Diels-Alder reactions and 1,3-dipolar cycloadditions

The cycloaddition of 3,5-disubstituted-4//-pyrazol-4-one iV.AT-dioxide (144) towards various unsaturated compounds like 1,4-epoxynaphthalene (145) was investigated 93JHC1557,94JCS(P2)1337 predominance of the exo-l,3-dipolar cycloadducts (146) was observed. 

It is known that 4-benzal-l-phenyl-A -pyrazolin-5-one reacts both like an heterodiene and an alkene in a Diels-Alder reaction towards 2,3-dimethylbutadiene 88T5229 . The 1,3-dipolar cycloaddition of nitrones to the N//-tautomer of l-phenyl-3-methyl-3-pyrazolin-5-one yields a nonisolated cycloadduct which rearranges to a 4-substituted derivative of the starting material (4-substituent = CH(Ar)— N(Ph)—OH) 94JCR(S)154 . 

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Wittkopp and Schreiner introduced the simple electron-deficient N,N -bis [3,5-(trif-luoromethyl)phenyl]thiourea 9 (Figure 6.3) as an efficient double hydrogen-bonding organocatalyst in a series of Diels-Alder reactions and 1,3-dipolar cycloadditions of... 

Carbonyl or cyano groups, of proven value in Diels-Alder reactions and 1,3-dipolar cycloadditions, are unsuitable for anionic cycloadditions owing to the pronounced nucleophilic and basic character of the anionic reagents. Instead of cycloaddition nucleophilic attack of these groups or deprotonation of the substrates would occur. This means that aromatic residues are indispensible which are practically unremovable after the cycloaddition and, unlike the carbonyl group, hardly unsuitable for subsequent synthetic steps. 

The distinction between Diels-Alder reactions and 1,3-dipolar cycloadditions is semantic for the five-membered rings Diels-Alder reaction at the F/B positions in (357) (four atom fragment) is... 

Diels-Alder reactions are sometimes classified as [4+2] cycloadditions, and 1,3-dipolar cycloadditions as [3+2], where the numbers identify the number of atoms involved in the two chains. This classification is not as useful as the one used throughout this book, which is based on the number of electrons involved. In this classification, both Diels-Alder reactions and 1,3-dipolar cycloadditions are [4+2] cycloadditions—4 electrons from the diene or... 

As measured by the criteria of stereospecificity, regioselectivity, kinetic isotope effects, and solvent effects [117-120, 541-543], 1,3-dipolar cycloaddition reactions represent orbital symmetry-allowed [n + n s] cycloadditions, which usually follow concerted pathways Diels-Alder reactions and 1,3-dipolar cycloadditions resemble each other, as demonstrated by the small solvent effects on their bimolecular rate constants. In going from nonpolar to polar solvents, the rate constants of 1,3-dipolar cycloadditions change only by a factor of 2... 10 [120, 131-134]. 

Diels-Alder reactions and 1,3-dipolar cycloadditions. 9.2 Photochemical cycloadditions... 

A common problem occurring with ketenes is their polymerization. Diaz-Ortiz et al. [80] reported hetero-Diels-Alder reaction and 1,3-dipolar cycloaddition of ketone acetals under microwave irradiation which was completed in 3 min. The product was isolated directly through cmde reaction mixture without the polymerization of ketene (Scheme 11.27). 

The Diels-Alder reactions and 1,3-dipolar cycloaddition reactions are known as [4 +2]-cycloaddition reactions because four electrons from diene or 1,3-dipole, and two electrons from the dienophile or dipolarophile are involved in these reactions. The 1,3-dipolar cycloaddition reactions are also called [3H-2]-cycloaddition... 

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