Furans 2-acyl— from

New mercaptoaryl- and mercaptoheteroaryl-phosphonates (139) have been prepared by or /io-lithiation of the corresponding O, O-diisopropyl S -aryl/hetero-aryl phosphorothioates 138 followed by sulfur to carbon rearrangement. A simple and efficient synthesis of the 2-substituted 3-diethylphosphono 5-methyl-furans 141 from the ketal phosphonate 140 has been reported. Acylation of a-... 

AIkyl-3-pyiidinols (54) aie lepoited to be formed from acyl furans and ammonia under pressure (66—68). 

Addition of acyl anions generated from acylsilanes to a,(3-unsaturated ketones using N-heterocyclic carbenes (NHCs) derived from thiazolium salts as catalyst produced 1,4-diketones, which cyclized to form the corresponding furans in good yields under an acidic condition <06JOC5715>. 

A very elegant formation of heptafulvenes from triafulvenes was found by Gompper294 utilizing the push-pull stabilized cyclobutadiene 511 in the case of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the 4-acyl group caused additional ring closure yielding cyclohepta(b)furan 514 ... 

Acyl azides 268, derived from furan, thiophene and selenophene, add slowly at room temperature to the strained double bond of 5-methylenebicyclo[2.2.1]hept-2-ene. Two regioisomeric triazolines, 269 and 270, which form in the first step, are unstable and decompose with elimination of nitrogen to provide aziridine derivatives 271. Products 271 are isolated in good yield (73-85%). It is worthy to note that not only the terminal, unstrained double bond in the starting material, 5-methylenebicyclo[2.2.1]hept-2-ene, is unaffected, but also the typical dipolarophiles like esters of crotonic, propiolic and byt-2-ynoic acids do not react with azides 268 under these conditions (Scheme 39) <2002J(P1)1420>. 

Table 9.41 Synthesis of furans from acyl chlorides and allenylsilanes.

Intramolecular C-C bond formation in the furan precursor is the main synthetic method for furobenzazepines. 2-Hydroxybenzonitrile 35c produces the corresponding benzofuran benzazepine dione 36c when reacted with o-carboxymethyl bromoacetophenone in refluxing DMF (Scheme 6, Section 2.1.1.2 (1991JHC379)). Alternatively, benzofurobenzazepinone 91 can be synthesized starting from benzofuran amino ester 90 by intramolecular acylation... 

Formylfuran reacts with potassium cyanide in alcohol solution to give a product containing two furan nuclei. What is this product and how does it form Can you suggest an alternative synthesis of the compound from 2-formylfuran using propane-1,3-dithiol as one reagent Hint consider an acyl anion equivalent approach). 

An interesting one-pot, five-component domino process using an intermolecular Diels-Alder reaction of furans with AT-phenylmaleimide as its final step has been used to construct the central core of indolo[2,3- ]carbazoles (Equation 86) <2002AGE4291>. Thus, aminooxazoles produced from an Ugi three-component reaction undergo acylation/intramolecular Diels-Alder/retro-Diels-Alder cycloreversion with pentafluorophenyl arylprop-2-ynoates to give furan derivatives. Subsequent Diels-Alder cycloaddition at elevated temperatures with A -phenylmaleimide produces carbazoles in good yields (Table 5). 

Triacycloxyborohydrides (62), derived from NaBH (1 equiv.) and (S)-N-acyl-proline (3 equiv.), were found to reduce 3,4-dihydropapaverine (89) in tetrahydro-furan to (S)-norlaudanosine (90) hydrochloride in 60% optical yield93. In some cases (90) was obtained in even higher optical purity (e.e. = 70-86 %). 

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

Diels-Alder reactions of oxazoles afford useful syntheses of pyridines (Scheme 53) (74AHC( 17)99). A study of the effect of substituents on the Diels-Alder reactivity of oxazoles has indicated that rates decrease with the following substituents alkoxy > alkyl > acyl >> phenyl. The failure of 2- and 5-phenyl-substituted oxazoles to react with heterodienophiles is probably due to steric crowding. In certain cases, bicyclic adducts of type (359) have been isolated and even studied by an X-ray method (87BCJ432) they can also decompose to yield furans (Scheme 54). With benzyne, generated at 0°C from 1-aminobenzotriazole and lead tetraacetate under dilute conditions, oxazoles form cycloadducts (e.g. 360) in essentially quantitative yield (90JOC929). They can be handled at room temperature and are decomposed at elevated temperatures to isobenzofuran. 

Examples of the reaction of pyrroles and indoles in acidic media with 77-electron-excessive heterocycles are discussed in Section 3.05.1.2.2 and elsewhere. However, for comparison of the reaction conditions, it is of interest to note, for example, the reaction of indole with an acyl chloride in the presence of a 7r-electron-excessive heterocycle, such as thiophene or furan, to yield the 3-acyl-2-heteroarylindoline (78KGS561, 79KGS493), the reaction of pyrroles and indoles with imidazole and benzimidazole in the presence of acetic anhydride to yield the iV,iV -diacetylimidazoline derivatives of the indoles and pyrroles (80T2505), and the formation of 2-heteroaryl-3,3-dimethylindolines from the reaction of 3,3-dimethyl-3Ef-indoles with indoles or pyrroles (77S343). 

The simple furan-3(2/f)-ones exist in the keto form but may be O-acylated with acetic anhydride and sodium acetate however, they undergo C-alkylation. They are usually stable to acid, merely being protonated. 4-Alkoxyfuran-3(2//)-ones are readily hydrolyzed to tetronic acids. Furan-3(2//)-ones are degraded by aqueous base which attacks in a conjugate fashion so that 2,5-dimethylfuran-3(2iT)-one, readily available from biacetyl, furnishes acetate and acetoin, but compounds with an ester group at the 4-position furnish tetronic acids (Scheme 109). 

The 3-oxo-2//-furans (260) obtained from the reaction of a-halogenoacyl halides with /3-ketonic esters at -20 °C show no tendency to enolize to the hydroxyfurans (261) but rather behave as unsaturated ketones (Scheme 71) (73RTC731). 2,4-Dialkylfurans (262) are obtained by condensation of acyl halides with allyl halides catalyzed by aluminum chloride (73KGS1434). The acid catalyzed condensation of 2-chlorocyclopentanone with dimethyl /3-oxoglutarate leads to the furan ester (263) (77JHC711). 

Furyl ketones can be easily obtained by Friedel-Crafts acylation of furans. They can also be obtained from derivatives of furan-2-carboxylic acid such as the amides, nitriles and chlorides by literature reactions. The 3-furyl ketones are also obtained by similar methods. 

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