Preparation and Structures of Cyclotetradeca‐4,11‐diynone, Cyclotetradeca‐4,11‐diyne‐1,8‐dione, Their Exomethylene Derivatives, and 1,8‐Dioxacyclotetradeca‐4,11‐diyne

The following 14‐membered ring diynes have been synthesized: cyclotetradeca‐4,11‐diynone ( 4 ), cyclotetradeca‐4,11‐diyne‐1,8‐dione ( 6 ), the corresponding exomethylene derivatives 5 and 7 , as well as 1,8‐dioxacyclotetradeca‐4,11‐diyne ( 8 ). The synthesis of the diketal of 4 was achieved by the reaction of the ethylene ketal of 1,5‐dibromopentan‐3‐one ( 9 ) with the dilithium salt of 1,8‐nonadiyne. Similarly, the diketal of 6 was prepared from the dilithium salt of the ethylene ketal of 1,8‐nonadiyn‐5‐one 11 with 9 . The unprotected ketones 4 and 6 were obtained by treating the ketals 10 and 14 , respectively, with HCl. The ketones 4 and 6 were transformed to the exomethylene derivatives by a Wittig reaction. The reaction of the dilithium salt of 5‐oxanona‐1,8‐diyne ( 16a ) with 1,5‐dibromo‐3‐oxapentane ( 15 ) yielded 8 . X‐ray investigations of single crystals of 6 and 8 revealed chair conformations with a zig–zag arrangement, employing two anti orientations of the two chains between the triple bonds. A zig–zag arrangement of the chain containing the carbonyl group was also found in 4 . In 3b and 7 C i symmetry was found, with only one anti conformation in each chain. Molecule 6 crystallizes with C 2 h symmetry and 8 with a center of symmetry.