Regular Diels-Alder Reaction

TL 27 5509 (1986) (3-sulfinyl-2-aUcenoate ester) 28 107 (1987) (3-sulfinyl-2-alkenoate ester) 30 3853 (2-suiiinyI-2-a]ken-l-oiie)l 4003 (sulfinyl quinone), 4227 (fumaric diamide), 6973 (2-alkoxycarbonyl-2-alkene-4-lactam), 6977 (2-alkoxycarbonyl-2-alkene-4-lactam) (1989) 32 947 (sulfinyl maleate), 2005 (2-acyl-2-alkenoate ester), 7751 (4-aIkoxy-3-sulfonyl-2-alken-4-olide) (1991) 

JOC 56 1983 (1991) (sulfinyl maleimide) 574664 (2-cyano-2-alkenoate ester), 6870 (sulfinyl quinone) (1992) 59 1499 (sulfinyl maleate), 2211 (l-nitro-2-suIfinyl-l-alkene), 7774 (2-cyano-2-alkenoate ester) (1994) 60 4962 (1995) (1,1-disuMmyl-l-aIkene) 

Inverse Electron-Demand Dlels-Alder Reaction 

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These authors found that the tetrazinylhydrazone derivative 46 when reacted with pyrrolidinoenamine 47 in methanol yields the cyclopenta-fused derivative of the title ring system 48 in 94% yield. A similar transformation was carried out successfully by using morpholine-enamine in somewhat poorer yield. When the transformation was tried in acetonitrile as a solvent, a totally different reaction was observed a regular Diels-Alder reaction between the tetrazine ring and the enamine double bond (of inverse electron demand) took place to yield pyridazines. 

Maynollo J., Kraeutler B. Diels-Alder Reactions of the [60]Fullerene From Regularly Functionalized Carbon Spheres to Cyclophanes Fullerene Sci. Technol. 1996 4 213-226... 

Some aspects of the chemistry of helicenes require still more attention. Since the interpretation of the mass spectrum of hexahelicene by Dougherty 159) no further systematic work has been done on the mass spectroscopy of helicenes, to verify the concept of an intramolecular Diels-Alder reaction in the molecular ion. Though the optical rotation of a number of helicenes is known and the regular increase of the optical rotation with increasing number of benzene rings has been shown, the dependence of the rotation on the helicity is still unknown. The asymmetric induction in the synthesis of helicenes by chiral solvents, or in liquid crystals, though small, deserves still more attention because application to other organic compounds will be promoted when the explanation of observed effects is more improved. 

Chiral oxazoborolidine systems have been regularly used as catalysts for the enantioselective Diels-Alder reaction [173]. Such a catalyst has been immobilized by copolymerization of a sulfonamide-modified styrene monomer to produce polystyrene beads and subsequently reacted with borane to furnish catalyst 49 (Scheme 4.77). A column of 49 was cooled to —30 °C and a 1 1.5 methylacrolein cyclopentadiene solution was flowed in. Following aqueous workup and column chromatography, the desired product 50 was isolated to yield 138 mmol of product in 95% yield and 71% ee by using only 5.7 mmol of catalyst. This result was found to be comparable with the heterogeneous batch reactions that were also attempted [174]. 

A detailed solvent and pressure study was reported for the Diels-Alder reaction between isoprene and methyl vinyl ketone [75]. The medium effect on the rate constant is recorded in Table 10.20. The theory of regular solutions predicts a linear correlation between In fc and the cohesive energy density [81]. Plotting ln(k/ko) (fe is the rate constant observed in dichloromethane) leads effectively to a linear plot (Fig. 10.4). 

In contrast, reaction with 2,3-dichlorobutadiene affords the regular Diels-Alder adduct in 70 % yield. 

Since the first step is a regular Lewis acid-assisted thermal Diels-Alder reaction, the scope of the reaction is limited to electron-deficient dienophiles. 

The types of cycloadditions discovered for enamines range through a regular sequence starting with divalent addition to form a cyclopropane ring, followed by 1,2 addition (i) of an alkene or an alkyne to form a cyclo-cyclobutane or a cyclobutene, then 1,3-dipolar addition with the enamine the dipolarophile 4), and finally a Diels-Alder type of reaction (5) with the enamine the dienophile. 

General forms are easily developed for other reactions. The machinery introduced in this section can then be utilized to write disconnection rules for other reactions. For example, consider the Diels-Alder adduct bicyclo[2.2.1]hept-2-ene. Using the regular expression notation described previously, the line notation for these types of compounds can be represented as... 

Bicyclo[3.3.0]oct-l(5)-ene 178 (Scheme 4.55) is a stable compound with a flattened alkene fragment and exhibits a regular pattern of reactivity. Computational studies revealed, however, that installation of a short 3,7-bridge should lead to noticeable pyramidalization of the double bond. Compounds like 179-181 were synthesized to check this prediction. Tricyclic hydrocarbon 179, with the smallest possible bridge, was generated as a transient species from diiodide 182. The formation of 179 is implicated by the isolation of its cyclodimer 183 (or respective Diels-Alder adduct if the reaction is carried out in the presence of a 1,3-diene). The next member of this series, 180, is more stable. In fact, the formation of 180 was ascertained not only from the structure of the final products (as was done for 179), but also by its matrix isolation and analysis of spectral data. The selenium derivative 181 was found to be stable at ambient temperature in the absence of oxygen. X-ray data confirmed a noticeable pyramidalization of the double bond in 181 but the distortion was different [Pg.372]

In the past few years solvent-free DA reactions have been regularly performed successfully under microwave irradiation conditions, reducing reaction times to a few minutes compared with several hours under conventional reflux conditions [3j]. Scheme 11.3, mentioned above, shows two recent examples of Diels-Alder cycloadditions performed by microwave dielectric heating [42, 43]. In both examples the diene and dienophile were reacted neat under solvent-free conditions. 

Along the columns of Table 13 no regular trend is immediately recognizable and the conclusion that dienophilic and dipolarophilic power are not synonymous must be drawn. A more accurate inspection shows that both electronegative and electron-rich substituents on the multiple bonds can accelerate some of these reactions, at variance with what had been found for Diels-Alder additions (Table 7). The lowest rate coefficients in (i) (iii) and (v) correspond to cyclohexene as dipolarophile. Styrene and phenylacetylene are one or two orders of magnitude more reactive. A similar or larger effect is observed with alkyl vinyl ethers. For a further increase in rate more polar substituents are required, and the best dipolarophiles appear to be acrylic, propiolic, acetylene dicarboxylic and similar esters, as well as 1-pyrrolidino-cyclohexene. 

Benzene is not a cyclohexatriene but a delocalized cyclic system of six it electrons. It is a regular hexagon of six sp -hybridized carbons. All six p orbitals overlap equally with their neighbors. Its unusually low heat of hydrogenation indicates a resonance energy or aromaticity of about 30 kcal mol (126 kJ mol ). The stability imparted by aromatic delocalization is also evident in the transition state of some reactions, such as the Diels-Alder cycloaddition and ozonolysis. 

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