Nickel-Catalyzed Regiodivergent Approach to Macrolide Motifs

Precatalyst-Enabled Selectivity: Enantioselective NiH-Catalyzed anti-Hydrometalative Cyclization of Alkynones to Endo- and Heterocyclic Allylic Alcohols

A highly enantioselective NiH-catalyzed hydrocyclization of alkynones with unparalleled anti- and endocyclic selectivities is described. The choice of the precatalysts has significant influence in tuning the regio- and enantioselectivity. Using Ni(OTs)₂ /Phox as a precatalyst and (EtO)₂ MeSiH as a hydride source, an array of enantioenriched O-, N-, and S-containing heterocyclic tertiary allylic alcohols are obtained in 24-81 % yields with 80:20-99:1 er. Mechanistic investigations and synthetic application are also carried out. This study represents an efficient access to a set of allylic alcohols that are unable to access by the state-of-the-art coupling reactions.

Divergent, C-C Bond Forming Macrocyclizations Using Modular Sulfonylhydrazone and Derived Substrates

A divergent approach to C-C bond forming macrocycle construction is described. Modular sulfonylhydrazone and derived pyridotriazole substrates with three key building blocks have been constructed and cyclized to afford diverse macrocyclic frameworks. Broad substrate scope and functional group tolerance have been demonstrated. In addition, site-selective postfunctionalization allowed for further diversification of macrocyclic cores.

Metric-Based Analysis of Convergence in Complex Molecule Synthesis

Convergent syntheses are characterized by the coupling of two or more synthetic intermediates of similar complexity, often late in a pathway. At its limit, a fully convergent synthesis is achieved when commercial or otherwise readily available intermediates are coupled to form the final target in a single step. Of course, in all but exceptional circumstances this level of convergence is purely hypothetical; in practice, additional steps are typically required to progress from fragment coupling to the target. Additionally, the length of the sequence required to access each target is a primary consideration in synthetic design.In this Account, we provide an overview of alkaloid, polyketide, and diterpene metabolites synthesized in our laboratory and present parameters that may be used to put the degree of convergence of each synthesis on quantitative footing. We begin with our syntheses of the antiproliferative, antimicrobial bacterial metabolite (-)-kinamycin F (1) and related dimeric structure (-)-lomaiviticin aglycon (2). These synthetic routes featured a three-step sequence to construct a complex diazocyclopentadiene found in both targets and an oxidative dimerization to unite the two halves of (-)-lomaiviticin aglycon (2). We then follow with our synthesis of the antineurodegenerative alkaloid (-)-huperzine A (3). Our route to (-)-huperzine A (3) employed a diastereoselective three-component coupling reaction, followed by the intramolecular α-arylation of a β-ketonitrile intermediate, to form the carbon skeleton of the target. We then present our syntheses of the hasubanan alkaloids (-)-hasubanonine (4), (-)-delavayine (5), (-)-runanine (6), (+)-periglaucine B (7), and (-)-acutumine (8). These alkaloids bear a 7-azatricyclo[4.3.3.0^1,6]dodecane (propellane) core and a highly oxidized cyclohexenone ring. The propellane structure was assembled by the addition of an aryl acetylide to a complex iminium ion, followed by intramolecular 1,4-addition. We then present our synthesis of the guanidinium alkaloid (+)-batzelladine B (9), which contains two complex polycyclic guanidine residues united by an ester linkage. This target was logically disconnected by an esterification to allow for the independent synthesis of each guanidine residue. A carefully orchestrated cascade reaction provided (+)-batzelladine B (9) in a single step following fragment coupling by esterification. We then discuss our synthesis of the diterpene fungal metabolite (+)-pleuromutilin (10). The synthesis of (+)-pleuromutilin (10) proceeded via a fragment coupling involving two neopentylic reagents and employed a nickel-catalyzed reductive cyclization reaction to close the eight-membered ring, ultimately providing access to (+)-pleuromutilin (10), (+)-12-epi-pleuromutilin (11), and (+)-12-epi-mutilin (12). Finally, we discuss our synthesis of (-)-myrocin G (13), a tricyclic pimarane diterpene that was assembled by a convergent annulation.In the final section of this Account, we present several paramaters to analyze and quantitatively assess the degree of convergence of each synthesis. These parameters include: (1) the number of steps required following the point of convergence, (2) the difference in the number of steps required to prepare each coupling partner, (3) the percentage of carbons (or, more broadly, atoms) present at the point of convergence, and (4) the complexity generated in the fragment coupling step. While not an exhaustive list, these parameters bring the strengths and weaknesses each synthetic strategy to light, emphasizing the key contributors to the degree of convergence of each route while also highlighting the nuances of these analyses.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Iodine/water-mediated deprotective oxidation of allylic ethers to access α,β-unsaturated ketones and aldehydes

The first iodine/water-mediated deprotective oxidation of allylic ethers to access α,β-unsaturated ketones and aldehydes was achieved. The reaction tolerates a wide range of functionalities. Furthermore, this protocol was found to be applicable to the oxidative transformation of allylic acetates. The proposed mechanism involves an oxygen transfer from solvent water to the carbonyl products.

The first iodine/water-mediated deprotective oxidation of allylic ethers to access α,β-unsaturated ketones and aldehydes was effectively achieved.

A Synthesis Strategy for the Production of a Macrolactone of Gulmirecin A via a Ni(0)-Mediated Reductive Cyclization Reaction

A synthesis strategy for the production of a key synthetic intermediate of gulmirecin A was described. The key reaction in the preparation of the 12-membered macrolactone is the Ni(0)-mediated reductive cyclization reaction of ynal using an N-heterocyclic carbene ligand and silane reductant. In addition, the α-selective glycosylation reaction of the macrolactone was performed to demonstrate the synthesis of gulmirecin and disciformycin precursors.

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.

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.

Stable, Well-Defined Nickel(0) Catalysts for Catalytic C-C and C-N Bond Formation

The synthesis and catalytic activity of several classes of NHC-Ni(0) pre-catalysts stabilized by electron-withdrawing alkenes are described. Variations in the structure of fumarate and acrylate ligands modulate the reactivity and stability of the NHC-Ni(0) pre-catalysts and lead to practical and versatile catalysts for a variety of transformations. The catalytic activity and efficiency of representative members of this class of catalysts have been evaluated in reductive couplings of aldehydes and alkynes and in N-arylations of aryl chlorides.

Development of a Modular Synthetic Route to (+)-Pleuromutilin, (+)-12-epi-Mutilins, and Related Structures

We describe the development of an enantioselective synthetic route to (+)-pleuromutilin (1), (+)-12-epi-mutilin, and related derivatives. A key hydrindanone was prepared in three steps and 48% overall yield from cyclohex-2-en-1-one. 1,4-Hydrocyanation provided a nitrile (53%, or 85% based on recovered starting material) that was converted to the eneimide 57 in 80% yield by the 1,2-addition of methyllithium to the nitrile function, cyclization, and in situ acylation with di-tert-butyldicarbonate. The eneimide 57 was employed in a 2-fold neopentylic coupling reaction with an organolithium reagent derived from the alkyl iodides (R)- or (S)-30, which contain the C11-C13 atoms of the target, to provide diastereomeric diketones in 60% or 48% yield (for coupling with (R)- or (S)-30, respectively). The diketone derived from (S)-30 contains the (S)-C12 stereochemistry found in pleuromutilin and was elaborated to an alkynylaldehyde. Nickel-catalyzed reductive cyclization of this alkynylaldehyde, to construct the eight-membered ring of the target, unexpectedly provided a cyclopentene (67%), which arises from participation of the C12-α-olefin in the transformation. The diketone derived from the enantiomeric C12-fragment (R)-30 underwent reductive cyclization to provide the desired product in 60% yield. This was elaborated to 12-epi-mutilin by a four-step sequence (39% overall). Installation of the glycolic acid residue followed by C12 epimerization (Berner et al. Monatsh. Chem. 1986, 117, 1073) generated (+)-pleuromutilin (1). (+)-12-epi-Pleuromutilin and (+)-11,12-di-epi-pleuromutilin were prepared by related sequences. This work establishes a convergent entry to the pleuromutilins and provides a foundation for the production of novel antibiotics to treat drug-resistant and Gram-negative infections.

NHC Ligands Tailored for Simultaneous Regio- and Enantiocontrol in Nickel-Catalyzed Reductive Couplings

An exceptionally hindered class of enantiopure NHC ligands has been developed. While racemic forms had previously been utilized, a scalable and practical route to the enantiopure form of this ligand class is described utilizing a Buchwald-Hartwig N,N-diarylation in a highly sterically demanding environment. Using this newly accessible ligand class, nickel-catalyzed enantioselective reductive coupling reactions of aldehydes and alkynes have been developed. These studies illustrate that the newly available NHC ligands are well suited for simultaneous control of regio- and enantioselectivity, even in cases with internal alkynes possessing only very subtle steric differences between two aliphatic substituents. The steric demand of the new ligand class enables a complementary regiochemical outcome compared with previously described enantioselective processes. Using this method, a number of allylic alcohol derivatives were efficiently obtained with high regioselectivity (up to >95:5) and high enantioselectivity (up to 94% ee). The reaction conditions can also be extended to the reaction of aldehydes and allenes, providing silyl-protected allylic alcohol derivatives possessing a terminal methylene substituent. Computational studies have explained the origin of the exceptional steric demand of this ligand class, the basis for enantioselectivity, and the cooperative relationship of the aldehyde, alkyne, and ligand in influencing enantioselectivity.

A modular and enantioselective synthesis of the pleuromutilin antibiotics

The tricyclic diterpene fungal metabolite (+)-pleuromutilin has served as a starting point for antibiotic development. Semisynthetic modification of its glycolic acid subunit at C14 provided the first analogs fit for human use, and derivatization at C12 led to 12-epi-pleuromutilins with extended-spectrum antibacterial activity, including activity against Gram-negative pathogens. Given the inherent limitations of semisynthesis, however, accessing derivatives of (+)-pleuromutilin with full control over their structure presents an opportunity to develop derivatives with improved antibacterial activities. Here we disclose a modular synthesis of pleuromutilins by the convergent union of an enimide with a bifunctional iodoether. We illustrate our approach through synthesis of (+)-12-epi-mutilin, (+)-11,12-di-epi-mutilin, (+)-12-epi-pleuromutilin, (+)-11,12-di-epi-pleuromutilin, and (+)-pleuromutilin itself in 17 to 20 steps.

Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes

In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)₂/SIPr·HBF₄/KO^tBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuterium-labeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

Discovery of a new class of 16-membered (2Z,11Z)-3,11-di(aryl/naphthyl)-1,13-dioxa-5,9-dithia-2,12-diazacyclohexadeca-2,11-dienes as anti-tumor agents

A series of new 16-membered macrocyclic compounds were synthesized and evaluation of in vitro anti-tumor activities on MDAMB-231 cell lines reveal that the macrocycles, 1a, 1f, 1g, 1i and 1k are promising anti-tumor agents.

Macrolactam analogues of macrolide natural products

The chemical modification of macrolide natural products into aza- or lactam analogues is a strategy employed to improve their metabolic stability and biological activity.

Catalytic Transformation of Aldehydes with Nickel Complexes through η(2) Coordination and Oxidative Cyclization

Chemists no longer doubt the importance of a methodology that could activate and utilize aldehydes in organic syntheses since many products prepared from them support our daily life. Tremendous effort has been devoted to the development of these methods using main-group elements and transition metals. Thus, many organic chemists have used an activator-(aldehyde oxygen) interaction, namely, η(1) coordination, whereby a Lewis or Brønsted acid activates an aldehyde. In the field of coordination chemistry, η(2) coordination of aldehydes to transition metals by coordination of a carbon-oxygen double bond has been well-studied; this activation mode, however, is rarely found in transition-metal catalysis. In view of the distinctive reactivity of an η(2)-aldehyde complex, unprecedented reactions via this intermediate are a distinct possibility. In this Account, we summarize our recent results dealing with nickel(0)-catalyzed transformations of aldehydes via η(2)-aldehyde nickel and oxanickelacycle intermediates. The combination of electron-rich nickel(0) and strong electron-donating N-heterocyclic carbene (NHC) ligands adequately form η(2)-aldehyde complexes in which the aldehyde is highly activated by back-bonding. With Ni(0)/NHC catalysts, processes involving intramolecular hydroacylation of alkenes and homo/cross-dimerization of aldehydes (the Tishchenko reaction) have been developed, and both proceed via the simultaneous η(2) coordination of aldehydes and other π components (alkenes or aldehydes). The results of the mechanistic studies are consistent with a reaction pathway that proceeds via an oxanickelacycle intermediate generated by the oxidative cyclization with a nickel(0) complex. In addition, we have used the η(2)-aldehyde nickel complex as an effective activator for an organosilane in order to generate a silicate reactant. These reactions show 100% atom efficiency, generate no wastes, and are conducted under mild conditions.

Mechanistic Basis for Regioselection and Regiodivergence in Nickel-Catalyzed Reductive Couplings

The control of regiochemistry is a considerable challenge in the development of a wide array of catalytic processes. Simple π-components such as alkenes, alkynes, 1,3-dienes, and allenes are among the many classes of substrates that present complexities in regioselective catalysis. Considering an internal alkyne as a representative example, when steric and electronic differences between the two substituents are minimal, differentiating among the two termini of the alkyne presents a great challenge. In cases where the differences between the alkyne substituents are substantial, overcoming those biases to access the regioisomer opposite that favored by substrate biases often presents an even greater challenge. Nickel-catalyzed reductive couplings of unsymmetrical π-components make up a group of reactions where control of regiochemistry presents a challenging but important objective. In the course of our studies of aldehyde-alkyne reductive couplings, complementary solutions to challenges in regiocontrol have been developed. Through careful selection of the ligand and reductant, as well as the more subtle reaction variables such as temperature and concentration, effective protocols have been established that allow highly selective access to either regiosiomer of the allylic alcohol products using a wide range of unsymmetrical alkynes. Computational studies and an evaluation of reaction kinetics have provided an understanding of the origin of the regioselectivity control. Throughout the various procedures described, the development of ligand-substrate interactions plays an essential role, and the overall kinetic descriptions were found to differ between protocols. Rational alteration of the rate-determining step plays a key role in the regiochemistry reversal strategy, and in one instance, the two possible regioisomeric outcomes in a single reaction were found to operate by different kinetic descriptions. With this mechanistic information in hand, the empirical factors that influence regiochemistry can be readily understood, and more importantly, the insights suggest simple and predictable experimental variables to achieving a desired reaction outcome. These studies thus present a detailed picture of the influences that control regioselectivity in a specific catalytic reaction, but they also delineate strategies for regiocontrol that may extend to numerous classes of reactions. The work provides an illustration of how insights into the kinetics and mechanism of a catalytic process can rationalize subtle empirical findings and suggest simple and rational modifications in procedure to access a desirable reaction outcome. Furthermore, these studies present an illustration of how important challenges in organic synthesis can be met by novel reactivity afforded by base metal catalysis. The use of nickel catalysis in this instance not only provides an inexpensive and sustainable method for catalysis but also enables unique reactivity patterns not accessible to other metals.

Exo-selective reductive macrocyclization of ynals

A general protocol for the highly exo-selective macrocyclization of ynals using a nickel/N-heterocyclic carbene catalyst system has been developed. A series of 10- to 21-membered macrocycles bearing an exomethylene substituent was synthesized in good yields with excellent regioselectivity (exo/endo >95:5). Very high levels of long-range diastereocontrol can also be achieved for some classes of macrocycles. Complementary to previously reported endo-selective macrocyclizations, this method provides accesses to exoalkylidene macrocycles from simple ynals in high selectivity.

Dimer involvement and origin of crossover in nickel-catalyzed aldehyde-alkyne reductive couplings

The mechanism of nickel(0)-catalyzed reductive coupling of aldehydes and alkynes has been studied. Extensive double-labeling crossover studies have been conducted. While previous studies illustrated that phosphine- and N-heterocyclic carbene-derived catalysts exhibited differing behavior, the origin of these effects has now been evaluated in detail. Many variables, including ligand class, sterics of the ligand and alkyne, temperature, and ring size being formed in intramolecular versions, all influence the extent of crossover observed. A computational evaluation of these effects suggests that dimerization of a key metallacyclic intermediate provides the origin of crossover. Protocols that proceed with crossover are typically less efficient than those without crossover given the thermodynamic stability and low reactivity of the dimeric metallacycles involved in crossover pathways.

A streamlined strategy for aglycone assembly and glycosylation

Multipurpose sugars: Carbohydrate-derived silane reagents are utilized as the reductant for nickel-catalyzed aldehyde-alkyne reductive coupling reactions and as the glycosyl donor for subsequent intramolecular glycosylation. The approach enables the assembly of the carbon-carbon framework and stereochemical features of an aglycone while simultaneously establishing the site of glycosylation.