Dienophiles, structures

Oxazoles represent the most widely recognized heteroaromatic azadiene capable of [4 + 2] cycloaddition reactions. The course of the oxazole Diels-Alder reaction and the facility with which it proceeds are dependent upon the dienophile structure (alkene, alkyne), the oxazole and dienophile substitution, and the reaction conditions. Alkene dienophiles provide pyridine products derived from fragmentation of the [4 + 2] cycloadducts which subsequently aromatize through a variety of reaction pathways to provide the substituted pyridines (Scheme 14). In comparison, alkyne dienophiles provide substituted fiirans that arise from the retro Diels-Alder reaction with loss of R CN from the initial [4 + 2] cycloadduct (Scheme 15,206 Representative applications of the [4 + 2] cycloaddition reactions of oxazoles are summarized in Table 14. Selected examples of additional five-membered heteroaromatic azadienes participatiitg in [4 + 2] cycloaddition reactions have been detailed and include the Diels-Alder reactions of thiazoles, - 1,3,4-oxadiazoles, isoxazoles, pyrroles and imidazoles. ... 

The course of and facility with which the Diels-Alder reaction of oxazoles proceed are dependent on the dienophile structure, the oxazole/dienophile substitution, as well as the reaction conditions. Olefinic dienophiles provide pyridine products derived from the fragmentation of the initial [4 + 2] cycloadducts 2 to provide 3 which subsequently aromatize to provide the substituted pyridines [Eq. (1)]. 

The axial methyl group at C(10) hinders the approach of the diene to the p-face of the rigid bicyclic dienophile 4.5. While both the rate and endo/exo (14a/14P) ratio are increased when this reaction was performed in water rather than organic media, the observed diastereofacial selectivity appears to derive solely from the dienophile structure since the same a-selectivity was observed when the reaction was performed in benzene. 

The Influence of Dienophile Structure on Reactivity and Stereoselectivity in [4+2] Cycloadditions of Nitroalkenes Various dienophiles have been used successfully in nitroalkene [4 + 2] cycloadditions, such as vinyl ethers [77], enamines [78], ketene acetals [79], unactivated alkenes [58, 80, 81], 1,3-dienes [47, 82], allylic silanes [83], and allenes [84] (Scheme 16.10). Because of their high... 

The Influence of Dienophile Structure on Reactivity and Stereoselectivity in Tandem Cycloadditions of 1,3,4-0xadiazoles The best dienophiles for the tandem cycloadditions of 1,3,4-oxadiazoles are electron-rich, unhindered, and strained alkenes. These components are used in excess because they also serve as the dipolarophiles in the next step. The yields are often moderate, probably, due to the harsh reaction conditions. Thus, cyclopentene reacts with oxadiazole 440 (Scheme 16.88) to provide the oxabi-cycloheptane derivative 443 in 33% yield [167a]. The product configuration has been established as synlanti with respect to the oxygen atom bridge. It is not clear which of the two steps ([4 + 2] or [3 - - 2]) produced which relationship. Ethylene has also been used as the dienophile in the earliest report. 

The rate constants for the catalysed Diels-Alder reaction of 2.4g with 2.5 (Table 2.3) demonstrate that the presence of the ionic group in the dienophile does not diminish the accelerating effect of water on the catalysed reaction. Comparison of these rate constants with those for the nonionic dienophiles even seems to indicate a modest extra aqueous rate enhancement of the reaction of 2.4g. It is important to note here that no detailed information has been obtained about the exact structure of the catalytically active species in the oiganic solvents. For example, ion pairing is likely to occur in the organic solvents. 

Chapter 5, may provide a rationale. Conclusions derived from a number of H-MVIR measurements indicate that cyclopentadiene has a high affinity for the interior of the micelles that were investigated, whereas the dienophile prefers the outer regions. In view of the structures of most dienes and dienophiles such a spatial separation can be expected for the majority of Diels-Alder reactions. This arrangement accounts for the unexpectedly small influence of micelles on the rates of Diels-Alder reactions as reported in the literature. 

The alkene that adds to the diene is called the dienophile Because the Diels-Alder reaction leads to the formation of a ring it is termed a cycloaddition reaction The prod uct contains a cyclohexene ring as a structural unit... 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

The bifunctionality of the bis-diene and bis-dienophile monomers is apparent from the condensation product, structure [XXI], which still contains a diene and a dienophile in the same molecule. This polymer is crystalline, indicating a high degree of stereoregularity in the condensed rings. It decomposes to a graphitic material before melting. 

Reduction of indolenines with sodium and ethanol gives indolines. The pentachloropyr-role, obtained by chlorination of pyrrole with sulfuryl chloride at room temperature in anhydrous ether, was shown by spectroscopic methods to have an a-pyrrolenine (2H-pyrrole) structure (222). It is necessary, however, to postulate that it is in equilibrium with small but finite amounts of the isomeric /3-pyrrolenine form (3//-pyrrole 223), since pentachloropyrrole functions as a 2-aza- rather than as a 1-aza-butadiene in forming a cycloadduct (224) with styrene (80JOC435). Pentachloropyrrole acts as a dienophile in its reaction with cyclopentadiene via its ene moiety (81JOC3036). 

A transition-state structure was proposed on the basis of the solid-state structure of [Ni((R,R)-DBF0X)(H20)3](C104)2 (Fig. 1.8). The catalyst-dienophUe complex is thought to be a square-bipyramidal structure containing an octahedral nickel ion. The dienophile adopts an s-cis conformation with the si face shielded by a phenyl group. 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

Activation of the Dienophile by Lewis Acids, Interactions, Reaction Course, and Transition-state Structures... 

In a combined experimental and theoretical investigation it was found that the / -alkyl group in the dienophile gave a steric interaction in the transition-state structure which supported the asynchronous transition-state structure for the Lewis acid-catalyzed carbo- and hetero-Diels-Alder reactions. The calculated transition-state energies were of similar magnitude as obtained in other studies of these BF3-catalyzed carbo-Diels-Alder reactions. 

Another stereochemical feature of the Diels-Alder reaction is that the diene and dienophile partners orient so that the endo product, rather than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyclic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be exo if it is anti (trans) to the larger of the other two bridges and is said to be endo if it is syn (cis) to the larger of the other two bridges. 

Since both oxepin and its valence isomer benzene oxide contain a x-tb-diene structure they are prone to Diels-Alder addition reactions. The dienophiles 4-phenyl- and 4-methyl-4//-l,2,4-triazole-3,5-dione react with substituted oxepins at room temperature to give the 1 1 adducts 7 formed by addition to the diene structure of the respective benzene oxide.149 190,222... 

Methyl-7-(trimethylsilyl)oxepin and 4-methyl-4//-l,2,4-triazole-3,5-dione as dienophile undergo a Diels-Alder reaction in which the 4,6-diene structure of the seven-membered ring react. Contrary to the aforementioned reactions, the primary adduct 12 is stable and does not rearrange to a carbonyl compound.222... 

The equilibrium between oxepin and benzene oxide created interest in performing Diels-Alder reactions trapping one or both isomeric structures.1 The reaction of maleic anhydride or maleic imide with oxepin and substituted derivatives gives products 1 derived from the addition of the dienophile to the benzene oxide structure.2-l4-126 14 9 156 158 228 231-259... 

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