2.4- Dinitrophenyl aromatic quaternary salt

Two experimental techniques have improved the observed participation of simple aromatic quaternary salts in [4+ + 2] cycloadditions. The addition of hydroquinone, which forms a stabilized 1 2 complex with the isoquinolinium salts, to the reaction mixture accelerates the rate of observed isoquinolinium salt [4+ + 2] cycloaddition.148b In addition, the use of 2,4-dinitrophenyl aromatic quaternary salts, prepared from the parent base and 2,4-dinitrophenyl bromide or chloride, accentuates the electron-deficient character of the quaternary salt and accelerates their participation in [4+ + 2] cycloadditions with electron-rich dienophiles.152 These observations of Falck and co-workers have proved useful for promoting the [4+ + 2] cycloaddition of isoquinolinium systems previously regarded unmanageable. The application of these observations in the total syntheses of 14-epicorynoline (16) and methyl arrnothanamide (17) have been detailed [Eq. (67)],152... 

The use of 2,4-dinitroi enyl aromatic quaternary salts prepared directly from the parent isoquinoline compound and 2,4-dinitrophenyl bromide or chloride enhances the electron-deficient nature of the iso-quinolinium salts, accelerates the rate of their [4 + 2] cycloaddition reactions with electron-rich alkenes, and has permitted the use of isoquinolinium s ts previously regarded as unmanageable. The application of these observations in the total syntheses of methylamottianamide and 14-epicorynoline is summarized in Scheme 26. ... 

Most examples of the Bradsher cycloaddition reaction have utilized fused polycyclic aromatics as the cationic aza-diene fragment. Falck and co-workers have reported that one can carry out this reaction using monocyclic quaternary aza-aromatics. The application of this methodology was illustrated using the A -(2,4-dinitrophenyl) salt of A, A -diethylnicotin-amide 3 and ethyl nicotinate 4 in conjunction with enol ethers. The reaction proceeded at room temperature to generate adducts 5. This was the result of the exo-addition at the C2-C5 positions of the pyridyl ring. The resultant iminium ion was then trapped by the methanolic solvent. 

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