Synthesis and Properties of 4,4,9,9‐Tetramethyl‐1‐oxa‐cycloundecane‐5,6,7,8‐tetrone and 5,5,10,10‐Tetramethyl‐1‐oxa‐cyclotridecane‐6,7,8,9‐tetrone

The synthesis of 4,4,9,9‐tetramethyl‐1‐oxa‐cycloundecane‐5,6,7,8‐tetrone ( 9 ) and 5,5,10,10‐tetramethyl‐1‐oxa‐cyclotridecane‐6,7,8,9‐tetrone ( 10 ) has been achieved in a multistep procedure. The key steps in this synthesis were the ring closure of 23 and 24 to 25 and 26 , respectively, and the oxidation of the triple bond with RuO 2 /NaIO 4 to the dihydroxydiketones 31 and 32 . Compound 9 is the first cyclic tetraketone for which an intramolecular donor‐acceptor stabilization has been found. A strong transannular interaction between the ether oxygen and the C 4 O 4 unit in 9 was detected by X‐ray studies on single crystals of 9 . The transannular distance is 2.7–2.8 Å. Further evidence for a strong transannular interaction was obtained from the comparison of the reduction potential and the first band in the UV/Vis spectrum with the corresponding values from other openchain tetraketones. These findings were substantiated by PE investigations on 9 . The crystal structures of the dihydroxy‐diketones 31 a and 32 a showed that, in the case of the eleven‐membered ring ( 31 a ), there are also short transannular distances between the ether oxygen and the C 2 O 2 moiety (2.5 Å and 2.9 Å). In the case of the thirteen‐membered ring ( 32 a ), no transannular interactions were found in the solid state.