RESUMO
Stereochemistry is information, and stereoselective reactions are the means by which that information may be communicated within and between molecules. The control of remote stereogenic centres can be achieved by stereochemical relay, and the use of thermodynamic control over conformational preference is turning out to be a very powerful method for long-range transmission of stereochemical information.
Assuntos
Compostos Orgânicos/química , Amidas/química , Ciclização , Cinética , Ligantes , Conformação Molecular , Estereoisomerismo , Termodinâmica , Xantenos/químicaRESUMO
Tertiary aromatic amides bearing stereogenic centres ortho to the amide group may adopt two diastereoisomeric conformations which interconvert slowly on the NMR timescale at ambient temperature, and are therefore detectable by NMR. Certain classes of stereogenic centre--particularly sulfoxides, ephedrine-derived oxazolidines, and proline-derived imidazolidines--strongly bias the population of the two conformers. We propose a model, supported by molecular mechanics calculations, which rationalises the sense and magnitude of the conformational selectivity attained in terms of the steric and electronic properties of the controlling centre. The control over conformation may be exploited either by trapping the favoured conformer as an atropisomer, or by using it to relay information about the stereochemistry of the controlling centre.