Reactivities in Diels-Alder reaction

It was found that the introduction of a sulfonyl substituent considerably enhances the furanone reactivity in Diels-Alder reaction. Thus, (55)-5-(reacted with cyclopentadi-ene at room temperature in benzene with complete conversion to the adduct 212. Also, the reaction of 211 with 2,3-dimethyl-1,3-butadiene was readily performed in refluxing benzene to give the adduct213 in 98% yield (Scheme 57) (91TL7751). 

Pentadiene is much more reactive in Diels-Alder reactions than 2,4-pentadienal. Why might this be ... 

Tamariz and coworkers [42] have described a versatile, efficient methodology for preparing N-substituted-4,5-dimethylene-2-oxazolidinones 42 (Figure 2.5) from a-diketones and isocyanates and have also studied their reactivity in Diels-Alder reactions. This is a method for synthesizing polycyclic heterocyclic compounds. Some of the reactions of diene 42 are summarized in Scheme 2.18. The nitrogen atom seems to control the regiochemistry of the reaction. 

As depicted in Scheme 5-10, iminium salts such as 42 have been found to exhibit substantially increased reactivity in Diels-Alder reactions.9 This means... 

The reactivity of dienes in Diels-Alder reactions is also controlled by the diene conformation. The two planar conformations of 1,3-butadiene are referred to as s-trans and s-cis (equation 16). Calculations have shown the s-trans conformation to be 2-5 kcalmol-1 more stable than the s-cis conformation. Open-chain dienes can only react in their cisoid conformation. Thus, 2-substituted dienes are generally more reactive than 1,3-butadiene due to their stronger preference for the s-cis conformation. 1 -Cis substituted 1,3-butadienes are almost exclusively in the s-trans conformation and are not reactive in Diels-Alder reactions. Highly substituted dienes may, however, be present in the s-cis conformation during a sufficient amount of time to participate in Diels-Alder reactions, even if a 1 -cis substituent is present62. 

Cyclic conjugated dienes that are s-cis conformation, e.g. cyclopentadiene and 1,3-cyclohexadiene, are highly reactive in Diels-Alder reactions. In fact, cyclopentadiene is reactive both as a diene and as a dienophile, and forms dicyclopentadiene at room temperature. When dicyclopentadiene is heated to 170 °C, a reverse Diels-Alder reaction takes place and reforms the cyclopentadiene. 

Diels-Alder reactions.1 This highly substituted silyloxydiene is comparable to 1-methoxy-3-trimethylsilyloxy-l,3-butadiene (6, 370 9, 303-304) in reactivity in Diels-Alder reactions. 

The intermediates (66), (67), upon in situ oxidation, afford 2(or 3)-pyridynes or 2(or 3)-quinolynes <71JCS(C)3948> which are highly reactive in Diels-Alder reactions, as shown in Schemes 10 and 11 for syntheses of ellipticine (69), isoellipticine (70) (84JOC4518,92T10645), and furo[3,4-c]pyridine (72) (77TL1741). Nitrosation of the aminotriazolopyridines affords the parent triazolopyridines. 

The synthesis of BENZYL ISOCYANIDE from benzaldehyde via reductive amination with 5-aminotetrazole followed by oxidation of the resultant amine with sodium hypobromite provides a general method for the synthesis of isocyanides. The preparation of BIS(2,2,2-TRICHLOROETHYL) AZODICARBOXYLATE makes available an alternative to dimethyl azodicarboxylate that is not only more reactive in Diels-Alder reactions but whose ester groups can be removed under neutral conditions. 

The reactivity in Diels-Alder reactions of some mono- and polycyclic metalloles containing silicon, germanium and tin was calculated <2006NJC1149>. 

Diels-Alder reactions. 1 is considerably more reactive in Diels-Alder reactions than the corresponding dimethyl-1,3-diene. It reacts readily with activated dienophiles (e.g., maleic anhydride). It also reacts regioselectively with monosubstituted dienophiles to give the so-called meta adduct. 

Cyclopropylidene ketones exhibit high reactivity in conjugate addition of nucleophiles . Dicyclopropylideneethane, 31 29,591 cyclopropylideneacetic ester show high reactivities in Diels-Alder reactions. 

Diels-Alder reactions. This reagent is more reactive in Diels-Alder reactions than cyclohexenone. Treatment of the /i-nitro ketone with DBN effects elimination of HNO, to form the a,/ -enone.2 Thus the final products correspond to adducts of a cyclohexynone, but with reversed regioselectivity. 3-Nitro-2-cyclopentenone, m.p. 69-71°, is also a useful dienophile (second example). 

It is well known that introduction of electron-withdrawing substituents increases electron deficiency in the 1,2,4-triazine substrate and enhances its reactivity in Diels-Alder reactions with electron-rich dienophiles (83T2869 86CRV781 87MI1). The reaction under consideration is the... 

Prepared by the action of methanolic potassium hydroxide on hexachlorocyclo-pentadiene, the dimethoxy compound is more reactive in Diels-Alder reactions than the 5,5-dichloro derivative and reacts with a number of dienophiles under relatively mild conditions. - The adducts are of interest because they can be aromatized by a simple reaction sequence, as illustrated for the acrylic acid adduct, prepared by... 

Aminosiloxy dienes have been found to exhibit unusually high reactivity in Diels-Alder reactions with a wide range of electron-deficient dienophiles.2 5 Determination of the second order rate constants with methacrolein showed the parent aminosiloxy diene to be about 103 more reactive than the analogous 1-alkoxy-3-siloxy diene.4 The reactions of aminosiloxy dienes with various dienophiles occur under mild conditions and afford the corresponding cycloadducts in high yields and with complete regioselectivity. An extensive study on the preparation and cycloadditions of amino siloxy dienes has been carried out.3... 

However, the assignment of structures to 21a and 21b relies ultimately on their different reactivity in Diels-Alder reactions 2-pyrone 21a acts as a diene, but 21b does not (60JCS502). The same behavior differences have been observed for related methyl ethers [68AC(R)664]. 

Dienes that contain electron-donating groups (activated dienes) are more reactive in Diels-Alder reactions than unsubstituted or electron-deficient dienes. In molecular orbital formalism, the substituents on the diene perturb the tT-electron density to cause an increase in the energy of the highest occupied molecular orbital (HOMO Figure 1). In a normal-demand Diels-Alder reaction this results in an increase in the interaction between the HOMO of the diene and the LUMO (lowest unoccupied molecular orbital) of the dienophile. This interaction, in turn, lowers the transition state energy of the reaction. Similar arguments have also been used to explain the increased reactivity of activated dienes towards heterodienophiles such as aldehydes. 

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