A simple, effective, new, palladium-catalyzed conversion of enol silanes to enones and enals

Comprehensive Mechanistic Analysis of Palladium- and Nickel-Catalyzed α,β-Dehydrogenation of Carbonyls via Organozinc Intermediates

Introducing degrees of unsaturation into small molecules is a central transformation in organic synthesis. A strategically useful category of this reaction type is the conversion of alkanes into alkenes for substrates with an adjacent electron-withdrawing group. An efficient strategy for this conversion has been deprotonation to form a stabilized organozinc intermediate that can be subjected to α,β-dehydrogenation through palladium or nickel catalysis. This general reactivity blueprint presents a window to uncover and understand the reactivity of Pd- and Ni-enolates. Within this context, it was determined that β-hydride elimination is slow and proceeds via concerted syn-elimination. One interesting finding is that β-hydride elimination can be preferred to a greater extent than C-C bond formation for Ni, more so than with Pd, which defies the generally assumed trends that β-hydride elimination is more facile with Pd than Ni. The discussion of these findings is informed by KIE experiments, DFT calculations, stoichiometric reactions, and rate studies. Additionally, this report details an in-depth analysis of a methodological manifold for practical dehydrogenation and should enable its application to challenges in organic synthesis.

Synthesis of nimbolide and its analogues and their application as poly(ADP-ribose) polymerase-1 trapping inducers

Nimbolide, a ring seco-C limonoid natural product, was recently found to inhibit the poly(ADP)-ribosylation (PARylation)-dependent ubiquitin E3 ligase RNF114. In doing so, it induces the 'supertrapping' of both PARylated PARP1 and PAR-dependent DNA-repair factors. PARP1 inhibitors have reshaped the treatment of cancer patients with germline BRCA1/2 mutations partly through the PARP1 trapping mechanism. To this end, modular access to nimbolide analogues represents an opportunity to develop cancer therapeutics with enhanced PARP1 trapping capability. Here we report a convergent synthesis of nimbolide through a late-stage coupling strategy. Through a sulfonyl hydrazone-mediated etherification and a radical cyclization, this strategy uses a pharmacophore-containing building block and diversifiable hydrazone units to enable the modular synthesis of nimbolide and its analogues. The broad generality of our synthetic strategy allowed access to a variety of analogues with their preliminary cellular cytotoxicity and PARP1 trapping activity reported.

Structure-Unit-Based Total Synthesis of (-)-Sinulochmodin C

Herein we report a structure-unit-based asymmetric total synthesis of sinulochmodin C, a norcembranoid diterpenoid bearing a transannular strained ether bridge β-keto tetrahydrofuran moiety. Our synthetic route features an intramolecular double Michael addition to construct stereospecifically the [7,6,5,5] tetracyclic skeleton, a vinylogous hydroxylation/oxidation procedure or a stereospecific epoxide opening/oxidation sequence to establish the γ-keto enone intermediate, a Lewis acid/Brønsted acid mediated transannular oxa-Michael addition to fuse the β-keto tetrahydrofuran moiety, a Mukaiyama hydration/Pd-C hydrogenation to reverse the C1-configuration of the isopropenyl unit, and a bioinspired transformation of sinulochmodin C into scabrolide A.

Total Synthesis of Taxol Enabled by Intermolecular Radical Coupling and Pd-Catalyzed Cyclization

Taxol (1) is a clinically used antineoplastic diterpenoid. The tetracyclic ring system comprises a 6/8/6-membered carbocycle (ABC-ring) and a fused oxetane ring (D-ring) embedded with a bridgehead double bond and decorated with multiple oxygen functionalities. Here, we report a convergent total synthesis of this exceedingly complex natural product. The C-ring fragment was designed to possess a bromocyclohexenone and an extra tetrahydrofuran ring to control the reactivity and selectivity, as well as to minimize functional group manipulations en route to 1. The α-alkoxyacyl telluride of the A-ring served as a radical precursor, and intermolecular radical coupling with the C-ring realized the installation of the C2- and C3-stereocenters and reductive removal of the bromide. After the C8-quaternary stereocenter was constructed by exploiting the three-dimensional shape of the intermediate, the C11-vinyl triflate of A-ring and the C8-methyl ketone of C-ring were utilized for Pd(0)-catalyzed cyclization of the central eight-membered B-ring with the bridgehead olefin. Adjustment of the oxidation level and attachment of the oxetane D-ring completed the total synthesis of 1 (28 steps, as the longest linear sequence). The fragment design principle and implementation of the powerful radical coupling reaction described in the present synthesis provide valuable information for planning and executing syntheses of diverse densely oxygenated terpenoids.

Deciphering the quest in the divergent total synthesis of natural products

The divergent synthesis of natural products is rapidly developing towards achieving the goal of efficiency and economy in total synthesis. However, presently, the sustainable development of the synthesis of natural products does not permit the linear synthesis of a single target. In this case, divergent total synthesis is based on the identification of an advanced intermediate with structural features that can be mapped in more than two molecules. However, the identification of this intermediate and its scalable synthesis in enantiopure form are challenging. Herein, we present the details of the ingenious efforts by researchers in the last six years toward the divergent synthesis of two to as many as eight natural products initially via a single route, and then diverging from a common intermediate and further branching out toward several natural products. The planning and strategies adopted can serve as guidelines for the future development of efficient divergent routes aimed at achieving higher efficiency toward multiple targets, causing divergent synthesis to become an accepted common practice.

Direct Regioselective Dehydrogenation of α-Substituted Cyclic Ketones

We disclose a highly regioselective, catalytic one-step dehydrogenation of α-substituted cyclic ketones in the presence of 2,3-dichlorobenzo-5,6-dicyano-1,4-benzoquinone (DDQ). The high regioselectivity originates from a phosphoric acid-catalyzed enolization, selectively affording the thermodynamically preferred enol, followed by the subsequent oxidation event. Our method provides reliable access to several α-aryl and α-alkyl substituted α,β-unsaturated ketones.

Examining the Scope of Deriving β-Aryl Enones from Enol Silanes as Ketone Equivalents via Pd(II)-Mediated Sequential Dehydrosilylation and Arylation

Silyl enol ethers were examined as a masked source of saturated ketones to derive β-aryl enones and their derivatives by dehydrosilylation to generate enones in situ and subsequent oxidative arylation with arylboronic acids as transmetallation coupling partners using relayed Pd(II) catalysis in one pot under base-free conditions. Oxygen was found to be an efficient and green oxidant to enable both dehydrosilylation of enol silanes and arylation. Additionally, arylation conditions can be custom-designed to take advantage of aryl halides as an alternative source of arylating agents. The preparative scope was investigated with 35 examples (up to 95% yield), and mechanistic studies implied a cationic Pd(II)-based catalytic system.

Asymmetric Total Synthesis of Brasiliquinones B and C via Oxidative Cyclization of a Hydroquinone-Silyl Enol Ether Hybrid

The asymmetric total synthesis of angucycline antibiotics (S)-brasiliquinones B and C was accomplished. The benz[a]anthraquinone core was constructed via oxidative cyclization of a hydroquinone-silyl enol ether hybrid. The resultant pentacyclic acetal was converted to the silyl enol ether, which was treated with Pd(II)/O₂ to afford brasiliquinone C, after multistep conversion including dehydrogenation, desilylation and deacetalization, and hydroquinone oxidation. The (S)-configuration of natural brasiliquinones was confirmed based on the stereochemical correlation with the synthetic products.

Selective desaturation of amides: a direct approach to enamides

C(sp3)-H bond desaturation has been an attractive strategy in organic synthesis. Enamides are important structural fragments in pharmaceuticals and versatile synthons in organic synthesis. However, the dehydrogenation of amides usually occurs on the acyl side benefitting from enolate chemistry like the desaturation of ketones and esters. Herein, we demonstrate an Fe-assisted regioselective oxidative desaturation of amides, which provides an efficient approach to enamides and β-halogenated enamides.

Total Synthesis of the Alleged Structure of (+)-Fimbricalyxoid A

An enantioselective total synthesis of the alleged structure of (+)-fimbricalyxoid A is reported. The synthetic strategy features a pyridine-N-oxidate-mediated S_N2' reaction to introduce an oxygen functionality at position C3 of the A-ring and a sequential three-step process via the cleavage of the C-O bonds and hemiketalization to form the 3,20-oxybridge. With this strategy, the target molecule was synthesized in 19% overall yield and 12 steps from our previously synthesized cis-fused octahydrophenanthrene (+)-6.

A Low-Cost and Scalable Synthesis of a Cyclohexanone-Related Bifunctional Building Block

A practical route to 2-(2-(2-methyl-1,3-dioxolan-2-yl)ethyl)cyclohexan-1-one was developed, featuring the use of inexpensive starting materials/reagents and readily attainable reaction conditions. The overall transformation was achieved in 53% yield with one chromatographic purification via NaOH-mediated aldol condensation, ethylene glycol protection of the ketone group in the presence of HC(OEt)₃/concd HCl, saturation of the C=C bond and the benzene ring with Al-Ni alloy in aqueous KOH, and oxidation of the intermediate cyclohexanol with aqueous NaClO/TEMPO/KBr.

The Ligand Free Palladium(II)-Catalyzed Regioselective 1,2-Addition of Enol Silanes to Quinones to Access 4-Hydroxy-4-(2-oxo-2-arylethyl)cyclohexadien-1-ones and Synthetic Applications

In contrast to the conventional 1,4-addition process, regioselective 1,2-addition of silyl enol ethers to quinones can now be achieved via a palladium(II) enolate pathway that provides access to 4-hydroxy-4-(2-oxo-2-arylethyl)cyclohexa-2,5-dien-1-one derivatives. This quinone alkylation protocol proceeds under mild reaction conditions at ambient temperature under open air and does not require either an external ligand for the palladium or the use of a base. Additionally, the cyclohexadienone products have been exploited as synthetic precursors for the construction of fused heteroaryl systems.

Synthesis of (+)-Panamonon B, 7-epi-Panamonon B, and Their (Z)-Isomers

(+)-Panamonon B was synthesized with the key quaternary center (of a predefined absolute configuration) installed using Stoltz asymmetric allylation. The C-5 ketone functionality and the cross-conjugated enone moiety in the side chain were introduced via a photosensitized [2+4] cycloaddition of singlet oxygen to diene silyl enol ether and an aldol condensation under the conditions of Sugiura, respectively. The ¹H and ¹³C NMR of the synthetic and natural samples were fully consistent with each other. However, because two samples showed opposite signs for optical rotations, they must be antipodes to one another. The synthesis also provided valuable chances to observe unexpected, yet rather intriguing, phenomena such as a bulky substituent in an axial position of a cyclohexane ring and (E)-and (Z)-isomers with opposite signs for optical rotations despite their identical stereogenic centers. The rare occurrence of a bulky substituent in an axial position of a cyclohexane ring is rationalized as a consequence of the presence of a quaternary center and formation of the five-membered lactone fused to the six-membered ring, while the so far unnoticed influence of C═C geometry on optical rotation is shown to be consistent with the information encapsulated in several discrete pairs of similar compounds retrieved from the literature.

Gold(I)-Catalyzed 7-exo-dig Cyclization: A Key Step to Access the Bicyclo[4.2.1]nonane Skeleton of Vibsatin A, a Neurotrophic Diterpenoid

Vibsatin A is a new neurotrophic vibsane-type diterpenoid comprising a bridged bicyclo[4.2.1]nonane skeleton. Inspired by Sawamura's works, we generated the bicyclic backbone through a Conia-ene-derived 7-exo-dig cyclization from an enantiomerically enriched TIPS-based silyl enol ether. The reaction, catalyzed by a sensitive gold(I) complex, was efficiently performed on a large scale by glovebox free techniques. Furthermore, the shape of this system was exploited for subsequent installation of all of the stereogenic centers.

Enantioselective Total Synthesis of (+)-Alstilobanine C, (+)-Undulifoline, and (-)-Alpneumine H

We report herein the enantioselective total synthesis of three monoterpene indole alkaloids, namely, (+)-alstilobanine C, (+)-undulifoline, and (-)-alpneumine H. The key features of our synthesis include: a) introduction of chirality via enantioselective deprotonation of a prochiral 4-substituted cyclohexanone; b) use of methoxymethyl (MOM) ether as both a hydroxyl protective group and a latent oxonium species for the formation of bridged oxepane and c) domino double reductive cyclization to build both the indole and the piperidine ring at the end of the synthesis. The synthesis confirmed the absolute configuration of these natural products assigned based on the biogenetic hypothesis.

Desymmetrization Approach to the Synthesis of Optically Active P-Stereogenic Phosphin-2-en-4-ones

Two synthetic protocols for the conversion of 1-phenylphosphinan-4-ones to novel P-stereogenic 1-phenylphosphin-2-en-4-ones by enantioselective deprotonation followed by oxidation and by asymmetric organocatalytic halogenation accompanied by elimination have been developed. These two-step one-pot transformations provide convenient access to optically active 1-phenylphosphin-2-en-4-one 1-sulfide and 1-phenylphosphin-2-en-4-one 1-oxide of 96 and 55% enantiomeric purities, respectively.

Saegusa Oxidation of Enol Ethers at Extremely Low Pd-Catalyst Loadings under Ligand-free and Aqueous Conditions: Insight into the Pd(II)/Cu(II)-Catalyst System

A highly efficient and practical Pd(II)/Cu(OAc)₂-catalyst system of Saegusa oxidation, which converts enol ethers to the corresponding enals with a number of diverse substrates at extremely low catalyst loadings (500 mol ppm) under ligand-free and aqueous conditions, is described. Its synthetic utility was demonstrated by large-scale applications of the catalyst system to important nature molecules. This work allows Saegusa oxidation to become a highly practical approach to preparing enals and also suggests new insight into the Pd(II)/Cu(II)-catalyst system for dehydrogenation of carbonyl compounds and decreasing Pd-catalyst loadings.

Dehydrogenative Pd and Ni Catalysis for Total Synthesis

The development of novel synthetic methods remains a cornerstone in simplifying complex molecule synthesis. Progress in the field of transition metal catalysis has enabled new mechanistic strategies to achieve difficult chemical transformations, increased the value of abundant chemical building blocks, and pushed the boundaries of creative and strategic route design to improve step economy in multistep synthesis. Methodologies to introduce an olefin into saturated molecules continue to be essential transformations because of the plethora of reactions available for alkene functionalization. Of particular importance are dehydrogenation reactions adjacent to electron-withdrawing groups such as carbonyls, which advantageously provide activated olefins that can be regioselectively manipulated. Palladium catalysis occupies a central role in the most widely adopted carbonyl dehydrogenation reactions, but limits to the scope of these protocols persist.In this Account, we describe our group's contributions to the area of transition-metal-catalyzed dehydrogenation using palladium catalysis and more sustainable and economical nickel catalysis. These metals are used in conjunction with allyl and aryl halides or pseudohalides that serve as oxidants to access a unique mechanistic approach for one-step α,β-dehydrogenation of various electron-withdrawing groups, including ketones, esters, nitriles, amides, carboxylic acids, and electron-deficient heteroarenes. The pivotal reaction parameters that can be modified to influence reaction efficiency are highlighted, including base and oxidant structure as well as ligand and salt additive effects. This discussion is expected to serve as a guide for troubleshooting challenging dehydrogenation reactions and provide insight for future reaction development in this area.In addition to enabling dehydrogenation reactions, our group's allyl-Pd and -Ni chemistry can be used for C-C and C-X bond-forming reactions, providing novel disconnections with practical applications for expediting multistep synthesis. These transformations include a telescoped process for ketone α,β-vicinal difunctionalization; an oxidative enone β-functionalization, including β-stannylation, β-silylation, and β-alkylation; and an oxidative cycloalkenylation between unstabilized ketone enolates and unactivated alkenes. These bond-forming methodologies broaden the range of transformations accessible from abundant ketone, enone, and alkene moieties. Both the dehydrogenation and C-C and C-X bond-forming methodologies have been implemented in our group's total synthesis campaigns to provide step-efficient synthetic routes toward diverse natural products.Through the lens of multistep synthesis, the utility and robustness of our dehydrogenation and dehydrogenative functionalization methodologies can be better appreciated, and we hope that this Account will inspire practitioners to apply our methodologies to their own synthetic challenges.

Synthesis of the bis(cyclohexenone) core of (-)-lomaiviticin A

(-)-Lomaiviticin A is a complex C ₂-symmetric bacterial metabolite comprising two diazotetrahydrobenzo[b]fluorene (diazofluorene) residues and four 2,6-dideoxy glycosides, α-l-oleandrose and N,N-dimethyl-β-l-pyrrolosamine. The two halves of lomaiviticin A are linked by a single carbon-carbon bond oriented syn with respect to the oleandrose residues. While many advances toward the synthesis of lomaiviticin A have been reported, including synthesis of the aglycon, a route to the bis(cyclohexenone) core bearing any of the carbohydrate residues has not been disclosed. Here we describe a short route to a core structure of lomaiviticin A bearing two α-l-oleandrose residues. The synthetic route features a Stille coupling to form the conjoining carbon-carbon bond of the target and a double reductive transposition to establish the correct stereochemistry at this bond. Two synthetic routes were developed to elaborate the reductive transposition product to the bis(cyclohexenone) target. The more efficient pathway features an interrupted Barton vinyl iodide synthesis followed by oxidative elimination of iodide to efficiently establish the enone functionalities in the target. The bis(cyclohexenone) product may find use in a synthesis of lomaiviticin A itself.

Divergent Synthesis of Antiviral Diterpenes Wickerols A and B

Wickerols A and B are diterpene natural products that have a novel fused 6-5-6-6 ring framework and exhibit potent antiviral activity against the H1N1 type A influenza virus. Herein, we report a divergent synthesis of wickerols A and B in 16 and 15 steps, respectively, from commercial sitolactone. The key reactions of the synthesis are a SmI₂-mediated intramolecular ketone-allylic acetate reductive cyclization, a Claisen rearrangement, and an intramolecular alkylation/aldol reaction that rapidly assembled the compact tetracyclic core framework in a stereocontrolled manner. The work described herein allowed us to confirm the absolute configurations of wickerols A and B.

Iodine-catalyzed α,β-dehydrogenation of ketones and aldehydes generating conjugated enones and enals

A transition metal-free α,β-dehydrogenation of ketones and aldehydes was developed.

Palladium-catalyzed α,β-dehydrogenation of acyclic ester equivalents promoted by a novel electron deficient phosphinooxazoline ligand

A unique example of Pd-catalyzed decarboxylative dehydrogenation of fully substituted N-acyl allyl enol carbonates is enabled by a new electron deficient phosphinooxazoline (PHOX) ligand. The reaction proceeds from the Z-enol carbonate to provide dehydrogenation products exclusively in high E/Z selectivity, while the E-enol carbonate provides the α-allylation product with only minor dehydrogenation. The reaction proceeds with a broad scope of (Z)-enol carbonates derived from N-acyl indoles to furnish acyclic formal α,β-unsaturated ester equivalents.

Diastereo- and Enantioselective Access to Stereotriads through a Flexible Coupling of Substituted Aldehydes and Alkenes

A flexible redox-neutral coupling of aldehydes and alkenes enables rapid access to stereotriads starting from a single stereocenter with perfect levels of enantio- and diastereoselectivity under mild conditions. The versatility of the method is highlighted by the installation of heteroatoms along the tether, which enables a route to structurally diverse building blocks. The formal synthesis of (+)-neopeltolide further demonstrates the synthetic utility of this approach.

Synthetic Strategy: Palladium-Catalyzed Dehydrogenation of Carbonyl Compounds

Palladium-catalyzed oxidative α,β-dehydrogenation (oxidative desilylation, decarboxylative dehydrogenation, direct dehydrogenation, and oxidative dehydroboration) of carbonyl compounds and their derivatives to α,β-unsaturated carbonyl compounds via palladium enolate intermediates is reviewed as a versatile synthetic method.

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

Antibacterial properties and clinical potential of pleuromutilins

Covering: up to 2018Pleuromutilins are a clinically validated class of antibiotics derived from the fungal diterpene (+)-pleuromutilin (1). Pleuromutilins inhibit bacterial protein synthesis by binding to the peptidyl transferase center (PTC) of the ribosome. In this review we summarize the biosynthesis and recent total syntheses of (+)-pleuromutilin (1). We review the mode of interaction of pleuromutilins with the bacterial ribosome, which involves binding of the C14 extension and the tricyclic core to the P and A sites of the PTC, respectively. We provide an overview of existing clinical agents, and discuss the three primary modes of bacterial resistance (mutations in ribosomal protein L3, Cfr methylation, and efflux). Finally we collect structure-activity relationships from publicly available reports, and close with some forward looking statements regarding future development.

An efficient synthesis of fluoro-neplanocin A analogs using electrophilic fluorination and palladium-catalyzed dehydrosilylation

An alternative and efficient approach to neplanocin A analogs 1b and 1d has been developed using electrophilic fluorination and Pd-catalyzed dehydrosilylation.

Helenalin Analogues Targeting NF-κB p65: Thiol Reactivity and Cellular Potency Studies of Varied Electrophiles

Helenalin is a pseudoguaianolide natural product that targets Cys38 within the DNA binding domain of NF-κB transcription factor p65 (RelA). Helenalin contains two Michael acceptors that covalently modify cysteines: a α-methylene-γ-butyrolactone and a cyclopentenone. We recently reported two simplified helenalin analogues that mimic the biological activity of helenalin and contain both electrophilic moieties. To determine the individual contributions of the Michael acceptors toward NF-κB inhibition, we synthesized a small library of helenalin-based analogues containing various combinations of α-methylene-γ-butyrolactones and cyclopentenones. The kinetics of thiol addition to a subset of the analogues was measured to determine the relative thiol reactivities of the embedded electrophiles. Additionally, the cellular NF-κB inhibitory activities of the analogues were determined to elucidate the contributions of each Michael acceptor to biological potency. Our studies suggest the α-methylene-γ-butyrolactone contributes most significantly to the NF-κB inhibition of our simplified helenalin analogues.

Synthesis of Cyclic Enones by Allyl-Palladium-Catalyzed α,β-Dehydrogenation

The use of allyl-palladium catalysis for the one-step α,β-dehydrogenation of ketones via their zinc enolates is reported. The optimized protocol utilizes commercially available Zn(TMP)₂ as base and diethyl allyl phosphate as oxidant. Notably, this transformation operates under salt-free conditions and tolerates a diverse scope of cycloalkanones.

Recent Advances in Steroid Synthesis: A Tribute to Sir Derek Barton

The synthesis of steroids and gaining an ultimate understanding of their reactivity was one of Sir Derek Barton’s most notable research areas. This highlight will focus on the construction of the steroid ring system from 2016 to 2018, and will include pathways that eventually led to natural product synthesis. For example, efficient syntheses of ent-pregnanolone sulfate and oestradiol methyl ether will be explained along with the total synthesis of cannogenol-3-O-α-l-rhamnoside.