Furan ring opening–indole ring closure: SnCl2-induced reductive transformation of difuryl(2-nitroaryl)methanes into 2-(2-acylvinyl)indoles

A simple and efficient method for the synthesis of 2-(2-acylvinyl)-3-(5-alkyl-2-furyl)indoles by reductive recyclization of bis(5-alkyl-2-furyl)(2-nitroaryl)methanes is reported. This transformation was carried out by heating the substrates with SnCl₂·2H₂O in ethanol. The intermediate nitrosoarene moiety interacted with the furan ring via electrophilic nitrogen attack onto the C(2) position of the furan ring. It was shown that the related bis(5-alkyl-2-thienyl)(2-nitroaryl)methanes under the same reaction conditions failed to undergo the analogous recyclization and were transformed into bis(5-alkyl-2-thienyl)(2-aminoaryl)methanes.     

Enzymatic diastereo- and enantioselective synthesis of α-alkyl-α,β-dihydroxyketones

An enzymatic strategy for the preparation of optically pure α-alkyl-α,β-dihydroxyketones is reported. Homo- and cross-coupling reactions of α-diketones catalyzed by acetylacetoin synthase (AAS) produce a set of α-alkyl-α-hydroxy-β-diketones (30-60%, ee 67-90%), which in turn are reduced regio-, diastereo-, and enantioselectively to the corresponding chiral α-alkyl-α,β-dihydroxyketones (60-70%, ee >95%) using acetylacetoin reductase (AAR) as catalyst. Both enzymes are obtained from Bacillus licheniformis and used in a crude form. The relative syn stereochemistry of the enantiopure α,β-dihydroxy products is assigned by NOE experiments, whereas their absolute configuration is determined by conversion of the selected 3,4-dihydroxy-3-methyl-pentan-2-one to the natural product (+)-citreodiol.     

Synthesis of α-(aminomethylene)-9H-purine-6-acetic acid derivatives

α-(Aminornethylene)-9H-purine-6-acetamide ( 3a ) and the corresponding ethyl acetate 9 have been synthesized by catalytic hydrogenation of 6-cyanomethylenepurine derivatives 2 and 7 which were obtained by the substitution of 6-chloropurine derivatives with α-cyanoacetamide and ethyl cyanoacetate, respectively. Substitution of α-(aminomethylene)-9-(tetrahydrofuran)-9H-purine-6-acetamide ( 3b ) with amines gave the corresponding N-alkyl- and N-arylamines 5 , which were treated with acid to give N-substituted α-(aminomethylene)-9H-purine-6-acetamides 6 . Substitution of 9 with amines gave the corresponding N-alkyl- and N-aryl substituted amines 10 .     

A new route to α-alkyl-α-fluoromethylenebisphosphonates

A new route to α-alkyl-α-fluoromethylenebisphosphonates, 2 has been developed starting from commercially available tetraethyl fluoromethylenebisphosphonate (1), and alkyl halides using either caesium carbonate in DMF or sodium dimsyl. De-esterification of 2 provided biologically important α-alkyl-α-fluoromethylenebisphosphonic acid, 3, while alkoxide-induced carbon-phosphorus bond cleavage of 2 gave α-fluorophosphonates, 4, which are useful synthons in organic synthesis.     

An efficient synthesis of optically pure α-alkyl-β-azido- and α-alkyl-β-aminoalanines via ring opening of 3-amino-3-alkyl-2-oxetanones

N-Boc-α-alkylserine β-lactones on ring opening with sodium azide provide N-Boc-α-alkyl-β-azidoalanines, as N-protected amino acids are suitable for direct incorporation into peptides. N-Boc-α-alkyl-β-azidoalanines can be transformed by catalytic hydrogenation into the corresponding N-Boc-α-alkyl-β-aminoalanines.     

Convenient preparation of highly active phase-transfer catalyst for catalytic asymmetric synthesis of α-alkyl- and α,α-dialkyl-α-amino acids: application to the short asymmetric synthesis of BIRT-377

A highly active phase-transfer catalyst was conveniently prepared from the known, easily available (S)-4,5,6,4′,5′,6′-hexamethoxybiphenyldicarboxylic acid. This catalyst exhibited the high catalytic performance (0.01-1 mol %) in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester and N-(p-chlorophenylmethylene)alanine tert-butyl ester compared to the existing chiral phase-transfer catalysts, thereby allowing to realize a general and useful procedure for highly practical enantioselective synthesis of structurally diverse natural and unnatural α-alkyl-α-amino acids as well as α,α-dialkyl-α-amino acids.     

Visible-light photoredox catalysis: dehalogenation of vicinal dibromo-, α-halo-, and α,α-dibromocarbonyl compounds

vic-Dibromo-, α-halo-, or α,α-dibromocarbonyl compounds can be efficiently dehalogenated using catalytic tris(2,2'-bipyridyl)ruthenium dichloride (Ru(bpy)(3)Cl(2)) in combination with 1,5-dimethoxynaphthalene (DMN) and ascorbate as sacrificial electron donor. For this process, a visible light promoted photocatalytic cycle is proposed that involves the reduction of carbon halogen bonds via free radical intermediates.     

Vanadium-catalyzed sulfenylation of indoles and 2-naphthols with thiols under molecular oxygen

Vanadium oxyacetylacetonate [VO(acac)(2)] works as a catalyst for the direct synthesis of 3-sulfanylindoles from indoles and thiols under an atmospheric pressure of molecular oxygen as a reoxidant. For example, the reaction of 2-phenylindole with benzenethiol in the presence of a catalytic amount of VO(acac)(2), potassium iodide, and 2,6-di-tert-butyl-p-cresol in chlorobenzene under molecular oxygen proceeds to afford 2-phenyl-3-(phenylsulfanyl)indole in 86% yield. This catalytic system can also be applied to 2-naphthols instead of indoles to give the corresponding 1-sulfanyl-2-naphthols in up to 57% yield.     

Reaction of aromatic aldehydes with 1-alkylindole-2-carboxylic acid and its derivatives

The effect of a substituent in benzaldehydes during reactions with 1-alkylindole-2-carboxylic acid and its esters and anilide is expressed in the formation of either di(3-indolyl)phenylmethanes or 1-alkyl-2-carboxy(carbomethoxy, carbanilido)-3-(α-X-benzyl)indoles. The possibility of replacement of X by OH, OAc, SC₂H₅, and hydrazine groups is shown.     

Chiral phosphoric acid catalyzed aza-Friedel-Crafts alkylation of indoles with cyclic aryl α-ketimino esters

A highly enantioselective aza-Friedel-Crafts alkylation of indoles with cyclic aryl α-ketimino esters catalyzed by a chiral phosphoric acid has been developed, the corresponding α,α-disubstituted unnatural α-amino ester derivatives were obtained in moderate to high yields (67–85%) with high enantioselectivities (up to 93% ee) under mild reaction conditions.     

An Ir-Pt catalyst for the multistep preparation of functionalized indoles from the reaction of amino alcohols and alkynyl alcohols

The 1,2,4-trimethyltriazolylidene (ditz) ligand allows the preparation of homo- and heterodimetallic complexes of Pt(2) and Ir-Pt. These two complexes have been characterized by means of spectroscopic and diffractommetric techniques. The catalytic activity of these complexes, together with that of other Pt-based compounds, has been explored in the cyclization-addition of alkynyl alcohols and indoles. The Ir-Pt complex [{PtI(2)(py)}(μ-ditz){IrI(2)(Cp*)}] (py=pyridine; Cp*=pentamethylcyclopentadienyl) allows the combination of an iridium-mediated oxidative cyclization of 2-(ortho-aminophenyl)ethanol to form indoles, with a further step employing a Pt-based multistep reaction that functionalizes indoles. Our results show that the Ir-Pt complex is a very active catalyst in this new multistep preparation of functionalized indoles from the reaction of an amino alcohol with alkynyl alcohols.     

Diarylprolinol in an asymmetric aldol reaction of an α-alkyl-α-oxo aldehyde as an electrophile

The direct aldol reaction of an α-alkyl-α-oxo aldehyde was catalyzed by trifluoromethyl-substituted diarylprolinol 1 to afford a γ-oxo-β-hydroxy-α-substituted aldehyde in good yield with excellent anti-selectivity and excellent enantioselectivity.     

Regioselective synthesis of 1-alkyl-5-(indol-3-yl- and -2-yl)pyrrolidin-2-ones from available reagents

The reaction under mild conditions of 1-alkyl-5-hydroxypyrrolidin-2-ones with different indoles having a free 3 position leads exclusively to 1-alkyl-5-(indol-3-yl)pyrrolidin-2-ones but if position 3 is occupied to 1-alkyl-5-(indol-2-yl)pyrrolidin-2-ones.     

Beckman rearrangement of oxime sulfonates by grignard reagents

The Beckmann rearrangement of oxime sulfonates by Grignard reagents provides an efficient and general entry to α-alkyl- and α,α-dialkylamines in good yields.     

General synthesis of exomethylene-α-lactones via the retro-diels-alder reaction

New synthetic methods of tulipalin A were deviced via the retro-Diels-Alder reaction and the generality was demonstrated by the synthesis of several α-methylene-α-alkyl-α-butyrolactones.     

Lipase-catalyzed kinetic resolution of α-hydroxymethylcycloalkanones with a quaternary carbon center. Chemoenzymatic synthesis of chiral pseudoiridolactones

The resolution of α-alkyl-α-hydroxymethylcyclopentanones 1 and cyclohexanones 3 has been efficiently achieved by using lipase-catalyzed transesterification reactions with vinyl acetate as the acylating agent. The enantiomeric selectivities were dependent on both the ring size of the cycloalkanone and the bulk of the carbon group located at the stereogenic quaternary center. The resolved α-alkyl-α-hydroxymethylcyclopentanones 1 were used as enantiopure (or enantioenriched) precursors for the synthesis of the optically active pseudoiridolactones 6–7.     

Catalytic asymmetric Friedel-Crafts/protonation of nitroalkenes and N-heteroaromatics

The catalytic asymmetric Friedel-Crafts/protonation of indoles and pyrroles with α-substituted nitroalkenes to give the corresponding adducts in a highly anti-selective manner was achieved by an imidazoline-aminophenol (L2)-Cu complex. The anti-adducts could be successfully transformed to biochemically important α-substituted β-heteroarylalkylamines.     

Synthesis of pseudoiridolactones

Bicyclic δ-lactones with a carbon group at the bicyclic junction C-7a, designed as pseudoiridolactones, were synthesized from α-alkyl-α-hydroxymethylcyclopentanones via an intramolecular Horner-Wadsworth-Emmons reaction.     

Synthesis and physicochemical properties of 1-(2-alkylamidoethyl)-2-alkyl-2-imidazolines based on α,α′-branched carboxylic acids

Condensation of diethylenetriamine with lauric, 2-ethylhexanoic, and α,α′-branched C_10,12-carboxylic acids results in 1-(2-alkylamidoethyl)-2-alkyl-2-imidazolines. A stability of the synthesized compounds relative to the acid and alkaline hydrolysis was studied. Their protonation constants were determined. The effect of the structure of the alkyl substituents on the distribution of the substance between the organic and aqueous phases was examined. The principal possibility of extracting Zn(II), Fe(III), Cu(II), Co(II), Mn(II) chlorides from hydrochloric acid solutions was shown. A mixture of 1-(2-alkylamidoethyl)-2-alkyl-2-imidazolines based on the α,α′-branched carboxylic acids was suggested as a potential extractant of the metal salts from hydrochloric acid and chloride solutions.     

Platinum/Copper Dual-Catalyzed Asymmetric Vinylogous Addition Reactions for the Synthesis of Functionalized Indoles

The development of multicomponent ligands to improve catalytic reactivity and selectivity remains a significant challenge for the synthesis of chiral building blocks. Here, a modular synthesis of structurally diverse multiligated platinum complexes, characterized by X-ray crystallography, was revealed to open up a previously inaccessible reaction space. A library of more than 16 binary component-ligated platinum complexes was identified as a practical toolkit to enable faster screening. The isolated bench-stable Pt^II(oxazoline)(phosphine) complex paired with a chiral copper complex exhibits fundamentally new cooperative reactivity. The newly designed Pt/Cu dual catalytic system contributed to the development of highly enantioselective vinylogous addition reactions between a Pt-activated electrophilic α,β-unsaturated carbene and a Cu-activated nucleophile, resulting in a reliable process for the asymmetric synthesis of valuable functionalized indoles in good yields and excellent enantioselectivities.