Walking a Fine Line with Sucrose Phosphorylase: Efficient Single‐Step Biocatalytic Production of l‐Ascorbic Acid 2‐Glucoside from Sucrose

The 2‐O‐α‐d ‐glucoside of l ‐ascorbic acid (AA‐2G) is a highly stabilized form of vitamin C, with important industrial applications in cosmetics, food, and pharmaceuticals. AA‐2G is currently produced through biocatalytic glucosylation of l ‐ascorbic acid from starch‐derived oligosaccharides. Sucrose would be an ideal substrate for AA‐2G synthesis, but it lacks a suitable transglycosidase. We show here that in a narrow pH window (pH 4.8–6.0, with sharp optimum at pH 5.2), sucrose phosphorylases catalyzed the 2‐O‐α‐glucosylation of l ‐ascorbic acid from sucrose with high efficiency and perfect site‐selectivity. Optimized synthesis with the enzyme from Bifidobacterium longum at 40 °C gave a concentrated product (155 g L^?1; 460 mm ), from which pure AA‐2G was readily recovered in ～50 % overall yield, thus providing the basis for advanced production. The peculiar pH dependence is suggested to arise from a “reverse‐protonation” mechanism in which the catalytic base Glu232 on the glucosyl–enzyme intermediate must be protonated for attack on the anomeric carbon from the 2‐hydroxyl of the ionized l ‐ascorbate substrate.     

Aldehyde‐Catalyzed Transition Metal‐Free Dehydrative β‐Alkylation of Methyl Carbinols with Alcohols

Different to the borrowing hydrogen strategy in which alcohols were activated by transition metal‐catalyzed anaerobic dehydrogenation, the direct addition of aldehydes was found to be an effective but simpler way of alcohol activation that can lead to efficient and green aldehyde‐catalyzed transition metal‐free dehydrative C‐alkylation of methyl carbinols with alcohols. Mechanistic studies revealed that the reaction proceeds via in situ formation of ketones by Oppenauer oxidation of the methyl carbinols by external aldehydes, aldol condensation, and Meerwein–Ponndorf–Verley (MPV)‐type reduction of α,β‐unsatutated ketones by substrate alcohols, affording the useful long chain alcohols and generating aldehydes and ketones as the by‐products that will be recovered in the next condensation to finish the catalytic cycle.     

Direct C‐2 Alkylation of Quinoline N‐Oxides with Ethers via Palladium‐Catalyzed Dehydrogenative Cross‐Coupling Reaction

An efficient and concise one‐pot strategy for the direct alkylation of quinoline N‐oxides via palladium‐catalyzed dual C H bonds activation has been developed. This methodology provides quinoline‐containing heterocyclic molecules in moderate to excellent yields.

     

Palladium(II) Acetate‐Catalyzed Dual C–H Functionalization and C–C Bond Formation: A Domino Reaction for the Synthesis of Functionalized (E)‐Bisindole‐2‐ones from Diarylbut‐2‐ynediamides

A domino reaction of palladium(II)‐catalyzed dual C–H functionalization with subsequent intramolecular annulation is presented. This method provides a convenient synthesis of a range of symmetrical and unsymmetrical biologically important (E)‐bisindole‐2‐ones under extremely mild reaction conditions – room temperature, green oxidant and no additive. The reaction mechanism is elucidated in light of the yield values as well as additional control experiment results.

     

sp3‐sp2 C‐C Bond Formation via Brønsted Acid Trifluoromethanesulfonic Acid‐Catalyzed Direct Coupling Reaction of Alcohols and Alkenes

A novel and efficient trifluoromethanesulfonic acid‐catalyzed sp³‐sp² C C bond formation reaction through the direct coupling of alcohols with alkenes has been realized under mild conditions. The present protocol provides an attractive approach to a diverse range of polysubstituted olefins in good to excellent yields with high stereo‐ and regioselectivities.     

Palladium‐Catalyzed Decarboxylative sp‐sp2 Cross‐Coupling Reactions of Aryl and Vinyl Halides and Triflates with α,β‐Ynoic Acids using Silver Oxide

Palladium‐catalyzed decarboxylative sp‐sp² cross‐coupling reactions of aryl and vinyl halides and triflates with α,β‐ynoic acids using silver oxide have been developed. A variety of α,β‐ynoic acids were readily decarboxylated in the presence of silver oxide and then, generated in situ, silver acetylides were coupled with electrophiles in the presence of a palladium(0) catalyst under neutral conditions, producing either symmetrical or unsymmetrical diarylacetylenes, arylalkylacetylenes and arylvinylacetylenes in good to excellent yields.     

Efficient Preparation of the Isoindoline Framework via a Six Component,Tandem Double A3‐Coupling and [2+2+2] Cycloaddition Reaction

The preparation of tetrasubstituted isoindolines from three alkyne units, two aldehyde units and a primary amine via three consecutive reactions, two aldehyde‐amine‐alkyne couplings (A³‐couplings) and a final [2+2+2] cycloaddition, in a single synthetic operation, is described. The A³‐couplings are catalyzed by copper bromide and the cycloaddition is catalyzed by Wilkinson’s catalyst. It was found that many catalysts known to be efficient at the cycloaddition step were not suitable when this step was part of the tandem reaction sequence. Wilkinson’s catalyst was found to be unique in its suitability for the overall domino reaction sequence.     

Copper(I)‐Catalyzed Coupling Cyclization of Methyl Perfluoroalk‐2‐ynoates with 2‐Aminobenzonitriles: Synthesis of 2‐Perfluoroalkylated Quinolines

An efficient route to 2‐perfluoroalkylated quinoline derivatives through the copper(I)‐mediated coupling–cyclization of 2‐aminobenzonitriles with methyl perfluoroalk‐2‐ynoates is described. Moderate to excellent yields have been achieved under mild conditions. The reaction mechanism is also discussed.     

A Novel Enantioselective Catalytic Tandem Oxa‐Michael–Henry Reaction: One‐Pot Organocatalytic Asymmetric Synthesis of 3‐Nitro‐2H‐chromenes

An enantioselective oxa‐Michael–Henry reaction of substituted salicylaldehydes with nitroolefins that proceeds though an aromatic iminium activation (AIA) has been developed by using a chiral secondary amine organocatalyst and salicylic acid as a co‐catalyst. The corresponding 3‐nitro‐2H‐chromenes were obtained in moderate‐to‐good yields with up to 91% ee under mild conditions. Based on the experimental results and ESI‐mass spectrometric detection of the intermediates, a plausible transition state has been proposed to explain the origin of the activation and the asymmetric induction.     

Cobalt‐Catalyzed Twofold Direct C(sp2)−C(sp3) Bond Coupling: Regioselective C‐3 Alkylation of Coumarins with (Cyclo)alkyl Ethers

A cobalt‐catalyzed regioselective C‐3 alkylation of coumarins was realized under mild reaction conditions, during which a variety of substituted coumarins including those containing sensitive functional groups could smoothly undergo the selective C(sp²)−C(sp³) bond formation with a series of cyclic or straight‐chain alkyl ethers.

     

Palladium on Calcium Carbonate Combined to 2‐Hydroxypropyl‐α/β‐cyclodextrins: A Selective Catalytic System for Aqueous Heck Coupling and Hydroarylation

An efficient, selective and recoverable catalytic system for ligand‐free aqueous Heck reactions using hydroxypropylated cyclodextrins (HPCDs) and palladium on calcium carbonate (Pd/CaCO₃) is highlighted. Remarkably, stereo‐ and chemoselectivities could be tuned by the cavity size of cyclodextrins, exploiting the relevance of host‐guest interactions. UV‐Vis experiments have led to strong evidence concerning an interplay between Pd(II) and α‐HPCD, possibly ascribed to a reduction/stabilization effect of CDs. Unexpectedly, hydroarylation was the favored pathway with acrylonitrile which provided access to 3‐phenylpropionitrile derivatives without usual hydride donors. Finally, determination of soluble Pd(0/II) via AAS enabled the definition of a predominant homogeneous mechanism in which TONs over 5000 were observed.     

Palladium‐Catalyzed Two‐Component Domino Coupling Reaction of (Z)‐β‐Bromostyrenes with Norbornenes: Synthesis of 1,5‐Enynes

Polyfunctional molecules, 1,5‐enynes, have been achieved via a palladium(0)‐catalyzed domino coupling reaction of (Z)‐β‐bromostyrenes with norbornenes in the presence of cesium carbonate and N,N‐dimethylformamide. The process involves a double Heck‐type procedure, two‐fold C(sp²) H activation and formation of two carbon‐carbon bonds. There are possibilities of diversified transformation for the domino coupling of (Z)‐β‐bromostyrenes with norbornenes, the procedure is successfully driven to 1,5‐enynes via accurate adjustment of the reaction conditions.

     

Synthesis of 2H‐Indazoles via Tandem Palladium‐Catalyzed Deacylative Cross‐Coupling and Denitrogenative Cyclization of 2‐Iodoazoarenes and 2‐Iodoaryltriazenes with Acyldiazoacetates in One‐Pot

A synthetic method to prepare a wide range of 2H‐indazoles was developed via a tandem palladium‐catalyzed deacylative cross‐coupling reaction of 2‐iodoazoarenes and 2‐iodoaryltriazenes with acyldiazoacetates and denitrogenative cyclization reaction of in situ generated diazoacetates having azoaryl and triazenylaryl moieties in one‐pot. Additionally, azoaryl‐substituted diazoacetates underwent palladium‐catalyzed denitrogenative cyclization to produce 2H‐indazoles. The present reaction is a good example in which a Pd(0)‐catalyst is involved in two catalytic cycles in one‐pot.

     

Iodine‐Mediated Copper‐Catalyzed Efficient α‐C(sp2)‐Thiomethylation of α‐Oxoketene Dithioacetals with Dimethyl Sulfoxide in One Pot

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.

     

Catching Elusive 2‐Furyl Carbenes with Silanes: A Metal‐Free Microwave‐Assisted Silicon‐Hydrogen Bond Functionalization

An efficient, metal‐free, silicon–hydrogen bond functionalization based on the microwave‐assisted reaction of readily available enynones and silanes is reported. This process seemingly proceeds through a 2‐furyl carbene species, a particularly elusive intermediate. Preliminary studies on the metal‐free oxygen–hydrogen and nitrogen–hydrogen bond functionalization of representative alcohols, azoles and sulfonamides are also provided.

     

Palladium(II)‐Catalyzed Sequential Aminopalladation and Oxidative Coupling with Acetylenes/Enones: Synthesis of Newly Substituted Quinolines from 2‐Aminophenyl Propargyl Alcohols

Palladium catalyzed conversion of 1‐(2‐aminophenyl)‐propargyl alcohols to 3‐alkynyl quinolines is realized via a cascade that involves aminopalladation, oxidative coupling with alkynes and dehydration. The method is shown to have a broad substrate scope with respect to propargyl alcohols as well as alkynes. Vinyl ketones as coupling partners in the same reaction afforded 3‐alkenyl quinolines with equal ease.

     

Ligand‐Free Copper‐Manganese Spinel Oxide‐Catalyzed Tandem One‐Pot C–H Amidation and N‐Arylation of Benzylamines: A Facile Access to 2‐Arylquinazolin‐4(3H)‐ones

An efficient ligand‐free copper‐manganese (Cu‐Mn) spinel oxide‐catalyzed direct tandem C−H oxygenation and N‐arylation of benzylamines has been developed. The method has been utilized for the synthesis of medicinally important 2‐arylquinazolin‐4(3H)‐ones. Salient features of this method include recyclable catalyst, no ligand, excellent product yields, shorter reaction times and a broad substrate scope.

     

Copper‐Mediated Tandem C(sp2)–H Sulfenylation and Annulation of Arenes with 2‐Mercaptoimidazoles: Regio‐ and Site‐selective Access to Polycyclic Fused Imidazo[2,1‐b][1,3]thiazinones

An efficient copper‐mediated tandem C(sp²)–H sulfenylation and annulation of arenes with 2‐mercaptoimidazoles to provide polycyclic fused imidazo[2,1‐b][1,3]thiazinones has been developed. This tandem reaction is likely initiated by C(sp²)–H thiolation of benzamide with 2‐mercaptoimidazole followed by intramolecular nucleophilic substitution of the amide carbonyl group. A notable feature of this reaction is that it can afford rather complex products in a single synthesis step from easily accessible starting materials using amide‐oxazoline as a removable bidentate directing group. A variety of benzamides and 2‐mercaptoimidazoles bearing different substituents are compatible with this transformation.