Reactivity of pyridines bearing EWG with bis-(TMS)ketene acetals. Substituent-induced lactonization reaction

The nucleophilic addition of bis-(TMS)ketene acetals to pyridines substituted by electron-withdrawing groups, activated with triflic anhydride, leads to their corresponding dihydropyridine carboxylic acids in a first instance. These acids can be efficiently turned into new bicyclic lactones by either of three different methods: first, bicyclic γ-lactones were successfully obtained by an unprecedented Michael-type ring closing procedure assisted by silica gel; second, bicyclic δ-bromolactones were achieved by a NBS-promoted process; and third, the unexpected spontaneous formation of new hydroxy γ-spirolactones was observed. Along the work presented herein, the scope, limitations, and regioselectivity of these lactonization reactions were studied, confirming the utility of bis-(TMS)ketene acetals as 1,3-dinucleophiles in this type of reactions, extending thus the range of synthesis of a wide variety of new aza-compounds.