Dienophile, definition

Alkynes substituted with one or two trifluoromethyl groups are also highly reactive dienophiles [9] Indeed, hexafluoro-2-butyne is used increasingly as a definitive acetylenic dienophile in "difficult Diels-Alder reactions. It was used, for example, to prepare novel inside-outside bicycloalkanes via its reaction with cir,trnns -l,3-undecadiene [74] (equation 67) and to do a tandem Diels-Alder reaction with a l,l-bis(pyrrole)methane [75] (equation 68) Indeed, its reactions with pyrrole derivatives and furan have been used in the syntheses of 3,4-bis(tri-fluoromethyl)pyrrole [76, 77] (equation 69) and ],4-bis(trifluoromethyl)benzene-2,3-oxide [78] (equation 70), respectively. 

Alkenes can add to double bonds in a reaction different from those discussed in 15-19, which, however, is still formally the addition of RH to a double bond. This is called the ene reaction or the ene synthesis For the reaction to proceed without a catalyst, one of the components must be a reactive dienophile (see 15-58 for a definition of this word) such as maleic anhydride, but the other (which supplies the hydrogen) may be a simple alkene such as propene. Cyclopropene has also been used. ° The reaction is compatible with a variety of functional groups that can be appended to the ene and dienophile. N,N-Diallyl amides give an ene cyclization. 

Recently, Denmark and coworkers have developed a new strategy for the construction of complex molecules using tandem [4+2]/[3+2]cycloaddition of nitroalkenes.149 In the review by Denmark, the definition of tandem reaction is described and tandem cascade cycloadditions, tandem consecutive cycloadditions, and tandem sequential cycloadditions are also defined. The use of nitroalkenes as heterodienes leads to the development of a general, high-yielding, and stereoselective method for the synthesis of cyclic nitronates (see Section 5.2). These dipoles undergo 1,3-dipolar cycloadditions. However, synthetic applications of this process are rare in contrast to the functionally equivalent cycloadditions of nitrile oxides. This is due to the lack of general methods for the preparation of nitronates and their instability. Thus, as illustrated in Scheme 8.29, the potential for a tandem process is formulated in the combination of [4+2] cycloaddition of a donor dienophile with [3+2]cycload-... 

Other dienes, even cyclopentadiene, perform poorly with this dienophile 38 but the azadiene 39 that does so well here is a very special case. It is stabilised by the phenyl groups at both ends of the molecule and it cannot tautomerise into an enamine. More generally useful 1-azadienes must be stabilised by conjugating substituents that are definitely electron-donating or withdrawing so that the HOMO or the LUMO is unambiguously selected for cycloaddition. 

Oxazole cycloadditions have been reported with alkyne dienophiles (tandem Diels-Alder addition and retro Diels-Alder loss of a nitrile leads on to furans), benzyne (the primary adduct can be isolated), and with typical alkene dienophiles. The primary adducts from addition of singlet oxygen rearrange, by a mechanism which is not definitely established, to form triamides, themselves useful synthetic intermediates. The only example of this sort of process with an imidazole is an intramolecular example, the product in this case being a pyr-role. ... 

Because the reversibility of the Diels—Alder reaction at accessible temperatures was key to the concept of re-mending a polymeric solid, we considered other diene dienophile combinations but were dissatisfied with aU we found in the literature for various reasons, but in the process realized that cyclopentadiene is a molecule that is a diene and a dienophile, a property that was a definite advantage over systems 2 and 3 + 4. This led to our first encounter with Hermann Staudinger. 

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