Electrohalogenation of Propargyl Acetates and Amides to Form 1,1-Dibromo-2-oxo Functionality and a Facile Synthesis of Fucaneol

Inokuchi, T.; Matsumoto, S.; Tsuji, M.; Torii, S. J. Org. Chem. 1992, 57, 5023 .

Oxidation of acetylenic bonds with metallic peroxides offers a practical access to 1,2-diketones. Alternative promising approaches to this functionality reported in the literature involve p-complexation of alkynes with mercury(II) ion followed by oxidation with Mo(VI) or W(VI) peroxo complexes. However, the undesired carbon-carbon bond cleavage of the 1,2-dicarbonyl framework is a drawback to this method which occurs as a result of overoxidation. In addition, hazardous mercury(II) acetate is inevitably required to facilitate the reaction. Consequently, finding a nonmercuric method of providing either 1,2-diketones or synthetic equivalents is a desirable goal in current organic synthesis. Our attention was focused on halogenative hydroxylation processes involving acetylenic bonds, because a,a-dihalo ketones or their analogues are synthons of 1,2-diketones which are versatile, highly functionalized intermediates. To this end, hypohalorites were employed as a source of positively charged halogen species, and their reactions with terminal alkynes or phenylacetylenes were studied. Although the regiochemical control of halogen addition to internal acetylenes has remained unexplored, we report in this paper that acetylenes 1, bearing 1-acetoxyalkyl(s) at one or both sites, can be converted into the corresponding a,a- dibromo ketones 2 or 3 by electrochemical bromination in media containing N-oxyl compound 5. Furthermore, this N-oxo-mediated brominatibe hydroxylation is of great interst not only from a mechanistic point of view, but also as a means of obtaining compounds bearing an array of different substituents at 1,2,3-positions. One of these compounds was shown to be useful for the synthesis of furaneol, a key flavoring ingredient used in foods.