Tetrasubstituted Carbon Stereocenters Research Articles

Construction of Fused Azacyclic Systems via Cascade Cyclizations of Vinyl Sulfonium Salts

AbstractIn this work cascade cyclizations of vinyl sulfonium salts with either indolyl chalcones or ortho‐amino trifluoroacetophenones are described. This protocol enables the assembly of fused 2,3‐dihyro‐1H‐pyrrolo[1,2‐a]indoles and fused tetrahydroquinolines containing a CF3‐substituted tetrasubstituted carbon stereocenters.

Asymmetric [3+3] Cycloaddition of N‐Vinyl Oxindole Nitrones with 2‐Indolylmethanols to Prepare Spirooxindole[1,2]oxazines

AbstractWe describe a chiral phosphoric acid (CPA) catalyzed asymmetric [3+3] cycloaddition of N‐viny oxindole nitrones with 2‐indolylmethanols to prepare various spirooxindole[1,2]oxazines in 43–93% yields and 88:12–97:3 enantiomeric ratios. Experimental results revealed that the addition of hexafluoroisopropanol (HFIP) played important roles to help CPA control the reaction reactivity and enantioselectivity. The present method features the construction of a tetrasubstituted chiral carbon stereocenter and the example of N‐vinyl nitrones in asymmetric [3+3] cycloaddition.

Asymmetric Paired Electrolysis: Enantioselective Alkylation of Sulfonylimines via C(sp3)-H Functionalization.

Enantioselective transformation of ubiquitous C(sp3)-H bonds into three-dimensional chiral scaffolds is of longstanding interest to synthetic chemists. Herein, an asymmetric paired electrolysis enables a highly efficient and sustainable approach to the enantioselective alkylation of sulfonylimines via C(sp3)-H functionalization. In this protocol, anodic oxidation for benzylic radical formation and Lewis acid-catalyzed sulfonylimine reduction on the cathode were seamlessly cross-coupled (up to 88 % yield). Enantioenriched chiral amines containing a tetrasubstituted carbon stereocenter are accessed with high enantioselectivity (up to 96 % ee). Mechanistic studies suggest that the amine generated in situ could serve as a base to deprotonate phenols and decrease the oxidation potential of the reaction, allowing phenols with lower potentials to be preferentially oxidized.

Asymmetric Paired Electrolysis: Enantioselective Alkylation of Sulfonylimines via C(sp3)−H Functionalization

AbstractEnantioselective transformation of ubiquitous C(sp3)−H bonds into three‐dimensional chiral scaffolds is of longstanding interest to synthetic chemists. Herein, an asymmetric paired electrolysis enables a highly efficient and sustainable approach to the enantioselective alkylation of sulfonylimines via C(sp3)−H functionalization. In this protocol, anodic oxidation for benzylic radical formation and Lewis acid‐catalyzed sulfonylimine reduction on the cathode were seamlessly cross‐coupled (up to 88 % yield). Enantioenriched chiral amines containing a tetrasubstituted carbon stereocenter are accessed with high enantioselectivity (up to 96 % ee). Mechanistic studies suggest that the amine generated in situ could serve as a base to deprotonate phenols and decrease the oxidation potential of the reaction, allowing phenols with lower potentials to be preferentially oxidized.

Stereoselective Electrophilic Sulfenylation of β,β-Disubstituted Enesulfinamides: Asymmetric Construction of Less Accessible Acyclic α,α-Disubstituted α-Sulfenylated Ketimines.

Current methods for the asymmetric α-sulfenylation of carbonyls cannot be applied to acyclic carbonyls that have two similar substituents at the α-position. This research demonstrated that the electrophilic sulfenylation of geometry-defined acyclic β,β-disubstituted enesulfinamides using S-aryl or S-alkyl benzenethiosulfonates can be highly stereoselective. This process results in enantioenriched α,α-disubstituted α-sulfenylated ketone surrogates with sulfur-containing acyclic tetrasubstituted carbon stereocenters bearing two electronically and sterically similar substituents (e.g., methyl and ethyl). Furthermore, by employing the corresponding stereoisomers of enensulfinamides, any of the four stereoisomers of α-sulfenylated ketimines can be selectively accessed.

Iron-Catalyzed Three-Component Asymmetric Carboazidation of Alkenes with Alkanes and Trimethylsilyl Azide

AbstractThe fusion of transition-metal catalysis with radical chemistry provides a versatile platform for the asymmetric radical carboazidation of alkenes to enable the rapid assembly of highly functionalized chiral azide compounds. Here, we present an iron-catalyzed asymmetric three-component radical carboazidation that processes electron-deficient alkenes by direct activation of aliphatic C–H bonds. This strategy provides access to a range of valuable chiral azides from readily available chemical feedstocks bearing a tetrasubstituted carbon stereocenter, and their synthetic potential is further showcased through straightforward transformations to provide other valuable enantioenriched building blocks.

One‐Pot Synthesis of 3‐Hydroxy‐1H‐Pyrrolo[3,4‐b]quinolin‐1‐one via Enaminamides

A multicomponent cascade cyclization reaction using aryl methyl ketones, aryl amines, and enaminamide reagents to construct 3‐hydroxy‐1H‐pyrrolo[3,4‐b]quinolin‐1‐one skeleton with good substrate compatibility (50 examples) has been reported. The one‐pot, metal‐free process formed two C‐C bonds, two C‐N bonds, and a tetrasubstituted carbon stereocenter containing a hydroxyl group. The reaction enabled modified transformation of drug molecules and derivatization of products.

Phosphine‐Catalyzed Domino Annulation Reactions of o‐Aminotrifluoroacetophenone Derivatives with Morita–Baylis–Hillman Carbonates: Construction of Tetrahydrofuro[3,2‐b]indolines

AbstractThe domino annulation reactions of o‐aminotrifluoroacetophenone derivatives and MBH carbonates enabled by PPh3 have been reported, which provide a series of tetrahydrofuro[3,2‐b]indolines containing a CF3‐substituted tetrasubstituted carbon stereocenters in yields of 74 to 95% and >20:1 dr. In this process, MBH carbonates were used as C1/C3 synthons. The MTT assay shows that the products exhibit the activity of inhibitory effect on proliferation on Hela cancer cell line.

Asymmetric Paired Electrocatalysis: Enantioselective Olefin-Sulfonylimine Coupling.

Asymmetric electrocatalysis offers exciting new strategies for the synthesis of chiral molecules through novel reaction pathways. However, simultaneous activation of reactants on both electrodes via asymmetric paired electrolysis, which is more energy efficient and economic than single half-electrode synthesis, remains a formidable challenge. Herein, an asymmetric olefin-sulfonylimine coupling via paired electrocatalysis is presented for the first time. In this protocol, Co-catalyzed hydrogen atom transfer on the anode and Ni-catalyzed sulfonylimine reduction on the cathode were seamlessly cross-coupled. The new catalytic system enables the formation of chiral amine products bearing a tetrasubstituted carbon stereocenter with a high enantioselectivity (up to 96% ee).

Aza‐Prins Cyclization on Ketones to Access Piperidines with Tetrasubstituted Carbon Stereocenters

AbstractThe aza‐Prins cyclization in presence of a variety of aliphatic, aromatic, and heterocyclic ketones is disclosed. The developed method allows an access to diverse C‐2 functionalized piperidines, bearing a tetrasubstituted or spiranic carbon stereocenter, in a range of 30 to 87% yields. When diastereomers were formed, the trans isomer was identified as the major product.

Photoinduced Synthesis of Polycyclic γ‐Sultines by a Radical‐Polar Crossover Cyclization

AbstractDescribed herein is a visible light‐mediated radical addition/SO2 insertion/anionic cyclization of benzo‐fused homoallylic tosylates, allowing access to a diverse array of otherwise challenging‐to‐access fluoromethylated polycyclic γ‐sultines with two adjacent tetrasubstituted carbon stereocenters in 49–95% yields, which can be further readily functionalized towards bulky two adjacent tetrasubstituted carbon‐containing benzo‐fused γ‐sultone and mercaptoalkanol. Sodium fluoroalkanesulfinates were utilized as bifunctionalization reagents for both fluoromethyl groups and SO2. The cyclization proceeds through a stepwise photoinduced radical addition followed by an intramolecular 5‐exo‐tet anionic substitution cyclization after SO2 insertion.

Scope and Mechanistic Probe into Asymmetric Synthesis of α-Trisubstituted-α-Tertiary Amines by Rhodium Catalysis.

Asymmetric reactions that convert racemic mixtures into enantioenriched amines are of significant importance due to the prevalence of amines in pharmaceuticals, with about 60% of drug candidates containing tertiary amines. Although transition-metal catalyzed allylic substitution processes have been developed to provide access to enantioenriched α-disubstituted allylic amines, enantioselective synthesis of sterically demanding α-tertiary amines with a tetrasubstituted carbon stereocenter remains a major challenge. Herein, we report a chiral diene-ligated rhodium-catalyzed asymmetric substitution of racemic tertiary allylic trichloroacetimidates with aliphatic secondary amines to afford α-trisubstituted-α-tertiary amines. Mechanistic investigation is conducted using synergistic experimental and computational studies. Density functional theory calculations show that the chiral diene-ligated rhodium promotes the ionization of tertiary allylic substrates to form both anti and syn π-allyl intermediates. The anti π-allyl pathway proceeds through a higher energy than the syn π-allyl pathway. The rate of conversion of the less reactive π-allyl intermediate to the more reactive isomer via π-σ-π interconversion was faster than the rate of nucleophilic attack onto the more reactive intermediate. These data imply that the Curtin-Hammett conditions are met in the amination reaction, leading to dynamic kinetic asymmetric transformation. Computational studies also show that hydrogen bonding interactions between β-oxygen of allylic substrate and amine-NH greatly assist the delivery of amine nucleophile onto more hindered internal carbon of the π-allyl intermediate. The synthetic utility of the current methodology is showcased by efficient preparation of α-trisubstituted-α-tertiary amines featuring pharmaceutically relevant secondary amine cores with good yields and excellent selectivities (branched-linear >99:1, up to 99% enantiomeric excess).

Enantioselective C−P Bond Formation through C(sp3)−H Functionalization

AbstractAn enantioselective C−P bond formation has been developed through a C(sp3)−H activation in an oxidation step followed by an organocatalyzed hydrophosphonylation protocol. The asymmetric organocatalytic Pudovik reaction has been achieved following a one‐pot strategy, starting from different benzylic and allylic alcohols and dibenzyl phosphite, using MnO2 as the oxidant and a chiral squaramide as organocatalyst. The scope of the reaction provides enantiomerically enriched α‐hydroxy phosphonates with yields from 40% to >95% and enantioselectivities from 64% to >99%. Furthermore, the use of this methodology has been demonstrated to form a tetrasubstituted carbon stereocenter, generating an acetophenone derivative in situ, using diphenyl phosphite. Therefore, this approach represents an asymmetric strategy for constructing chiral C−P bonds, which are of interest to the pharmaceutical industry.

Asymmetric [3 + 2] Photocycloaddition of β-Keto Esters and Vinyl Azides by Dual Photoredox/Nickel Catalysis.

Photocatalytic [3 + 2] cycloadditions and control of stereochemistry have remained a substantial challenge, particularly in the context of heterocycle synthesis; sporadic successful examples have involved enantioselective [3 + 2] photocycloaddition between redox-active direct group-containing cyclopropanes and alkenes for creation of cyclopentanes. Herein, we report a cooperative catalytic system comprising a chiral nickel Lewis acid catalyst and an organic photocatalyst fueled by visible-light irradiation that allows for the hitherto elusive asymmetric [3 + 2] photocycloaddition of β-keto esters with vinyl azides under redox-neutral conditions. This protocol enables highly enantioselective construction of polycyclic densely substituted 3,4-dihydro-2H-pyrrole heterocycles featuring two contiguous tetrasubstituted carbon stereocenters, including a useful chiral N,O-ketal motif that is not easily accessible with other catalytic methods. Mechanistic studies revealed that the overall reactivity relies on the seamless integration of dual roles of nickel catalysts by the catalytic formation of the substrate/Ni complex, assisting both photoredox event and enantioselective radical addition.

Photo‐Induced Functionalization of para‐Benzoquinones: Access to Dihydroquinoxalines with a Tetrasubstituted Carbon Stereocenter

Visible light induced activation of para-benzoquinone, and its sequential coupling with alkynes and ortho-phenylenediamines/ortho-aminophenols is reported. The protocol provides a route for synthesizing value-added quinoxaline derivatives with tetrasubstituted carbon stereocenter at room temperature in 36–75% yields. Mechanistic investigation suggests that the reaction proceeds via in-situ generation of para-quinone methide through direct excitation of para-benzoquinone in blue light. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Cu(I)-Catalyzed Chemo- and Enantioselective Desymmetrizing C-O Bond Coupling of Acyl Radicals.

Transition-metal-catalyzed enantioselective functionalization of acyl radicals has so far not been realized, probably due to their relatively high reactivity, which renders the chemo- and stereocontrol challenging. Herein, we describe Cu(I)-catalyzed enantioselective desymmetrizing C-O bond coupling of acyl radicals. This reaction is compatible with (hetero)aryl and alkyl aldehydes and, more importantly, displays a very broad scope of challenging alcohol substrates, such as 2,2-disubstituted 1,3-diols, 2-substituted-2-chloro-1,3-diols, 2-substituted 1,2,3-triols, 2-substituted serinols, and meso primary 1,4-diols, providing enantioenriched esters characterized by challenging acyclic tetrasubstituted carbon stereocenters. Partnered by one- or two-step follow-up transformations, this reaction provides a convenient and practical strategy for the rapid preparation of chiral C3 building blocks from readily available alcohols, particularly the industrially relevant glycerol. Mechanistic studies supported the proposed C-O bond coupling of acyl radicals.

Copper-Catalyzed Diastereo- and Enantioselective Decarboxylative [3 + 2] Cyclization of Alkyne-Substituted Cyclic Carbamates with Azlactones: Access to γ-Butyrolactams Bearing Two Vicinal Tetrasubstituted Carbon Stereocenters

A copper-catalyzed diastereo- and enantioselective decarboxylative [3 + 2] cyclization reaction of alkyne-substituted cyclic carbamates with azlactones has been established. A range of optically pure γ-butyrolactams bearing two vicinal tetrasubstituted carbon stereocenters were obtained in high yields with good to excellent stereoselectivities (up to 99% yield, 99:1 dr, and 99% ee). This is the first example of asymmetric synthesis γ-butyrolactams containing sterically congested vicinal tetrasubstituted stereocenters via a decarboxylative cyclization pathway.

Rhodium-Catalyzed 1,4-Aryl Rearrangement of Sulfur Ylide for the Synthesis of 2-Pyridyl Thioethers

We report a novel rhodium-catalyzed rearrangement involving N-substituted 2-thiopyridones and diazoesters. This reaction proceeds through the rhodium-catalyzed formation of sulfur ylides, followed by a direct C-N bond cleavage to achieve N-to-C 1,4-pyridyl migration. The protocol can be used to construct various thiopyridines possessing tetrasubstituted carbon stereocenters in moderate to excellent yields, which expands the transformation pattern of sulfur ylide intermediates in rearrangement reactions.

Enantioselective Construction of Vicinal Quaternary-Tetrasubstituted Carbon Stereocenters by Copper-Catalyzed Decarboxylative Propargylic Substitution.

Enantioselective construction of vicinal tetrasubstituted carbon stereocenters is a formidable challenge in organic synthesis. A copper-catalyzed asymmetric decarboxylative propargylic substitution with 3-amino oxindoles as trisubstituted carbon nucleophiles and propargylic cyclic carbonates as tertiary carbon electrophiles was developed. A range of 3-amino-3,3'-disubstituted oxindoles bearing vicinal quaternary-tetrasubstituted carbon stereocenters were obtained in high yields and good to excellent stereoselectivities (up to 98% yield, >20:1 dr, and 98.5:1.5 er).

Tandem Asymmetric Cycloaromatization/ Intramolecular Pictet‐Spengler‐Type Reaction. An Entry to Polycyclic Pyrroles

AbstractThe asymmetric synthesis of tetracyclic pyrroles bearing a tetrasubstituted carbon stereocenter has been accomplished through a one‐pot sequence that comprises a cross metathesis reaction followed by a tandem cycloisomerization/ intramolecular Pictet‐Spengler‐type reaction. This sequence took place on conveniently functionalized chiral amino acids to render the final products in good overall yields and diastereoselectivities up to 20:1. Those pyrrole‐containing scaffolds are present in a variety of biologically relevant compounds.magnified image