Furans zinc chloride

A review on fliran and its derivatives in the synthesis of other heterocycles was published <95CHE1034>. Furan decomposes on Pd(lll) at 300 K to form H, CO and CjH, which can dimerize to benzene at 350 K <96JA907>. Again, a considerable number of Diels-Alder reactions with furan and fiiran derivatives was reported. The synthesis of 2-pyridinyl-7-oxabicyclo[2.2.1]heptanes (e.g., 3, 4) was accomphshed via zinc chloride-mediated Diels-Alder reaction of furan with 2-vinylpyridines <96SL703>. 

The Cu(I)-catalyzed cyclization for the formation of ethyl ( )-tetrahydro-4-methylene-2-phenyl-3-(phenylsulfonyl)furan-3-carboxylate 82 has been accomplished starting from propargyl alcohol and ethyl 2-phenylsulfonyl cinnamate. Upon treatment with Pd(0) and phenylvinyl zinc chloride as shown in the following scheme, the methylenetetrahydrofuran 82 can be converted to a 2,3,4-trisubstituted 2,5-dihydrofuran. In this manner, a number of substituents (aryl, vinyl and alkyl) can be introduced to C4 <00EJO1711>. Moderate yields of 2-(a-substituted N-tosyIaminomethyl)-2,5-dihydrofurans can be realized when N-tosylimines are treated with a 4-hydroxy-cis-butenyl arsonium salt or a sulfonium salt in the presence of KOH in acetonitrile. The mechanism is believed to involve a new ylide cyclization process <00T2967>. 

Hsung and co-workers described the first epoxidation of 1-amidoallenes leading to highly reactive intermediate 292 (Scheme 8.76) [159]. Formation of bicyclic products 291 occurs via iminium enolate 293, which was trapped by cyclopentadiene 290 (X = CH2) or furan 290 (X = 0). [4+3] Cycloaddition of the intermediate 293furnished 291 in good yield as a mixture of mdo-diastereoisomers (-75 25). The best dia-stereoselectivity was found when the reaction was performed in the presence of 2 equiv. of zinc chloride (>96 4). 

Benzo[6]furan may be alkylated with f-butyl chloride and zinc chloride, the products being the 2- and 3-substituted compounds in the ratio 1 2. Benzo[6]furan is chloromethylated at the 2-position. 2-Methyl- and 2-phenyl-benzo[6]furan are chloromethylated in the 3-position. 

The preparation of furans by acid catalyzed dehydration of 1,4-diketones followed by cyclization of the monoenol has been comprehensively reviewed (69RCR389). This method of furan preparation is useful for all 1,4-diketones which are not sterically hindered with the highly hindered l,4-bis(2,4,6-triisopropylphenyl)-l,4-butanedione no cyclization occurs (50JA5754). Although sulfuric acid is normally used for cyclization, hydrochloric acid was employed in the conversion of a 1,2-diacylethylene to a furylacrylic acid derivative (59MIP31200). Other reagents used for this purpose include polyphosphoric acid, phosphorus trichloride, zinc chloride and DMSO. Both DMSO and phosphoric esters provide neutral... 

Monosaccharides react with a variety of 1,3-dicarbonyl compounds in the presence of zinc chloride in ethanolic or aqueous solution to yield substituted furans (Scheme 69) (56MI31200). The reaction of ethyl acetoacetate with D-glucose and D-mannose yielded the trisubstituted furan (252) in 20% yield, while D-fructose under similar conditions yielded (253 7%). These products have been used for the synthesis of dehydromuscarones (63HCA1259). Oxidation of the tetrahydroxybutyl side chains with lead tetraacetate gives the aldehyde, which can be converted to the corresponding acid with alkaline silver oxide. 

Carboxy-4-methyl-5-pentyl-2-furanpropanoic acid (273), isolated from blood and urine, is a hitherto unknown class of metabolic compound. The structure of (273) has recently been confirmed by synthesis (80CB699). 2,4,5-Trialkyl substituted furan-3-carboxylic acids have been synthesized from acyloin and /3-ketoesters by treatment with zinc chloride. By analogy with this synthetic route, the reaction of acetoin with 3-oxoadipic acid dimethyl ester was found to yield the 2,3-dimethylfuran (274). The dimethyl ester (275) was prepared by condensation of 3-chloro-2-octanone with 3-oxoadipic acid dimethyl ester and was shown to be identical with the dimethyl ester of the natural product. 

Zinc chloride was reported to be a suitable catalyst as compared to gold in cycloisomerization of homopropargylic ketones. Thus, reaction of 1,4-disubstituted and 1,2,4-trisubstituted but-3-yn-l-ones in the presence of zinc chloride gave the corresponding 2,5-di- and 2,3,5-trisubstituted furans in high yields <07OLl 175>. 

Negishi coupling with methyl zinc chloride to regioselectively afford a trisubstituted benzo[3]furan 99, as illustrated in Scheme 53 <2002TL9125>. 

Disubstituted furans are available from a,/3-unsaturated enones in a two-step sequence. At first, conjugate addition of a cuprate generates an enolate, which undergoes an aldol reaction with (tetrahydropyranyloxy)acetalde-hyde under zinc chloride catalysis (Scheme 19) <20000 L4095>. Treatment of the reaction product with acid affords the disuhstituted furans in good yields. 

Butyllithium (9.5 mL, 15 mmol, 1.6 M solution in hexane) was added to a solution of furan (1.1 mL, 15 mmol) in THF (15 mL) at 0 °C. After stirring at 0 °C for 1 h and room temperature for 1 h, the yellow solution was slowly transferred via cannula into a solution of zinc chloride (15 mL, 15 mmol, 1 M solution in Et20) at 0°C. The mixture was stirred for 1 h at 0 °C and then added to a solution of l-bromo-2-iodobenzene (1.3 mL, 10 mmol) and bistriphenylphosphine palladium chloride (0.21 g, 0.3 mmol) in THF (2 mL). After stirring for 12 h at room temperature, the reaction was added to IM HCl aqueous solution (10 mL) and the product was extracted into ether and washed with saturated aqueous sodium bicarbonate solution. The ether solution was dried (MgS04) and the solvent was removed in vacuo. The crude material was purified by flash chromatography (hexane) to give 2.10 g (95%) of the l-bromo-2-(2 -furyl)-benzene 5.3a as a colorless oil. 

In the presence of zinc chloride, [4 + 3] cycloadducts between the allylic cation formed from 29 and furan are obtained, Eq. 18. However, the major product of the reaction arises from electrophilic addition to furan [46]. 

Treating succinic anhydrides (64) with triethylamine, zinc chloride and trimethylchlorosilane in acetonitrile gives 2,5-bis(trimethylsiloxy)furans (65). ° The relative ease of silylation in these cases demonstrates one of the differences between enol silyl ether chemistry and classical enolate anion chemistry. It would have been quite difficult to generate the dianion of succinic anhydride. 

Acetic anhydride and aluminum chloride in caibon disulfide gives a high yield of the para-acylated product with thioanisole, and in dichloromethane the same reagents give an almost quantitative yield of 3-acetyl-1-benzenesulfonylindole. Acylation of more nucleophilic heterocycles can be achieved using milder catalysts, such as zinc chloride. It has been known for some time that furan can be acylated very efficiently using acetic anhydride and zinc chloride. The Paal-Knorr furan synthesis (1,4-diketone, acetic anhydride and zinc chloride) can sometimes result in acylation as well as cyclization (equation 40). - Equations (41) and (42) further exemplify the acylation of furan derivatives that have been used in the synthesis of cytotoxic furanonaphthoquinones. 

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