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[摘要]：Aza-Nazarov reactions starting from 1-azapenta-1,4-dien-3-ones offer an attractive route to five-membered nitrogen heterocycles such as N-substituted pyrroles. It has now been discovered that N-indolinylhydrazone-derived 1-azapenta-1,4-dien-3-one 9a gives different products under conditions of varying acidity. Under strongly acidic conditions (2 equiv. of triflic acid, dilute solution, electrophilic conditions) it simply undergoes an aza-Nazarov reaction to give N-indolinylpyrrole 15a after workup with acetic anhydride. If, however, a large excess of triflic acid is used (710 equiv., concentrated solution, superelectrophilic conditions), 9a has been found to undergo an aza-Nazarov reaction cascade followed by NN bond cleavage to give NH-pyrrole 10a and acetylated indole 8 as the final products (after workup with acetic anhydride). High-level quantum chemical calculations were used to explain this acid-concentration-dependent reaction cascade. The formation of the reaction products can be explained in terms of an electrocyclization reaction of the protonated starting material 9a and a subsequent NN bond cleavage reaction involving either mono- or dicationic species under strongly acidic conditions. |
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