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An efficient synthetic strategy towards beta‐lactams, amides, and esters involving “in situ” generation of ketenes and subsequent trapping with nucleophiles is presented. Carbonylation of carbene radical intermediates using the cheap and highly active cobalt(II) tetramethyltetraaza[14]annulene catalyst [Co(MeTAA)] provides a convenient one‐pot synthetic protocol towards substituted ketenes. N‐tosylhydrazones are used as carbene precursors, thereby bridging the gap between aldehydes and ketenes. Activation of these carbene precursors by the metalloradical cobalt(II) catalyst affords CoIII‐carbene radicals, which subsequently react with carbon monoxide to form ketenes. In the presence of a nucleophile (imine, alcohol, or amine) in the reaction medium the ketene is immediately trapped, resulting in the desired products in a one‐pot synthetic protocol. The β‐lactams formed upon reaction with imines are produced in a highly trans‐selective manner. [ABSTRACT FROM AUTHOR]

Asymmetric total synthesis of (+)-sphaerodiol (2) has been achieved. A key step is an intramolecular [2 + 2] cycloaddition of alkyl(phenylthio)ketene for rapid assembly of the decalin ring. [ABSTRACT FROM AUTHOR]

The influence of ketene as possible intermediate for the reaction of methanol to aromatics was investigated over HZSM‐5 catalysts (Si/Al ratio of 15 and 9) using diketene‐acetone (2,2,6‐trimethyl‐4H‐1,3‐dioxin‐4‐one) as ketene precursor under atmospheric pressure at 300 °C. The physicochemical properties of HZSM‐5 catalysts were investigated by NH3‐TPD, BET, XRD and ICP analysis. The distribution of products was measured by GC‐Mass spectrometer. These catalysts exhibit high reactivity and selectivity for the conversion of methanol and/or ketene to hydrocarbons (mostly high octane number xylenes, trimethylbenzenes, tetramethylbenzenes and ≤1.5% benzene). The conversion of neat diketene‐acetone over these catalysts produced aromatics compounds and significant amounts of oxygenated compounds (ketone, carboxylic acid, phenol, furan, pyran and others) depending on Si:Al ratio. These catalysts with high acid content produced small amount of saturated and unsaturated hydrocarbons without releasing significant amount of gas. The ketene and produced oxygenates from ketene appears to act as intermediates for the formation of aromatic compounds. [ABSTRACT FROM AUTHOR]

The stereoselective properties of modified thiourea organocatalysts were tested in the Friedel–Crafts alkylation of indole with 5-arylidene-2,2-dimethyl-1,3-dioxane-4,6-diones, which produces chiral 5-((1H-indol-3-yl)(aryl)methyl)-2,2-dimethyl-1,3-dioxane-4,6-diones. Based on a tentative reaction mechanism for ((S)-N-benzyl-2-(3-(3,5-bis(trifluoromethyl)phenyl)thioureido)-N,3,3-trimethylbutanamide organocatalysts, modifications were applied in four selected regions. Systematic structure-stereoselectivity relationship study allowed designing the best efficient organocatalyst for the investigated Friedel–Crafts alkylation of indole with 5-arylidene-2,2-dimethyl-1,3-dioxane-4,6-diones. [ABSTRACT FROM PUBLISHER]

Regioselective synthesis of 1,1-bis(methylthio)-6-nitro-5-arylhex-1-en-3-one has been achieved from α-alkenoylketene dithioacetals with nitromethane using l-proline catalyst via Michael addition. The synthesized compounds 3a- j were well characterised by NMR and mass spectral techniques. One of the structures was confirmed by single crystal XRD. The enantioselectivity and its specific rotation of synthesized compounds were analyzed. All the compounds were screened for their anti-oxidant property, most of the compounds having the ability to scavenge free radicals. Graphical Abstract: [ABSTRACT FROM AUTHOR]

Stereoselective synthesis of novel monocyclic trans-3-halogenated-4-pyrazolyl- β -lactams 5 is described. The reaction of ketene derived from α -bromo/chloroethanoic acids 4 using POCl3 and Et3N with pyrazolyl substituted imines 3a–d in refluxing toluene resulted exclusive formation of trans- β -lactams through [2 + 2] through cycloaddition reaction. The chemical structures of all the newly synthesized β -lactams were verified on the basis of spectroscopic techniques such as FTIR, 1H NMR, 13C NMR, and elemental analysis (CHN). The trans configuration of β -lactams 5 was assigned with respect to position of C3-H and C4-H. The novel β -lactams 5 are potential synthons for azetidines, aziridines, 3-unsubstituted azetidinones, 3-alkyl-halo-azetidinones, and promising biologically active agents. [ABSTRACT FROM AUTHOR]

Non-crystal microporous starch (NCMS) containing microporous and amorphous structures was prepared from native corn starch by heat treatment and solvent exchange. NCMS can be used as fillers, coatings, and raw materials in the preparation of various denatured starch because of its specific surface area and amorphous region. However, the hydrophilicity of NCMS limits its applications in papermaking. Thus, in this study, NCMS was reacted with alkyl ketene dimer (AKD) to prepare hydrophobic NCMS (H-NCMS), which is more stable, and convenient for storage and use in surface sizing. The optimal preparation conditions were selected using single-factor tests. The product, which was prepared at 55 °C for 3 h with an AKD dosage of 80%, had a sizing degree of 67 s. Characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy confirmed that NCMS, H-NCMS, and sized paper were obtained. The thermal stability and hydrophobicity of the paper were measured using thermogravimetric analysis (TGA) and water contact angles, respectively. The results indicated that the sized paper has excellent thermal stability and hydrophobicity after surface sizing with 0.9% H-NCMS. Food wrapping paper with excellent strength and hydrophobicity was successfully prepared using H-NCMS and ultrasonic-assisted wheat straw pulp (UWP). [ABSTRACT FROM AUTHOR]

Enantioselective allylic substitution with enolates derived from aliphatic esters under mild conditions remains challenging. Herein we report iridium-catalyzed enantioselective allylation reactions of silyl ketene acetals, the silicon enolates of esters, to form products containing a quaternary carbon atom at the nucleophile moiety and a tertiary carbon atom at the electrophile moiety. Under relatively neutral conditions, the allylated aliphatic esters were obtained with excellent regioselectivity and enantioselectivity. These products were readily converted into primary alcohols, carboxylic acids, amides, isocyanates, and carbamates, as well as tetrahydrofuran and γ-butyrolactone derivatives, without erosion of enantiomeric purity. [ABSTRACT FROM AUTHOR]

A novel five-component strategy involving commercially available diketene, primary amines, malononitrile and various benzaldehydes for the synthesis of densely functionalized 1,4-dihydropyridines in good yields was achieved. The reaction pathway involves a sequential ring-opening of diketene/enamine formation/Knoevenagel-condensation/Michael addition and 6- exo-tet cyclization, resulting in multiple bond-formation events including two C–C and three C–N bonds ultimately leading to the formation of the respective 1,4-dihydropyridines. [ABSTRACT FROM AUTHOR]

Causticized calcium carbonate (CCC), a solid waste derived from kraft black recovery process, can be used as an alternative for the conventional precipitated calcium carbonate (PCC). However, the application of the CCC has been limited due to its low sizing efficiency in its filled paper. In this study, the characteristics of the CCC were studied aiming to improve the alkyl ketene dimer (AKD) sizing performances of the CCC filled papers, and the results were compared with those from PCC filled papers. The results showed that the CCC had higher pore structure, higher specific surface area, and more negative charge density than the PCC, thus leading to a higher cationic AKD adsorption onto the CCC filler. The lower AKD sizing efficiency in the CCC filled paper can be explained by the combination of higher AKD adsorption and migration, both of which resulted in preferred AKD adsorption onto/into the CCC fillers, rather than the cellulose fibers. Based on the above, the prior addition of polyamide-polyamine epichlorhydrin (PAE) resin to the CCC filler system was proposed to remedy the related issues, thus improving the sizing efficiency. [ABSTRACT FROM AUTHOR]

An I2-catalyzed synthesis of dialkenyl oxindoles from isatins andα-cyano ketene ethylene dithioacetal is described. Both electron-withdrawing groups (EWGs) and alkylthio groups exert effects on the reactivities of ketene dithioacetals. Density functional theory (DFT) calculations suggested that the highest negative charge density on theα-carbon ofα-cyano ketene ethylene dithioacetal and the largest positive charge on C(3) of the related key intermediate are both responsible for the superior activity ofα-cyano ketene ethylene dithioacetal. The cationic intermediate derived from 2-(1,3-dithian-2-ylidene)acetonitrile is the most stable but the least positive, thus the corresponding alkenylhydroxyoxindole is the thermally stable and separable product. Other ketene dithioacetals are less nucleophilic, and their corresponding cationic intermediates are probably not positive enough to enable further transformation. [ABSTRACT FROM PUBLISHER]

β-Dicarbonyl compounds with the ketene dithioacetyl fragment were obtained by condensation of acetyl ketene dithioacetals with ethyl polyfluorocarboxylates and diethyl oxalate in the presence of NaH in Et2O at room temperature in 65-94% yields. These compounds can be used for the synthesis of 6-polyfluoroalkyl(ethoxycarbonyl)-2-methylsulfanyl-4-pyrones. [ABSTRACT FROM AUTHOR]

The reactions of butadienylketene with variety of 1,4-diazabuta-1,3-dienes are studied. The reactions resulted in the formation of previously unknown functionalized cis butadienyl-4-iminomethyl-azetidin-2-ones and butenylidene-butadienyl-[2,2′-biazetidine]-4,4′- diones. Butadienyl ketene reacts in fashion with both iminic portion of 1,4- diazabuta-1,3-dienes and competitive [4+2] of 1,4-diazabuta-1,3-dienes as 4π component with butadienyl ketene as 2π component are not observed. [ABSTRACT FROM AUTHOR]

A new BINOL-derived chiral phosphoric acid bearing 2,4,6-trimethyl-3,5-dinitrophenyl substituents at the 3,3′-positions was developed. The utility of this chiral phosphoric acid is demonstrated by a highly enantioselective ( ee up to >99 %) and diastereoselective ( syn/ anti up to >99:1) asymmetric Mukaiyama-Mannich reaction of imines with a wide range of ketene silyl acetals. Moreover, this method was successfully applied to the construction of vicinal tertiary and quaternary stereogenic centers with excellent diastereo- and enantioselectivity. Significantly, BINOL-derived N-triflyl phosphoramide constitutes a complementary catalyst system that allows the title reaction to be applied to more challenging imines without an N-(2-hydroxyphenyl) moiety. [ABSTRACT FROM AUTHOR]

Geometric isomerization can expand the scope of biological activities of natural products. The observed chemical diversity among the pseurotin-type fungal secondary metabolites is in part generated by a trans to cis isomerization of an olefin. In vitro characterizations of pseurotin biosynthetic enzymes revealed that the glutathione S-transferase PsoE requires participation of the bifunctional C-methyltransferase/epoxidase PsoF to complete the trans to cis isomerization of the pathway intermediate presynerazol. The crystal structure of the PsoE/glutathione/presynerazol complex indicated stereospecific glutathione-presynerazol conjugate formation is the principal function of PsoE. Moreover, PsoF was identified to have an additional, unexpected oxidative isomerase activity, thus making it a trifunctional enzyme which is key to the complexity generation in pseurotin biosynthesis. Through the study, we identified a novel mechanism of accomplishing a seemingly simple trans to cis isomerization reaction. [ABSTRACT FROM AUTHOR]

Trapping of the ketene generated from the thermolysis of 2-methyl-2-phenyl-1,3-dioxane-4,6-dione-keto-dioxinone at 50 °C with primary, secondary, or tertiary alcohols gave the corresponding dioxinone β-keto-esters in good yield under neutral conditions. These intermediates were converted by palladium(0)-catalyzed decarboxylative allyl migration and aromatization into the corresponding β-resorcylates. These transformations were applied to the syntheses of the natural products (±)-cannabiorcichromenic and (±)-daurichromenic acid. [ABSTRACT FROM AUTHOR]

A novel and facile one-pot synthesis of 1-substituted tetrahydro- β-carbolines by cyclocondensation of ketene S, S-acetals with tryptamine in presence of InCl and TFA as co-catalysts by Bischler-Napieralski cyclization is described. The reaction involves formation of one C-N bond, one C-C bond and a new ring annulation over an indole moiety. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Efficient protocols enabling the rapid installation of trifluoromethyl, as well as further functionalized fluoroalkyl groups by an electrophilic perfluoroalkylation of lactam-derived ketene silyl amides (KSAs) using hypervalent iodine reagents 1 and 2 have been developed. [ABSTRACT FROM AUTHOR]

The synthesis of donor-stabilized silacyclobutanone 2 was successfully realized by the reaction of silacyclopropylidene 1 with benzaldehyde in the presence of a Lewis acid catalyst. Of particular interest, silacyclobutanone 2 evolves at room temperature via a retro-[2 + 2]-cycloaddition reaction, leading to an original NHC-stabilized 1-silaketene 4 and cis-stilbene. [ABSTRACT FROM AUTHOR]

The condensation of organic salt adducts generated by cyclic thioureas and 2-di(methylsulfanyl)methylenemalononitrile with dialkyl acetylenedicarboxylates selectively leads to either tetrasubstituted thiophenes (2 examples) or hitherto unreported bicyclic guanidine (13 examples) derivatives, with controlling factors being ring size, the substituent on the S -methylisothiourea and the bulk of the substituent on the dialkyl acetylenedicarboxylates. Green solvents, short reaction time; easy purification, high yield and selectivity, are key feature of this protocol in the condensation step. The performances of the reactions in the presence of free-base and a catalyst at room temperature is unprecedented in the synthesis of thiophene frameworks and functionalized bicyclic guanidine scaffolds. [ABSTRACT FROM AUTHOR]

In the presence of a strong carbon acid, the sequential Mukaiyama–Michael reaction using two different Michael acceptors proceeded and the reaction of ketene silyl acetal derived from EtOAc with α-pyrones as primal acceptors yielded the corresponding cyclic ketene silyl acetals, which were reactive enough to undergo the following reaction with second acceptors. [ABSTRACT FROM AUTHOR]

A series of 3,7-diaryl-6,7-dihydro-5 H-6-substituted-thiazolo[3,2-a]pyrimidin-5-ones ( 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h, 3i, 3j) were synthesized by the [4+2] of 2-arylideneamino-4-arylthiazoles (2a-j) with in situ generated monosubstituted ketenes under PTC conditions coupled with ultrasonication in good to excellent yields. All the synthesized compounds were characterized on the basis of their spectral (IR, PMR, and Mass) and analytical data. The synthesized compounds were also screened for their insecticidal activity against Helicoverpa armigera and some of them showed promising activity. [ABSTRACT FROM AUTHOR]

Aromatic polyketides make up a large class of natural products with diverse bioactivity. During biosynthesis, linear poly-β-ketone intermediates are regiospecifically cyclized, yielding molecules with defined cyclization patterns that are crucial for polyketide bioactivity. The aromatase/cyclases (ARO/CYCs) are responsible for regiospecific cyclization of bacterial polyketides. The two most common cyclization patterns are C7-C12 and C9-C14 cyclizations. We have previously characterized three monodomain ARO/CYCs: ZhuI, TcmN, and WhiE. The last remaining uncharacterized class of ARO/CYCs is the di-domain ARO/CYCs, which catalyze C7-C12 cyclization and/or aromatization. Di-domain ARO/CYCs can further be separated into two subclasses: "nonreducing" ARO/CYCs, which act on nonreduced poly-β-ketones, and "reducing" ARO/CYCs, which act on cyclized C9 reduced poly-β-ketones. For years, the functional role of each domain in cyclization and aromatization for di-domain ARO/CYCs has remained a mystery. Here we present what is to our knowledge the first structural and functional analysis, along with an in-depth comparison, of the nonreducing (StfQ) and reducing (BexL) di-domain ARO/CYCs. This work completes the structural and functional characterization of mono- and di-domain ARO/CYCs in bacterial type II polyketide synthases and lays the groundwork for engineered biosynthesis of new bioactive polyketides. [ABSTRACT FROM AUTHOR]

The direct α-Csp2--H functionalization and thiomethylation of α-oxoketene dithioacetals (DTAs) has been accomplished with dimethyl sulfoxide (DMSO) in the presence of iodine and a copper(I) salt for the first time. A prerequisite is the in situ iodination of the α-Csp2 atom of dithioacetals that could offer other reaction channels. The operationally simple one-pot protocol includes region-defined consecutive iodination and sulfenylation of the challenging α-Csp2--H bond of dithioacetals employing cheap and readily available reagents. DMSO here plays a dual role as thiomethyl source and solvent. [ABSTRACT FROM AUTHOR]

Stereo- and chemodivergent enantioselective reaction pathways are observed upon treatment of alkylarylketenes and trichloroacetaldehyde (chloral) with N-heterocyclic carbenes, giving selectively either β-lactones (up to 88:12 dr, up to 94 % ee) or α-chloroesters (up to 94 % ee). Either 2-arylsubstitution or an α-branched iPr alkyl substituent within the ketene favours the chlorination pathway, allowing chloral to be used as an electrophilic chlorinating reagent in asymmetric catalysis. [ABSTRACT FROM AUTHOR]

Straightforward synthetic protocols for the synthesis of new ethyl-substituted dichlorinated chondramides with different methyl-substitution patterns in the polyketide fragment have been developed. The methyl groups at the ε-position can be removed completely without a significant influence on the biological activity. In contrast, after removal of the α-methyl group, a significant drop in activity is observed. This is also observed if the configuration of the α-methyl group is inverted. [ABSTRACT FROM AUTHOR]

A novel C–N cleavage strategy for the regioselective synthesis of 2-nitroso heterocyclic ketene aminals (HKAs) has been established. In this procedure, the C–N bond of nitrobenzenyl nitroethene is cleaved by the oxidation reaction and selective nitroso reaction of the α-carbon in HKAs. The presented synthetic route features excellent selectivity, straightforward purification and simple starting materials. [ABSTRACT FROM AUTHOR]

In addition to spirangiens A ( 1) and B ( 2), 11 new spirangiens have been isolated from the myxobacterium Sorangium cellulosum (strain So ce90). Spirangiens A and B consist of a polyketide backbone with a spiroketal core, 13 stereocenters, and a conjugated pentaene chromophore with a terminal carboxylic acid. The structures of the derivatives were elucidated on the basis of NMR and MS data. The stereochemistry was determined by 1D and 2D NMR experiments as well as by biosynthetic studies. Four derivatives 6- 9 differ in the cis/ trans double-bond geometry of the pentaene chromophore side-chain and four of them ( 4- 6 and 10) are characterized by the absence of methoxy groups. Derivative 13 does not exhibit the spiroketal core due to reduction of the acetalic carbonyl group. In addition, 11 and 12 exhibit the opposite configuration at C-20 compared with the known spirangiens A and B. Derivative 3 contains an additional hydroxy group at the end of the side-chain. All the new spirangien derivatives, as well as spirangiens A and B, were tested in cytotoxicity assays against two different cell lines and showed strong cytotoxic activity. Even though spirangien A was identified as the most cytotoxic compound, other derivatives, such as 3, 4, 6, and 10, showed significantly higher activity against the KB-3-1 cell line than against the nontransformed cells (L-929). [ABSTRACT FROM AUTHOR]

The first formal [3+2] cycloaddition using a-trifluoromethyl-(indol- 3-yl)methanols as the trifluoromethylated C3 1,3-dipoles for the construction of the five-membered carbocycle of 1-trifluoromethylated cyclopenta[b]indole alkaloids is described. An unprecedented step-wise dehydrative alkenylation of α-trifluoromethyl alcohols was revealed as the crucial transformation. [ABSTRACT FROM AUTHOR]

CuCl2-mediated intramolecular C-H/C-H cross-dehydrogenative coupling (CDC) of thioalkyl-substituted α-acetyl or α-aroyl ketene N,S-acetals afforded 2-thioalkyl indoles. Tunable C--S bond transformations of the resultant indoles led to highly functionalized N-heterocyclic compounds. A β-thioalkyl is necessary to activate the N,S-acetal substrate and enable the CDC reaction to occur, and the relevant mechanism studies revealed that the CDC reaction follows a radical pathway. [ABSTRACT FROM AUTHOR]

An efficient and simple procedure for the synthesis of ketene imines via a one-pot three-component reaction of cyclohexyl isocyanide, diethyl acetylenedicarboxylate, and 1,4-dihydropyridines under ultrasonic irradiation is described. The remarkable advantages are the simplicity of the experimental procedures, high yields, and short reaction times. [ABSTRACT FROM AUTHOR]

Aroylketenes generated by thermolysis of 6-aryl-2,2-dimethyl-4 H-1,3-dioxin-4-ones reacted with ( Z)-2-[4a,7,10-trimethyl-1,3,4,4a,5,10b-hexahydrophenanthridin-6(2 H)-ylidene]acetamide to give ( Z)-3-aryl-3-hydroxy- N-{(Z)-2-[4a,7,10-trimethyl-1,3,4,4a,5,10b-hexahydrophenanthridin-6(2 H)-ylidene]acetyl}acrylamides whose structure was confirmed by X-ray analysis. [ABSTRACT FROM AUTHOR]

Sonochemical method is an innovative task for the sustainable chemical research industry. In this work, an attempt was made to synthesize bis-azetidin-2-ones ( 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j) by ([2 + 2] ketene-imine ) in the presence of zeolite by using both conventional and under ultrasound irradiation. The synthesized compounds have been elucidated on the basis of their elemental analysis and spectral data (IR, 1H NMR, 13C NMR, and mass spectra) to evaluate its performance obtained under ultrasonic waves. It was observed that complete conversion to azetidin-2-ones occurred in 1-2 h by sonochemical method and in 15-46 h by conventional method. Finally, it has been observed that 2-azetidinones synthesis using sonochemical method is an energy efficient and environmentally friendly technique over the conventional method. [ABSTRACT FROM AUTHOR]

A series of novel ketene dithioacetals has been synthesized by condensation of active methylene compounds with carbon disulfide in the presence of dialkyl acetylenedicarboxylates. This regioselective reaction provides products in good yield by a Michael addition reaction followed by 5-endo-trig intramolecular cyclization. Structures of synthesized compounds have been characterized by elemental analysis, FT-IR,1H NMR,13C NMR, and mass spectra. The crystal structure of compound (C17H22O6S2) has been determined. [ABSTRACT FROM AUTHOR]

The catalytic asymmetric synthesis of alkyl fluorides, particularly α-fluorocarbonyl compounds, has been the focus of substantial effort in recent years. While significant progress has been described in the formation of enantioenriched secondary alkyl fluorides, advances in the generation of tertiary alkyl fluorides have been more limited. Here, we describe a method for the catalytic asymmetric coupling of aryl alkyl ketenes with commercially available N-fluorodibenzenesulfonimide (NFSI) and C6F5ONa to furnish tertiary α-fluoroesters. Mechanistic studies are consistent with the hypothesis that the addition of an external nucleophile (C6F5ONa) is critical for turnover, releasing the catalyst (PPY*) from an N-acylated intermediate. The available data can be explained by a reaction pathway wherein the enantioselectivity is determined in the turnover-limiting transfer of fluorine from NFSI to a chiral enolate derived from the addition of PPY* to the ketene. The structure and the reactivity of the product of this proposed elementary step, an α-fluoro-Nacylpyridinium salt, have been examined. [ABSTRACT FROM AUTHOR]

Bis-naphtho-γ-pyrones are an important group of aromatic polyketides derived from fungi. They have a variety of biological activities including cytotoxic, antitumor, antimicrobial, tyrosine kinase and HIV-1 integrase inhibition properties, demonstrating their potential applications in medicine and agriculture. At least 59 bis-naphtho-γ-pyrones from fungi have been reported in the past few decades. This mini-review aims to briefly summarize their occurrence, biosynthesis, and structure, as well as their biological activities. Some considerations regarding to synthesis, production, and medicinal and agricultural applications of bis-naphtho-γ-pyrones are also discussed. [ABSTRACT FROM AUTHOR]

Theconversion of dithianylium cations into α-azo ketenedithioacetals via addition of polymer-bound diazonium precursors isshown. This new procedure allows the synthesis of α-azo ketenedithioacetals in one step within 2–90 min at rt and yieldshighly pure compounds that do not have to be purified in most cases.The α-azo ketene dithioacetals obtained have been shown to bevaluable intermediates for the synthesis of hydrazones, α-halogenatedα-azo ketene dithioacetals, and azo-functionalized dienes. [ABSTRACT FROM AUTHOR]

Heterocyclic ketene aminals (HKAs) have proven to be a class of fascinating and versatile synthetic building blocks for use in the construction of heterocyclic or fused heterocyclic compounds. The bicyclic, tricyclic, and tetracyclic nitrogen-containing analogues with HKA skeleton widely exist in natural and synthetic drugs. To understand and to master the reaction properties of HKAs and to construct other new fused heterocycles in the future, it is very important and necessary to investigate HKAs. This review covers the relevant contributions of the last twenty years with respect to the developments and applications of HKAs, categorized by reaction type. [ABSTRACT FROM AUTHOR]

A total of 11 new α,α -dioxoketene - N,S -acetals ( 2a- 2k) and two new α,α -dioxoketene - N,N -acetals ( 3j and 3k) have been synthesised by treating 3 -[bis(methylthiol)methylene]pentane -2,4 -dione ( 1) with increasing mole ratios of secondary aliphatic amines at room temperature, in either toluene or ethanol. Eight non -cyclic N -methylalkyl and N -ethylalkyl amines and the azacyclopentane of pyrrolidine yielded exclusively mono -substituted N, S -acetals ( 2a- 2i), while the azacyclohexanes of piperidine and morpholine yielded the mono -substituted N, S -acetals 2j and 2k and the double -substituted N, N -acetals 3j and 3k. The conversion yields for the reactions in ethanol are considerably higher than those in toluene. Furthermore, the secondary aliphatic amines with an N -methylalkyl moiety, which have one primary α -carbon and less steric crowding around the nucleophilic nitrogen, appear to be more reactive towards 1 than those with the N -ethylalkyl group, which have two primary α -carbons; further, the latter amines are more reactive than the amines with secondary α -carbons. [ABSTRACT FROM AUTHOR]

ABSTRACT The continued evolution of functional materials that contribute to pressing societal challenges requires the development of powerful synthetic methodologies in polymer systems. Since their discovery by Staudinger in the early 20th century, the unique chemistry of ketenes have fascinated synthetic chemists and been the driver of revolutionary applications in photolithography, medicinal chemistry, and commodity materials. The versatile chemistry of ketenes, specifically their ability to act as an electrophile and/or undergo cycloaddition reactions, has recently been shown to provide a powerful platform for the design of next-generation materials. This Highlight focuses on the history of ketenes in materials science and their recent renaissance in polymer chemistry, with specific focus being given to methodologies that provide reliable access to this important functional group in polymer systems. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3769-3782 [ABSTRACT FROM AUTHOR]

The formation of ketene (H2CCO, ethenone) in polar and apolar ices was studied with in situ 0.8 MeV proton irradiation, far-UVphotolysis, and infrared spectroscopic analyses at 10–20 K. Using isotopically enriched reagents, unequivocal evidencewas obtained for ketene synthesis in H2O-rich and CO2-rich ices, and several reaction products were identified. Results from scavenging experiments suggested that ketene was formed by free-radical pathways, as opposed to acid–base processes or redox reactions. Finally, we use our results to draw conclusions about the formation and stability of ketene in the interstellar medium. [ABSTRACT FROM AUTHOR]

Highlights: [·] A series of trimethoxy aryl S,N-ketene acetals has been synthesized. [·] Compounds were evaluated against extracellular promastigotes and intracellular amastigotes of Leishmania donovani. [·] Of the six screened compounds, 7a and 7b have shown interesting antiamastigote activity with IC50s of 1.2 and 2.1μM and selectivity indices of 34 and 30, respectively. [·] Compounds 7a and 7b showed significant inhibition of parasite multiplication, 72% and 83%, respectively, against L. donovani/hamster model. [·] Compound 7b exhibited moderate metabolic stability with three metabolites (M1, M2 and M3) formed by CYP enzyme. [ABSTRACT FROM AUTHOR]

An efficient catalytic and highly enantioselective protonation of silyl ketene imines is described. The reaction is catalyzed by the chiral phosphoric acids TRIP or STRIP in the presence of a stoichiometric amount of methanol as the proton source and silyl acceptor. A variety of substituted racemic silyl ketene imines have been transformed into highly enantioenriched nitriles. [ABSTRACT FROM AUTHOR]

A process for the chemical modification of Chinese fir with alkyl ketene dimer (AKD) was studied. The hydrophobicity of the resulting products was evaluated by characterization of the equilibrium moisture content, water-surface contact angle, water absorption coefficient, and anti-swelling efficiency. The results indicated that when 5% AKD solution was used, the modified wood generally showed good hydrophobicity and improved dimensional stability. The result of boiling water and hexane extraction tests showed that the modified fir possessed very stable hydrophobicity. [ABSTRACT FROM AUTHOR]

The annuloselectivity defined as the annulation selectivity between [2 + 2] cycloaddition and two kinds of novel cascade [2 + 2 + 2] cycloadditions (2 ketenes + imine and ketene + 2 imines) in a Staudinger reaction to afford three classes of annulation products has been studied in depth with the density functional theory (DFT) calculations. The computed results indicate that the cascade [2 + 2 + 2] reaction of ketene 4 and ketimine 5 initiates the dimerization of the ketene as the rate-determining step, affording a lactone that further converts to α-acetylketene, followed by the [4 + 2] cycloaddition with imine 5 to furnish a 2,3-dihydro-1,3-oxazin-4-one derivative. That is very competitive to the normal Staudinger reaction. The alternative [2 + 2 + 2] cycloaddition undergoes the hetero-Diels-Alder (HDA) cycloaddition of the zwitterionic intermediates generated from ketenes and conjugated imine 11 with less steric hindrance as a good dienophile to afford 2,3,4,5-tetrahydropyrimidin-6(1H)-ones, which is the most favorable pathway in the case of the Staudinger reaction system. The HDA process is supported and confirmed experimentally by X-ray crystallography via analysis of the stereochemistry of the cycloadducts. The further investigation into the nature of the frontier molecular orbitals accounts well for the origin of the annuloselectivity. The extensive studies on ketenes containing various representative substituents reveal that ketenes with electron-donor and conjugated monosubstituents are inclined to dimerization, preferring the [2 + 2 + 2] cycloaddition of two molecules of ketenes and one molecule of imines, while less steric bulky imines with ketenes are apt to the [2 + 2 + 2] cycloaddition of one molecule of ketenes and two molecules of imines. [ABSTRACT FROM AUTHOR]

The nature of the m/z 104 ions formed by loss of CO[sub 2] or Ph-O-NCO from the molecular ions of phthalic anhydride, N-phenoxyphthalimide, and N-phenoxyisophthalimide was investigated by means of ion/molecule reactions with acetone. This allows a clear-cut differentiation of the so-obtained ions from the isomeric molecular ions of cyclopentadienylideneketene. The different intrinsic chemical reactivities of ionized cyclopentadienylideneketene and its distonoid isomer towards neutral acetone were investigated on a large-scale hybrid mass spectrometer and confirmed by density functional theory calculations. The isomeric radical cations 1[sup +.] and 8[sup +.] derived from cyclopentadienylideneketene and phthalic anhydride, respectively, are differentiated by ion/molecule reactions with acetone. [ABSTRACT FROM AUTHOR]

The photochemistry of matrix-isolated isoxazole, induced by narrowband tunable UV-light, was investigated by infrared spectroscopy, with the aid of MP2/6-311++G(d,p) calculations. The isoxazole photoreaction starts to occur upon irradiation at λ = 240 nm, with the dominant pathway involving decomposition to ketene and hydrogen cyanide. However, upon irradiation at λ = 221 nm, in addition to this decomposition, isoxazole was also found to isomerize into several products: 2-formyl-2H-azirine, 3-formylketenimine, 3-hydroxypropenenitrile, imidoylketene, and 3-oxopropanenitrile. The structural and spectroscopic assignment of the different photoisomerization products was achieved by additional irradiation of the λ = 221 nm photolyzed matrix, using UV-light with λ ≥ 240 nm: (i) irradiation in the 330 ≤ λ ≤ 340 nm range induced direct transformation of 2-formyl-2H-azirine into 3-formylketenimine; (ii) irradiation with 310 ≤ λ ≤ 318 nm light induced the hitherto unobserved transformation of 3-formylketenimine into 3-hydroxypropenenitrile and imidoylketene; (iii) irradiation with λ = 280 nm light permits direct identification of 3-oxopropanenitrile; (iv) under λ = 240 nm irradiation, tautomerization of 3-hydroxypropenenitrile to 3-oxopropanenitrile is observed. On the basis of these findings, a detailed mechanistic proposal for isoxazole photochemistry is presented. [ABSTRACT FROM AUTHOR]

A green and highly efficient method for the regioselective synthesis of imidazo[1,2-a]pyridine derivatives has been developed by annulation of heterocyclic ketene aminals (HKAs) and β-oxodithioesters (ODEs) as building blocks with aldehydes under solvent-free conditions using Et3N as the catalyst. The present green synthesis shows fascinating properties such as high regioselectivity, concise one-pot methodology, short reaction times, easy purification and avoids the use of transition metals. This methodology provides an alternative approach for easy access to highly substituted imidazo[1,2-a]pyridines in good yields. [ABSTRACT FROM AUTHOR]

Regioselective synthesis of functionalized tetrahydrobenzo[b]pyrans has been developed by multicomponent reactions (MCRs) and tandem [3 + 3] annulations of β-benzoylthioacetanilides or β-(2-haloaroyl)thioacetanilides as valuable sources with aromatic aldehydes and 5,5-dimethyl-1,3-cyclohexanedione catalyzed by triethylamine. This MCR followed by a postcondensation cyclization via an intramolecular SNAr in the presence of K2CO3 led to an unprecedented novel chromeno[2,3-b]quinoline framework containing an important chromene moiety in good yields. The reactions were very mild, convenient, and o-selective to form new fused tetracyclic target molecules. [ABSTRACT FROM AUTHOR]

Synthesis of a nonnitrogenous phenanthrene-based precursor of oxiranyl carbene yielded a photochemical source for diphenyl ketene after an in situ intramolecular rearrangement. The initial carbene precursor targeted rearranged from a putative spiro-oxiranyl intermediate to a cyclobutanone. Photolysis of the phenanthrene–cyclobutanone generated an infrared signal centered at 2100 cm −1 consistent with diphenyl ketene formation. Time-resolved infrared spectroscopy followed the reaction progress and measured bimolecular rate constants ( k obs = 9.1 × 10 6 M −1 s −1 ; butyl amine) consistent with diphenyl ketene formation. Quenching experiments combined with Stern–Volmer kinetics suggests a radical pathway is likely involved in the formation of diphenyl ketene from the phenanthrene-based precursor. The generality of the rearrangement is also discussed. [ABSTRACT FROM AUTHOR]