Dienes in the Diels-Alder reaction,

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). 

Although furan is usually a poor diene in the Diels-Alder reaction, the chiral copper reagent 24b promotes its asymmetric addition to acryloyloxazolidinone to afford the 7-oxabicyclo[2.2.1]hept-2-ene derivative in high optical purity (Scheme 1.40). Because a retro-Diels-Alder reaction occurs above -20 °C, the reaction must be performed at low temperature (-78 °C) to obtain a high optical yield. The bicy-... 

Thiophene 1,1-dioxide (61) is too unstable to isolate and dimerizes with loss of S02 to give 3a, 7a-dihydrobenzothiophene 1,1-dioxide (172) in 34%113. However, alkyl-substituted thiophene 1,1-dioxides can serve as dienes in the Diels-Alder reaction, since the aromatic properties of the thiophene nucleus are lost completely and the n-electrons of the sulfur atom are used for forming the bond with oxygen. The sulfones 173-178 are found to react with two moles of maleic anhydride at elevated temperature to give bicyclic anhydrides114. Thus, at high reaction temperature, S02 is split off to give cyclohexadiene... 

Fringuelli F. and Taticchi A. Dienes in the Diels- Alder Reaction, Wiley, New York, 1990. 

As a follow-up to our book Dienes in the Diels-Alder Reaction (1990) and in light of our personal experience as well as the reviews and books that have been published on this topic to date, we decided that a book collecting and describing the experimental methods that have been developed to perform the Diels-Alder reaction would be a useful tool for researchers working in organic synthesis. 

Azadienes undergo Diels-Alder reactions to form pyridine, dihydro- and tetrahydropyridine derivatives. N-Vinyl lactim ethers undergo Diels-Alder reactions with a limited set of dienophiles. " Thioketones react with dienes to give Diels-Alder cycloadducts. The carbonyl group of lactams have also been shown to be a dienophile. Certain heterocyclic aromatic rings (among them furans) can also behave as dienes in the Diels-Alder reaction. Some hetero dienes that give the reaction are -C=C-C=0, 0=C-C=0, and N=C-C=N. ... 

Fig. 6.1. Interaction between LUMO of dienophile and HOMO of diene in the Diels-Alder reaction.

Sensitive dienol ether functionality in the diene carboxylate was shown to be compatible with the conditions of the aqueous Diels-Alder reaction (Eq. 12.26).84 The dienes in the Diels-Alder reactions can also bear other water-solubilizing groups such as the sodium salt of phosphoric acid and dienyl ammonium chloride (Eq. 12.27).85 The hydrophilic acid functionality can also be located at the dienophile.86... 

As the name implies it is related to the diene cycloaddition reaction, the Diels-Alder reaction. The ene component the olefin with the allylic hydrogen, takes much the same part as the diene in the Diels-Alder reaction. 

The starting material is called dicyclopentadiene . Cyclopentadiene itself is not stable it dimerizes to dicyclopentadiene slowly at room temperature by a Diels-Alder reaction. It does this even though it is not an electron-deficient dienophile, demonstrating the enormous reactivity of cyclopentadiene as a diene in the Diels-Alder reaction. 

Trapping of an intermediate. In some cases, the suspected intermediate is known to be one that reacts in a given way with a certain compound. The intermediate can then be trapped by running the reaction in the presence of that compound. For example, benzynes (p. 646) react with dienes in the Diels-Alder reaction (5-47). In any reaction where a benzyne is a suspected intermediate, the addition of a diene and the detection of the Diels-Alder adduct indicate that the benzyne was probably present. 

Markl has shown (68TL3257) that l-phenyl-2,5-dimethylarsole acts as a diene in the Diels-Alder reaction. When acetylenedicarboxylates are used as the dienophile the intermediate arsolene is unstable. The adduct with tetracyanoethene was, however, isolated as the stable crystalline solid (50 m.p. 157 °C) (reaction 10). At higher temperatures 1,2,5-triphenylarsole reacts similarly as a diene with diphenylacetylene (81TL4713). 

The dibenzarsenins (125) are prepared by the pyrolysis of a 10-benzyl-5,10-dihy-drodibenz[6,e]arsenin (equation 27). This reaction proceeds via a radical abstraction process. The parent (125 R = H) is unstable and could not be isolated, but the phenyl (125 R = Ph), tolyl (125 R = 4-MeCeH4) and p-methoxyphenyl (125 R = 4-MeOCeH4) derivatives were isolated. Like arsenin itself, (125) can act as a diene in the Diels-Alder reaction and maleic anhydride adducts were prepared for each of the above (77RTC265). 

The reactivities of dienes in the Diels-Alder reaction depend on the number and kind of substituents they possess. The larger the substituents are, or the more of them, at the ends of the conjugated system, the slower the reaction is likely to be. There also is a marked difference in reactivity with diene configuration. Thus trans- i, 3-pentadiene is substantially less reactive toward a given dienophile (such as maleic anhydride) than is cis-1,3-pentadiene. In fact,... 

F. Ftinguelli, A. Taticchi, Dienes in the Diels-Alder Reaction, J. Wiley and Sons, New York-Chichester (1990)... 

The instability of 1-azabuta-1,3-dienes arising from the imine moiety and the low reactivity as dienes in the Diels-Alder reaction are sometimes problems. Benzylidene(cyano)methyl-l,3-benzothia/oxazoles, featuring a stabilized imine moiety in the form of a heteroaromatic ring... 

A theoretical study of thieno[3,4-rf]thiepine and furo[3,4-rf]thiepine as dienes in the Diels-Alder reaction has been published <95JHC1499>. 

In contrast, the double bonds of cyclopentadiene are held in the s-cis conformation. This makes cyclopentadiene so reactive as a diene in the Diels-Alder reaction that it... 

Although oxazole possesses a sextet of 7r-electrons, all its properties indicate that the delocalization is quite incomplete hence it has but little aromatic character. There is considerable bond fixation (see Section 4.18.2.3.1), hydroxyoxazoles are unstable relative to their oxo tautomers, oxazolediazonium salts are unknown, oxazoles function as dienes in the Diels-Alder reaction (see Section 4.18.3.1.2(vii)) and electrophilic substitution is rare. The chemistry of oxazole is dominated by its tendency to undergo ring-opening rather than preserve its type. 

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