6-Endo-Dig vs. 5-Exo-Dig Ring Closure in o-Hydroxyaryl Phenylethynyl Ketones. A New Approach to the Synthesis of Flavones and Aurones

Article abstract of DOI:10.1021/jo00373a016

The aryl phenylpropynoates 3a-c, prepared by esterification of phenylpropynoic acid with the corresponding phenols, by means of N,N'-dicyclohexylcarbodiimide, give upon irradiation the o-hydroxyaryl phenylethynyl ketones 4a-c.The cyclization of these compounds in basic media follows two alternative pathways: 6-endo-dig ring closure, to give the flavones 8a-c, and /or 5-exo-dig ring closure , to give the aurones 9a-c.The predominance of one or the other cyclization mode is strongly influenced by the reaction conditions.Thus, the use of potassium carbonate in acetoneas cyclizing reagent favors the 6-endo-dig process, while the 5-exo-dig process becomes clearly enhanced when using sodium ethoxide or potassium carbonate in ethanol.The mechanistic implications of these facts are discussed within the framework of Baldwin's rules.From the preparative point of view, the above results disclose the synthetic possibilities of the key ketones 4 as precursors of flavones and aurones.