Catalytic enantioselective [3+2] and [4+2]-annulation of cyclic N-sulfonyl ketimines with γ- or δ-hydroxy-α,β-unsaturated ketones

A highly efficient asymmetric [3+2] and [4+2]-annulation of cyclic N-sulfonyl ketimines with γ- or δ-hydroxy-α,β-unsaturated ketones has been developed. This innovative reaction employs an organocatalytic approach, utilizing a hydrogen-bonding bifunctional squaramide-based catalyst. The process enables precise synthesis of chiral polyheterotricyclic oxazolidines and 1,3-oxazinane derivatives, revealing intricate structures with incorporated chiral quaternary centers. Remarkably, this method delivers high yields and exceptional enantioselectivities and diastereoselectivities, achieving up to 99% yield, >20 : 1 dr, and >99% ee.

SnCl2-catalyzed multicomponent coupling: synthesis of 1,3-oxazolidine derivatives using paraformaldehyde as a C1 feedstock

SnCl2 catalyzed the three-component coupling of aniline, epoxide, and paraformaldehyde, resulting in the synthesis of 1,3-oxazolidine derivatives. The reaction is simple and does not require any additives, bases, or oxidants, and proceeds at moderate temperature with good functional group tolerance. The scope of the utilization of paraformaldehyde as the methylene source was further extended to the synthesis of benzothiazole and 4,4'-methylenebis(N,N-dimethylaniline) using the same catalyst. A catalytic pathway was proposed based on the control experiments.

Organocatalytic Asymmetric [3 + 2]-Annulations of γ-Sulfonamido/γ-Hydroxy-α,β-Unsaturated Ketones with Cyclic N-Sulfimines: Synthesis of Chiral Polyheterotricyclic Imidazolidines and Oxazolidines

The first organocatalytic asymmetric [3 + 2]-annulation of γ-sulfonamido-α,β-unsaturated ketones with cyclic N-sulfimines has been developed, and enantioenriched functionalized polyheterotricyclic imidazolidines were obtained in good yields and with excellent enantioselectivities. This approach was also extended to the asymmetric [3 + 2]-annulation of γ-hydroxy-α,β-unsaturated ketones, affording enantioenriched polyheterotricyclic oxazolidines. In addition, base-catalyzed [3 + 2]-annulations of γ-sulfonamido/γ-hydroxy-α,β-unsaturated ketones with cyclic N-sulfimines were re-investigated under mild reaction conditions for the synthesis of racemic polyheterotricyclic imidazolidines and oxazolidines with excellent diastereoselectivities.

Diastereoselective Hydroxylation of N-tert-Butanesulfinyl Imines with 2-(Phenylsulfonyl)-3-phenyloxaziridine (Davis Oxaziridine)

The diastereoselective α-hydroxylation of N-tert-butanesulfinyl metallodienenamine and metalloenamines with Davis oxaziridine affords α-hydroxy N-sulfinyl imines with 50-88% yield and up to 98:2 diastereomeric ratio. Dramatic changes in diastereoselectivity and stereoselectivity were observed by choice of metal bases. The mechanistic understanding for the switch in diastereoselectivity was assisted by DFT computational modeling, which suggests the facial approach is governed by aza-enolate geometry. A one-pot protocol for the asymmetric synthesis of 1,2-amino alcohols is described.

Simple Tandem Olefin Isomerization/Intramolecular Hydroamination of Alkenyl Amines with Various Allylic Tethers

A simple and efficient AgOTf-promoted tandem olefin isomerization/intramolecular hydroamination of 1,1-disubstituted alkenyl amines has been developed. This one-pot process represents a facile and attractive route for the synthesis of diverse 2-alkyl-substituted 1,3-X,N-heterocycles through chemo- and regioselective C(sp³)-N bond formation with atom economy. Advantages such as the operationally simple and practical procedure that uses a readily available catalyst under aerobic conditions, good to excellent chemical yields, the high functional group tolerance, the broad substrate scope, and high efficiency and selectivity are noteworthy.

Synthesis of β2,2-Amino Acids by Stereoselective Alkylation of Isoserine Derivatives Followed by Nucleophilic Ring Opening of Quaternary Sulfamidates

Chiral bicyclic N,O-acetal isoserine derivatives have been synthesized by an acid-catalyzed tandem N,O-acetalization/intramolecular transcarbamoylation reaction between conveniently protected l-isoserine and 2,2,3,3-tetramethoxybutane. The delicate balance of the steric interactions between the different functional groups on each possible diastereoisomer controls their thermodynamic stability and hence the experimental product distribution. These chiral isoserine derivatives undergo diastereoselective alkylation at the α position, proceeding with either retention or inversion of the configuration depending on the relative configuration of the stereocenters. Quantum mechanical calculations revealed that a concave-face alkylation is favored due to smaller torsional and steric interactions at the bicyclic scaffold. This synthetic methodology gives access to chiral β^2,2-amino acids, attractive compounds bearing a quaternary stereocenter at the α position with applications in peptidomimetic and medicinal chemistry. Thus, enantiopure α-alkylisoserine derivatives were produced upon acidic hydrolysis of these alkylated scaffolds. In addition, α-benzylisoserine was readily transformed into a five-membered ring cyclic sulfamidate, which was ring opened regioselectively with representative nucleophiles to yield other types of enantiopure β^2,2-amino acids such as α-benzyl-α-heterofunctionalized-β-alanines and α-benzylnorlanthionine derivatives.

Pd-Catalyzed Regioselective Intramolecular Allylic C-H Amination of 1,1-Disubstituted Alkenyl Amines

Pd-Catalyzed intramolecular allylic C-H amination of 1,1-disubstituted alkenyl amines with various allylic tethers (X = O, NMs, CH₂) was developed. This process allows for the divergent synthesis of 1,3-X,N-heterocycles through a regioselective allylic C-H cleavage and π-allylpalladium formation. Particularly noteworthy is the use of substrates containing a labile allylic moiety and new simple catalytic systems capable of promoting highly chemo- and regioselective allylic C-H amination by overcoming significant challenges.

CsPbBr3 perovskite quantum dots as a visible light photocatalyst for cyclisation of diamines and amino alcohols: an efficient approach to synthesize imidazolidines, fused-imidazolidines and oxazolidines

We report the synthesis of highly stable CsPbBr3QD based photocatalysts using dibromoisocyanuric acid (DBI) as a benign non-toxic bromide precursor.

Organocatalytic Asymmetric Synthesis of Spirooxindole Embedded Oxazolidines

The first organocatalytic asymmetric synthesis of spirooxindole embedded oxazolidines has been developed via a domino reaction involving hemiaminal formation, followed by an unprecedented aza-Michael reaction between isatin derived N-Boc ketimines and γ-hydroxy enones. A quinine derived bifunctional squaramide catalyst was found to be efficient for this reaction, and the products were obtained in good diastereoselectivity and with high enantioselectivity.

Amphipathic 1,3-oxazolidines from N-alkyl glucamines and benzaldehydes: stereochemical and mechanistic studies

Chiral N,O-heterocycles appended to a non-reducing carbohydrate chain, which are valuable synthons, reveal further stereodynamic implications.

Palladium-catalyzed stereoselective (3 + 2) cycloaddition of vinylethylene carbonates with cyclic N-sulfonyl ketimines

A diastereoselective (3 + 2) cycloaddition of N-sulfonyl ketimines with vinylethylene carbonates (VECs) in the presence of Pd2dba3·CHCl3 and PPh3 has been developed. The reaction of various substituted VECs and diverse cyclic N-sulfonyl ketimines proceeded smoothly under mild conditions, giving highly functionalized oxazolidine frameworks in good to excellent yields with moderate to good diastereoselectivities. With the use of spiroketal-based diphosphine SKP as a chiral ligand, an asymmetric version of the current (3 + 2) cycloaddition was achieved, and chiral products were obtained in >99% ee in most cases.

Electrochemical tandem trifluoromethylation of allylamines/formal (3 + 2)-cycloaddition for the rapid access to CF3-containing imidazolines and oxazolidines

A straightforward and environmentally friendly synthesis of CF₃-containing imidazolines and oxazolidines has been developed through an electrochemical three-component reaction among allylamines, the Langlois reagent, and nitrile or carbonyl compounds.

Graphite/gold nanoparticles electrode for direct protein attachment: characterization and gas sensing application

In this work, graphite/gold nanoparticles (G/AuNPs) were synthesized through a facile chemical method, and its potential application for direct protein attachment for electrochemical detection of carbon monoxide (CO) was investigated. The preparation of G/AuNPs electrodes was optimized by synthesizing the nanoparticles in different concentration of HAuCl₄.3H₂O at various temperatures. The G/AuNPs electrode was subsequently modified by four types of mercaptopropionic acid, including 1-mercaptopropionic, 3-mercaptopropionic, 6-mercaptopropionic, and 11-mercaptopropionic acid, to achieve the best structure for protein attachment. Visible absorption and electrochemical studies showed that 3-mercaptopropionic acid possesses the best performance regarding the electrical conductivity between electrode and protein redox center. The cyclic voltammetry results revealed that the modified electrode has an appropriate performance for CO detection at very low concentrations while keeping a linear response. The limit of detection for the modified electrode was calculated to be about 0.2 ppb. Finally, the interactions of cytochrome C and carbon monoxides were simulated using molecular dynamics (MD), and the effect of protein conformation changes on the electrochemical signal was thoroughly examined. The simulation results suggested that the proposed electrochemical sensor has an acceptable performance for the detection of CO due to less fluctuation of amino acids near the protein chain in the presence of CO molecules.

Construction of 1,3-Oxazolidines through a Three-Component [3+2] Cycloaddition of Tetrahydroisoquinolines, Aldehydes, and Ethyl Ketomalonate

A chemoselective and diastereoselective synthesis of fused oxazolidines was achieved by a three-component cascade reaction of tetrahydroisoquinolines (THIQs), α,β-unsaturated aldehydes, and diethyl 2-oxomalonate. Probably due to the reactivity difference between the aldehyde and the ketone, the reaction proceeded through the condensation of THIQs with α,β-unsaturated aldehydes and 1,3-dipolar cycloaddition of the generated azomethine ylide intermediate with 2-oxomalonate. The key features are easily available starting materials, mild reaction conditions, broad substrate scope, and high chemo- and diastereoselectivity.

Invisible Inks for Secrecy and Anticounterfeiting: From Single to Double-encryption by Hydrochromic Molecules

Secret information recorded by traditional single-encrypted invisible inks is easily cracked because the inks can switch only between "NONE" and "TRUTH". Developing double-encrypted systems makes the information reversibly switchable between "FALSE" and "TRUTH", which is helpful to ensure the safety of the secret information during transport. Here, we prepared heat-developed invisible inks by hydrochromic molecules donor-acceptor Stenhouse adducts (DASAs) and oxazolidines (OXs) and promoted the invisible inks from single to double encryption. DASAs coordinate with water molecules and form stable colorless cyclic DASA·xH₂O molecules, which lose coordinated water molecules after heating and switch to colored linear DASAs. In contrast, OXs are colored with water and are colorless after heating. Single-encrypted secrecy was realized by DASA invisible inks. The information is invisible under the encrypted state and becomes bright purple after heating. Vapor treating re-encrypted the information in ∼5 min. Furthermore, the single-encryption was promoted to double-encryption by a DASA/OX invisible inks system. Heating and vapor treating switch the information between the "FALSE" and "TRUTH" reversibly. The DASA/OX invisible ink system is applied for secrecy of texts, graphic images, and quick response (QR) codes.

Stereoselective Syntheses of Highly Functionalized Imidazolidines and Oxazolidines via Ring-Opening Cyclization of Activated Aziridines and Epoxides with Amines and Aldehydes

A mild one-pot stereospecific synthetic route to highly functionalized imidazolidines and oxazolidines via S_N2-type ring-opening of the corresponding activated aziridines and epoxides with amines followed by p-toluenesulfonic acid (PTSA)-catalyzed intramolecular cyclization with aldehydes has been developed. The methodology tolerates a variety of functional groups and furnishes the desired products in high yields (up to 92%) with excellent stereoselectivities (de, ee > 99%). Interestingly, imidazolidines were formed as the cis-isomers, whereas oxazolidines were produced as trans-isomers exclusively.

Divergent synthesis of oxazolidines and morpholines via PhI(OAc)2-mediated difunctionalization of alkenes

Herein we describe the PhI(OAc)₂-mediated 1,1- and 1,2-difunctionalization of alkenes with N-tosyl amino alcohols to form oxazolidine and morpholine derivatives.

Organocatalytic synthesis of chiral CF3-containing oxazolidines and 1,2-amino alcohols: asymmetric oxa-1,3-dipolar cycloaddition of trifluoroethylamine-derived azomethine ylides

A series of CF₃-containing oxazolidines were constructed via organocatalytic asymmetric oxa-1,3-dipolar cycloaddition. These oxazolidines could undergo facile conversion to CF₃-containing 1,2-amino alcohols with vicinal stereogenic centers.

Catalytic Enantioselective Ynamide Additions to Isatins: Concise Access to Oxindole Alkaloids

The highly enantioselective addition of terminal ynamides to a variety of isatins, catalyzed by a bisoxazolidine copper complex under mild, base-free reaction conditions, is described. The reaction is broad in scope, scalable, applicable to unprotected isatins, and provides efficient access to 3-hydroxyoxindoles carrying a tetrasubstituted chiral center with excellent yields and enantioselectivities. Synthetically versatile, multifunctional 3-hydroxyindolinones are obtained by hydration, partial hydrogenation, or hydroxyacyloxylation of the ynamide moiety at room temperature and exhaustive hydrogenation followed by reductive detosylation and spontaneous cyclization affords cinchonamidine alkaloids.

Benzyne-mediated trichloromethylation of chiral oxazolines

A three-component reaction between benzyne, oxazolines and chloroform was developed for the synthesis of trichloromethylated chiral oxazolidines.

Transition metal complexes of a versatile polyalkoxy oxazolidine-based ligand derived from in situ cyclization

One-pot reaction between 8-hydroxyquinoline-2-carboxaldehyde (HQC) and tris(hydroxymethyl)aminomethane (TRIS) followed by in situ cyclization yielded an oxazolidine based ligand which produced four mononuclear complexes of MnII(1), CoII(2), NiII(3), ZnII(4), a tetranuclear iron (FeIII4) complex (5) and a trinuclear cobalt (CoIICoIII2) complex (6). Magnetic studies show dominant antiferromagnetic interaction in tetranuclear iron (FeIII4) complex 5 and presence of the slow relaxation of magnetisation in 6. The compounds were also studied for their antibacterial properties. The oxazolidine ligand (H3L2) of this study showed good antimicrobial activity not only against Gram-positive bacteria but against Gram-negative bacteria too. The antimicrobial efficacy of the metal complexes (1-6) is also reported.

Diversity-Oriented Approach to N-Heterocyclic Compounds from α-Phenyl-β-enamino Ester via a Mitsunobu-Michael Reaction Sequence

Herein we delineate a novel route for the diastereoselective construction of diversely substituted N-heterocyclic ring systems as valuable scaffolds for natural products and pharmaceuticals, starting from an easily accessible prochiral α-phenyl-β-enamino ester. The reaction sequence relies on the unexplored reactivity of α-phenyl-β-enamino ester as a nucleophilic partner in the Mitsunobu reaction to forge the N-tethered alkene-alcohol/thiol/amine intermediate, which was subjected to an intramolecular hetero-Michael addition reaction under mild conditions to furnish the respective N-heterocyclic compounds embedded with an exocyclic chiral center in high yields and excellent diastereoselectivities. The methodology is amenable for a broad range of substrates based on a metal-free approach.

Enantioselective Synthesis of β-Allenoates via Phosphine-Catalyzed and ZnI2-Promoted Preparation of Oxazolidines and Propargylamines Using Chiral Amines, 1-Alkynes, and Propiolates

Diphenylphosphinoethane (DPPE)-catalyzed and ZnI₂-promoted in situ formation of oxazolidine, alkynyl zinc, and propargylamine intermediates from 1-alkynes, chiral (S)-diphenyl(pyrrolidin-2-yl)methanol, and propiolates gave the corresponding chiral (R)-β-allenoates in 40-72% yield with up to >99% ee. The intermediate propargylamine was isolated in 50% yield and converted to give the β-allenoate 10aa in 68% yield and 96% ee upon reaction with ZnI₂. The results are discussed considering a mechanism involving oxazolidine and iminium ions formed in situ followed by addition of alkynyl zinc complex to produce the propargylamine that gives the corresponding allenoate via a 1,5-hydrogen shift in the presence of ZnI₂.

Expedient stereospecific Co-catalyzed tandem C–N and C–O bond formation of N-methylanilines with styrene oxides

The Co(ii)-catalyzed stereospecific sequential C–N and C–O bond formation of styrene oxides with N-methylanilines has been developed. Optically active epoxides can be coupled with high enantiomeric purity.

Two Catalytic Methods of an Asymmetric Wittig [2,3]-Rearrangement

Two different approaches for asymmetric catalytic Wittig [2,3]-rearrangement were developed. Allyloxymalonate derivatives were converted into homoallyl alcohols via organocatalytic or Ca²⁺-catalyzed pathways in moderate to high enantioselectivities.

Palladium-catalyzed three-component tandem cyclization of buta-2,3-dien-1-ol, aryl iodides, and imines: an efficient protocol for the synthesis of oxazolidine derivatives

An efficient three-component tandem cyclization reaction for the synthesis of highly substituted oxazolidines was achieved through the Pd⁰-catalyzed cyclization of buta-2,3-dien-1-ol with aryl iodides and imines.

Tandem additions of 3,4-dihydroisoquinolines to γ-hydroxy-α,β-unsaturated ketones: a green and new access to oxazolo[2,3-a]tetrahydroisoquinolines

Addition reactions of various 3,4-dihydroisoquinoline derivatives with γ-hydroxy-α,β-unsaturated ketones were carried out in 15 min at room temperature without an additive, providing oxazolo[2,3-a]tetrahydroisoquinoline derivatives in high yields.