A versatile and stereoselective synthesis of functionalized cyclobutenes

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.

Ni-Catalyzed Stereoconvergent Reductive Dimerization of Bromocyclobutenes

A nickel-catalyzed reductive dimerization of bromocyclobutenes to produce unusual and unprecedented cyclobutene dimers was developed. In a stereoconvergent procedure, various bromocyclobutenes were readily dimerized in good yields, with good diastereoselectivities and broad functional group tolerance. Notably, the presence of a carbonyl group in the starting material appears to dictate diastereoselectivity.

Photochemical α-selective radical ring-opening reactions of 1,3-disubstituted acyl bicyclobutanes with alkyl halides: modular access to functionalized cyclobutenes

Although ring-opening reactions of bicyclobutanes bearing electron-withdrawing groups, typically with β-selectivity, have evolved as a powerful platform for synthesis of cyclobutanes, their application in the synthesis of cyclobutenes remains underdeveloped. Here, a novel visible light induced α-selective radical ring-opening reaction of 1,3-disubstituted acyl bicyclobutanes with alkyl radical precursors for the synthesis of functionalized cyclobutenes is described. In particular, primary, secondary, and tertiary alkyl halides are all suitable substrates for this photocatalytic transformation, providing ready access to cyclobutenes with a single all-carbon quaternary center, or with two contiguous centers under mild reaction conditions.

Concise Synthesis of (±)-Myrioneurinol Enabled by Sequential [2+2] Cycloaddition/Retro-Mannich Fragmentation/Mannich Reaction

A concise total synthesis of (±)-myrioneurinol has been achieved in 14 steps. An efficient AgSbF₆ /t-BuCl-catalyzed intramolecular [2+2] cycloaddition reaction of the alkynone-tethered enamine was developed to prepare the highly strained cyclobutene. It was used in combination with a subsequent retro-Mannich fragmentation/Mannich reaction to efficiently construct the tricyclic core of myrioneurinol.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Advancing the Logic of Chemical Synthesis: C-H Activation as Strategic and Tactical Disconnections for C-C Bond Construction

The design of synthetic routes by retrosynthetic logic is decisively influenced by the transformations available. Transition-metal-catalyzed C-H activation has emerged as a powerful strategy for C-C bond formation, with myriad methods developed for diverse substrates and coupling partners. However, its uptake in total synthesis has been tepid, partially due to their apparent synthetic intractability, as well as a lack of comprehensive guidelines for implementation. This Review addresses these issues and offers a guide to identify retrosynthetic opportunities to generate C-C bonds by C-H activation processes. By comparing total syntheses accomplished using traditional approaches and recent C-H activation methods, this Review demonstrates how C-H activation enabled C-C bond construction has led to more efficient retrosynthetic strategies, as well as the execution of previously unattainable tactical maneuvers. Finally, shortcomings of existing processes are highlighted; this Review illustrates how some highlighted total syntheses can be further economized by adopting next-generation ligand-enabled approaches.

Scalability of photochemical reactions in continuous flow mode

Continuous flow photochemistry as a field has witnessed an increasing popularity over the last decade in both academia and industry. Key drivers for this development are safety, practicality as well as the ability to rapidly access complex chemical structures. Continuous flow reactors, whether home-built or from commercial suppliers, additionally allow for creating valuable target compounds in a reproducible and automatable manner. Recent years have furthermore seen the advent of new energy efficient LED lamps that in combination with innovative reactor designs provide a powerful means to increasing both the practicality and productivity of modern photochemical flow reactors. In this review article we wish to highlight key achievements pertaining to the scalability of such continuous photochemical processes.

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Interlaboratory Comparison of Untargeted Mass Spectrometry Data Uncovers Underlying Causes for Variability

Despite the value of mass spectrometry in modern natural products discovery workflows, it remains very difficult to compare data sets between laboratories. In this study we compared mass spectrometry data for the same sample set from two different laboratories (quadrupole time-of-flight and quadrupole-Orbitrap) and evaluated the similarity between these two data sets in terms of both mass spectrometry features and their ability to describe the chemical composition of the sample set. Somewhat surprisingly, the two data sets, collected with appropriate controls and replication, had very low feature overlap (25.7% of Laboratory A features overlapping 21.8% of Laboratory B features). Our data clearly demonstrate that differences in fragmentation, charge state, and adduct formation in the ionization source are a major underlying cause for these differences. Consistent with other recent literature, these findings challenge the conventional wisdom that electrospray ionization mass spectrometry (ESI-MS) yields a simple one-to-one correspondence between analytes in solution and features in the data set. Importantly, despite low overlap in feature lists, principal component analysis (PCA) generated qualitatively similar PCA plots. Overall, our findings demonstrate that comparing untargeted metabolomics data between laboratories is challenging, but that data sets with low feature overlap can yield the same qualitative description of a sample set using PCA.

Uncommon Four-Membered Building Blocks - Cyclobutenes, Azetines and Thietes

Strained ring systems have gained considerable importance over the last few years for their implication in natural product syntheses or in drug discovery programs. We present herein a recollection of our work on the construction and functionalization of unsaturated four-membered carbo- and heterocycles in the context of the literature, as well as their applications in further reactions.

Access to substituted cyclobutenes by tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition of dipropargylphosphonates under Ag/Co relay catalysis

We present herein an unconventional tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition of simple dipropargylphosphonates to deliver a range of bicyclic polysubstituted cyclobutenes and cyclobutanes under Ag/Co relay catalysis. An interesting switch from allene–allene to allene–alkyne cycloaddition was observed based on the substitution of the substrates, which further diversified the range of compounds accessible from this practical method. Significantly, preliminary biological screening of these new compounds identified promising candidates as suppressors of cellular proliferation.

In situ generation of allenes through [3,3]-sigmatropic rearrangement of propargylphosphonates. Divergent allene–allene or allene–alkyne cycloaddition by Ag/Co relay catalysis. Products as promising suppressors of cellular proliferation.

Recent advances in the total synthesis of cyclobutane-containing natural products

Recent developments of strategies on the construction of cyclobutanes and their application in complex natural product synthesis are discussed.

Catalytic Enantioselective Synthesis of Cyclobutenes from Alkynes and Alkenyl Derivatives

Discovery of enantioselective catalytic reactions for the preparation of chiral compounds from readily available precursors, using scalable and environmentally benign chemistry, can greatly impact their design, synthesis, and eventually manufacture on scale. Functionalized cyclobutanes and cyclobutenes are important structural motifs seen in many bioactive natural products and pharmaceutically relevant small molecules. They are also useful precursors for other classes of organic compounds such as other cycloalkane derivatives, heterocyclic compounds, stereodefined 1,3-dienes, and ligands for catalytic asymmetric synthesis. The simplest approach to make cyclobutenes is through an enantioselective [2 + 2]-cycloaddition between an alkyne and an alkenyl derivative, a reaction which has a long history. Yet known reactions of this class that give acceptable enantioselectivities are of very narrow scope and are strictly limited to activated alkynes and highly reactive alkenes. Here, we disclose a broadly applicable enantioselective [2 + 2]-cycloaddition between wide variety of alkynes and alkenyl derivatives, two of the most abundant classes of organic precursors. The key cycloaddition reaction employs catalysts derived from readily synthesized ligands and an earth-abundant metal, cobalt. Over 50 different cyclobutenes with enantioselectivities in the range of 86-97% ee are documented. With the diverse functional groups present in these compounds, further diastereoselective transformations are easily envisaged for synthesis of highly functionalized cyclobutanes and cyclobutenes. Some of the novel observations made during these studies including a key role of a cationic Co(I)-intermediate, ligand and counterion effects on the reactions, can be expected to have broad implications in homogeneous catalysis beyond the highly valuable synthetic intermediates that are accessible by this route.

Identifying Amidyl Radicals for Intermolecular C-H Functionalizations

Recent studies have demonstrated the capabilities of amidyl radicals to facilitate a range of intermolecular functionalizations of unactivated, aliphatic C-H bonds. Relatively little information is known regarding the important structural and electronic features of amidyl and related radicals that impart efficient reactivity. Herein, we evaluate a diverse range of nitrogen-centered radicals in unactivated, aliphatic C-H chlorinations. These studies establish the salient features of nitrogen-centered radicals critical to these reactions in order to expedite the future development of new site-selective, intermolecular C-H functionalizations.

An Efficient Approach to Regio- and Stereodefined Fully-Substituted Alkenylsilanes by Pd-Catalyzed Allenic C(sp3 )-H Oxidation

A highly efficient palladium-catalyzed functionalization of allenylsilanes to give regio- and stereodefined fully-substituted alkenylsilanes has been developed. This oxidative coupling reaction showed good functional group compatibility with exclusive regio- and stereoselectivity. The pending olefin on the silyl group was shown to be an indispensable element for the initial allenic C(sp³ )-H bond cleavage, and performs as the directing group to control the overall selectivity. The addition of substoichiometric amounts of Et₃ N was found to increase the reaction rate leading to a higher reaction yield. The reaction can be easily scaled up and applied for the late-stage functionalization of natural products and pharmaceutical compounds, including amino acids and steroid derivatives. The newly introduced functional groups include aryl, alkynyl, and boryl groups. The highly strained four-membered ring, silacyclobutene was obtained when B₂ pin₂ was employed as the coupling partner. Mechanistic studies, including kinetic isotope effects, showed that the allenic C(sp³ )-H bond cleavage is the rate-limiting step.

Towards a Scalable Synthesis of 2-Oxabicyclo[2.2.0]hex-5-en-3-one Using Flow Photochemistry

Cyclobutene lactones hold great potential as synthetic building blocks, yet their preparation by photochemical rearrangement in batch can often be a bottleneck in synthetic studies. We report the use of flow photochemistry as a tool to enable a higher-throughput approach to the synthesis of 2-oxabicyclo[2.2.0]hex-5-en-3-one, which reduces reaction times from 24 h to 10 min. Accordingly, a significantly improved throughput of 144 mg/h (vs 14-21 mg/h in batch) was achieved. Scale-out experiments showed problematic reactor fouling and steps were taken to explore and minimize this effect.

On the Stability of Disubstituted Cyclobutenes - A Computational Study

A computational study of the electrocyclic ring-opening of 2-substituted cyclobutenecarboxylic acids is presented. Detailed calculations suggest a model to predict whether the product of nucleophilic alkylation of a bicyclic lactone electrophile will be a cyclobutenecarboxylic acid or its dienoic acid isomer, based on the used nucleophile.

Harnessing the Polarizability of Conjugated Alkynes toward [2 + 2] Cycloaddition, Alkenylation, and Ring Expansion of Indoles

Reported is the utilization of electronically biased conjugated alkynes in the development of highly diastereo- and regioselective dearomative [2 + 2] cycloadditions, alkenylations, and ring expansions of electron-rich indoles. Regioselective protonations of cross- and linear-conjugated alkynes were found to be crucial for accessing various cyclobutene-fused indoline and alkenylated indole derivatives. Furthermore, the facile ring expansion of [2 + 2] keto adducts, which were successfully synthesized from ynones, provided 1 H-benzo[ b]azepine scaffolds.

Thiazolium salt-catalyzed [3 + 2 + 1] cyclization for the synthesis of trisubstituted 2-pyrones using arylglyoxals as a carbonyl source

A new thiazolium salt-catalyzed [3 + 2 + 1] cyclization of acetylenedicarboxylates with arylglyoxals has been developed, enabling organocatalytic umpolung to access trisubstituted 2-pyrones with good yields via C–C bond cleavage.

Catalytic approaches to assemble cyclobutane motifs in natural product synthesis

New strategies based on transition-metal catalysis or organocatalysis have provided new perspectives into the total synthesis of cyclobutane-containing natural products.

Expeditious synthesis of polyacetylenic water hemlock toxins and their effects on the major GABAA receptor isoform

Classical synthetic approaches to highly unsaturated polyene/yne natural products rely on iterative cross-coupling of linear fragments.

Lewis Acid-Catalyzed Selective [2 + 2]-Cycloaddition and Dearomatizing Cascade Reaction of Aryl Alkynes with Acrylates

Combined Lewis acid, consisting of two or more Lewis acids, sometimes shows unique catalytic ability, and it may promote reactions which could not be catalyzed by any of the Lewis acids solely. On the other hand, the development of efficient methods for the facile synthesis of cyclobutenes and densely functionalized decalins is an attractive target for synthetic chemists due to their versatile synthetic utilities and widespread occurrence in natural products. Herein, we wish to report an efficient method for the assembly of cyclobutenes and densely functionalized decalin skeletons through In(tfacac)₃-TMSBr catalyzed selective [2 + 2]-cycloaddition and dearomatizing cascade reaction of aryl alkynes with acrylates with high chemo- and stereoselectivity. The obtained cyclobutene could be easily converted into cyclobutane as well as synthetically useful 1,4- and 1,5-diketones with high chemo- and stereoselectivity. On the basis of mechanistic studies, plausible reaction mechanisms were proposed for both the [2 + 2]-cycloaddition and the dearomatizing cascade reaction. Finally, the computational studies of reaction mechanisms were conducted, and the results suggest that the combined Lewis acid could efficiently promote both reactions.

Zirconocene catalyzed diastereoselective carbometalation of cyclobutenes

The regio- and diastereoselective zirconocene-catalyzed carbomagnesiation of cyclobutenes is herein reported to afford configurationally stable cyclobutylmagnesium species that could subsequently react with a large variety of electrophiles to give polysubstituted cyclobutane species as a single diastereoisomer.

Cyclobutenes: At a Crossroad between Diastereoselective Syntheses of Dienes and Unique Palladium-Catalyzed Asymmetric Allylic Substitutions

The rich chemistry of cyclobutanes is underpinned by a large body of synthetic literature devoted to their synthesis and decoration. This is motivated by the widespread representation of cyclobutane moieties in biologically active natural products and man-made molecules. Surprisingly, this vast array of knowledge finds no parallel in the chemistry of cyclobutenes, their unsaturated analogues. In particular, a dearth of methods to synthesize enantioenriched cyclobutenes is apparent upon cursory investigation of the literature. As a leading example, the photocycloaddition of maleic anhydride to acetylene or dichloroethylene, probably a benchmark of cyclobutene synthesis, delivers a meso cyclic anhydride which can be further converted to a cyclobutene product by enantioselective desymmetrization by ring opening. Nonetheless, such an approach delivers products with a rather inflexible substitution pattern around the four-membered ring. The lack of general approaches has motivated our group and others to develop novel routes to cyclobutene scaffolds, leading to the development of a strategy that combines photochemistry and catalysis. Indeed, we have coupled the simple and efficient photochemical isomerization of 2-pyrone into a strained bicyclo[2.2.0] lactone with palladium-catalyzed allylic alkylation as a simple and versatile access to functionalized cyclobutenes. Several nucleophiles can be added to the activated, strained intermediate, including malonate anions and azlactones. The products are mono- and bicyclic building blocks richly decorated with functional groups. Importantly, they are formed with high levels of diastereoselectivity as expected by the tenets of palladium-catalyzed allylic alkylation, which posit that the oxidative addition and nucleophilic capture steps proceed with inversion of configuration, resulting in overall retention (inversion + inversion). However, the transposition of the methodology to an asymmetric version subsequently led to the surprising discovery of a family of highly enantioselective, diastereodivergent catalytic processes. Indeed, we observed a ligand-dependent stereochemical outcome for a range of palladium-catalyzed allylic alkylations affording either overall retention or overall inversion of configuration, and that with very high levels of enantio- and diastereoselectivity. The new family of diastereodivergent reactions enables the conversion of the aforementioned racemic bicyclo[2.2.0] lactone into each of 4 stereoisomeric products, at will. Although the mechanistic details at the origin of this unusual stereodivergence are not yet fully elucidated, it became clear through our studies that unique Pd-allyl complexes, residing preferentially as their σ-(monohapto)-bound isomers, are at the heart of the process. The cyclobutenes prepared can also engage in electrocyclic ring-opening reactions (often spontaneous depending on the substitution pattern) that link this chemistry with that of diene and polyene frameworks. Using the strategies laid out above, our group was then able to harness the high stereospecificity of electrocyclic reactions and design modular syntheses of several natural products and natural product fragments. We believe that the methods presented herein shall soon pave the way for the streamlined synthesis of more complex polyenic natural products.

Highly Efficient Cascade Reaction for Selective Formation of Spirocyclobutenes from Dienallenes via Palladium-Catalyzed Oxidative Double Carbocyclization-Carbonylation-Alkynylation

A highly selective cascade reaction that allows the direct transformation of dienallenes to spirocyclobutenes (spiro[3.4]octenes) as single diastereoisomers has been developed. The reaction involves formation of overall four C–C bonds and proceeds via a palladium-catalyzed oxidative transformation with insertion of olefin, olefin, and carbon monoxide. Under slightly different reaction conditions, an additional CO insertion takes place to give spiro[4.4]nonenes with formation of overall five C–C bonds.

Palladium-Catalyzed Oxidative Carbocyclization-Borylation of Enallenes to Cyclobutenes

A highly efficient palladium-catalyzed oxidative borylation of enallenes was developed for the selective formation of cyclobutene derivatives and fully-substituted alkenylboron compounds. Cyclobutenes are formed as the exclusive products in MeOH in the presence of H2 O and Et3 N, whereas the use of AcOH leads to alkenylboron compounds. Both reactions showed a broad substrate scope and good tolerance for various functional groups, including carboxylic acid ester, free hydroxy, imide, and alkyl groups. Furthermore, transformations of the borylated products were conducted to show their potential applications.

Applications of C-H functionalization logic to cyclobutane synthesis

The application of C-H functionalization logic to target-oriented synthesis provides an exciting new venue for the development of new and useful strategies in organic chemistry. In this article, C-H functionalization reactions are explored as an alternative approach to access pseudodimeric cyclobutane natural products, such as the dictazole and the piperarborenine families. The use of these strategies in a variety of complex settings highlights the subtle geometric, steric, and electronic effects at play in the auxiliary guided C-H functionalization of cyclobutanes.

Peptide-catalyzed conversion of racemic oxazol-5(4H)-ones into enantiomerically enriched α-amino acid derivatives

We report the development and optimization of a tetrapeptide that catalyzes the methanolytic dynamic kinetic resolution of oxazol-5(4H)-ones (azlactones) with high levels of enantioinduction. Oxazolones possessing benzylic-type substituents were found to perform better than others, providing methyl ester products in 88:12 to 98:2 er. The mechanism of this peptide-catalyzed process was investigated through truncation studies and competition experiments. High-field NOESY analysis was performed to elucidate the solution-phase structure of the peptide, and we present a plausible model for catalysis.

Stereoselective synthesis of dienyl-carboxylate building blocks: formal synthesis of inthomycin C

A direct synthesis of stereodefined halodienes is reported. Those key building blocks enable a concise access to polyenic products, as demonstrated in a modular synthesis of Inthomycin C.

An atom-economical and stereoselective domino synthesis of functionalised dienes

Open sesame: A direct synthesis of functionalised and stereodefined dienes, relying on a domino allylic alkylation/electrocyclic ring-opening sequence, is reported. This method allows concise access to doubly vinylogous esters. A further systematic study of ring-opening rates of carbon-substituted cyclobutenes allowed the design of substrates amenable to sequential pericyclic reactions (see scheme).