The Angucyclines Rearrangement of Angular to Linear Tetracycles

Many of the polycyclic antibiotics are based on linear assemblies of three or more six-membered rings, such as the tetracycline and anthracycline antibiotics (examples are chlortetracycline and 

1 -Acyl-2-alkyl-2,3-dihydro-4( 1 //)-pyridones A are readily available heterocycles in both racemic and chiral form. l-Acyl-2-alkyl-l,2-dihydropyridines B are much less readily accessible, especially enantiopure, but are much sought after building blocks for alkaloid synthesis. A very efficient (83-96%) and simple procedure has now been developed for the A — B transformation, illustrated as follows treatment of l-allyloxycarbonyl-2-cyclohexyl-2,3-dihydro-4-pyridone with one equivalent of the Vilsmeier reagent in trichloroethylene at room temperature gave l-allyloxycarbonyl-4-chloro-2-cyclohexyl-l,2-dihydropyridine in 92% yield. 

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