Synthesis, X-ray Crystallography, and Computational Analysis of 1-Azafenestranes

Abstract

The tandem [4+2]/[3+2] cycloaddition of nitroalkenes has been employed in the synthesis of 1-azafenestranes, molecules of theoretical interest because of planarizing distortion of their central carbon atoms. The synthesis of c,c,c,c-[5.5.5.5]-1-azafenestrane was completed in good yield from a substituted nitrocyclopentene, and its borane adduct was analyzed through X-ray crystallography, which showed a moderate distortion from ideal tetrahedral geometry. The syntheses of two members of the [4.5.5.5] family of 1-azafenestranes are also reported, including one with a trans fusion at a bicyclic ring junction which brings about considerable planarization of one of the central angles (16.8 degrees deviation from tetrahedral geometry). While investigating the [4.5.5.5]-1-azafenestranes, a novel dyotropic rearrangement that converts nitroso acetals into tetracyclic aminals was discovered. Through conformational analysis, a means to prevent this molecular reorganization was formulated and realized experimentally with the use of a bulky vinyl ether in the key [4+2] cycloaddition reaction. Finally, DFT calculations on relative strain energy for the 1-azafenestranes, as well as their predicted central angles, are disclosed.