New macrocyclic ligands. XI. Synthesis, X-ray structure and computational studies of a new N2O4S2-Heteroatom cage

The synthesis and single-crystal X-ray structure determination of a new cag e molecule containing four oxygen, two sulfur, and two nitrogen heteroatoms are described. The structure determination shows that a twist occurs about each (tribenzyl) nitrogen bridgehead so that an overall helical configurat ion within putative quasi-2 symmetry is generated; both nitrogens have thei r lone pairs orientated exo. Single crystals of this compound are homochira l. Semiempirical MO calculations have been used to probe the topological ri gidity of this system relative to the more symmetrical analogue incorporati ng six oxygen and two nitrogen heteroatoms reported previously. The introdu ction of the sulfur atoms appears to restrict the facile interconversion be tween enantiomers that was predicted for the N2O6-analogue mentioned above. The implications of the structure for the design of larger cages are discu ssed.