Diels-Alder reactions definition

Chemists usually learn about reactions according to fiinctional groups for example, How can I make an aldehyde and what reactions are known for aldehydes " This is clearly not a very good starting point for classifying reactions. The poor state of affairs in the definition of reaction types is further quite vividly illustrated by the fact that many chemical reactions are identified by being named after their inventor Diels-Alder reaction, Michael addition, Lobry-de Bruyn-van Ekenstein rear-... 

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. 

An impressive combination of two Diels-Alder reactions is also described by Winkler [4] for the synthesis of the taxane skeleton, though two different Lewis acids must be used for the two cycloadditions. Thus, it does not strictly match the definition of a domino reaction. 

On the basis of available experimental data, it is impossible to choose a definite pathway of elimination of silanol. However, study of silylation of methyl P -nitropropionate (411) with BSA in the presence of trapping agents rigorously proved that silyl nitronate D is initially formed. This compound can be detected in the [3 + 2]-cycloaddition reaction with methyl acrylate product (413). If silylation of AN (411) is performed in the presence of ethyl vinyl ether, a-nitrosoalkene E can be successfully trapped in as heterodiene a Diels-Alder reaction. Dihydroox-azine (414) is formed, and its silylation affords isolable product (415). 

Olefins can add to double bonds in a reaction different from those discussed in 5-15, which, however, is still formally the addition of RH to a double bond. This reaction is called the ene synthesis44,1 and bears a certain similarity to the Diels-Alder reaction (5-47). For the reaction to proceed without a catalyst, one of the components must be a reactive dienophile (see 5-47 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. There has been much discussion of the mechanism of this reaction, and both concerted pericyclic (as shown above) and stepwise mechanisms have been suggested. The reaction between maleic anhydride and optically active PhCHMeCH=CH2 gave an optically active product,441 which is strong evi-... 

More recently, however, it has been suggested that carbocation intermediates might result from the addition of an amminium ion (through the aromatic ring) to a jr-bond114. Recently, this controversy seems to have been definitively settled Most hole-catalyzed Diels-Alder reactions actually do involve cation radicals115, except for the addition of tetracyanoethylene to electron-rich alkenes116. 

Here we are primarily concerned with the fact that this ortho -adduct may occur in the form of two diastereomers. The diastereomers are formed as a 57 43 cis/trans-mixtme in the absence of A1C13, but a 95 5 cis/trans-mixture is obtained in the presence of A1C13. In the latter case, thus, one is dealing with a Diels-Alder reaction that exhibits a substantial simple diastereoselectivity (see Section 11.1.3 for a definition of the term). Here, the simple diastereoselectivity is due to kinetic rather than thermodynamic control, since the preferentially formed ds-disubstituted cyclohexene is less stable than its irans-isomer. 

A more definitive synthesis of the unsubstituted cyclopropaphenanthrene 17 has been accomplished using a retro-Diels-Alder reaction (see Section 5.2.2.6.). ... 

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