Pyrrole-2.3-quinodimethane intermediates

CATEGORY llegf CYCLIZATIONS - CYCLOADDITIONS INVOLVING PYRROLE-2,3-QUINODIMETHANE INTERMEDIATES AND EQUIVALENTS... 

This category corresponds to the construction of the carbocyclic ring by 2 + 4 cycloaddition with pyrrole-2,3-quinodimethane intermediates. Such reactions can be particularly useful in the synthesis of 5,6-disubstituted indoles. Although there are a few cases where a pyrrolequinodimethane intermediate is generated, the most useful procedures involve more stable surrogates. Both 1,5-di-hydropyrano[3,4-b]pyrrol-5(lf/)-ones[l] and l,6-dihyropyrano[4,3-b]pyrrol-6-(in)-ones[2] can serve as pyrrole-2,3-quinodimethane equivalents. The adducts undergo elimination of CO2. 

Another strategy for annelation of pyrroles and indoles involves cycloaddition reactions. Because the heteroaromatic rings have substantial aromatic stabilization, the C2—C3 bond is not very reactive toward cycloaddition and there are only a few examples of the heterocycles acting as dienophiles. The carbocyclic ring of indole, like benzene, is unreactive toward cycloaddition. However, vinylpyrroles and indoles are quite reactive and react as electron-rich dienes because of the electron-donating nature of the ring. Other cycloaddition reactions involve the 2,3-dimethylene derivatives of pyrrole and indole, the so-called quinodimethanes, which are very reactive dienes. These intermediates, and more stable synthetic equivalents, are useful in cycloadditions with a variety of dienophiles. 27/-Isoindole, which itself can be thought of as a quinodimethane, is a very reactive diene. 

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