Highly Functionalized Tris‐(σ)π‐Homobenzenes — Nine‐Membered Ring Compounds with Special Substitution Patterns — En Route to cis,cis,cis‐2,5,8‐Cyclononatriene‐1,4,7‐trione

In the pursuit of the tris‐(σ)π‐homobenzene chemistry, the synthesis of potentially tri(hexa)podal, C3v‐symmetrical, polyfunctionalized 9‐membered ring systems is being explored. A rather “expensive” tris‐σ‐homobenzene (13b) is the starting material for the tris‐σ‐isomer (14b) from which 1,4,7‐trimethylene‐cyclononatriene (2), its tri(hexa)epoxides (18, 20) and triepoxytriketo derivative (19) are prepared through generally highly selective transformations. An alternative approach toward the C3v 2,5,8‐cyclononatriene‐1,4,7‐trione [4, C6H6(CO)3] starts from the relatively “cheap” all‐exo‐1,4,7‐cyclononatriene‐trioxide (32), relies on selective epoxide opening reactions, and is complicated by typical mediumring transanular cyclizations; the rather efficient six‐step sequence does not reach the stated goal but carries attractive preparative prospects. For a central intermediate in each approach (trimethylenetrioxide 18 and bromo‐hydroxy‐diepoxycyclononane 50), X‐ray structural data are presented.