Synthesis, structure and AM1 conformational study of [3]paracyclo[3](1,3)indolophane, a novel chiral cyclophane.

The title compound 5 was synthesized in seven steps with an overall yield of 22%. The two bridges were formed by a conjugate addition of indole to a Michael acceptor and by an intramolecular N-alkylation, respectively. A dynamic NMR study revealed the presence of a conformational process with an activation barrier of 10.9+/-0.2 kcal/mol, which was assigned to a ring flipping process that interconverts two enantiomeric sets of equilibrating bridge conformers. In the crystal structure of 5, both bridges adopt the pseudo-chair conformation, in accord with the lowest energy conformation identified by an AM1 conformational search. (C) 1999 Elsevier Science Ltd. All rights reserved.