Diene in Diels-Alder reaction

The Boekelheide reaction has found utility in other synthetic methodology. An approach to 2,3-pyridynes made use of this chemistry in the preparation of the key intermediate 30. Treatment of 28 with acetic anhydride produced the desired pyridone 29. Lithiation was followed by trapping with trimethylsilyl chloride and exposure to triflic anhydride gave the pyridyne precursor 30. Fluoride initiated the cascade of reactions that resulted in the formation of 2,3-pyridyne 31 that could be trapped with appropriate dienes in Diels-Alder reactions. 

Due to the two electron-donating groups in the bicyclic product 150 and the unhydrolyzed precursor of 148, they should be quite reactive dienes in Diels-Alder reactions. However, such [4+2] cycloadditions were observed only for the cyclohexane-annelated cyclopentadienes 151b, which equilibrate with the more reactive isomers 154 by 1,5-hydrogen shifts (Scheme 33). The [4+2] cycload-... 

Dicyclopropylideneethane (2) is a reactive diene in Diels-Alder reactions, despite the steric congestion at its bonding centers. Relief of the strain appears to contribute to the enhanced reactivity of 2 which underwent cycloaddition with... 

Thus BCP seems to follow two competitive pathways in the cycloaddition with dienes (i) a stepwise diradical process giving the [2 + 2] adduct, or (ii) a concerted pathway giving the [4 + 2] adduct. Accordingly, the proportion of the latter increases with the reactivity of diene in Diels-Alder reactions. Conversely, the reaction with 2,3-dicyanobutadiene (529), generated in situ by electrocyclic ring-opening of 1,2-dicyanoeyelobutene [142], furnishes selectively the [2 + 2] cycloadduct 530 (Table 42, entry 4) due to the presence of substituents able to stabilize the diradical intermediate [13b],... 

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. 

Furthermore, the synthetic utility of 2,6-divinyl-l,4-dithiin 68 as a reactive diene in Diels-Alder reactions was reported with tetracyanoethylene, maleic anhydride, A -phenylmaleimide, and dimethyl acetylenedicarboxylate (DMAD) and allowed the preparation of various dihydrothianthrene derivatives (Equation 9) <2003S849>. 

Show the products expected from the Diels-Alder reactions. Use orbital interaction theory to develop a bonding scheme for rectangular cyclobutadiene and explain why rectangular cyclobutadiene may be exceptionally reactive as a diene in Diels-Alder reactions. 

Diets-Alder catalysis.2 This cation radical enhances the reactivity of a neutral or electron-rich eis-1,3-diene in Diels-Alder reactions. Thus 1,3-cyclohexadiene undergoes Diels-Alder dimerization only at temperatures around 200°. The presence of 5-10 mole % of this salt effects dimerization even at —78°, with the usual endo/ exo selectivity (5 1). It also permits facile condensation of 1,3-cyclohexadiene with a hindered dienophile such as 2,5-dimethyl-2,4-hexadiene (equation 1) the dimer of the former diene is a minor product (20% yield). 

Dihydropyridines are isomerized photochemically to 2-azabicyclohexenes (520). 2/7-Thiins behave as dienes in Diels-Alder reactions (Scheme 59). 

K. M. Sun, R. Guiliano, and B. Fraser-Reid, Diacetone glucose derived dienes in Diels-Alder reactions. Products and transformations, J. Org. Chem. 50 4114 (1985). 

Triazines are reactive electron-deficient dienes in Diels-Alder reactions with inverse electron demand. They react with alkenes, strained double bonds, electron-rich and electron-deficient alkynes and C=N double bonds. In most cases it is found that the dienophile addition occurs across the 3- and 6-positions of the triazine ring, but ynamines can also add across the 2- and 5-positions. The reactions are still under active theoretical and practical investigation. 

Tetrazines (624) are reactive dienes in Diels-Alder reactions with inverse electron demand. They react with both C—C and C—N multiple bonds. Cycloaddition of (624) with imidates thus affords 1,2,4-triazines (625) which are formed via the bicyclic intermediates (626) and the dihydro-1,2,4-triazines (627) (69JHC497). Further studies have been made on the limitations of this reaction. 

The double bond of a dihydropyran in conjunction with an exocyclic double bond conjugated with it acts as a diene in Diels-Alder reactions. In this way, the dihydrochroman... 

F. Fringuelli and A. Tatticchi, Dienes in Diels-Alder Reactions, Wiley, New York, 1990. 

Diels-Alder reactions.2 This diene provides an alternative to l-methoxy-3-trimethysilyloxy-1,3-butadiene (Danishefsky diene) in Diels-Alder reactions. Examples ... 

Diels-Alder reactions of a,fi-unsaturatedN,N-dimethylhydrazones.1 These readily available hydrazones can function as 1-amino-l-aza-l,3-dienes in Diels-Alder reactions. Thus, 1 undergoes regioselective cycloaddition with various electrophilic dienophiles to give tetrahydropyridines such as 2 and 3. Unfortunately, removal of the dimethylamino group with zinc and acetic acid (or other reagents) also effects reduction of the double bond. The initial adduct from cycloaddition of 1 with naphthoquinone is unstable and undergoes spontaneous elimination of the elements of dimethylamine to give the aromatic adduct 4. 

Vinylbenzofurans also react as dienes in Diels-Alder reactions with acyclic olefins. The 3-vinylbenzofiirans 174,175, and 176 gave the normal adducts resulting from (4ir+27r)-cycloaddition in its reaction with TCNE. A by-product from the reaction of the diene 174 and TCNE was the cyclobutane 177 (91AJC1085). 

Benzene and all of the group 5 heterobenzenes except pyridine behave as dienes in Diels-Alder reactions n). Qualitatively reactivity increases with the atomic number of the heteroatom. For example, benzene reacts with hexafluorobutyne at 180° 80), while phosphabenzene gives adduct 44b at 100°. Arsabenzene reacts at 25° while stibabenzene and bismabenzene react at 0°. 

Oxazole A is often used as a diene in Diels-Alder reactions. However, isoxazole B appears to be inert. Their different reactivities are surprising, considering that their MOs are similar. Propose an explanation. Bond lengths and strengths CC 1.54 A and 82.6 kcalmoU1 C=C 1.34 A and 144 5 kcalmoU1 CN 1.47 A and 72.8 kcalmol-1 C=N 1.27 A and 147 kcal/mol-1 NO 1.40 A and 48 kcalmol-1.86... 

Pyrrole has also been utilized to some extent as a diene in Diels-Alder reactions to give functionalized 7-azabicylo[2.2.1]heptenes and 7-azabi-cyclo[2.2.1]heptadienes.4 While the synthetic utility of this reaction is limited by the aromatic stability of the pyrrole ring, the use of Lewis acids, electron-withdrawing groups on the pyrrole, alkyne dienophiles, and high pressures have allowed pyrroles to be employed in the synthesis of several azanorbomane targets.4... 

Five-membered heterocycles act as dienes in Diels-Alder reactions... 

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