Organocatalytic Enantioselective Diels-Alder Reaction of 2-Trifluoroacetamido-1,3-dienes with α,β-Unsaturated Ketones

We report herein the first examples of chiral phosphoric acid-catalyzed enantioselective Diels-Alder reactions between 2-trifluoroacetamido-1,3-dienes 1 and α,β-unsaturated carbonyl compounds 2. Polysubstituted 1-acetamido cyclohexenes 3 were formed in high yields with excellent diastereo- and enantioselectivities. The reaction proceeds through a stepwise process as shown by deuterium labelling experiments. A catalytic enantioselective three-component reaction of 1, 2 and ortho-hydroxybenzhydryl alcohols 4 was subsequently developed furnishing the densely functionalized hexahydroxanthenes 5 in a highly stereoselective manner. This multicomponent reaction generates four chemical bonds with concurrent creation of five contiguous stereocenters.

Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.

Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst‐Controlled Cyclization of ortho ‐Quinone Methide Iminiums

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel–Crafts 6π electrocyclization through ortho‐quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho‐quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion‐pairing catalysis over ortho‐quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Multicomponent Reactions (MCRs) with o-Quinone Methides

This article presents a comprehensive overview of multicomponent reactions (MCRs) that proceed via ortho-quinone methide intermediates (o-QM) generated in the reaction medium. Examples of applications involving these highly reactive intermediates in organic synthesis and biological processes (e. g., biosynthetic pathways, prodrug cleavage and electrophilic capture of biological nucleophiles) are also described. QMs are often generated by eliminative processes of phenol derivatives or by photochemical reactions, including reversible generation in photochromic substances. This class of compounds can undergo various reaction types, including nucleophilic attack at the methide carbon, with subsequent rearomatization, and react with electron-rich dienophiles in inverse-electron demand hetero-Diels-Alder reactions. Its versatile reactivity has been explored in the context of cascade reactions for the construction of several classes of substances, including complex natural products.

Enantioconvergent Substitution Reactions of Racemic Electrophiles by Organocatalysis

Over the past decades, the development of enantioselective catalysis using organocatalysts has evolved into an active research field and a number of enantioselective transformations have been established. However, despite their being a highly desirable process for the synthesis of organic molecules in an enantioenriched form, the enantioconvergent substitution reactions of racemic electrophiles using organocatalysts still present several challenges. Although intrinsic difficulties in the catalytic stereocontrol abound due to the initial chiral information of racemic electrophiles, in recent years, mechanistically diverse enantioconvergent processes have been intensively investigated in organocatalysis. This Minireview focuses on recent achievements in the development of enantioconvergent substitution reactions of racemic electrophiles using organocatalysts. The contents are classified on the basis of the mechanistic types of enantioconvergent processes.

Divergent Asymmetric Reactions of ortho-Quinone Methides with α-Thiocyanato Indanones for the Synthesis of Spiro- and Fused-Indanones

Reported in this work is a water triggered chemo-divergent enantioselective spiro-annulation and cascade reaction of ortho-quinone methides (o-QMs) with α-thiocyanato indanones catalyzed by a chiral organic base. In the case of spiro-annulation, the use of trace amount of water as additive is critical to achieve high enantioselectivity (up to 96 % ee). We found that a cascade reaction was enabled by just tuning the ratio of water in solvent. Accordingly, two new highly efficient asymmetric reactions for the divergent synthesis of spiro- and fused-indanone scaffolds with excellent enantioselectivities (up to 99 % ee) were developed. Mechanistic investigations suggest that interfacial hydrogen bonding may play an important role in achieving the switchable reaction pathways.

Brønsted Acid-Catalysed Dehydrative Substitution Reactions of Alcohols

The direct, catalytic dehydrative substitution of alcohols is a challenging, yet highly desirable process in the development of more sustainable approaches to organic chemistry. This review outlines recent advances in Brønsted acid-catalysed dehydrative substitution reactions for C-C, C-O, C-N and C-S bond formation. The wide range of processes that are now accessible using simple alcohols as the formal electrophile are highlighted, while current limitations and therefore possible future directions for research are also discussed.

Allylsilane Reagent-Controlled Divergent Asymmetric Catalytic Reactions of 2-Naphthoquinone-1-methide

Herein, the first allylsilane reagent-controlled divergent asymmetric Hosomi-Sakurai conjugate allylation and hetero-Diels-Alder (HDA) with 2-naphthoquinone-1-methide (2-Nap-Q-1-M) under the catalysis of Sc^III /Feng ligand complex is reported. With these methods, a variety of uniquely substituted chiral allyl-functionalized diaryl compounds and naphthopyran products were obtained in a straightforward and highly stereoselective (up to 96.5:3.5 e.r.) manner under mild conditions. Moreover, it is demonstrated that 2-Nap-Q-1-M can serve as an efficient diene for a side asymmetric Diels-Alder (D-A) reaction. This principle can provide a straightforward access to hydrophenalene in an optically active form, which represents a structural core of various natural products and bioactive molecules.

Chiral Brønsted Acid Catalyzed Enantioconvergent Propargylic Substitution Reaction of Racemic Secondary Propargylic Alcohols with Thiols

Despite the significant progress of the enantioselective reaction using chiral catalysts, the enantioselective nucleophilic substitution reaction at the chiral sp³ -hybridized carbon atom of a racemic electrophile has not been largely explored. Herein, we report the enantioconvergent propargylic substitution reaction of racemic propargylic alcohols with thiols using chiral bis-phosphoric acid as the chiral Brønsted acid catalyst. The substitution products were formed in high yields with high enantioselectivities in most cases. The cation-stabilizing effect of the sulfur functional group introduced at the alkynyl terminus is the key to achieving the efficient enantioconvergent process, in which chiral information originating from not only the racemic stereogenic center but also the formed contact ion pair is completely eliminated from the present system.

Cooperative Catalysis for the Highly Diastereo- and Enantioselective [4+3]-Cycloannulation of ortho-Quinone Methides and Carbonyl Ylides

We describe herein a highly diastereo- and enantioselective [4+3]-cycloannulation of ortho-quinone methides and carbonyl ylides to furnish functionalized oxa-bridged dibenzooxacines with excellent yields and stereoselectivity in a single synthetic step. The combination of rhodium and chiral phosphoric acid catalysis working in concert to generate both transient intermediates in situ provides direct access to complex bicyclic products with two quaternary and one tertiary stereogenic centers. The products may be further functionalized into valuable and enantiomerically highly enriched building blocks.

Catalytic Enantioselective Formal (4+2) Cycloaddition by Aldol-Aldol Annulation of Pyruvate Derivatives with Cyclohexane-1,3-Diones to Afford Functionalized Decalins

The decalin structure is found in bioactive molecules. We have developed catalytic enantioselective formal (4+2) cycloaddition reactions via aldol-aldol cascade reactions between pyruvate-derived diketoester derivatives and cyclohexane-1,3-dione derivatives that afford highly functionalized decalin derivatives. The reactions were performed using a quinidine-derived catalyst under mild conditions. Decalin derivatives bearing up to six chiral carbon centers including tetrasubstituted carbon centers were synthesized with high diastereo- and enantioselectivities. Five to six stereogenic centers were generated from achiral molecules with the formation of two C-C bonds in a single transformation resulting in the formation of the decalin system.

Catalytic Asymmetric N-Alkylation of Indoles and Carbazoles through 1,6-Conjugate Addition of Aza-para-quinone Methides

Catalytic asymmetric N-alkylation of indoles and carbazoles represents a family of important but underdeveloped reactions. Herein, we describe a new organocatalytic strategy in which in situ generated aza-para-quinone methides are employed as the alkylating reagent. With the proper choice of a chiral phosphoric acid and an N-protective group, the intermolecular C-N bond formation with various indole and carbazole nucleophiles proceeded efficiently under mild conditions with excellent enantioselectivity and functional-group compatibility. Control experiments and kinetic studies provided important insight into the reaction mechanism.

Regioselective α-Addition of Deconjugated Butenolides: Enantioselective Synthesis of Dihydrocoumarins

The enantioselective α-addition of deconjugated butenolides has rarely been exploited, in contrast to the well-studied γ-addition of deconjugated butenolides. In this study, an unprecedented asymmetric α-addition/transesterification of deconjugated butenolides with ortho-quinone methides generated in situ afforded a series of functionalized 3,4-dihydrocoumarins containing two contiguous stereogenic centers with excellent diastereo- and enantioselectivity. DFT calculations suggested that the rarely observed regioselectivity was due to the distortion energy that resulted from the interaction between the nucleophilic dienolate and the electrophilic ortho-quinone methide.

Chiral N,N'-Dioxide-Scandium(III) Complex-Catalyzed Asymmetric Friedel-Crafts Alkylation Reaction of ortho-Hydroxybenzyl Alcohols with C3-Substituted N-Protected Indoles

The first Lewis acid catalyzed asymmetric Friedel-Crafts alkylation reaction of ortho-hydroxybenzyl alcohols with C3-substituted indoles is described. A chiral N,N'-dioxide Sc(OTf)₃ complex served not only to promote formation of ortho-quinone methides (o-QMs) in situ but also induced the asymmetry of the reaction. This methodology enables a novel activation of ortho-hydroxybenzyl alcohols, thus affording the desired chiral diarylindol-2-ylmethanes in up to 99 % yield and 99 % ee. A range of functional groups were also tolerated under the mild reaction conditions. Moreover, this strategy gives concise access to enantioenriched indole-fused benzoxocines.