Conformational Assignement, Absolute Configuration and Chiral Separation of all the Stereoisomers Created by the Combined Presence of Stereogenic Centers and Stereogenic Conformational Axes in a Highly Hindered 1,5-Naphthyl Sulfoxide.

The presence of two stereogenic centers and of two stereogenic conformational axes in 2,6-dimethyl-1,5-bis(2-methyl-2-propylsulfinyl)naphthalene (1) entails the existence of 10 stereoisomers. In particular, both the meso form (1a) and the racemic form (1b) are constituted by three atropisomers; in the case of the latter (1b) each of them entails a pair of enantiomers (total of six species), whereas owing to the symmetry only one of the three atropisomers of the meso form (1a) yields a pair of enantiomers (a total of four species). Despite the low conformational interconversion barrier (18.5 kcal/mol) all of them have been separated by low temperature (-45°C) chiral HPLC. Their configurational and conformational assignment has been achieved by a combined use of NMR (both in solution and solid state) and on-line CD-detected chiral HPLC. The single crystal X-ray diffraction yielded the absolute configuration of one of the stereoisomers ((ZR,ER)-1b) from which all the others could be obtained by CD relationship

A 1,5-DIARYLPYRROLE DERIVATIVE

The structure of the 1,5-diarylpyrrole derivative, ethyl 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-2-methyl-3-pyrrolecarboxylate, C20H16Cl3NO2, has been determined. The orientation assumed by the phenyl rings corresponds to the deepest of the two minima obtained from ab initio calculations. All bond distances and angles are in the expected ranges

CONFORMATION IN 2,3-DIFLUOROBUTANES

A conformation anal. for meso- and d,l-2,3-difluorobutanes has been carried out, employing 1H and 19F NMR and theor. calcns. The real configurations of the two isomers were assigned by specific optical rotation measurements of the products coming from an optically active precursor. Gauche conformations were predominant for meso (E) isomer, while the d,l (T) isomer showed all possible staggered rotamers almost equally populated. Finally remarks for NMR peak assignment of homo- and copolymers partially fluorinated by using gauche additive effects are given

X-Ray crystal structure of a p-hydroxycalix[6]arene derivative

The first molecular structure of a p-hydroxycalix[6]arene 6 has been determined by a single crystal X-ray diffraction study. The calix[6]arene molecule assumes a 1,2,3-alternate conformation with all OH groups at the upper rim engaged in H-bonds with pyridine molecules. The stacking of molecules of p-hydroxycalix[6]arene 6 along the a and c axes gives rise to a solvent pseudo-cylindrical cavity at the centre of the cell