Superelectrophilic Intermediates
in the Synthesis of Novel Indenole Derivatives via 1,5-Cyclization
Reactions of 5-Aryl-1-azapenta-1,4-dien-3-ones
作者:Ernst-Ulrich Würthwein、Nugzar Ghavtadze、Roland Fröhlich、Klaus Bergander
DOI:10.1055/s-0028-1083188
日期:——
Oximes of 5,5-disubstituted 1-azapenta-1,4-dien-3-ones 3 underwent 1,5-cyclization reactions yielding indenole derivatives 4 in moderate to good yields upon treatment with an excess of trifluoromethanesulfonic acid. Similarly, the hydrazone 3g led to indenole 4f upon protonation. The reaction could also be successfully applied to the synthesis of benzo[b]cyclopenta[d]thiophene 5 starting from 3f. Products of a competing aza-Nazarov reaction were not observed. The reaction mechanism is explained by assuming the involvement of dicationic intermediates, generated by superelectrophilic solvation, similar to that of known Nazarov and aza-Nazarov-type transformations. Quantum chemical calculations of reaction energies and activation barriers are discussed with respect to the experimental results.
5,5-取代的1-氮杂戊-1,4-二烯-3-酮的亚胺在过量三氟甲磺酸处理下发生了1,5-环化反应,生成了中等至良好产率的茚烯衍生物。同样,肼缩合物在质子化后也生成了茚烯。该反应还成功应用于从3f合成苯并[b]环戊[d]噻吩。未观察到竞争性氮杂纳扎罗夫反应的产物。反应机理通过假设参与了二阳离子中间体来解释,这些中间体通过超电亲性溶剂化生成,类似于已知的纳扎罗夫和氮杂纳扎罗夫型转化。讨论了反应能量和活化能的量子化学计算结果,并与实验结果进行了比较。