1 (321)

Prévia do material em texto

318 Organic Chemistry Solutions Manual Problem 10 Treatment of cyclohexa-1,3-dione with this acetylenic amine gives a stable enamine in good yield. Refluxing this enamine in nitrobenzene gives a pyridine after a remarkable series of reactions. Fill in the details: give mechanisms for the reactions, structures for any intermediates, and suitable explanations for each pericyclic step. A mechanism is not required for the last step (nitrobenzene acts as an oxidant). 0 NaOH PhNO₂ + H2O 195 °C 74% yield N 0 NH₂ H 0 0 0 PhNO₂ NH N N H Purpose of the problem Practice in unravelling complicated reaction sequences involving pericyclic steps. Suggested solution The formation of the enamine requires only a patient adding and subtracting of protons. 0 0 0 0 0 H 0 N N N N H H2O H H H + The cascade of reactions in hot nitrobenzene starts with a [3,3]-sigmatropic rearrangement that unusual in that it forms an allene but is otherwise straightforward. To get to the next interme the stable conjugated primary enamine, we must enolize and go back to the ketone again but the double bond into conjugation. H 0 0 0 0 H / N NH NH H + NH₂ H K. Berg-Nielsen and L. Skattebol, Now we can transfer a proton from nitrogen to the middle of the allene. This is formally a Acta Chem. Scand., 1978, B32, shift and is, of course, allowed but it may be the movement of a proton as nitrogen is involved. 553. gives a diene that can twist round for a six-electron electrocyclic reaction. This would no disrotatory but we can't tell as there is no stereochemistry. 0 0 o six-electron disrotatory [1,5]H electrocyclic H H N NH NH H