And the Diels-Alder reaction

Note that the stereochemistry comes out right. H s a and b are cis because they were cis in the starting quinone and the Diels-Alder reaction is stereospecific in this respect. H is also cis to and H " because the Diels-Alder reaction is stereoselectively endo. These points are described in more detail in Norman p.284-6 and explained in Ian Fleming Frontier Orbitals and Organic Chemical Reactions, Wiley 1976, p. 106-109. How would you make diene A ... 

Maleimides have three principal reaction pathways. These are radical addition to vinyl compounds the Michael addition with compounds having active hydrogens and the Diels-Alder reaction with dienes (Fig. 3). Any of the three can be tools for forming thermosetting adhesives. 

An interesting parallel was found while the microwave-enhanced Heck reaction was explored on the C-3 position of the pyrazinone system [29]. The additional problem here was caused by the capability of the alkene to undergo Diels-Alder reaction with the 2-azadiene system of the pyrazinone. An interesting competition between the Heck reaction and the Diels-Alder reaction has been noticed, while the outcome solely depended on the substrates and the catalyst system. Microwave irradiation of a mixture of pyrazinone (Re = H), ethyl acrylate (Y = COOEt) and Pd(dppf)Cl2 resulted in the formation of a mixture of the starting material together with the cycloaddition product in a 3 1 ratio (Scheme 15). On the contrary, when Pd(OAc)2 was used in combination with the bulky phosphine ligand 2-(di-t-butylphosphino)biphenyl [41-44], the Heck reaction product was obtained as the sole product. When a mixture of the pyrazinone (Re = Ar) with ethyl acrylate or styrene and Pd(dppf)Cl2 was irradiated at 150 °C for 15 min, both catalytic systems favored the Heck reaction product with no trace of Diels-Alder adduct. 

Acetylenic diester (45) is available, the synthesis of (44) is known, and the Diels-Alder reaction goes in excellent yield. 

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

There also appears to be great similarity between 1, 3 dipolar addition and the Diels-Alder reaction (which is 4 + 2 cycloaddition) described in the later pages. 

The real reactions that most resemble these are the production of cyclobutane from two ethylene molecules ((112)2) and the Diels-Alder reaction between butadiene and ethylene ((H2)3). Even these cannot be made to react in the bare forms, but fairly simple activation by substituents will allow the (H2)3 analog to proceed. Apparently, no form of the (1 2)2 analog has ever been observed. Our analysis suggests that there is a fundamental difference between the four-electron and six-electron systems that produces the effect. The book by Woodward and Hoffrnan[58] may be consulted for a rationalization of these results based upon MO theory. 

Review. The varied uses of singlet oxygen in organic syntheses have been reviewed (70 references). The best known are the ene reaction of alkenes, which results in an allylic hydroperoxide, and the Diels-Alder reaction with 1,3-dienes to form 1,4-endoperoxides. 

A similar stereochemical problem arises with the dialdehyde 16 needed for alkaloid synthesis. Reconnecting the 1,6-diCO relationship 17 and removing the acetal reveals an obvious Diels-Alder adduct 18 from the enone 19 and butadiene. The only substituent on the nearly flat enone 19 is the methyl group and the Diels-Alder reaction does indeed give the right diastereomer.6... 

Finally, 10 mol-% of the lanthanide perfluoroalkylsulfone amide complex Yb(N(S02C4F9)2)3 in QH5CF3 catalyzes both Friedel-Crafts acylation and the Diels-Alder reaction [26]. The high Lewis acidity of the metal complexes seems to be crucial. 

A more versatile approach uses an inner-outer ring diene system such as compound 496, which reacted with iV-methylmaleimide in toluene at 145 °C in a sealed tube to give cycloadduct 497 in 91% yield <2004TL7255>. Similar procedures with different electrophiles have been reported by other workers who were able to combine consecutively the preparation of the inner-outer diene by RCM reaction (see Section 2.04.9.3) and the Diels-Alder reaction in a one-pot process, which gave increased yields in most cases for the polycyclic compounds when compared with the two-pot approach <2004S2665>. 

The multipole expansion model has seen use in the examination of solvation effects on both reaction coordinates and conformational equilibria, including the isomerization of push-pull ethylenes o (e,g., nitroenamines), the ketene-imine [2-f 2]-cycloaddition to form p-lactam,24i and the Diels-Alder reaction.242,243 Again, only the ENP terms are considered in general. 

Mukaiyama Aldol Condensation. As expected, the chiral titanium complex is also effective for a variety of carbon-carbon bond forming processes such as the aldol and the Diels-Alder reactions. The aldol process constitutes one of the most fundamental bond constructions in organic synthesis. Therefore the development of chiral catalysts that promote asymmetic aldol reactions in a highly stereocontrolled and truly catalytic fashion has attracted much attention, for which the silyl enol ethers of ketones or esters have been used as a storable enolate component (Mukaiyama aldol condensation). The BINOL-derived titanium complex BINOL-TiCl2 can be used as an efficient catalyst for the Mukaiyama-ty pe aldol reaction of not only ketone si ly 1 enol ethers but also ester silyl enol ethers with control of absolute and relative stereochemistry (eq 11). ... 

Hie Diels-Alder reaction of the conjugated dienes (32) is well established. Hie following reaction sequence illustrates the potential synthetic utility of a combination of the bicyclization and the Diels-Alder reaction (equation 29). ... 

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