Maleic anhydride-furane adduct

Recent work shows that less reactive furans, where longer reaction times and higher temperatures are necessary, give rise to exo adducts. In contrast, with more active dienes, and in reactions at lower temperatures, more endo adducts are formed. It has been shown that the reactions are seldom completely stereospecific. This is illustrated by the work of Stockmann.259 He employed a new titration technique with hypoiodite, and found that the maleic anhydride-furan adduct which was prepared in an ether medium contained about 5-6% of the exo isomer. 

A reverse Diels-Alder reaction, the decomposition of 2-methylfuran-maleic anhydride ejro-adduct (in iso-octane) was the object of an extensive study of isotope effects . Here rehybridisation is from ip to sp and therefore a ratio knlko > 1 is to be anticipated. This has, in fact, been found (/ h/ d)50 c = 108 per D atom, both for all possible deuterations on the maleic anhydride moiety (see below), either at X or at Y or at X and at Y, and for deuteration of the furan moiety at Z. 

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). 

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. 

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... 

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. 

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). 

Novitskii, Khachaturova and Yur ev obtained 4i/,6/f-thieno[3,4-c]-furan (128) from 3,4-bischloromethylfuran (127) and sodium sulfide. The thienofuran (128) was converted into an adduct (129) with maleic anhydride and into 3,4-dimethylfuran (130) by desulfurization with Raney nickel [Eq. (40)]. 

The parent compound (4), a derivative of o-quinodimethane, is unstable in solution it reacts almost instantaneously with typical dienophiles like maleic anhydride and methyl vinyl ketone to give the corresponding Diels-Alder adducts (Section III). Simple HMO calculations show that the total ji-electron energy of 4 is smaller than that of benzo[h]furan (8). It can be... 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

Thermally unstable azetidines may be obtained by irradiation of furan in the presence of oxazolinones or 3-ethoxyisoindolone by [7r2s+ .2s] cycloaddition (Scheme 90) (75JA7298). Benzophenone photosensitized [ 2S+W2S] cycloaddition of furan to dimethylmaleic anhydride yields an adduct (34%). This reaction has been extended to 2,5-dimethylfuran and other maleic anhydrides and maleimides (76CA(84)120785). 

---