Furane-maleic anhydride

The radical-catalyzed polymerization of furan and maleic anhydride has been reported to yield a 1 1 furan-maleic anhydride copolymer (89,91). The stmcture of the equimolar product, as shown by nmr analyses, is that of an unsaturated alternating copolymer (18) arising through homopolymerization of the intermediate excited donor—acceptor complex (91,92). 

The radical catalyzed homopolymerization of the furan-maleic anhydride (F-MAH) Diels-Alder adduct yields a saturated homopoly-mer at temperatures below 60 C, and an unsaturated equimolar alternating copolymer at elevated temperatures, due to retrograde dissociation of the adduct (10, 11). The copolymerization of monomeric furan and maleic anhydride yields the same unsaturated alternating copolymer, independent of temperature (1C)). 

It has previously been proposed that the saturated products from the homopolymerization of norbornene (17) and the CPD-MAH Diels-Alder adduct (12-15), and probably from the furan-maleic anhydride Diels-Alder adduct (11), have rearranged structures. An analogous structure would arise from the homopolymerization of the cyclopentadiene-N-phenylmaleimide CPD-NPMI adduct, as follows ... 

Development of synthetic routes to the anthracycline antibiotics is important because of the cytotoxic properties of many of these compounds. A short route " to possible intermediates proceeds via double Diels-Alder addition to (16). The problem of the availability of (16) has been overcome " by efficient palladium-catalysed carbomethoxylation of the furan-maleic anhydride Diels-Alder adduct to give (122) in 92% yield. Elaboration of Diels-Alder adducts of (16) to anthracyclines requires rupture of the ether bridge. Acid-catalysed ringopening " in simple 7-oxabicyclo[2.2.1]heptanes has been studied. 

Sol 15. The majority of Diels—Alder reactions yield endo products. In most of the cases, the exo product is thermodynamically more stable, but the endo adduct forms much more rapidly (kinetic control). However, [4 + 2] cycloaddition of furan and maleic anhydride gives predominantly exo adduct. Such a stereochemistry of the furan-maleic anhydride adduct is due to the fact that the initially formed endo compound readily reverses into the reactants whereas the exo cycloaddition gives a relatively stable adduct that is the product of the thermodynamic control. 

The balance between aromatic and aUphatic reactivity is affected by the type of substituents on the ring. Furan functions as a diene in the Diels-Alder reaction. With maleic anhydride, furan readily forms 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyhc anhydride in excellent yield [5426-09-5] (4). 

Furan and maleic anhydride undergo the Diels-Alder reaction to form the tricycHc 1 1 adduct, 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyHc anhydride (4) in exceUent yield. Other strong dienophiles also add to furan (88). Although both endo and exo isomers are formed initially, the former rapidly isomerize to the latter in solution, even at room temperature. The existence of a charge-transfer complex in the system has been demonstrated (89,90). 

Furan can be catalyticaHy oxidized in the vapor phase with oxygen-containing gases to maleic anhydride (93). Oxidation with bromine or in an electrochemical process using bromide ion gives 2,5-dimethoxy-2,5-dihydrofuran [332-77-4] (19) which is a cycHc acetal of maleic dialdehyde (94—96). 

The success of the cycloaddition reaction of maleic anhydride varies gready depending on which heterocyclic diene is used. The cycloaddition of maleic anhydride to furan [110-00-9] occurs ia a few seconds under ambient conditions (42,43). Although the endo adduct (14) is favored kiaeticaHy, the exo adduct (13) is isolated. 

Metal-Induced Cycloadditions. The effect of coordination on the metal-iaduced cyclo additions of maleic anhydride and the isostmctural heterocycles furan, pyrrole, and thiophene has been investigated (47). Each heterocycle is bound to an Os(II) center in the complex... 

NH2)50s(2,3-T -L)], where L = furan, pyrrole, and thiophene. Although neither the furan nor thiophene complexes react with maleic anhydride over a period of 10 days, the pyrrole complex (15) reacts rapidly at room temperature and 101.3 kPa to form a mixture of endo (17) and exo (16) complexes. An a2omethine ylide intermediate was postulated as the key intermediate through which maleic anhydride added to the 2- and 5-positions of the coordinated pyrrole ring. 

Aqueous ring-opening metathesis polymerization (ROMP) was first described in 1989 (90) and it has been appHed to maleic anhydride (91). Furan [110-00-9] reacts in a Diels-Alder reaction with maleic anhydride to give exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3—dicarboxylate anhydride [6118-51 -0] (24). The condensed product is treated with a soluble mthenium(Ill) [7440-18-8] catalyst in water to give upon acidification the polymer (25). Several apphcations for this new copolymer have been suggested (91). 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

Soon after the discovery of the addition reaction between diene-ophiles and dienes which now bears their names, Diels and Alder extended their investigations to include potential heterocyclic dienes. In 1929 the first compound investigated, furan, was observed to combine with maleic anhydride, like butadiene in a typical Diels-Alder reaction, across the 2,5-positions yielding a 1 1 molar adduct... 

Copolymerization of Furan and Homologues with Maleic Anhydride... 

Vinyl- and 3-vinylthiophene (73b and 77) are less reactive than the corresponding furans and show a notable preference for extra-annular addition due to the higher reactivity of the diene system, including the side-chain double bond. 2-Vinylthiophene is less reactive than 3-vinylthiophene. Whereas 2-vinylthiophene (73b) reacted with maleic anhydride and 1,4-benzoquinone to give cycloadducts in reasonable yield, 3-vinylthiophene (77) gave a higher yield of the cycloadduct [76, 77] (Scheme 2.31). 

The investigation on the use of K-10 montmorillonite under free solvent conditions was then extended to inner ring dienes such as furan and its 2,5-dimethyl derivative [9] (Table 4.3). The cycloadditions generally proceed slowly, and Zn(II)-doped clay and microwave irradiation were used to accelerate the reactions. The reaction with maleic anhydride preferentially affords the thermodynamically favored exo adduct. 

The more reactive furan (139a) undergoes thermal Diels-Alder reaction [52] with reactive dienophiles such as maleic anhydride and maleimide (Scheme 5.21). Whereas the cycloaddition with the maleic anhydride afforded the exoadduct at room temperature, the stereochemistry of the reaction of maleimide depends on the reaction temperature. 

Whereas maleic anhydride can react with furan (139a) at ambient pressure, citraconic anhydride (140) reacts only at high pressures due to the strong deactivating effect of the methyl group (Schemes 5.21 and 5.22). The two-step synthesis [53] of the palasonin (141), in an overall yield of 96 %, is a good example of the acceleration of the Diels-Alder by high pressure (Scheme 5.21). Previous synthesis [54] based on the thermal Diels-Alder reaction of furan with methoxy carbonyl maleic anhydride required 12 steps. 

The cycloaddition between furan and maleic anhydride was the first uncatalyzed aqueous Diels-Alder reaction reported in the literature and was studied by Diels and Alder themselves [11]. This cycloaddition was successfully revised by Woodward and Baer [12] and some years later by De Koning and coworkers [13]. The aqueous medium was also used in the cycloaddition of aromatic diazonium salts with methylsubstituted 1,3-butadienes [14]. 

Maleic anhydride is an important industrial fine chemical (see original citations in [43]). The oxidation of C4-hydrocarbons in air is a highly exothermic process, therefore carried out at low hydrocarbon concentration (about 1.5%) and high conversion. The selectivity of 1-butene to maleic anhydride so far is low. The reaction is composed of a series of elementary reactions via intermediates such as furan and can proceed to carbon dioxide with even larger heat release. As a consequence, hot spots form in conventional fixed-bed reactors, decrease selectivity and favor other parallel reactions. 

Arylidine-furan-2-ones 439 undergo [4+2] cycloaddition with maleic anhydride 440 to give the tricyclic adducts 441 (Equation 119) <1997CHE910>. 

Reactions of furan (5) under solvent-free conditions, catalyzed by Montmorillonite K10, have been described by Cintas [27]. The reaction with methyl vinyl ketone (32) produced Michael addition in positions 2 and 5, whereas reaction with symmetrically substituted cyclic dienophiles produced a mixture of the endo and exo adducts with the kinetically favored endo adduct predominating, except when maleic anhydride (39) was used as the dienophile (Scheme 9.2). 

The student is recommended to carry out the Diels and Alder diene synthesis when making preparations from the original literature. For example, he should condense cyclo-hexadiene with quinone (Annalen. 1933, 507, 288) or furane with maleic anhydride (Ber., 1929, 62, 554). 

Anodic oxidation of fiirans in acetic acid leads to the 2,5-diacetoxy-2,5-dihydro-furan 58 [185, 186]which is readily converted to 2-acetoxyfiiran, This has proved a valuable intermediate for the synthesis of butenolides [187]. Reactions in moist acetonitrile yield the 2,5-dihydro-2,5-dihydroxyfurans which can be oxidised to the maleic anhydride 59 [188], Oxidation of furan-2-carboxylic acid in methanol and sulphuric acid is a route to the ester of a-ketoglutaric acid [189]. 

The mechanism is not well understood, but the intermediates in the process are butadiene and furan. The uses of maleic anhydride are... 

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