4 /7-Pyrazole, aromaticity Diels-Alder reaction

The distinction between these two classes of reactions is semantic for the five-membered rings Diels-Alder reaction at the F/B positions in (269) (four atom fragment) is equivalent to 1,3-dipolar cycloaddition in (270) across the three-atom fragment, both providing the 47t-electron component of the cycloaddition. Oxazoles and isoxazoles and their polyaza analogues show reduced aromatic character and will undergo many cycloadditions, whereas fully nitrogenous azoles such as pyrazoles and imidazoles do not, except in certain isolated cases. 

A similar use of maleimide was seen in Yu s hetero-Diels-Alder reactions. The Ar-ethyl pyrazole reacted under aqueous conditions at room temperature, with air oxidation to furnish the fully aromatized product <2001TL8931>. Other 1,3-substituted pyrazoles required heating in acetic acid at 50°C for 24h, with product yields of 42-67%. 

The reaction of l-aryl-4,6,6-trimethyl-3-phenyl-l,6-dihydropyrano[2,3-cjpyrazoles 88a-d with DMAD in dimethylformamide (DMF) at reflux is the first example of a Diels-Alder reaction involving the pyrazole ring. The reaction afforded an indazole 89 by elimination of a molecule of acetone from the intermediate cycloadduct (83S852). The s-cis conformation of the reactive diene fixed by the dihydropyran and the aromatization process to afford the indazole by elimination of acetone could be the driving force for this reaction. 

Pyrazol-3-one derivatives have taken part in both 1,3-dipolar cycloaddition and Diels-Alder reactions. The 1,3-dipolar cycloaddition between (Z)-pyrazol-3-ones 694a g with an excess of ethyl vinyl ether gave the pyrazol-3-one-4-spiro-3-isoxazolidines 695a-g, in nearly quantitative yield (82G483) (Scheme 203). The kinetics of this reaction was studied by quantitative spectroscopic analysis. The rate of reaction increases with the electron-withdrawing character of the substituent on the aromatic ring and a linear relationship is obtained between logk and op constants. The LUMO nitrone-HOMO vinyl ether is taken as the dominant interaction. 

There is no experimental evidence supporting the employment of 2H-pyrazoles as dienes for the Diels-Alder reactions as far as we know. The report of their participation in a [4-h2] cycloaddition has been shown to be incorrect [61]. There are three possible tautomers (Scheme 12). The first is aromatic while the two... 

From a synthetic chemist s point of view, it is not sufficient to demonstrate that "frozen" anti-aromatic tautomer of 1,2-pyrazole can be a good diene for the Diels-Alder reaction. It is of equal importance to illustrate that we can prepare functionalized 1,2-diazolse, which can be used as starting materials for the preparation of many different classes of organic compounds. For instance, it is possible to synthesize vinyl chloride as a valuable organic synthetic intermediate by taking advantage of the chemical transformations outlined in Scheme 14. 

Although in theory vinylpyrazoles can react with dienophiles to give Diels-Alder adducts, the reaction destroys the aromaticity of the pyrazole ring, and these dienic systems show a very low reactivity toward dienophiles. 

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