Furanes synthesis

The ring closure of 1,4-enediones has also been used for furan synthesis even though the precursor is at the wrong oxidation level. The examples in Scheme 14 indicate some of the ways in which the necessary reduction has been effected. 

The acid promoted cyclization of AT-(2-chloroallyl)enaminones (Scheme 35a) provides the expected 3-methyltetrahydroindoles, whereas similar treatment of iV-(2-chloroallyl)anilines yields unexpectedly 2-, rather than 3-, methylindoles (Scheme 35b) (75JCS(Pl)U46). The course of the latter cyclization is not resolved although various intermediates, such as those shown, have been considered. The ring closure in the furan synthesis shown in Scheme 35c is catalyzed by mercury(II) ion (79JCs(Pl)316l). 

H-Benzo[a]carbazole, 4,4a,5,l 1,1 la,l Ib-hexahydro-synthesis, 4, 283 Benzo[b]carbazole, N-acetyl-photochemical rearrangements, 4, 204 Benzo[/]chroman-4-one, 9-hydroxy-2,2-dimethyl-synthesis, 3, 851 Benzochromanones synthesis, 3, 850, 851, 855 Benzochromones synthesis, 3, 821 Benzocinnoline-N-imide ring expansion, 7, 255 Benzocinnolines synthesis, 2, 69, 75 UV, 2, 127 Benzocoumarins synthesis, 3, 810 Benzo[15]crown-5 potassium complex crystal stmcture, 7, 735 sodium complex crystal stmcture, 7, 735 Benzo[ 18]cr own-6 membrane transport and, 7, 756 Benzo[b]cyclohepta[d]furans synthesis, 4, 106 Benzocycloheptathi azoles synthesis, 5, 120... 

Paal synthesis thiophenes, 4, 884-885 Paal-I orr synthesis furan synthesis by, 4, 97 in heterophane synthesis, 7, 770 pyrroles, 4, 118... 

Spirofuran-2(5H),2 (3 H)furo[2,3-i]furan synthesis, 4, 659 Spirofuranones synthesis, 3, 734 Spiro[3.3]heptane... 

FEIST - SENARY Furan Synthesis Synthesis ol lurans by condensation ol an a-halocarbonyl compound with an enol. 

The Feist-Benary furan synthesis occurs when an a-halocarbonyl (1) reacts with a P-dicarbonyl (2) in the presence of a base. The resulting product (3) is a 3-furoate that incorporates substituents present in the two starting materials. ... 

Chatteijea showed that cyclic ot-halocarbonyls are acceptable substrates for the Feist-Benary furan synthesis by combining 1-chlorocyclohexanone (34) with 1,3-cyclohexanedione (30) to yield octahydrodibenzofuran 35. ... 

Several modifications of the Feist-Benary furan synthesis have been reported and fall into two general classes 1) reactions that yield furan products 2) reactions that yield dihydrofuran products. One variant that furnishes dihydrofiirans uses substrates identical to the traditional Feist-Benary furan synthesis with a slight modification of the reaction conditions. The other transformations covered in this section involve the combination of P-dicarbonyls with reagents that are not simple a-halocarbonyls. Several reactions incorporate a-halocarbonyl derivatives while others rely on completely different compounds. 

The final variation of the Feist-Benary furan synthesis encompasses reactions of 1,3-dicarbonyls with 1,2-dibromoethyl acetate (52). For example, treatment of ethyl acetoacetate (9) with sodium hydride followed by addition of 52 at 50°C yields dihydrofuran 53. The product can be easily converted into the corresponding 2-methyl-3-furoate upon acid catalyzed elimination of the acetate, thus providing another strategy for the synthesis of 2,3-disubstituted furans. 

Treatment of 1,4-dicarbonyls (1) with catalytic acid yields substituted furans (2) and is called the Paal-Knorr furan synthesis. This method is used extensively to produce a variety of mono-, di-, tri-, and tetrasubstituted furans. ... 

Ibers used the Paal-Knorr furan synthesis to prepare a key intermediate for the synthesis of novel porphyrin-like aromatic macrocycles. Bis yrolyljfuran 27 was available in good yield via the acid catalyzed condensation of diketone 26. ... 

The Paal-Knorr furan synthesis can also be used to prepare 2,5-arylalkylfurans, as illustrated in the following example. Salimbeni produced furan 29 from dione 28 and subsequently used the furan as an intermediate for the production of angiotensin II receptor antagonists. ... 

Separately, Paal and Knorr described the initial examples of condensation reactions between 1,4-diketones and primary amines, which became known as the Paal-Knorr pyrrole synthesis. Paal also developed a furan synthesis in related studies. The central theme of these reactions involves cyclizations of 1,4-diketones, either in the presence of a primary amine (Paal-Knorr pyrrole synthesis), in the presence of a sulfur(II) source (Paal thiophene synthesis), or by dehydration of the diketone itself (Paal furan synthesis). 

Two sequential pericyclic reactions are involved in the following furan synthesis. Identify them, and propose a mechanism for the transformation. 

The palladium-catalyzed arylation of alkenes by haloarenes, and applications in furan synthesis have been described earlier (see Eq. 14 in Section IV,B, 1). By employing o-aminoiodoarenes and appropriately substituted (Z)-alkenes it has proved possible to develop an efficient synthetic route to quinolin-2-ones (Scheme 140).213... 

Gold as an efficient catalyst has widely been used in furan synthesis. One example is shown below, in which the double hydroarylation of unactivated alkyne using 2-methylfuran afforded a difuranylmethane derivative in a moderate yield <06EJOC4340>. 

As can be seen in the scheme below, the catalytic activity of gold species was also shown in a multi-substituted furan synthesis. Cyclization of allenones in the presence of Au(III)-porphyrin gave rise to the corresponding substituted furan in good to high yields. The catalyst can be recycled several times and still maintain the same catalytic activity <06OL325>. 

A similar substituted furan synthesis was realized via a Pd-catalyzed tandem carbonylation-arylation using an a,p-acetylenic ketone, carbon monoxide, and bromothiophene [104],... 

The most unique feature of furan synthesis using palladium chemistry is heteroannulation. Enones, ynones and ynols all have been annulated into furans and benzofurans. More importantly, trapping the reactive Pd(II) intermediates at different stages with electrophiles offers unique opportunities to synthesize substituted furans and benzofurans. 

Dimethylprop-2-ynylsulfonium bromide, preparation, intermediate in furan synthesis, 53, 1... 

Shea, K. M. Feist—Benary Furan Synthesis In Name Reactions in Heterocyclic Chemistry, Li, J. J. Corey, E. J., Eds. Wiley Sons Hoboken, NJ, 2005, 160-167. (Review). 

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