Construction of functionalized 2,3-dihydro-1,4-benzoxazines via [5 + 1] annulations of 2-halo-1,3-dicarbonyl compounds with imines

A series of functionalized 2,3-dihydro-1,4-benzoxazines were obtained in moderate to excellent yields via domino [5 + 1] annulations of 2-halo-1,3-dicarbonyl compounds 2 with imines 1 under mild conditions and the application of this method in the synthesis of bioactive analogues, such as functionalized tetracyclic-1,4-benzoxazines which contain two new heterocyclic rings and one quaternary carbon center has also been developed.     

A mild and efficient aza-Diels-Alder reaction of N-benzhydryl imines with trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene catalyzed by Yb(OTf)3

The aza-Diels-Alder reaction of trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene with N-benzhydryl imines in the presence of Yb(OTf)₃ in toluene at room temperature gave the corresponding 2,5-disubstituted 2,3-dihydro-4-pyridones in high yields. This reaction can also be carried out with diene, aldehydes, and amine in a three-component one-pot reaction manner in moderate to high yields under solvent-free conditions. The relationship between Lewis acids and activity, solvents and catalyst loading were studied. Some investigation towards the reaction mechanism was discussed.     

An organocatalytic synthesis of N-sulfonyl imines using chloramine-T in aqueous medium

N-Sulfonyl imines have been synthesized in good to excellent yields from aldehydes and chloramine-T using proline as an organocatalyst in aqueous medium at ambient temperature. The protocol is applicable to a wide range of aldehydes, especially enals exhibit remarkable efficiency in the reaction. The reaction presumably occurs via iminium activation and opens new avenues for the synthesis of N-sulfonyl imines under environmentally benign and mild conditions.     

A highly efficient and direct approach for synthesis of enantiopure beta-amino alcohols by reductive cross-coupling of chiral N-tert-butanesulfinyl imines with aldehydes

A highly efficient and practical approach for the synthesis of optically pure beta-amino alcohols by the SmI2-induced reductive cross-coupling of chiral N-tert-butanesulfinyl imines with aldehydes was developed. This method allows the preparation of a broad range of chiral beta-amino alcohols, including functionalized ones under mild conditions. It provides a straightforward access to enantiopure beta-amino alcohols that are widely applicable in asymmetric synthesis.     

Low-valent Titanium Induced Simultaneous Reduction of Nitro Group and S-S Bond in Nitrodisulfides: A Novel Method For the Synthesis of Benzothiazoline,Benzothiazoles and 2,3-Dihydro- 1,5-benzothiazepines

The simultaneous reduction of nitro group and S-S bond in nitrodisulfides by TiCl₄/Sm system leds to the active intermediates 2, which were “living” double-anions (nitride anions and sulfide anions) in situ. These new anion species reacted smoothly with aldehydes or ketones, acid chlorides or acid anhydrides and α,β-unsaturated ketones respectively to afford the desired benzothiazolines, benzothiazoles and 2,3-dihydro-1,5-benzothiazepines in good yields under mild and neutral conditions.     

Hetero Diels-Alder reactions of 1-amino-3-siloxy-1,3-butadienes under strictly thermal conditions

The hetero Diels-Alder reaction of 1-amino-3-siloxy-1,3-butadiene (1a) with a range of unactivated aldehydes proceeds readily under remarkably mild conditions: at room temperature and in the absence of Lewis acid catalysts. The cycloadducts are formed in good yields and can be converted directly to the corresponding dihydro-4-pyrones using acetyl chloride. Ketones and imines are also reactive in hetero Diels-Alder reactions with this diene.     

Phosphomolybdic acid-catalyzed efficient one-pot three-component aza-Diels-Alder reactions under solvent-free conditions: a facile synthesis of trans-fused pyrano- and furanotetrahydroquinolines

trans-Fused pyrano- and furanotetrahydroquinoline derivatives have been synthesized via a one-pot three-component aza-Diels-Alder reaction of aryl imines formed in situ from aromatic amines and arylaldehydes with cyclic enol ethers,such as 3,4-dihydro-2H-pyran and 2,3-dihydrofuran,in the presence of 1 mol % of phosphomolybdic acid under solvent-free conditions at room temperature in good to excellent yields.     

N-Phenacylpyridinium bromides in the one-pot synthesis of 2,3-dihydrofurans

A one-pot three-component process for the synthesis of highly functionalized 2,3-dihydrofurans from 1,3-dicarbonyl compounds, aromatic aldehydes, and N-phenacylpyridinium bromides has been developed. Benzylidene 1,3-dicarbonyl compounds could be generated in situ from aromatic aldehydes and 1,3-dicarbonyl compounds and then reacted smoothly with N-phenacylpyridinium bromides to produce 2,3-dihydrofurans in moderate to good yields. Piperidine/acetonitrile is the optimal condition for this process. To increase the efficiency of this reaction, this one-pot process was also conducted solvent-free under classical heating conditions and grinding conditions.     

Access to 2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyridin]-2-ones via amino acid derived phosphine-catalyzed asymmetric [4+2] annulation with easily available oxindole-derived α,β-unsaturated imines

The phosphine-catalyzed asymmetric [4+2] annulation of vinyl ketones with more easily available oxindole-derived α,β-unsaturated imines has been further developed in the presence of an easily available multifunctional thiourea-phosphine catalyst derived from natural amino acid, providing the enantioselective synthesis of 2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyridin]-2-ones in good yields and moderate de values with higher enantioselectivities under mild conditions.     

Convenient Syntheses of Functionalized Dialkyl Ketones and Alkanoylsilanes: 1-(Benzotriazol-1-yl)-1-phenoxyalkanes as Alkanoyl Anion Equivalents

(Benzotriazol-1-yl)-1-phenoxyalkanes 10, prepared by two-step transformations of the corresponding aldehydes, are readily deprotonated at the methine group by BuLi. Subsequent reactions with alkyl halides, aldehydes, ketones, and imines yield the corresponding substituted derivatives that undergo hydrolysis under acidic conditions to afford the expected functionalized ketones 13, 15,17, 19, 21, 24, and25. Two successive lithiations of (benzotriazolyl)phenoxymethane, each followed by reaction with a trialkylsilyl chloride, alkyl halide, aldehyde, or ketone, generate similar intermediates 27, 29, 31,33, and 36. Subsequent hydrolyses of 27, 29, 31, 33, and 36 yield the functionalized ketones 28, 30, and 32 and the alkanoylsilanes 34 and 37 in good yields.     

Cu0-catalysed 1,3-dipolar cycloadditions of α-amino acid derived N,N-cyclic azomethine imines to ynones

A series of 20 CuAIAC reactions between eight 4-acylamino substituted pyrazolidine-3-one-1-azomethine imines and four terminal ynones were performed using Cu⁰ as catalyst. The corresponding fluorescent cycloadducts were obtained in very high yields upon simple workup. Thus, Cu-metal turned out to be a better catalyst than Cu^I in terms of yield and ease of isolation. Availability of azomethine imines, mild reaction conditions, and simple workup enable a “click” access to libraries of densely substituted 2,3-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-ones. Reactivity of differently substituted dipoles was evaluated experimentally and by quantum chemical methods (DFT).     

FeCl3-promoted formation of C–C bonds: synthesis of substituted quinolines from imines and electron-deficient alkynes

An efficient method for the synthesis of substituted quinolines is described. By FeCl₃-promoted C–C bond formation, 2,3-disubstituted and 2,3,4-trisubstituted quinolines were successfully synthesized from the reaction between imines and electron-deficient alkynes in good yields. The reaction can also be performed in a sequential three-component manner by using aromatic amines and aromatic aldehydes instead of the corresponding imines.     

Reactivity of lithium 2,3-dihydro-1-benzothiophene-1-oxide toward aldehydes and imines and DFT calculations

The sulfinyl carbanion derived from 2,3-dihydro-1-benzothiophene-1-oxide and its lithium salt has been investigated by DFT calculations. The lithium carbanion was treated with aldehydes and imines to give chiral hydroxy and amino derivatives, with high stereoselectivity at the carbon α to the sulfoxide group (trans diastereoisomers), but with low diastereoselectivity at the hydroxyl or amine group. DFT calculations were used to rationalize the different stereochemical behavior of cyclic and acyclic lithiated sulfoxides in the reaction with aldehydes and azomethines.     

Synthesis of Dendritic Schiff Base Ligands for Titanium Catalyzed Enantioselective HDA Reaction of Danishelfsky‘ s Diene with Aldehydes

A new type of dendritic NOBIN derived Schiff base ligands has been synthesized and applied to titanium catalyzed hetero-Diels-Alder reaction of Danigheifsky‘ s diene and aldehydes, affording the corresponding 2-substituted 2,3-dihydro-4H-pyran-4-one in good yields and moderate enantioselectivities (up to 59.2% ee). It was found that the size of dendron attached to the tridentated ligands has slight impact on the enantioselectivtty of the reaction and the second generation of dendritic ligand exhibited the best enantioselectivity.     

Ionic Liquid–Catalyzed One-Pot Synthesis of 1H-Pyrano[2,3-d]pyrimidin-2(8aH)-one Derivatives via Three-Component Reaction under Solvent-Free Conditions

A series of 1H-pyrano[2,3-d]pyrimidin-2(8aH)-one derivatives were synthesized via one-pot, three-component reaction of aromatic aldehydes, urea or thiourea, and 3,4-dihydro-2H-pyran using the green and inexpensive Brønsted acidic ionic liquid 1-methyl-2-pyrrolidinone hydrosulfate ([Hnmp]HSO₄) as catalyst under solvent-free conditions. The method has several advantages such as mild conditions, shorter reaction time, good yields, and environmentally benign procedure. Moreover, the catalyst could be recovered conveniently and reused at least four times without evident loss of activity.     

An effective method to prepare imines from aldehyde, bromide/epoxide, and aqueous ammonia

A three-component reaction of aldehydes, alkyl bromides, and ammonia to form imines was studied. Aqueous ammonia was applied as the nitrogen source and solvent in the reaction. For the aromatic aldehyde, the product yields are good to excellent and the reaction conditions are mild to be compatible with a range of functional groups. The reaction of aldehydes and aqueous ammonia with epoxides was also studied and imines bearing a vicinal hydroxyl group can be obtained efficiently and regioselectively. And studies showed that this method allows the synthesis of primary amines and especially 1,2-amino alcohol selectively in high yield. It is proposed that the reaction pathway might involve a key intermediate of hydrobenzamide.     

Cation-exchange resin as an efficient hetero-geneous catalyst for one-pot three-component synthesis of 2,3-dihydro-4(1H)-quinazolinones

Various mono- and disubstituted 2,3-dihydro-4(1H)-quinazolinones were synthesized efficiently by a one-pot three-component condensation of isatoic anhydride, aromatic aldehydes, and ammonium salts or primary amines using a strong acidic cation-exchange resin as the catalyst in EtOH–H₂O solution. The catalyst is cheap, efficient, stable, and reusable under the reaction conditions. The novel method offers several advantages, such as excellent yields, environmentally friendly reaction media, and simple procedure.     

A convenient and versatile approach to 2,3-dihydro-4H-pyran-4-ones via tandem aldol reaction-conjugate addition

Aldol reaction of enones 1a-c with aldehydes followed by intramolecular conjugate addition proceeded smoothly to afford corresponding 2,3-dihydro-4H-pyran-4-ones in high yields under mild conditions.     

Efficient synthesis of highly functionalized indazoles and 2,3-dihydro-1,2-benzisoxazoles by reaction of stable Fischer dienyl carbenes and isocyanides

A range of stable chromium and tungsten Fischer dienyl carbenes have been prepared by [3+2] cycloaddition of alkenylethynyl carbene complexes with nitrones or diazoalkanes. Treatment of these systems with isocyanides gives entry to highly functionalized 2,3-dihydro-1,2-benzisoxazoles and indazoles in a completely regioselective fashion, under mild conditions, and with high yields. This methodology can be also applied to the preparation of analogous naphthoisoxazoles starting from arylethynyl Fischer complexes. Reductive cleavage of the isoxazole moiety in the prepared heterocycles also enables the efficient synthesis of highly substituted p-aminophenols.     

l-Proline catalyzed synthesis of densely functionalized pyrido[2,3-d]pyrimidines via three-component one-pot domino Knoevenagel aza-Diels-Alder reaction

Highly functionalized hitherto unreported pyrido[2,3-d]pyrimidines have been concisely synthesized in good yields via l-proline catalyzed one-pot three-component domino coupling of 6-amino-1,3-dimethyluracil, aldehydes, and dialkyl acetylenedicarboxylates under mild conditions for the first time. The MCR process involves Knoevenagel condensation followed by [4+2] cycloaddition reaction. No co-catalyst or activator is required for this MCR. The molecular structures of two representative pyrido[2,3-d]pyrimidines 4a and 4h were confirmed by single crystal X-ray diffraction.