With Other Dienophiles

R = Ph, CgHi3, m-H0CgH4, m-Me0CgH4,p-N02C6H4 Ar = Ph, p-MeCgH4, j-C1C6H4, 

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A.2.6. Asymmetric Diels-Alder Reactions with Other Dienophiles... 

DMAD), and a 1 1 mixture of the diesters 2a (endocyclic product) and 4a (exo-endo product) was obtained when the reaction was carried out at room temperature. The overall yield (10%) was very low due to polymerization of the 2-vinylfuran. In contrast with reactions with other dienophiles (see Section II,B,1), in which the exo-endo diene of 2-vinylfurans proved to be more reactive than the furan system itself, the reaction with DMAD resulted in reduced selectivity, probably due to equilibration of the more soluble adducts in this case (68TL4589 73AJC1059). 

The reaction of 85 with other dienophiles, however, proved to be less effective /V-mcthacryloyloxazolidinonc 104 gave no reaction whereas methacrolein gave product 105 in lower yield and ee (Scheme 28). 

The reaction of methacrolein with cyclopentadiene catalyzed by a chiral menthoxyaluminum complex gives adducts with ee s of up to 72%, but with other dienophiles little, if any, induction was noted.9495 A chiral cyclic amido aluminum complex 2 catalyzes the cycloaddition of cyclopentadiene with the fran.v-crotyl derivative 3 in good yield and enantioselectivity (Scheme 26.2).47 This chiral catalyst can also be easily recovered. 

Precomplexation of a,/ -unsaturated ketone 163 with ATPH in CH2CI2 at -78 °C, then cyclization with cyclopentadiene, resulted in the stereochemical reversal to furnish exo-164 as a major product, as shown in Sch. 128. Similarly, the Diels-Alder reaction with other dienophiles complexed with ATPH resulted in exo selectivity [168],... 

Benzvalene also reacts with other dienophiles. ... 

The stable, monomeric 0-enaminodithioesters, like their thioketone counterparts, have been shown to participate in regiospecific, intermolec-ular Diels-Alder reactions with a range of representative electron-deficient and reactive dienophiles (Scheme 8-1V).19-20 Cinnamic acid thioamides exist in equilibrium with their dimers at room temperature, although to date no reports of their cycloaddition with other dienophiles have been detailed [Eq. (9)].21... 

It should be noted that because of the extreme reactivity of arynes, other reactions such as 2+2 cycloaddition or the ene reaction may complete with the Diels-Alder mode of addition more frequently than is the case with less reactive dienophiles. This is especially true if the diene component is not conformationally predisposed to the 4+2 cycloaddition mode. Also, as with other dienophiles, Diels-Alder reactions of arynes are stereoselective. 

Another important group of dienophiles of the a,p-unsaturated carbonyl class are quinones. 1,4-Benzoquinone reacts readily with butadiene at room temperature to give a high yield of the mono-adduct, tetrahydronaphthaquinone (3.8) under more vigorous conditions a bis-adduct is obtained which can be converted into anthraquinone by oxidation of an alkaline solution with atmospheric oxygen. As with other dienophiles, alkyl substitution on the double bond leads to a decrease in activity and cycloaddition of monoalkyl 1,4-benzoquinones with dienes occurs preferentially at the unsubstituted double bond. In addition to steric effects, electronic effects can play a part, such that cycloaddition occurs at the more electron-deficient double bond of the benzoquinone. The first step in an approach to the steroid ring system makes use of such selectivity (3.9). ... 

The same group of authors has reported on the formation of similar DNBF cycloadducts with other dienophiles, in particular, with ethyl vinyl ether [116]. When the reaction is carried out in the presence of 2.5 equivalents of... 

The above results show that in reactions with arynes, pyrrolinediones act as synthetic equivalents of azadienes. One therefore wonders how they behave in reactions with other dienophiles. Is it possible to generalize the method described above to obtain a new synthetic route to pyridones and pyridines As a step in this direction, we have found that the pyrrolinedione 29b reacts rapidly with dimethyl acetylene dicarboxylate (DMAD) to give the emetine 42 in 75% yield (Scheme 14)(Ref. 21). 

Besides rran -cyclooctenes and norbomenes, 1,2,4,5-tetrazines also react with other dienophiles that include cyclooctynes [83], cyclopropenes [84, 85] and N-... 

Inukai and Kojima have shown that in the reaction of dialkyl maleate with cyclopentadiene, the aluminum-chloride-catalyzed reaction yields a higher endojexo ratio in the product (Table 4.8) than the uncatalyzed reaction. Similar results were also obtained with other dienophiles. A possible explanation is that by complexing with AICI3, the dienophiles are rendered more electrophilic and hence a rate enhancement is obtained. Stronger interaction in the transition state would favor the endo geometry. 

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