Dienophiles cycloadditions

Maleic thioanhydride and its dichloro derivative function as dienophiles. Cycloaddition with cyclopentadiene and butadiene has been reported (Scheme 155) (72AHC(14)33l). Succinic thioanhydride undergoes bis-Wittig condensations (Scheme 156) the product from ethoxycarbonylmethylenetriphenylphosphorane aromatizes to the thiophene-2,5-bis-acetic ester (454) (75LA1967). 

Few reactions of the parent oxazole with the usual alkenic and alkynic dienophiles have been reported. Most oxazoles which yield Diels-Alder adducts contain electron-releasing substituents, the order of reactivity being alkoxy> alkyl 4-phenyl > acetyl > ethoxycarbonyl. This sequence suggests that the oxazole functions as the electron-rich component and that the reaction is governed by interaction of the highest occupied molecular orbital of the oxazole and the lowest unoccupied orbital of the dienophile. Cycloadditions with inverse electron demand of electron-deficient oxazoles with electron-rich dienophiles can be envisaged. 

Cationic ansa metallocenes can be utilized as chiral catalysts in Diels-Alder reactions. For example, in the presence of the cationic zirconocene complex [(ebthi)Zr(Ot-Bu) thf]+, the [4 + 2] cycloaddition of acrolein and cyclopentadiene proceeds efficiently to afford endo and exo cycloadducts (equation 71). In reactions in which methyl acrylate is used as the dienophile, cycloadditions occur with lower levels of enan-tioselection (23% ee), but with significantly higher degrees of diastereoselectivity (17 1 endo, exo). In these processes, recent studies demonstrated the great influence of chiral metallocene structure and the dramatic solvent effect. ... 

W-Acylsultams of glyoxylic acid 1.134 (R = CHO) and nitroso compound 1.134 (R = NO) are valuable dienophiles. Cycloadditions of 1.134 (R = CHO) with 1-methoxybutadiene mediated by Eu(fod)3 are highly selective [466] (Figure 9.36), and take place via a chelated complex. The reactions of the nitroso compound 1.134 (R = NO) with cyclopentadiene and cydohexadiene give useful results [467], though heterodienes 9.65 do not. The observed results ware interpreted in a fashion similar to that shown in Figure 9.32. 

Nonauxiliary-Controlled Dienophiles Cycloaddition to Chiral Aldehydes... 

The first examples of intramolecular Af-sulfinyl dienophile cycloadditions have recently been reported by Weinreb et al. Treatment of ( , )-... 

A number of representative examples of N-sulfinyl dienophile cycloadditions can be found in Table 1-1. As noted above, some adducts undergo... 

Dimethylenecycloalkanes and ortho-quinodimethanes The cisoid conformation of the diene is fixed in 1,2-dimethylenecycloalkanes and this type of diene reacts readily with dienophiles. Cycloaddition provides a convenient route to polycyclic compounds and often occurs in high yield. Thus, 1,2-dimethylenecyclohexane reacts with benzoquinone to give the fcis-adduct 52 (3.47). This product is readily converted into the corresponding aromatic hydrocarbon pentacene. 

The Diels-Alder reaction of 2-azadienes also benefits from the presence of an electron-donating substituent, to enhance the reactivity with electron-deficient dienophiles. Cycloaddition with alkynyl dienophiles and aromatization leads to substituted pyridines (3.54). Silyloxy-substituted 2-azadienes such as 68 are effective dienes and have been used to prepare substituted 2-pyridones and piperidones after methanolysis (3.55). Asymmetric hetero Diels-Alder reactions with the chiral Lewis acid catalyst 66 provide access to the piperidone products with very high enantioselectivity. ... 

Direct Wittig reaction of Ph PCHCOn,Me with the four unsubstituted D-aldopentoses followed by acetylation provides convenient preparative access to acyclic seven-carbon trans-2.3-unsaturated sugar derivatives. These products served as dienophiles for a detailed comparative study in Diels—Alder cycloaddition with cyclopentadiene. Related syntheses afforded analogous cis-dienophiles. Cycloaddition under uncatalyzed thermal conditions gave mixtures of the four possible stereoisomeric norbornene adducts. The endo, exo ratios, and diastereofacial selectivities of the adducts were determined by NMR spectroscopy and by chemical transformations, supplemented by selected X-ray crystallographic analyses. Different distributions of isomers were encountered when a Lewis acid was used to catalyze the cycloaddition. The reaction can be controlled to provide preparative access to selected isomers and thus constitutes a versatile method for chirality transfer from the precursor sugar to four new asymmetric centers in a carbocyclic framework. 

A typical reaction of 1,3,5-triazines 32 is the inverse electron demand Diels-Alder reaction with electron-rich dienophiles. Cycloaddition in this case initiates a cascade of transformations including elimination of the... 

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