Enantioselective C(sp2)-C(sp3) Bond Construction by Ni Catalysis

ConspectusAfter decades of palladium dominating the realm of transition-metal-catalyzed cross-coupling, recent years have witnessed exciting advances in the development of new nickel-catalyzed cross-coupling reactions to form C(sp3) centers. Nickel possesses distinct properties compared with palladium, such as facile single-electron transfer to C(sp3) electrophiles and rapid C-C reductive elimination from NiIII. These properties, among others, make nickel particularly well-suited for reductive cross-coupling (RCC) in which two electrophiles are coupled and an exogenous reductant is used to turn over the metal catalyst. Ni-catalyzed RCCs use readily available and stable electrophiles as starting materials and exhibit good functional group tolerance, which makes them appealing for applications in the synthesis of complex molecules. Building upon the foundational work in Ni-catalyzed RCCs by the groups of Kumada, Durandetti, Weix, and others, as well as the advancements in Ni-catalyzed enantioselective redox-neutral cross-couplings led by Fu and co-workers, we initiated a program to explore the feasibility of developing highly enantioselective Ni-catalyzed RCCs. Our research has also been driven by a keen interest in unraveling the factors contributing to enantioinduction and electrophile activation as we seek new avenues for advancing our understanding and further developing these reactions.In the first part of this Account, we organize our reported methods on the basis of the identity of the C(sp3) electrophiles, including benzylic chlorides, N-hydroxyphthalimide (NHP) esters, and α-chloro esters and nitriles. We highlight how the selection of specific chiral ligands plays a pivotal role in achieving high cross-selectivity and enantioselectivity. In addition, we show that reduction can be accomplished not only with heterogeneous reductants, such as Mn0, but also with the soluble organic reductant tetrakis(dimethylamino)ethylene (TDAE), as well as electrochemically. The use of homogeneous reductants, such as TDAE, is well suited for studying the mechanism of the transformation. Although this Account primarily focuses on RCCs, we also highlight our work using trifluoroborate (BF3K) salts as radical precursors for enantioselective dual-Ni/photoredox systems.At the end of this Account, we summarize the relevant mechanistic studies of two closely related asymmetric reductive alkenylation reactions developed in our laboratory and provide a context between our work and related mechanistic studies by others. We discuss how the ligand properties influence the rates and mechanisms of electrophile activation and how understanding the mode of C(sp3) radical generation can be used to optimize the yield of an RCC. Our research endeavors to offer insights on the intricate mechanisms at play in asymmetric Ni-catalyzed RCCs with the goal of using the rate of electrophile activation to improve the substrate scope of enantioselective RCCs. We anticipate that the insights we share in this Account can provide guidance for the development of new methods in this field.

Electrocatalytic Asymmetric Nozaki-Hiyama-Kishi Decarboxylative Coupling: Scope, Applications, and Mechanism

The first general enantioselective alkyl-Nozaki-Hiyama-Kishi (NHK) coupling reactions are disclosed herein by employing a Cr-electrocatalytic decarboxylative approach. Using easily accessible aliphatic carboxylic acids (via redox-active esters) as alkyl nucleophile synthons, in combination with aldehydes and enabling additives, chiral secondary alcohols are produced in a good yield with high enantioselectivity under mild reductive electrolysis. This reaction, which cannot be mimicked using stoichiometric metal or organic reductants, tolerates a broad range of functional groups and is successfully applied to dramatically simplify the synthesis of multiple medicinally relevant structures and natural products. Mechanistic studies revealed that this asymmetric alkyl e-NHK reaction was enabled by using catalytic tetrakis(dimethylamino)ethylene, which acts as a key reductive mediator to mediate the electroreduction of the CrIII/chiral ligand complex.

Dataset Design for Building Models of Chemical Reactivity

Models can codify our understanding of chemical reactivity and serve a useful purpose in the development of new synthetic processes via, for example, evaluating hypothetical reaction conditions or in silico substrate tolerance. Perhaps the most determining factor is the composition of the training data and whether it is sufficient to train a model that can make accurate predictions over the full domain of interest. Here, we discuss the design of reaction datasets in ways that are conducive to data-driven modeling, emphasizing the idea that training set diversity and model generalizability rely on the choice of molecular or reaction representation. We additionally discuss the experimental constraints associated with generating common types of chemistry datasets and how these considerations should influence dataset design and model building.

A Pyridine Dearomatization Approach for the Gram Scale Synthesis of (±)-Sparteine

Both enantiomers of sparteine have suffered from pricing and supply chain variability, which has inspired efforts toward efficient chemical synthesis. Here, we build upon our reported synthesis of the matrine-type lupin alkaloids in order to synthesize (±)-sparteine. Specifically, selective quenching of the cyclization between glutaryl chloride and pyridine with methanol provides a functionalized quinolizidine core that was elaborated to (±)-sparteine in six additional steps on gram scale. This synthesis provides a scalable route to sparteine from inexpensive commodity chemicals utilizing a dearomative cyclization. In addition, this route provides concise access to (±)-lupinine.

Electronic Structures of Nickel(II)-Bis(indanyloxazoline)-dihalide Catalysts: Understanding Ligand Field Contributions That Promote C(sp2)-C(sp3) Cross-Coupling

NiII(IB) dihalide [IB = (3aR,3a'R,8aS,8a'S)-2,2'-(cyclopropane-1,1-diyl)bis(3a,8a-dihydro-8H-indeno[1,2-d]-oxazole)] complexes are representative of a growing class of first-row transition-metal catalysts for the enantioselective reductive cross-coupling of C(sp2) and C(sp3) electrophiles. Recent mechanistic studies highlight the complexity of these ground-state cross-couplings but also illuminate new reactivity pathways stemming from one-electron redox and their significant sensitivities to reaction conditions. For the first time, a diverse array of spectroscopic methods coupled to electrochemistry have been applied to NiII-based precatalysts to evaluate specific ligand field effects governing key Ni-based redox potentials. We also experimentally demonstrate DMA solvent coordination to catalytically relevant Ni complexes. Coordination is shown to favorably influence key redox-based reaction steps and prevent other deleterious Ni-based equilibria. Combined with electronic structure calculations, we further provide a direct correlation between reaction intermediate frontier molecular orbital energies and cross-coupling yields. Considerations developed herein demonstrate the use of synergic spectroscopic and electrochemical methods to provide concepts for catalyst ligand design and rationalization of reaction condition optimization.

Electrochemical Preparation of Sm(II) Reagent Facilitated by Weakly Coordinating Anions

Samarium diiodide (SmI2 ) is widely used as a strong one-electron reducing agent and is often employed to form C-C bonds in complex systems. Despite their utility, SmI2 and related salts suffer from several drawbacks that render the use of Sm reducing agents in large-scale synthesis impractical. Here, we report factors influencing the electrochemical reduction of Sm(III) to Sm(II), towards the goal of electrocatalytic Sm(III) reduction. We probe the effect of supporting electrolyte, electrode material, and Sm precursor on Sm(II)/(III) redox and on the reducing power of the Sm species. We find that the coordination strength of the counteranion of the Sm salt affects the reversibility and redox potential of the Sm(II)/(III) couple and establish that the counteranion primarily determines the reducibility of Sm(III). Electrochemically generated SmI2 performs similarly to commercial SmI2 solutions in a proof-of-concept reaction. The results will provide fundamental insight to facilitate the development of Sm-electrocatalytic reactions.

Development of a Nickel-Catalyzed N-N Coupling for the Synthesis of Hydrazides

A nickel-catalyzed N-N cross-coupling for the synthesis of hydrazides is reported. O-Benzoylated hydroxamates were efficiently coupled with a broad range of aryl and aliphatic amines via nickel catalysis to form hydrazides in an up to 81% yield. Experimental evidence implicates the intermediacy of electrophilic Ni-stabilized acyl nitrenoids and the formation of a Ni(I) catalyst via silane-mediated reduction. This report constitutes the first example of an intermolecular N-N coupling compatible with secondary aliphatic amines.

Mechanistic Investigation of Ni-Catalyzed Reductive Cross-Coupling of Alkenyl and Benzyl Electrophiles

Mechanistic investigations of the Ni-catalyzed asymmetric reductive alkenylation of N-hydroxyphthalimide (NHP) esters and benzylic chlorides are reported. Investigations of the redox properties of the Ni-bis(oxazoline) catalyst, the reaction kinetics, and mode of electrophile activation show divergent mechanisms for these two related transformations. Notably, the mechanism of C(sp³) activation changes from a Ni-mediated process when benzyl chlorides and Mn⁰ are used to a reductant-mediated process that is gated by a Lewis acid when NHP esters and tetrakis(dimethylamino)ethylene is used. Kinetic experiments show that changing the identity of the Lewis acid can be used to tune the rate of NHP ester reduction. Spectroscopic studies support a Ni^II-alkenyl oxidative addition complex as the catalyst resting state. DFT calculations suggest an enantiodetermining radical capture step and elucidate the origin of enantioinduction for this Ni-BOX catalyst.

A convergent fragment coupling strategy to access quaternary stereogenic centers

The formation of quaternary stereogenic centers via convergent fragment coupling is a longstanding challenge in organic synthesis. Here, we report a strategy for the formation of quaternary stereogenic centers in...

Enantioselective Synthesis of N-Benzylic Heterocycles by Ni/Photoredox Dual Catalysis

An asymmetric cross-coupling of α-N-heterocyclic trifluoroborates with aryl bromides using Ni/photoredox dual catalysis has been developed. This C(sp2)-C(sp3) cross-coupling provides access to pharmaceutically relevant chiral N-benzylic heterocycles in good to excellent enantioselectivity when bioxazolines (BiOX) are used as the chiral ligand. High-throughput experimentation significantly streamlined reaction development by identifying BiOX ligands for further investigation and by allowing for rapid optimization of conditions for new trifluoroborate salts.

Nickel-Catalyzed Reductive Alkylation of Heteroaryl Imines

The preparation of heterobenzylic amines by a Ni-catalyzed reductive cross-coupling between heteroaryl imines and C(sp3 ) electrophiles is reported. This umpolung-type alkylation proceeds under mild conditions, avoids the pre-generation of organometallic reagents, and exhibits good functional group tolerance. Mechanistic studies are consistent with the imine substrate acting as a redox-active ligand upon coordination to a low-valent Ni center. The resulting bis(2-imino)heterocycle⋅Ni complexes can engage in alkylation reactions with a variety of C(sp3 ) electrophiles, giving heterobenzylic amine products in good yields.

A Pyridine Dearomatization Approach to the Matrine-Type Lupin Alkaloids

(+)-Matrine and (+)-isomatrine are tetracyclic alkaloids isolated from the plant Sophora flavescens, the roots of which are used in traditional Chinese medicine. Biosynthetically, these alkaloids are proposed to derive from three molecules of (-)-lysine via the intermediacy of the unstable cyclic imine Δ1-piperidine. Inspired by the biosynthesis, a new dearomative annulation reaction has been developed that leverages pyridine as a stable surrogate for Δ1-piperidine. In this key transformation, two molecules of pyridine are joined with a molecule of glutaryl chloride to give the complete tetracyclic framework of the matrine alkaloids in a single step. Using this dearomative annulation, isomatrine is synthesized in four steps from inexpensive commercially available chemicals. Isomatrine then serves as the precursor to additional lupin alkaloids, including matrine, allomatrine, isosophoridine, and sophoridine.

Cobalt-electrocatalytic HAT for functionalization of unsaturated C-C bonds

The study and application of transition metal hydrides (TMHs) has been an active area of chemical research since the early 1960s¹, for energy storage, through the reduction of protons to generate hydrogen^2,3, and for organic synthesis, for the functionalization of unsaturated C-C, C-O and C-N bonds^4,5. In the former instance, electrochemical means for driving such reactivity has been common place since the 1950s⁶ but the use of stoichiometric exogenous organic- and metal-based reductants to harness the power of TMHs in synthetic chemistry remains the norm. In particular, cobalt-based TMHs have found widespread use for the derivatization of olefins and alkynes in complex molecule construction, often by a net hydrogen atom transfer (HAT)⁷. Here we show how an electrocatalytic approach inspired by decades of energy storage research can be made use of in the context of modern organic synthesis. This strategy not only offers benefits in terms of sustainability and efficiency but also enables enhanced chemoselectivity and distinct, tunable reactivity. Ten different reaction manifolds across dozens of substrates are exemplified, along with detailed mechanistic insights into this scalable electrochemical entry into Co-H generation that takes place through a low-valent intermediate.

Expanding the Chiral Monoterpene Pool: Enantioselective Diels-Alder Reactions of α-Acyloxy Enones

An enantioselective Diels-Alder (DA) reaction of α-acyloxy enones has been developed to synthesize chiral oxidized cyclohexenes. Yttrium(III) triflate, in conjunction with a chiral pyridinebisimidazoline (PyBim) ligand, was found to catalyze the asymmetric [4 + 2] cycloaddition with a variety of dienes and α-acyloxy enone dienophiles. Using this method, terpinene-4-ol, a key intermediate in the synthesis of commercial herbicide cinmethylin, can be prepared in four steps from isoprene. A combination of kinetic data and NMR studies support a mechanism involving reversible binding of a dienophile to a yttrium catalyst followed by cycloaddition with a diene as the rate-determining step.

Enantioselective Synthesis of (-)-10-Hydroxyacutuminine

An enantioselective synthesis of (-)-10-hydroxyacutuminine is reported. Central to our strategy is a photochemical [2+2] cycloaddition that forges two of the quaternary stereocenters present in the acutumine alkaloids. A subsequent retro-aldol/Dieckmann sequence furnishes the spirocyclic cyclopentenone. Efforts to chlorinate the acutumine scaffold at C10 under heterolytic or radical deoxychlorination conditions led to the synthesis of an unexpected cyclopropane-containing pentacycle.

Plugging the Leak: Empowering Women in Organic Chemistry

Held June 24-25, 2021, the third annual Empowering Women in Organic Chemistry (EWOC) conference gathered organic chemists at all stages of the career pipeline for rich professional development opportunities and a showcase of recent scientific achievements. This Meeting Review outlines the program.

Total Syntheses of the C19 Diterpenoid Alkaloids (-)-Talatisamine, (-)-Liljestrandisine, and (-)-Liljestrandinine by a Fragment Coupling Approach

The C19 diterpenoid alkaloids (C19 DTAs) are a large family of natural products, many of which modulate the activity of ion channels in vivo and are therefore of interest for the study of neurological and cardiovascular diseases. The complex architectures of these molecules continue to challenge the state-of-the art in chemical synthesis, particularly with respect to efficient assembly of their polcyclic ring systems. Here, we report the total syntheses of (-)-talatisamine, (-)-liljestrandisine, and (-)-liljestrandinine, three aconitine-type C19 DTAs, using a fragment coupling strategy. Key to this approach is a 1,2-addition/semipinacol rearrangement sequence which efficiently joins two complex fragments and sets an all-carbon quaternary center.

Electrochemical Nozaki-Hiyama-Kishi Coupling: Scope, Applications, and Mechanism

One of the most oft-employed methods for C-C bond formation involving the coupling of vinyl-halides with aldehydes catalyzed by Ni and Cr (Nozaki-Hiyama-Kishi, NHK) has been rendered more practical using an electroreductive manifold. Although early studies pointed to the feasibility of such a process, those precedents were never applied by others due to cumbersome setups and limited scope. Here we show that a carefully optimized electroreductive procedure can enable a more sustainable approach to NHK, even in an asymmetric fashion on highly complex medicinally relevant systems. The e-NHK can even enable non-canonical substrate classes, such as redox-active esters, to participate with low loadings of Cr when conventional chemical techniques fail. A combination of detailed kinetics, cyclic voltammetry, and in situ UV-vis spectroelectrochemistry of these processes illuminates the subtle features of this mechanistically intricate process.

Nickel-catalyzed asymmetric reductive cross-coupling of α-chloroesters with (hetero)aryl iodides

An asymmetric reductive cross-coupling of α-chloroesters and (hetero)aryl iodides is reported. This nickel-catalyzed reaction proceeds with a chiral BiOX ligand under mild conditions, affording α-arylesters in good yields and enantioselectivities. The reaction is tolerant of a variety of functional groups, and the resulting products can be converted to pharmaceutically-relevant chiral building blocks. A multivariate linear regression model was developed to quantitatively relate the influence of the α-chloroester substrate and ligand on enantioselectivity.

A Ni-catalyzed enantioselective reductive cross-coupling of α-chloroesters and (hetero)aryl iodides is reported. A MLR model was developed to quantitatively relate the influence of the α-chloroester substrate and ligand on enantioselectivity.

Total Synthesis of Ritterazine B

The first total synthesis of the cytotoxic alkaloid ritterazine B is reported. The synthesis features a unified approach to both steroid subunits, employing a titanium-mediated propargylation reaction to achieve divergence from a common precursor. Other key steps include gold-catalyzed cycloisomerizations that install both spiroketals and late stage C-H oxidation to incorporate the C7' alcohol.

Synthesis of Complex Diterpenes: Strategies Guided by Oxidation Pattern Analysis

With complex molecular architectures, intriguing oxidation patterns, and wide-ranging biological activities, diterpene natural products have greatly impacted research in organic chemistry and drug discovery. Our laboratory has completed total syntheses of several highly oxidized diterpenes, including the ent-kauranoids maoecrystal Z, trichorabdal A, and longikaurin E; the antibiotic pleuromutilin; and the insecticides ryanodol, ryanodine, and perseanol. In this Account, we show how analysis of oxidation patterns and inherent functional group relationships can inform key C-C bond disconnections that greatly simplify the complexity of polycyclic structures and streamline their total syntheses. In articulating these concepts, we draw heavily from the approaches to synthetic strategy that were codified by Evans, Corey, Seebach, and others, based on the formalism that heteroatoms impose an alternating acceptor and donor reactivity pattern upon a carbon skeleton. We find these ideas particularly useful when considering oxidized diterpenes as synthetic targets.In the first part of the Account, we describe the use of reductive cyclizations as strategic tactics for building polycyclic systems with γ-hydroxyketone motifs. We have leveraged Sm-ketyl radical cyclizations as "reactivity umpolungs" to generate γ-hydroxyketones in our total syntheses of the Isodon ent-kauranoid diterpenes (-)-maoecrystal Z, (-)-longikaurin E, and (-)-trichorabdal A. Following this work, we identified the same γ-hydroxyketone pattern in the diterpene antibiotic (+)-pleuromutilin, which again inspired the use of a SmI₂-mediated reductive cyclization, this time to construct a bridging eight-membered ring. This collection of four total syntheses highlights how reductive cyclizations are particularly effective umpolung tactics when used to simultaneously form rings and introduce 1,4-dioxygenation patterns.In the second part of the Account, we detail the syntheses of the complex and highly oxidized ryanodane and isoryanodane diterpenes and present the oxidation pattern analysis that guided our synthetic designs. We first discuss our 15-step total synthesis of (+)-ryanodol, which incorporated five of the eight oxygen atoms in just two transformations: a dihydroxylation of (S)-pulegone and a SeO₂-mediated trioxidation of the A-ring cyclopentenone. This latter transformation gave rise to an independent investigation of SeO₂-mediated peroxidations of simple bicyclic cyclopent-2-en-1-ones. The syntheses of (+)-ryanodine and (+)-20-deoxyspiganthine are also presented, which required modified end-game strategies to selectively incorporate the key pyrrole-2-carboxylate ester. Finally, we describe our fragment coupling approach to prepare the isoryanodane diterpene (+)-perseanol. Using a similar oxidation pattern analysis to that developed in the synthesis of ryanodol, we again identified a two-stage strategy to install the five hydroxyl groups. This strategy was enabled by a Pd-mediated carbopalladation/carbonylation cascade and leveraged unexpected, emergent reactivity to sequence a series of late-stage oxidations.While each of the diterpene natural products discussed in this Account present unique synthetic questions, we hope that through their collective discussion, we provide a conceptual framework that condenses and summarizes the chemical knowledge we have learned and inspires future discourse and innovations in strategy design and methodology development.

Multilabel Classification Models for the Prediction of Cross-Coupling Reaction Conditions

Machine-learned ranking models have been developed for the prediction of substrate-specific cross-coupling reaction conditions. Data sets of published reactions were curated for Suzuki, Negishi, and C-N couplings, as well as Pauson-Khand reactions. String, descriptor, and graph encodings were tested as input representations, and models were trained to predict the set of conditions used in a reaction as a binary vector. Unique reagent dictionaries categorized by expert-crafted reaction roles were constructed for each data set, leading to context-aware predictions. We find that relational graph convolutional networks and gradient-boosting machines are very effective for this learning task, and we disclose a novel reaction-level graph attention operation in the top-performing model.

Synthesis and Biological Evaluation of Pyrroloindolines as Positive Allosteric Modulators of the α1β2γ2 GABAA Receptor

γ-Aminobutyric acid type A (GABA_A) receptors are key mediators of central inhibitory neurotransmission and have been implicated in several disorders of the central nervous system. Some positive allosteric modulators (PAMs) of this receptor provide great therapeutic benefits to patients. However, adverse effects remain a challenge. Selective targeting of GABA_A receptors could mitigate this problem. Here, we describe the synthesis and functional evaluation of a novel series of pyrroloindolines that display significant modulation of the GABA_A receptor, acting as PAMs. We found that halogen incorporation at the C5 position greatly increased the PAM potency relative to the parent ligand, while substitutions at other positions generally decreased potency. Mutagenesis studies suggest that the binding site lies at the top of the transmembrane domain.

A copper-catalyzed asymmetric oxime propargylation enables the synthesis of the gliovirin tetrahydro-1,2-oxazine core

The bicyclic tetrahydro-1,2-oxazine subunit of gliovirin is synthesized through a diastereoselective copper-catalyzed cyclization of an N-hydroxyamino ester. Oxidative elaboration to the fully functionalized bicycle was achieved through a series of mild transformations. Central to this approach was the development of the first catalytic, enantioselective propargylation of an oxime to furnish a key N-hydroyxamino ester intermediate.

The bicyclic tetrahydro-1,2-oxazine subunit of gliovirin is synthesized through a diastereoselective copper-catalyzed cyclization of an N-hydroxyamino ester.

Nickel-Catalyzed Enantioselective Reductive Cross-Coupling Reactions

Nickel-catalyzed reductive cross-coupling reactions have emerged as powerful methods to join two electrophiles. These reactions have proven particularly useful for the coupling of sec-alkyl electrophiles to form stereogenic centers; however, the development of enantioselective variants remains challenging. In this Perspective, we summarize the progress that has been made toward Ni-catalyzed enantioselective reductive cross-coupling reactions.

Diversity-Oriented Enzymatic Synthesis of Cyclopropane Building Blocks

While biocatalysis is increasingly incorporated into drug development pipelines, it is less commonly used in the early stages of drug discovery. By engineering a protein to produce a chiral motif with a derivatizable functional handle, biocatalysts can be used to help generate diverse building blocks for drug discovery. Here we show the engineering of two variants of Rhodothermus marinus nitric oxide dioxygenase (RmaNOD) to catalyze the formation of cis- and tran- diastereomers of a pinacolboronate-substituted cyclopropane which can be readily derivatized to generate diverse stereopure cyclopropane building blocks.

Nickel-Catalyzed Conversion of Enol Triflates into Alkenyl Halides

A Ni-catalyzed halogenation of enol triflates was developed and it enables the synthesis of a broad range of alkenyl iodides, bromides, and chlorides under mild reaction conditions. The reaction utilizes inexpensive, bench-stable Ni(OAc)2 ⋅4 H2 O as a precatalyst and proceeds at room temperature in the presence of sub-stoichiometric Zn and either 1,5-cyclooctadiene or 4-(N,N-dimethylamino)pyridine.

Enantioselective Electroreductive Coupling of Alkenyl and Benzyl Halides via Nickel Catalysis

An electrochemically-driven enantioselective nickel-catalyzed reductive cross-coupling of alkenyl bromides and benzyl chlorides is reported. The reaction forms products bearing allylic stereogenic centers with good enantioselectivity under mild conditions in an undivided cell. Electrochemical activation and turnover of the catalyst mitigate issues posed by metal powder reductants. This report demonstrates that enantioselective Ni-catalyzed cross-electrophile couplings can be driven electrochemically.

A modular approach to prepare enantioenriched cyclobutanes: synthesis of (+)-rumphellaone A

A modular synthesis of enantioenriched polyfunctionalized cyclobutanes was developed that features an 8-aminoquinolinamide directed C-H arylation reaction. The C-H arylation products were derivatized through subsequent decarboxylative coupling processes. This synthetic strategy enabled a 9-step enantioselective total synthesis of the antiproliferative meroterpenoid (+)-rumphellaone A.

Radical Deoxychlorination of Cesium Oxalates for the Synthesis of Alkyl Chlorides

A radical deoxychlorination of cesium oxalates has been developed for the preparation of hindered secondary and tertiary alkyl chlorides. The reaction tolerates a number of functional groups, including ketones, alcohols, and amides, and provides complementary reactivity to standard deoxychlorination reactions proceeding by heterolytic mechanisms. Preliminary studies demonstrate that the developed conditions can also be applied to deoxybromination and deoxyfluorination reactions.

An Oxidative Dearomatization Approach To Prepare the Pentacyclic Core of Ryanodol

An approach to synthesize the pentacyclic framework of the polyol diterpenoid ryanodol is reported. The ABC tricycle was constructed by a Co-mediated Pauson-Khand reaction, and both radical and anionic cyclization pathways were developed to form the E-ring. In addition, a reaction sequence involving SeO2-mediated enone oxidation and hydroxyl-directed oxy-Michael addition was developed to introduce the A-ring oxidation. The feasibility of forming the bridging D-ring by an oxidative dearomatization was established.

Evolution of a Strategy for the Enantioselective Total Synthesis of (+)-Psiguadial B

(+)-Psiguadial B is a diformyl phloroglucinol meroterpenoid that exhibits antiproliferative activity against the HepG2 human hepatoma cancer cell line. This full account details the evolution of a strategy that culminated in the first enantioselective total synthesis of (+)-psiguadial B. A key feature of the synthesis is the construction of the trans-cyclobutane motif by a Wolff rearrangement with in situ catalytic, asymmetric trapping of the ketene. An investigation of the substrate scope of this method to prepare enantioenriched 8-aminoquinolinamides is disclosed. Three routes toward (+)-psiguadial B were evaluated that featured the following key steps: (1) an ortho-quinone methide hetero-Diels-Alder cycloaddition to prepare the chroman framework, (2) a Prins cyclization to form the bridging bicyclo[4.3.1]decane system, and (3) a modified Norrish-Yang cyclization to generate the chroman. Ultimately, the successful strategy employed a ring-closing metathesis to form the seven-membered ring and an intramolecular O-arylation reaction to complete the polycyclic framework of the natural product.

Total Synthesis of (+)-Pleuromutilin

An 18-step synthesis of the antibiotic (+)-pleuromutilin is disclosed. The key steps of the synthesis include a highly stereoselective SmI2-mediated cyclization to establish the eight-membered ring and a stereospecific transannular [1,5]-hydrogen atom transfer to set the C10 stereocenter. This strategy was also used to prepare (+)-12-epi-pleuromutilin. The chemistry described here will enable efforts to prepare new mutilin antibiotics.

Synthesis of Enantioenriched Allylic Silanes via Nickel-Catalyzed Reductive Cross-Coupling

An asymmetric Ni-catalyzed reductive cross-coupling has been developed to prepare enantioenriched allylic silanes. This enantioselective reductive alkenylation proceeds under mild conditions and exhibits good functional group tolerance. The chiral allylic silanes prepared here undergo a variety of stereospecific transformations, including intramolecular Hosomi-Sakurai reactions, to set vicinal stereogenic centers with excellent transfer of chirality.

Chemical Synthesis of (+)-Ryanodine and (+)-20-Deoxyspiganthine

(+)-Ryanodine is a natural product modulator of ryanodine receptors, important intracellular calcium ion channels that play a critical role in signal transduction leading to muscle movement and synaptic transmission. Chemical derivatization of (+)-ryanodine has demonstrated that certain peripheral structural modifications can alter its pharmacology, and that the pyrrole-2-carboxylate ester is critical for high affinity binding to ryanodine receptors. However, the structural variation of available ryanodine analogues has been limited by the challenge of site-specific functionalization of semisynthetic intermediates, such as (+)-ryanodol. Here we report a synthetic strategy that provides access to (+)-ryanodine and the related natural product (+)-20-deoxyspiganthine in 18 and 19 steps, respectively. A key feature of this strategy is the reductive cyclization of an epoxide intermediate that possesses the critical pyrrole-2-carboxylate ester. This approach allows for the direct introduction of this ester in the final stage of the synthesis and provides a framework for the synthesis of previously inaccessible synthetic ryanoids.

Nickel-Catalyzed Enantioselective Cross-Coupling of N-Hydroxyphthalimide Esters with Vinyl Bromides

An enantioselective Ni-catalyzed cross-coupling of N-hydroxyphthalimide esters with vinyl bromides is reported. The reaction proceeds under mild conditions and uses tetrakis(N,N-dimethylamino)ethylene as a terminal organic reductant. Good functional group tolerance is demonstrated, with over 20 examples of reactions that proceed with >90% ee.