New synthetic method for benzofurans from 2-(cyanomethyl)phenyl derivatives

A convenient and mild synthetic method of synthesizing benzofuran from various 2-(cyanomethyl)phenyl carbonyl compounds under reduced oxygen conditions is reported. Nine C-2 substituted benzofurans were synthesized at 52–89%.     

Catalytic alkylation of benzofurans

The alkylation of benzofuran with tert-butyl chloride in various solvents with a zinc chloride catalyst was studied. It was found that the alkylation proceeds primarily at the 3-position. The ratio of the 2- and 3-tert-butylbenzofurans is 12 regardless of the nature of the solvent.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 156–159, February, 1971.     

Ionic hydrogenation of benzofurans

Benzofurans are arranged in the order 2-methylbenzofuran > 3-methylbenzofuran > benzofuran with respect to the case with which they undergo ionic hydrogenation. Benzofuran and 2-methylbenzofuran are protonated in the 3 position of the heterocyclic ring, whereas 3-methylbenzofuran is protonated in the 2 position. Individual monodeuterated 2,3-dihydrobenzofurans can be obtained in high yields by ionic hydrogenation.     

Catalytic alkylation of benzofurans

The alkylation of benzofuran by isobutylene and tert-butyl alcohol in an autoclave in a flow system on ZnCl₂ applied to Al₂O₃ was studied. The major reaction products are 2- and 3-tert-tert-butylbenzofurans, the ratio of which depends on the conditions used to carry out the reaction. The product yield depends on the acidity of the catalyst used. The predominant formation of 2-tert-butylbenzofuran in the flow system is explained by the isomerization of 3-tert-butylbenzofuran via an intermolecular mechanism.See [1,2] for communications I and II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1020–1022, August, 1971.     

An isomerization-ring-closing metathesis strategy for the synthesis of substituted benzofurans

Twelve substituted benzofurans were synthesized from their corresponding substituted 1-allyl-2-allyloxybenzenes using ruthenium-mediated C- and O-allyl isomerization followed by ring-closing metathesis.     

Synthesis of some condensed benzofurans

The condensation of O-phenylhydroxylamine (Ia) with tetrahydro-4-thiopyrone, 1-methyl-4-piperidone (IIb), 1,2,5-trimethyl-4-piperidone, and 4-thiochromanone and of O-(4-tolyl)-hydroxylamine with IIb was used to synthesize the corresponding benzofuran derivatives (IIIa-e), from which IIIa and IIIb were obtained by deamination of 8-amino-3,4-dihydro-1H-thiopyrano-[4,3-b]benzofuran and 2-methyl-8-amino-1,2,3,4-tetrahydrobenzofuro[3,2-c]pyridine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 740–742, June, 1972.     

Development of a one-pot sequential Sonogashira coupling for the synthesis of benzofurans

An efficient one-pot protocol was developed for the construction of the benzofuran system from aryl halides and protected iodophenols using carbinol-based acetylene sources. The sequence includes alternating palladium-catalyzed Sonogashira couplings and deprotection steps concluded by a ring closure. The developed one-pot procedure was compared with the stepwise approach and its efficiency was also demonstrated by the total synthesis of vignafuran, a benzofuran natural product.     

A new method for the reduction of nitronyl nitroxides

Reduction of nitronyl nitroxides using hexamethylenetetramine is a very convenient method for preparing the corresponding imino nitroxides and 2-imidazoline N-oxides.     

Boronate titanium alkylidene reagents for diversity-based synthesis of benzofurans

[reaction: see text] Novel titanium benzylidenes (Schrock carbenes) bearing an arylboronate group are generated from thioacetals with low valent titanium species, Cp(2)Ti[P(OEt)(3)](2), and alkylidenate Merrifield resin-bound esters to give enol ethers. Treatment with 1% TFA gives 2-substituted (benzo[b]furan-5-yl)boronates, and solid-phase Suzuki cross-coupling gives 2,5-disubstituted benzofurans. Steps in the syntheses of thioacetal substrates include selective lithiation-boronation, hydrolysis of a MOM group without affecting a boronate ester, and cross-coupling with bis(pinacolato)diboron.     

Palladium-catalyzed direct arylation of polysubstituted benzofurans

An efficient access to 2-substituted 3-arylbenzofurans through a palladium-catalyzed C3 direct arylation of 2-substituted benzofurans with aryl bromides is described. The scope and limitation of this reaction was studied. The method tolerates a variety of functional groups on the aryl halide and has been successfully extended to polysubstituted benzofurans to obtain the corresponding 3-arylbenzofurans with good to excellent yields.     

Natural benzofurans: Synthesis of isopterofuran

A synthesis of the fungistatic phytoalexin, isopterofuran is described. In the key step, the benzofuran heterocycle is constructed by reaction of an o-iodophenol with a cuprous arylacetylide.     

Catalytic asymmetric brominative dearomatization reaction of benzofurans

A catalytic asymmetric brominative dearomatization reaction of benzofuran derivatives was achieved by using hydroquinidine 1,4-phthalazinediyl diether [(DHQ)₂PHAL] as the catalyst and N-bromoacetamide (NBAc) as the brominating reagent. A series of brominated spiro[benzofuran-2,5'-oxazoles] bearing two contiguous stereogenic centers were obtained in high yields (up to 99%) with excellent enantioselectivity (up to 97% ee).     

Microwave-mediated solvent free Rap-Stoermer reaction for efficient synthesis of benzofurans

The Rap-Stoermer reaction of salicylaldehydes with diverse phenacyl bromide and phenacyl iodides proceeded cleanly to afford various functionalized benzofurans in excellent yields under base-mediated solvent free microwave irradiation conditions.     

A superior method for the reduction of secondary phosphine oxides

[reaction: see text] Diisobutylaluminum hydride (DIBAL-H) and triisobutylaluminum have been found to be outstanding reductants for secondary phosphine oxides (SPOs). All classes of SPOs can be readily reduced, including diaryl, arylalkyl, and dialkyl members. Many SPOs can now be reduced at cryogenic temperatures, and conditions for preservation of reducible functional groups have been found. Even the most electron-rich and sterically hindered phosphine oxides can be reduced in a few hours at 50-70 degrees C. This new reduction has distinct advantages over existing technologies.     

A rapid and efficient method for the reduction of quinoxalines

Mono and di‐substituted alkyl and aryl quinoxalines are rapidly reduced in high yield to their respective 1,2,3,4‐tetrahydro‐derivatives by borane in THF solution. In the case of the 2,3‐di‐substituted compounds, reduction is stereoselective yielding exclusively the cis‐isomers. Sodium borohydride in acetic acid also reduces alkyl and aryl quinoxalines, but proceeds with lower yields and often produces side products. Sodium borohydride in ethanol reduces quinoxaline and 2‐methylquinoxaline in high yield; however, the reaction is very slow, whereas 2,3‐dialkyl and 2‐aryl quinoxalines are not efficiently reduced by sodium borohydride in ethanol.     

A mild and efficient method for the reduction of nitriles

A simple and useful method for the reduction of nitriles into the corresponding amines using a tetramethyldisiloxane/titanium(IV) isopropoxide reducing system is described. The synthetic approach is straightforward and provides primary amines as hydrochloride salt in almost quantitative yield. Other advantages of this method, such as easy-to-handle hydride source, inert by-products, that is, TiO₂ and oligomeric siloxanes, make it very attractive to prepare primary amines.