Bond formations by intermolecular and intramolecular trappings of acylketenes and their applications in natural product synthesis

Distinction of Plasmonic Intrananoparticle and Internanoparticle Molecular Reaction Rates at the Three-Phase Contact Line of an Evaporating Sessile Droplet

Molecular reactions on the surface of a plasmonic nanoparticle (intrananoparticle) and between nanoparticles (internanoparticle) may differ in kinetics, dynamics, and product selectivity. We report that the difficulty in distinguishing the kinetics in a dispersion medium could be overcome by probing the reactions at the three-phase contact line (TPCL) of an evaporating sessile droplet using surface-enhanced Raman spectroscopy (SERS). Thus, when an evaporating aqueous droplet on glass containing 4-aminothiophenol-stabilized Ag nanoparticles was monitored by SERS at the TPCL, dimerization into 4,4'-dimercaptoazobenzene followed two steps, each preceded by the loss of H-bonded water accordingly. On the basis of the results, we assigned the first step with a higher relative kinetic rate (∼3 times) to be an intrananoparticle reaction and the second one as an internanoparticle reaction. In D2O medium, the ratio of the rates was ∼1.8. The observed vibrational signatures of the losses of water molecules before reactions and product formations were accounted for by using density functional theoretical calculations.

Cyclization Reactions of In Situ-Generated Acyl Ketene with Ynones to Form Oxacycles

Acyl ketenes react with polar unsaturated functional groups to give unique heterocyclic rings, yet reactions with unpolarized unsaturated functional groups have not been reported. Herein, we describe two effective ring-forming reactions between acetyl ketene and electron-deficient alkynes. The first reaction involves in situ tethering between acetyl ketene and nucleophile-containing 1,3-diynones, which promotes sequential intramolecular 1,6/1,4-additions to generate 2-methylene-2H-pyrans in various yields (24-91%). The other involves a zwitterionic intermediate generated from acetyl ketene and DABCO, which undergoes a Michael addition with terminal alkynyl ketones to generate 3-acyl-4-pyrones (11-79%).

Total Synthesis of the 2,5-Disubstituted γ-Pyrone E1 UAE Inhibitor Himeic Acid A

The first total synthesis of the E1 ubiquitin-activating enzyme inhibitor, himeic acid A, is reported. A McCombie reaction was used to form the core γ-pyrone via a 6π-electrocyclization. A dioxenone ring-opening/acyl ketene trapping reaction with a primary amide provided the unusual unsymmetrical imide functionality. Other key steps include the use of an Evans auxiliary alkylation (d.r. ≥ 95:5) to install the (S)-2-methyl succinic acid fragment and a cross-metathesis to install the unsaturated side-chain.

Total synthesis of justicidin B, justicidin E, and taiwanin C: A general and flexible approach toward the synthesis of natural arylnaphthalene lactone lignans

Lignans are widely present in traditional medicinal plants. Many natural arylnaphthalene lactone lignans (NALLs) isolated from the genera Justicia, Haplophyllum, and Phyllanthus possess interesting biological activities. Herein, we report a general strategy for the total synthesis of this kind of lignans. Features of this new approach are an aryl-alkyl Suzuki cross-coupling to introduce the dioxinone unit, a cation-induced cyclization to construct the aryl dihydronaphthalene, and base-mediated oxidative aromatization to furnish the arylnaphthalene core. By incorporating these key transformations, the total syntheses of justicidins B and E and taiwanin C covered type I and type II NALLs were accomplished.

Recent applications of dioxinone derivatives for macrocyclic natural product and terpenoid synthesis

Dioxinone derivatives, a class of acetoacetate derivatives, have attracted widespread attention because of their multiple reactive sites, high reactivity, unique chemical properties, and potential synthetic applications. The dioxinone group is also stable under a wide range of reaction conditions, including strong acids, as well as a variety of transition-metal-catalysed processes, such as olefin metathesis and Pd-mediated cross-coupling. The inherent reactivity and diverse applications of dioxinones make them valuable reactive intermediates in organic synthesis. The conversion of dioxinones to acylketenes and their subsequent nucleophilic capture is also an excellent strategy for synthesising β-keto acid derivatives, which can be applied even in complex molecular synthesis. This review focuses on the recent advances in the application of dioxinones in synthetic method research and the total synthesis of natural products, highlighting the exceptional utility of these synthetic methodologies in the construction of macrocyclic cores and terpenoid skeletons. In particular, successful transformations of dioxinone fragments are discussed.

Continuous Flow Generation of Acylketene Intermediates via Nitrogen Extrusion

A flow chemistry process for the generation and use of acylketene precursors through extrusion of nitrogen gas is reported. Key to the development of a suitable continuous protocol is the balance of reaction concentration against pressure in the flow reactor. The resulting process enables access to intercepted acylketene scaffolds using volatile amine nucleophiles and has been demonstrated on the gram scale. Thermal gravimetric analysis was used to guide the temperature set point of the reactor coils for a variety of acyl ketene precursors. The simultaneous generation and reaction of two reactive intermediates (both derived from nitrogen extrusion) is demonstrated.

Total Synthesis of the Acetyl CoA Carboxylase Inhibitor Soraphen A: Asymmetric Tsuji Reduction Enables Successive Olefin Metathesis

The total synthesis of soraphen A, a myxobacterial metabolite and inhibitor of acetyl CoA carboxylase, was completed in 11 steps (longest linear sequence), less than half the steps previously required. Seven metal-catalyzed processes were deployed to unlock step-economy (comprising five asymmetric processes and four C-C bond formations). The present route does not utilize chiral auxiliaries, and four of five C-C bond formations exploit non-premetalated partners. To maximize convergency, an asymmetric Tsuji reduction was developed using a Pd-AntPhos catalyst that allows a metathesis-inactive allylic carbonate to serve as a masked terminal olefin, thereby enabling successive olefin metathesis events.

Phosphine-Catalyzed Annulations Based on [3+3] and [3+2] Trapping of Ketene Intermediates with Thioamides

With the aim of developing novel annulations via ketene intermediates, allenyl imide and alkynoates bearing good leaving groups are used for their function in a tandem conjugate addition-elimination reaction (SN2' type) promoted by nucleophilic phosphine catalysts. By utilizing thioamides as 1S,3N-bis-nucleophiles, [3+3] and [3+2] annulations have been established to allow rapid access to 1,3-thiazin-4-ones and 5-alkenyl thiazolones in high yields, respectively. Furthermore, the possible reaction mechanisms are proposed on the basis of deuterium labeling experiments and density functional theory calculations.

Acyl(imidoyl)ketenes: Reactive Bidentate Oxa/Aza-Dienes for Organic Synthesis

Polyfunctional building blocks are essential for the implementation of diversity-oriented synthetic strategies, highly demanded in small molecule libraries’ design for modern drug discovery. Acyl(imidoyl)ketenes are highly reactive organic compounds, bearing both oxa- and aza-diene moieties, conjugated symmetrically to the ketene fragment, enabling synthesis of various skeletally diverse heterocycles on their basis. The highlights of reactions utilizing acyl(imidoyl)ketenes are high yields, short reaction time (about several minutes), high selectivity, atom economy, and simple purification procedures, which benefits the drug discovery. The present review focuses on the approaches to thermal generation of acyl(imidoyl)ketenes, patterns of their immediate transformations via intra- and intermolecular reactions, including the reactions of cyclodimerization, in which either symmetric or dissymmetric heterocycles can be formed. Recent advances in investigations on mechanisms, identifications of intermediates, and chemo- and regioselectivity of reactions with participation of acyl(imidoyl)ketenes are also covered.

Expedient Total Syntheses of Pladienolide-Derived Spliceosome Modulators

Atom and step economical total syntheses of spliceosome modulating natural products pladienolides A and B are described. The strategic functionalization of an unsaturated macrolide precursor enabled the most concise syntheses of these natural products to date and provides convenient, flexible access to stereodefined macrolides to streamline medicinal chemistry explorations. Notably, this synthetic route does not depend on protecting group manipulations that traditionally define synthesis planning for polyhydroxylated natural products of polyketide origin. Its utility is further demonstrated by the enantioselective total synthesis of H3B-8800, a hitherto semisynthetic pladienolide-derived spliceosome modulator undergoing clinical trials for hematological malignancies.

Periselectivity in the Aza-Diels–Alder Reaction of 1-Azadienes with α-Oxoketenes: A Combined Experimental and Theoretical Study

Inversions in the periselectivity of formal aza-Diels–Alder cycloadditions between α-oxoketenes generated by a thermally-induced Wolff rearrangement and 1-azadienes were observed experimentally as a function of the α-oxoketene and the 1-azadiene, as well as the reaction temperature and time. Some unexpected inversion in the diastereoselectivity was observed, too. These variations in selectivities were fully rationalized by computational modeling using density functional theory (DFT) methods.

Periselectivity in the aza-Diels-Alder Cycloaddition between α-Oxoketenes and N-(5-Pyrazolyl)imines: A Combined Experimental and Theoretical Study

The thermal 6π aza-Diels-Alder cycloadditions between α-oxoketenes, in situ derived from a thermally induced Wolff rearrangement of 2-diazo-1,3-diketones, and N-(5-pyrazolyl)imines as prototypical electron-rich 2-azadienes lead to two distinct sets of products, essentially as a function of the nature of the α-oxoketenes involved. For instance, cyclic five-membered α-oxoketenes lead preferentially to spiro hydropyridin-4-ones, which involves the α-oxoketenes as the 2π partners at their C═C double bond and the N-(5-pyrazolyl)imines as the 4π partners at their 2-azadiene moiety. In contrast, other cyclic and acyclic α-oxoketenes lead preferentially to 1,3-oxazin-4-ones, which now involves the α-oxoketenes as the 4π partners at their 1-oxadiene moiety and the N-(5-pyrazolyl)imines as the 2π partners at their C═N double bond. A computational modeling study using DFT methods allowed rationalizing this change of periselectivity: the formation of spiro hydropyridin-4-ones is under thermodynamic control while the formation of 1,3-oxazin-4-ones is kinetically controlled, and slightly thermodynamically disfavored in the five-membered ring series. The competing cyclodimerization of the α-oxoketenes is also studied in detail.

Exploring the generation and use of acylketenes with continuous flow processes

The generation and use of acyl ketenes under continuous flow reaction conditions is reported. Several reaction classes of these reactive intermediates have been studied. Under zero headspace conditions, a ketone exchange process is possible between volatile ketones. The process can be readily scaled to deliver gram quantities of product.

Stereoconfining macrocyclizations in the total synthesis of natural products

This review covers selected examples of point chirality-forming macrocyclizations in natural product total synthesis in the past three decades.

Synthesis of pyrimido[1,6-a]quinoxalines via intermolecular trapping of thermally generated acyl(quinoxalin-2-yl)ketenes by Schiff bases

Acyl(quinoxalin-2-yl)ketenes generated by thermal decarbonylation of 3-acylpyrrolo[1,2-a]quinoxaline-1,2,4(5H)-triones react regioselectively with Schiff bases under solvent-free conditions to form pyrimido[1,6-a]quinoxaline derivatives in good yields.

Normal, Abnormal, and Cascade Wittig Olefinations of α-Oxoketenes

α-Oxoketenes generated in situ by a thermal Wolff rearrangement have been found to participate as 1,2- and 1,4-ambident C-electrophilic/O-nucleophilic reagents towards donor/acceptor carbonyl-stabilized Wittig ylides. This resulted in the very direct and practical syntheses of functionalized allenes by a normal Wittig olefination, 4H-pyran-4-ones by an abnormal Wittig olefination, or 4H-pyranylidenes following a Wittig/abnormal Wittig cascade sequence as a function of the substrates combination employed. Mechanistic experimental and computational studies provided a full rational for these reactivity switches. Some unusual mechanistic features for Lewis acid-free Wittig olefinations were identified in this series such as the involvement of betaine intermediates and some degree of reversibility in the normal Wittig olefination. The abnormal Wittig olefination was fully uncovered.

Synthesis and Biological Evaluation of the Antimicrobial Natural Product Lipoxazolidinone A

Natural products have historically been a major source of antibiotics and therefore novel scaffolds are constantly of interest. The lipoxazolidinone family of marine natural products, with an unusual 4-oxazolidinone heterocycle at their core, represents a new scaffold for antimicrobial discovery; however, questions regarding their mechanism of action and high lipophilicity have likely slowed follow-up studies. Herein, we report the first synthesis of lipoxazolidinone A, 15 structural analogues to explore its active pharmacophore, and initial resistance and mechanism of action studies. These results suggest that 4-oxazolidinones are valuable scaffolds for antimicrobial development and reveal simplified lead compounds for further optimization.

Total Synthesis of Pyrophen and Campyrones A-C

The first total syntheses of the natural products pyrophen and campyrones A-C, isolated from the fungus Aspergillus niger, have been achieved in six steps starting from commercially available N-Boc amino acids. Key steps in this sequence include a vinylogous Claisen condensation to achieve fragment coupling and a dioxinone thermolysis/cyclization cascade to form the α-pyrone ring. The route described herein afforded the natural products in 15-25% overall yield, furnishing sufficient material for testing in biological assays.

A Unique (3+2) Annulation Reaction between Meldrum's Acid and Nitrones: Mechanistic Insight by ESI-IMS-MS and DFT Studies

The fragile intermediates of the domino process leading to an isoxazolidin-5-one, triggered by unique reactivity between Meldrum's acid and an N-benzyl nitrone in the presence of a Brønsted base, were determined thanks to the softness and accuracy of electrospray ionization mass spectrometry coupled to ion mobility spectrometry (ESI-IMS-MS). The combined DFT study shed light on the overall organocatalytic sequence that starts with a stepwise (3+2) annulation reaction that is followed by a decarboxylative protonation sequence encompassing a stereoselective pathway issue.

A Biocatalytic Route to Highly Enantioenriched β-Hydroxydioxinones

A novel biocatalytic system to access a wide variety of β-hydroxydioxinones from β-ketodioxinones employing commercial engineered ketoreductases has been developed. This practical system provides a remarkably straightforward solution to limitations in accessing certain chemical scaffolds common in β-hydroxydioxinones that are of great interest due to their diversification capabilities. A few highlights of this system are that it is high yielding, highly enantioselective, and chromatography-free. We have demonstrated both a wide substrate scope and a high degree of scalability.

Methodology for the Construction of the Bicyclo[4.3.0]nonane Core

The bicyclo[4.3.0]nonane scaffold, commonly known as a hydrindane, is a common structural motif found in many terpenoid structures and one that remains a challenge for synthetic chemists to elaborate with appropriate regio- and stereo-selectivity. Over the course of the study of terpene natural products, the elaboration of the hydrindane structure has seen progress on the utilization of both old and newer methods to achieve the desired outcomes. This review seeks to serve as a general overview of these methods, and detail specific examples.

Diastereoselective syntheses of substituted cis-hydrindanones featuring sequential inter- and intramolecular Michael reactions

The hydrindane (bicyclo[4.3.0]nonane) structural motif (1) and related cis-1-hydrindanone skeleton (2) are common substructures in many natural products. Herein, we describe efficient access to substituted cis-1-hydrindanones enabled by a sequence of Michael reactions. A copper-catalyzed intermolecular Michael addition of a cyclic silyl ketene acetal to a β-substituted-α-alkoxycarbonyl-cyclopentenone enables construction of a quaternary center and is followed, after incorporation of an additional Michael acceptor, by a second, intramolecular addition of a nucleophilic β-ketoester. This strategy affords stereoselective access to substituted bicyclic cis-hydrindanone ring systems containing up to three contiguous stereocenters.

Total Synthesis and Complete Stereostructure of a Marine Macrolide Glycoside, (-)-Lyngbyaloside B

We have described in detail the total synthesis of both the proposed and correct structures of (-)-lyngbyaloside B, which facilitated the elucidation of the complete stereostructure of this natural product. Our study began with the total synthesis of 13-demethyllyngbyaloside B, in which an esterification/ring-closing metathesis (RCM) strategy was successfully used for the efficient construction of the macrocycle. We also established reliable methods for the introduction of the conjugated diene side chain and the l-rhamnose residue onto the macrocyclic framework. However, the esterification/RCM strategy proved ineffective for the parent natural product because of the difficulties in acylating the sterically encumbered C-13 tertiary alcohol; macrolactionization of a seco-acid was also extensively investigated under various conditions without success. We finally completed the total synthesis of the proposed structure of (-)-lyngbyaloside B by means of a macrolactonization that involves an acyl ketene as the reactive species. However, the NMR spectroscopic data of our synthetic material did not match those of the authentic material, which indicated that the proposed structure must be re-examined. Inspection of the NMR spectroscopic data of the natural product and molecular mechanics calculations led us to postulate that the configuration of the C-10, C-11, and C-13 stereogenic centers had been incorrectly assigned in the proposed structure. Finally, our revised structure of (-)-lyngbyaloside B was unambiguously verified through total synthesis.

Addition of silylated nucleophiles to α-oxoketenes

A general evaluation of silylated nucleophiles to intercept transient α-oxoketenes generated by microwave-assisted Wolff rearrangement of 2-diazo-1,3-dicarbonyl compounds is presented.

Exploiting ortho-substitution effect on formation of oxygen-containing [10]paracyclophane through ring-closing metathesis

The synthesis of strained paracyclophanes is facilitated byortho-substitution effect on phenyl ring.

Dioxanone-fused dienes enable highly endo-selective intramolecular Diels-Alder reactions

Intramolecular Diels-Alder reactions of dioxanone-fused (Z,Z)-dienes, available in a stereoselective manner from the corresponding alcohols are described. These dienes exhibited high reactivity and high levels of endo selectivity, giving functionalized trans-fused bicyclic compounds.

Synthetic Studies to Lyngbouilloside: A Phosphate Tether-Mediated Synthesis of the Macrolactone Core

A concise synthetic pathway to the originally assigned structure of lyngbouilloside macrolactone (3) is reported. The core macrocycle 3 was synthesized via a phosphate tether-mediated, one-pot, sequential RCM/CM/chemoselective hydrogenation reaction, Roskamp homologation, and a high yielding Boeckman acylketene cyclization.

β-Keto-dioxinones and β,δ-diketo-dioxinones in biomimetic resorcylate total synthesis

Resorcylates are a large group of bioactive natural products that are biosynthesized from acetate and malonate units via the intermediacy of polyketides. These polyketides undergo cyclization reactions to introduce the aromatic core. The bioactivities of the resorcylates including resorcylate macrocyclic lactones include anticancer, antimalarial, mycotoxicity, antifungal, and antibiotic properties, and several compounds in the series are already in use in medicine. Examples are prodrugs derived from mycophenolic acid as immunosuppressants and the Hsp-90 inhibitor, AT13387, which is in phase-II clinical trials for the treatment of small cell lung cancer and melanoma. In consequence of these biological activities, methods for the concise synthesis of diverse resorcylates are of considerable importance. In natural product chemistry, biomimetic total synthesis can have significant advantages including functional group tolerance in key steps, the minimization of the use of protection and deprotection reactions and the shortening of the total number of synthetic steps. This Account provides a description of our adaption of the dioxinone chemistry of Hyatt, Clemens, and Feldman for the synthesis and retro-Diels-Alder reactions of diketo-dioxinones. Such dioxinones, which were synthesized by a range of C-acylation reactions, were found to undergo retro-Diels-Alder reactions on heating to provide the corresponding triketo-ketenes with the loss of acetone. The ketene reactive intermediates were rapidly trapped both inter- and intramolecularly with alcohols to provide the corresponding β,δ,ζ-triketo-esters. These compounds, which consist of keto-enol mixtures, readily undergo cycloaromatization to produce resorcylate esters and macrocyclic lactones. We have established the use of diketo-dioxinones as key general intermediates for the synthesis of diverse resorcylate natural products and for the synthesis of new classes of compounds for the generation of medicinal chemistry lead structures. Many of the methods used were found to be tolerant of multiple sensitive functional groups. These include enolate C-acylations with acyl chlorides, 1-acyl-benzotriazoles, acyl imidazolides, or Weinreb amides to prepare diketo-dioxinones and their subsequent use to prepare β,δ,ζ-triketo-esters and lactones and hence resorcylates. In addition, in most cases, phenol protection was avoided. As an alternative to the synthesis of β,δ,ζ-triketo-esters, diketo-dioxinones were also found to undergo cycloaromatization with retention of the ketal entity via a nonketene pathway. Finally, diketo-dioxinones with an allyl, prenyl, geranyl, or other 2-alkenyl carboxylate esters at the γ-carbon underwent decarboxylative rearrangement with tetrakis(triphenylphosphine)palladium catalysis to produce α-substituted diketo-dioxinones and resorcylates with 3-allyl, prenyl, geranyl, or other 2-alkenyl groups. Such diketo-dioxinone chemistry was used in the total synthesis of natural products including aigialomycin, cruentaren A, and the oligomeric resorcylate antibiotics ent-W1278 A, B, and C. Additionally, tandem use of the decarboxylative rearrangement process and cycloaromatization was used in the total synthesis of natural products including the methyl ester of cristatic acid, mycophenolic acid, and hongoquercin B. The methodology was also applied to the synthesis of 9,10-anthraquinones, o-aminoalkyl resorcylates, dihydroxyisoindolinones, oligomers, and resorcinamides. The development of this methodology is described in this Account, showcasing its applicability and versatility for the synthesis of complex resorcylate products.

Gold-catalyzed formal [4π+2π]-cycloadditions of tert-butyl propiolates with aldehydes and ketones to form 4H-1,3-dioxine derivatives

Novel gold-catalyzed [4π+2π]-cycloadditions of tert-butyl propiolates with aldehydes and ketones have been developed to form 4H-1,3-dioxine derivatives.

Ester coupling reactions – an enduring challenge in the chemical synthesis of bioactive natural products

In this review we investigate the use of complex ester fragment couplings within natural product total syntheses. Using examples from the literature up to 2014 we illustrate the state-of-the-art as well as the challenges within this area of organic synthesis.

Total synthesis, stereochemical reassignment, and biological evaluation of (-)-lyngbyaloside B

(-)-Lyngbyaloside B is a 14-membered macrolide glycoside isolated from the marine cyanobacterium Lyngbya sp. as a cytotoxic substance by Moore and co-workers. The first total synthesis of (-)-lyngbyaloside B and the reassignment of its stereostructure is described. The synthesis features an Abiko-Masamune aldol reaction, a vinylogous Mukaiyama aldol reaction, and a macrocyclization involving an acyl ketene intermediate for the construction of the macrocyclic backbone, which contains an acylated tertiary alcohol. The antiproliferative activity of selected compounds against a small panel of human cancer cell lines is also reported.