TIPS-protected enoldiazoacetates with α-acyl sulfur ylides, has been developed. Removal of the TIPS group to form the isolable β-keto ester precedes the strain-induced ring-opening. Various amines, alcohols, thiols, and amino acid derivatives are effective nucleophiles, and their products are formed in very high yields via stoichiometric reactions. The chirality of the reactant donor-acceptor cyclobutenes is fully retained in the ring-opening reactions. The 3-acylglutaric acid products are converted to various valuable structures, including amido-diols, γ-aminobutyric acid (GABA) derivatives, and heterocycles.

Resolution of Racemic Mixtures by Phase Transition of PEGylated Resolving Agents

A novel, efficient, and simple method for the resolution of racemic mixtures is presented in which PEGylated resolving agents are subjected to diastereomeric complex formation in alcohols. The resulting complexes then undergo temperature-assisted phase transition, affording a precipitate that is enriched in one enantiomer and separable by filtration. In an aqueous solution, phase transition can be caused by the methods used in the precipitation of poly(ethylene glycol) (e.g., addition of ammonium sulfate). A number of racemic amines have been successfully resolved using this method. The first cycle of resolution affords the amines with an optical purity of 72-85% from their corresponding racemic mixture in good yields (78-90%). An additional cycle improved the optical purity to 87-95%. The PEGylated resolving agents can be recovered and reutilized without the loss of resolution efficiency.

Enantioselective Synthesis of Cyclobutenes by Intermolecular [2+2] Cycloaddition with Non-C2 Symmetric Digold Catalysts

The enantioselective intermolecular gold(I)-catalyzed [2+2] cycloaddition of terminal alkynes and alkenes has been achieved using non-C2-chiral Josiphos digold(I) complexes as catalysts, by the formation of the monocationic complex. This new approach has been applied to the enantioselective total synthesis of rumphellaone A.

Cyclobutene vs 1,3-Diene Formation in the Gold-Catalyzed Reaction of Alkynes with Alkenes: The Complete Mechanistic Picture

The intermolecular gold(I)-catalyzed reaction between arylalkynes and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which takes place stepwise, first by formation of cyclopropyl gold(I) carbenes, followed by a ring expansion. However, 1,3-butadienes are also formed in the case of ortho-substituted arylalkynes by a metathesis-type process. The corresponding reaction of alkenes with aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively to cyclobutenes. A comprehensive mechanism for the gold(I)-catalyzed reaction of alkynes with alkenes is proposed on the basis of density functional theory calculations, which shows that the two pathways leading to cyclobutenes or dienes are very close in energy. The key intermediates are cyclopropyl gold(I) carbenes, which have been independently generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1H-cyclopropa[a]naphthalenes.

A Mixed-Ligand Chiral Rhodium(II) Catalyst Enables the Enantioselective Total Synthesis of Piperarborenine B

A novel, mixed-ligand chiral rhodium(II) catalyst, Rh2(S-NTTL)3(dCPA), has enabled the first enantioselective total synthesis of the natural product piperarborenine B. A crystal structure of Rh2(S-NTTL)3(dCPA) reveals a "chiral crown" conformation with a bulky dicyclohexylphenyl acetate ligand and three N-naphthalimido groups oriented on the same face of the catalyst. The natural product was prepared on large scale using rhodium-catalyzed bicyclobutanation/ copper-catalyzed homoconjugate addition chemistry in the key step. The route proceeds in ten steps with an 8% overall yield and 92% ee.