Abstract
Attempted synthesis of [2.2] (2,5)pyrrolophanes by 1,6-Hofmann elimination proved unsuccessful and led only to dipyrrolmethanes. [2.2] (2,5)Pyrroloparacyclophane (11a), [2.2](2,5)pyrrolo(l,4)naphthalenophane (12a), and [2.2] (2,5)pyrrolo(2,5)furanophane (13a) were prepared by Paal-Knorr cyclization of 3,6-diketo[8]paracyclophane (10a), 3,6-diketo[8](l,4)naphthalenophane (10b), and 3,6-diketo[8](2,5)furanophane (10c), respectively, with am- monia. Preparation of the analogous N-methyl derivatives lib, 12b, and 13b by Paal-Knorr cyclization using me- thylamine was successful only for the synthesis of N-methyl[2.2](2,5)pyrroloparacyclophane (lib). JV-Methyl- [2.2] (2,5)pyrrolophane (17a) and JV-benzyl[2.2] (2,5)pyrrolophane (17b) were also synthesized by Paal-Knorr cycli- zation of 1,4,7,10-cyclododecatetraone by successive treatment with the appropriate alkylamine and ammonia. Re- ductive cleavage of the benzyl group in 17b with sodium afforded the parent [2.2](2,5)pyrrolophane (1). The spec- tral properties and structural assignment of the above pyrrolophanes are discussed. Variable temperature NMR studies on the above pyrrolophanes indicated that all aromatic rings in the above phanes are conformationally rigid on the NMR time scale with the exception of the pyrrole ring in 11a. In this pyrrolophane the barrier to pyrrole ring rotation is 17 kcal/mol.