Comparison of the steric barriers in three- and two-bladed propeller crowns

Abstract

The propeller crowns (2) and (4)(dinaphthopolyoxacycloalkanins) have been characterised by single crystal X-ray structure determinations. Only one enantiomeric pair of isomers is present in each crystal structure. The presence of the trimethoxyphenyl substituent in (4) has remarkably little effect on the conformation of the ether ring. The propeller skeleton of (4) is different from that observed previously in the o-methoxyphenyl propeller; these two skeletal isomers are related by a two-ring flip of one naphthyl and the phenyl ring. Room-temperature ¹H n.m.r. spectra of (4) and of the 6-t-butyldinaphthylpolycycloalkanin correspond to one ‘averaged’ species with a rapid averaging of naphthyl rings via a two-ring flip, slowed at 220 K. The relative steric demands of the propeller and the ether moieties are discussed.