Unexpected One-Step Formation of Iodo[1,3]dioxolo[4,5-c]pyridine Derivatives by a Hofmann-Löffler-Freytag Reaction Studies towards the Synthesis of a Pyridine Containing Macrocycle
T. Lechel, G. Podolan, B. Brusilowskij, C.A. Schalley, H.-U. Reissig – 2012
During attempts to prepare functionalized 5-iodopyridine derivatives the unexpected formation of iodo[1,3]dioxolo[4,5-c]pyridines was discovered. The conversion of 3-alkoxypyridin-4-ols into the corresponding 5-iodo compounds was achieved by reaction with one equivalent of iodine or tetramethylammonium dichloroiodate under basic conditions. When three equivalents of iodine were used in chlorinated solvents, after 5-iodination, subsequent reaction of the 3-alkoxy group took place to form a 1,3-dioxolane ring with the 4-hydroxyl group. Generation of the resulting iodo[1,3]dioxolo[4,5-c]pyridines is explained by a radical process known as the Hofmann–Löffler–Freytag reaction. Two 6-ethynylpyridine derivatives were examined in the iodination process to establish a route to pyridine-containing macrocycles. The pentasubstituted 5-iodopyridine derivative 21 could be prepared; however, attempts to achieve cyclotrimerization of this building block under different conditions were not successful. Reaction of 21 with copper chloride allowed isolation of a copper acetylide 22, which aggregates to a triangular trimeric complex containing four copper(I) ions such as [23·Cu]+ as monitored by ESI mass spectrometry.