Furan, 2,5-dialkyl

Furan, 2,5-dialkoxy-2-( 1 -hydroxyalkyl)-2,5-dihydro-ring expansion, 1, 425 Furan, 2,5-dialkoxytetrahydro-pyrrole synthesis from, 4, 330 Furan, 2,4-dialkyl-synthesis, 4, 661, 685 Furan, 2,5-dialkyl-... 

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

Benzo[b]furan, 2-aryl-2,3-dihydro- H NMR, 4, 570 Benzo[b]furan, 2-bromo-nitration, 4, 604 Benzo[b]furan, 5-cinnamoyl-properties, 4, 708 Benzo[b]furan, 2-cyano-photochemical reactions, 4, 636 Benzo[b]furan, 2,3-dialkyl-synthesis, 4, 710 Benzo[b]furan, 2,3-dihydro-aromaticity... 

The most common use of the Paal-Knorr condensation begins with a 1,4-diketone and yields a 2,5-disubstituted furan. This method has been used to produce dialkyl and disilyl furans however, the most popular use of this strategy is for the production of 2,5-diaryl furans. In addition to their utility as synthetic intermediates, these compounds are under investigation for novel electronic and pharmaceutical applications. 

It is also possible to use the Paal-Knorr condensation to prepare 2,5-dialkyl-3,4-dicarbonyl substituted furans. For example, Zaleska converted diketone 73 into furan 74 in 92%... 

In contrast with the relatively facile thermal rearrangement of sulfinates to sulfones discussed in the preceding section, the reverse process is relatively, rarely encountered and is usually observed only at elevated temperatures. One of the first thermal sulfone to sulfinate isomerizations has been invoked by Fields and Meyerson to occur during the pyrolysis of dibenzothiophene S, S-dioxide (26) to dibenzofuran, through elimination of sulfur monoxide from the sultine intermediate 27 (equation 27). More recently, the flash vapor-phase pyrolysis of various 2,5-dialkyl and diaryl thiophene-S, S-dioxides has also been shown to involve SO extrusion and formation of the corresponding furans in good yields . 

Tandem reaction of aromatic aldehydes with electron-deficient acetylenes and dialkyl acetylenedicarboxylates in the presence of I it iN led to the formation of fully substituted furans in moderate yields. One appropriate example is shown below <06EJOC5174>. 

Nair and coworkers have described the [8 + 2] cycloaddition reactions of 2H-cyclohep-ta[fr]furan-2-ones such as 521 in several reports311. The reactions of 521 with alkenes yield azulene derivatives upon extrusion of carbon dioxide. Table 30 summarizes the results of the reactions between 521 and some 6,6-disubstituted fulvenes 522 (equation 151)311b. In the case of 6,6-dialkyl fulvenes 522a-c, the [8 + 2] cycloadducts 523 were the major adducts obtained, the Diels-Alder adducts 524 only being formed in trace amounts. 

Furan und 2-Methyl-furan reagieren mit N,N-Dialkyl-methaniminium-chloriden (auf ver-schiedenen Wegen hergestellt) in Acetonitril unter Dialkylaminomethylierung in 2- bzw. 5-Stellung1. 

Furans can be prepared by acid catalyzed cyclization of masked 1,4-diketones. /3-Chloroallyl ketones which are obtained by alkylation of enamines or enolate ions behave as masked 1,4-diketones and afford furans on treatment with acid (67JA4557). 2,4-Dialkyl-furans (40) have been prepared by cyclization of the 3-chloroallyl ketone (39), which may be obtained by acylation of allyl chlorides (73KGS1434). 

Tosylpropanal ethylene acetal (41) is a useful synthon in the preparation of 3-substituted furans (78BCJ930). Dendrolasin [3-(4,8-dimethyl-3,7-monodienyl)furan] was prepared from (41) in good yield. General methods for the preparation of 2- and 3-alkylated and 2,3-dialkylated furans starting from 3-tosylpropanal ethylene acetal (41) have been developed <78H(10)105, 78CL717). The preparation of 2-substituted furans is illustrated in Scheme 8. 

Malonyl radicalsThiophenes and furans undergo substitution at C2 when treated with a dialkyl malonate and ceric sulfate at 25°. The reaction is considered to involve oxidative generation of malonyl radicals by the cerium salt. Yields are... 

Alkylation of compound (200) readily gave its 4-alkyl and 4,6-dialkyl derivatives, selectively. Reaction of 4-alkylated compounds with a variety of dienophiles led to fused furan compounds... 

Oxidation of 2,5-dialkylfurans with pyridinium chlorochromate results in high yields of, -unsaturated -dicarbonyl compounds. Similar results are obtained by peracid oxidation of furans, most frequently CPBA. It is assumed that the first step involves epoxidation as shown in Scheme 67. Mono-, di-, and trisubstituted furans are oxidized to (Z)-enediones by methyltrioxorheniumureaH202 <1998TL5651>. Mo(CO)6-catalyzed oxidation of 2,5-dialkyl furans by cumyl hydroperoxide provides (A)-enediones selectively. In the presence of Na2C03, the corresponding (Z)-isomers are obtained <2003TL835>. 

The formation of dialkyl benzo[f]furan-l-yl phosphonates by Lewis acid-promoted reaction of o-phthalaldehyde with trialkyl phosphites has been described (Equation 127) <2006S4124>. 

The adduct of furan and dibenzoyldiazene, previously assigned a diazetidine structure9, is instead an oxadiazine 1010. Furans, however, act as dienes in [4 + 2] cycloaddition reactions with dialkyl diazenedicarboxylates to give unstable adducts. 

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