Stereocontrolled Synthesis of Kalihinol C

Unified Synthesis of 2-Isocyanoallopupukeanane and 9-Isocyanopupukeanane through a "Contra-biosynthetic" Rearrangement

Herein, we describe our synthetic efforts toward the pupukeanane natural products, in which we have completed the first enantiospecific route to 2-isocyanoallopupukeanane in 10 steps (formal synthesis), enabled by a key Pd-mediated cyclization cascade. This subsequently facilitated an unprecedented bio-inspired "contra-biosynthetic" rearrangement, providing divergent access to 9-isocyanopupukeanane in 15 steps (formal synthesis). Computational studies provide insight into the nature of this rearrangement.

Annulative Nonaromatic Newman-Kwart-Type Rearrangement for the Synthesis of Sulfur Heteroaryls

By employing enaminones and thiuram disulfides as starting materials, the frontiers of Newman-Kwart rearrangement have been expanded to the alkenyl system for the first time. In addition, instead of leading to the formation of simple carbamothioates, the rearrangement has led to the unprecedented construction of S-heteroaryls. Depending on the differences in the enaminone structure, the efficient synthesis of functionalized isothiazoles and thiophenes has been achieved.

A Rapid Aza-Bicycle Synthesis from Dendralenes and Imines

The diene-transmissive 2-fold Diels-Alder sequence between carbon-based dienophiles and [3]dendralenes is becoming an established method for polycarbocycle synthesis. Here, we demonstrate for the first time that imines are competent participants in intermolecular formal [4 + 2] cycloadditions with dendralenes. After a second Diels-Alder process with a carbadienophile, hexahydro- and octahydro-isoquinoline structures are formed. The formal aza-Diels-Alder reaction, which requires Lewis acid promotion, proceeds in high regio- and stereoselectivity under optimized conditions. ωB97XD/Def2-TZVP//M06-2X/6-31+G(d,p) calculations reveal a stepwise ionic mechanism for the formal aza-dienophile cycloadditions and also explain an unexpected Z → E olefin isomerization of a non-reacting C═C bond in the first formal cycloaddition.

An Enantiospecific Synthesis of Isoneoamphilectane Confirms Its Strained Tricyclic Structure

We describe a total synthesis of the rare isocyanoterpene natural product isoneoamphilectane and two of its unnatural diastereomers. The significantly strained ring system of the reported natural product─along with a hypothesis about a biosynthetic relationship to related family members─inspired us to consider a potential misassignment in the structure's relative configuration. As a result, we initially targeted two less strained, more accessible, stereoisomers of the reported natural product. When these compounds failed to exhibit spectroscopic data that matched those of isoneoamphilectane, we embarked on a synthesis of the originally proposed strained structure via an approach that hinged on a challenging cis-to-trans decalone epimerization. Ultimately, we implemented a novel cyclic sulfite pinacol-type rearrangement to generate the strained ring system. Additional features of this work include the application of a stereocontrolled Mukaiyama-Michael addition of an acyclic silylketene acetal, an unusual intramolecular alkoxide-mediated regioselective elimination, and an HAT-mediated alkene hydroazidation to forge the C-N bond of the tertiary isonitrile. Throughout this work, our synthetic planning was heavily guided by computational analyses to inform on key issues of stereochemical control.

Three-Component Coupling of Anilines, Amines, and Difluorocarbene to Access Formamidines

A one-pot three-component reaction of two anilines (or one aniline and one alkylamine) and in situ-generated difluorocarbene is developed herein to enable efficient construction of formamidines. Crucial formimidoyl fluoride intermediate RN═CHF is proposed from the reaction of a primary aniline and difluorocarbene. Ensuing nucleophilic iminyl substitution of this intermediate with a second amine allows cross-condensation of the two amines to produce formamidines. When only one type of primary aniline is used as the substrate, the difluoromethylated homo-condensation products can also be produced under a 1:1 molar ratio of aniline/difluorocarbene. Intramolecular variant of this method allows concise synthesis of benzimidazoquinazolines and nitrogen-fused/spirocyclic compounds, showing the potential of this method in organic synthesis. More interesting reactions are anticipated by exploiting the reactivity of difluorocarbene and primary amines to isocyanides or the formimidoyl fluoride intermediates.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Catalytic Enantioselective Construction of Decalin Derivatives by Dynamic Kinetic Desymmetrization of C2-Symmetric Derivatives through Aldol-Aldol Annulation

We have developed and investigated a catalytic desymmetrization reaction strategy that affords functionalized decalin derivatives with high enantioselectivities from C2-symmetric derivatives through aldol-aldol annulation. We identified the structural moieties of the catalyst necessary for the formation of the decalin derivative with high enantioselectivity. We elucidated the mechanisms of the catalyzed reactions: the first aldol reaction step was reversible, and the second aldol step was rate-limiting and stereochemistry-determining and was enantioselective. Using theoretical calculations guided by the experimental results, we identified the interactions between the catalyst and the transition state that led to the major enantiomer. The information obtained in this study will be useful for the development of catalysts and chemical transformations.

Rh-Catalyzed [4 + 2] Annulation with a Removable Monodentate Structure toward Iminopyranes and Pyranones by C-H Annulation

The Rh-catalyzed reactions of N-pyridinyl enaminones with internal alkynes leading to the synthesis of iminopyranes via a key C-H bond activation and subsequent tautomeric O-H bond cleavage are reported. Moreover, the pyridine ring in the amino group acts as an auxiliary monodentate site for this annulation and can be easily removed by a simple hydrolysis to afford pyranones.

Metal-free enaminone C-N bond cyanation for the stereoselective synthesis of (E)- and (Z)-β-cyano enones

A highly practical method for C-CN bond formation by C-N bond cleavage on enaminones leading to the efficient synthesis of β-cyano enones is developed. The reactions take place efficiently to provide (E)-β-cyano enones with only a molecular iodine catalyst. In addition, the additional employment of oxalic acid enables the selective synthesis of (Z)-β-cyano enones.

Tunable Trifunctionalization of Tertiary Enaminones for the Regioselective and Metal-Free Synthesis of Discrete and Proximal Phosphoryl Nitriles

This paper reports an unprecedented trifunctionalization of tertiary enaminones for the synthesis phosphoryl nitriles by the reactions of enaminones with diarylphosphine oxides and trimethylsilyl cyanide (TMSCN) without the use of any metal reagent. Employing tetrabutyl ammonium hydroxide (TBAH) as the catalyst (0.2 equiv) enables discrete cyanophosphonation. On the other hand, selective proximal cyanophosphonation has been realized in the presence of acetic acid only (AcOH).

Radical-Promoted Distal C-H Functionalization of C(sp3 ) Centers with Fluorinated Moieties

Due to their unique properties, fluorinated scaffolds are pivotal compounds in pharmaceuticals, agrochemicals, and materials science. Over the last years, the development of versatile strategies for the selective synthesis of fluorinated molecules by direct C-H bond functionalization has attracted a lot of attention. In particular, the design of novel transformations based on a radical process was a bottleneck for distal C-H functionalization reactions, offering synthetic solutions for the selective introduction of fluorinated groups. This Minireview highlights the major contributions in this blossoming field. The development of new methodologies for the remote functionalization of aliphatic derivatives with various fluorinated groups based on a 1,5-hydrogen atom transfer process and a β-fragmentation reaction will be showcased and discussed.

Epoxy-Tethered Diels-Alder Reaction toward the Tricyclic Core of Kalihinols

A chiral-template-driven intramolecular Diels-Alder reaction has been used to build the tricyclic core of kalihinols, a group of antimalarial marine natural products. The key starting materials are commercially available nerol and sulcatone.

Nucleophilic Isocyanation

Isonitriles are frequently employed as both substrates for organic transformations and ligands for organometallic chemistry. However, despite the wide application of the isonitriles, their synthesis generally depends on the traditional dehydration of N-formamide. "Nucleophilic isocyanation" using cyanide as an N-nucleophile is another straightforward strategy affording the corresponding isonitriles. This method has been available since the 19th century but is still an immature procedure and is therefore more rarely used. In this review, we summarize the concepts and recent progress in nucleophilic isocyanation, including the relatively rare examples of catalytic isocyanation.

Development of a Strategy for Linear-Selective Cu-Catalyzed Reductive Coupling of Ketones and Allenes for the Synthesis of Chiral γ-Hydroxyaldehyde Equivalents

We report the development of a stereoselective method for the allylation of ketones utilizing N-substituted allyl equivalents generated from a chiral allenamide. By choice of the appropriate ligand for the Cu-catalyst, high linear selectivity can be obtained with good diastereocontrol. This methodology allows access to chiral γ-hydroxyaldehyde equivalents that were applied in the synthesis of chiral γ-lactones and 2,5-disubstitued tetrahydrofurans.

The High Chemofidelity of Metal-Catalyzed Hydrogen Atom Transfer

The implementation of any chemical reaction in a structurally complex setting ( King , S. M. J. Org. Chem. 2014 , 79 , 8937 ) confronts structurally defined barriers: steric environment, functional group reactivity, product instability, and through-bond electronics. However, there are also practical barriers. Late-stage reactions conducted on small quantities of material are run inevitably at lower than optimal concentrations. Access to late-stage material limits extensive optimization. Impurities from past reactions can interfere, especially with catalytic reactions. Therefore, chemical reactions on which one can rely at the front lines of a complex synthesis campaign emerge from the crucible of total synthesis as robust, dependable, and widely applied. Trost conceptualized "chemoselectivity" as a reagent's selective reaction of one functional group or reactive site in preference to others ( Trost , B. M. Science 1983 , 219 , 245 ). Chemoselectivity and functional group tolerance can be evaluated quickly using robustness screens ( Collins , K. D. Nat. Chem. 2013 , 5 , 597 ). A reaction may also be characterized by its "chemofidelity", that is, its reliable reaction with a functional group in any molecular context. For example, ketone reduction by an electride (dissolving metal conditions) exhibits high chemofidelity but low chemoselectivity: it usually works, but many other functional groups are reduced at similar rates. Conversely, alkene coordination chemistry effected by π Lewis acids can exhibit high chemoselectivity ( Trost , B. M. Science 1983 , 219 , 245 ) but low chemofidelity: it can be highly selective for alkenes but sensitive to the substitution pattern ( Larionov , E. Chem. Commun. 2014 , 50 , 9816 ). In contrast, alkenes undergo reliable, robust, and diverse hydrogen atom transfer reactions from metal hydrides to generate carbon-centered radicals. Although there are many potential applications of this chemistry, its functional group tolerance, high rates, and ease of execution have led to its rapid deployment in complex synthesis campaigns. Its success derives from high chemofidelity, that is, its dependable reactivity in many molecular environments and with many alkene substitution patterns. Metal hydride H atom transfer (MHAT) reactions convert diverse, simple building blocks to more stereochemically and functionally dense products ( Crossley , S. W. M. Chem. Rev. 2016 , 116 , 8912 ). When hydrogen is returned to the metal, MHAT can be considered the radical equivalent of Brønsted acid catalysis-itself a broad reactivity paradigm. This Account summarizes our group's contributions to method development, reagent discovery, and mechanistic interrogation. Our earliest contribution to this area-a stepwise hydrogenation with high chemoselectivity and high chemofidelity-has found application to many problems. More recently, we reported the first examples of dual-catalytic cross-couplings that rely on the merger of MHAT cycles and nickel catalysis. With time, we anticipate that MHAT will become a staple of chemical synthesis.

Diverse secondary C(sp3)-H bond functionalization via site-selective trifluoroacetoxylation of aliphatic amines

We describe a coinage-metal-catalyzed site-selective oxidation of secondary C(sp3)-H bonds for aliphatic amine substrates. Broad amine scope, good functional compatibility and late-stage diversification are demonstrated with this method. The steric demand of the β-substituents controlled diastereoselectivities under this catalytic system. The site selectivity favors secondary C(sp3)-H bonds over tertiary ones underscoring the unique synthetic potential of this method.

Scalable synthesis of enaminones utilizing Gold's reagents

Several Gold's reagents were synthesized from cyanuric chloride and N,N-dialkylformamides. These synthetic equivalents of N,N-dimethylformamide dimethyl acetal were used in an optimized and scalable procedure for the regioselective synthesis of a variety of enaminones from ketone starting materials, whose utility was demonstrated by the stereoselective synthesis of Rawal-type dienes.

General Approaches to Structurally Diverse Isocyanoditerpenes

Since their discovery in the 1970s, the striking architectures and the unusual isonitrile functional groups of the isocyanoterpenes have attracted the interest of many organic chemists. The more recent revelation of their potent in vitro antiplasmodial activity sparked new endeavors to synthesize members of this family of secondary metabolites. In this Synopsis, we discuss three distinct strategies that each address multiple structurally different members of the isocyanoterpenes, ending with some of our group's recent contributions in this area.