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

• Published in: Tetrahedron 2014-03, Vol.70 (10), p.1861-1871
• Main Authors: Rivera-Hernández, Alejandro, Chans, Guillermo M., Rudler, Henri, López Cortés, José G., Toscano, R. Alfredo, Álvarez-Toledano, Cecilio
• Format: Article
• Language: English
• Subjects: Acetals; Anhydrides; Carboxylic acids; Cyclization; Lactones; Nucleophilic additions; Pyridines; Silica gel; Silyl enol ethers; Tetrahedrons; Utilities; γ- and δ-Lactones
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/j.tet.2014.01.044

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. [Display omitted]