An assignment of relative configurations has been achieved for the diastereomeric racernates(lR2R,lS2S) and (1R2S, 1S2R) of 3,3-dimethyl-l,2-diphenylbuta~-l-ol through the comparative analysis of the respective chemical shifts induced by Eu(fod), in the 'H and '"C NMR spectra, and the corresponding conformational distribution.
INTRODUCCION
The assignment of the relative configurations to diastereomeric compounds is usually carried out by analysis of the observed values for 'H NMR vicinal coupling constants and the corresponding conformational di~tribution.'-~ The application of lanthanide shift reagents in 'H and I3C NMR has provided a new means for the conformational analysis and assignment of relative configurations of diastereomeric compound^^-^ through the study of the chemical shifts induced by the lanthanide (LIS) and the application of the McConnell-Robertson equation.*
In this paper we report the application of the method of the 'H NMR chemical shifts induced by Eu(fod), for the assignment of relative configurations to diastereomeric acyclic carbinols with two asymmetric carbon atoms. We have applied this method to the assignment of the relative configurations, (1R2R, 1S2S) and (1R2S, 1S2R), to the diastereomeric racemates of 3,3-dimethyl-1,2-diphenylbutan-1-01, taking into account a prior estimate of the significant conformational populations in both diastereomers. We use, for the first time, the comparative analysis of the limiting LIS values in 'H and 13C NMR for this purpose.
The agreement between the assignment deduced by this analysis and by the study of the respective vicinal coupling constants validates the method.
CONFORMATIONAL ANALYSIS
The study of Dreiding models for the (1R2R) and (1R 2s) isomers of 3,3-dimethyl-1,2-diphenylbutan-1-* Author to whom correspondence should be addressed. i-Part X of 'Conformational Study of Diastereomers.' For Part IX of this series, see Ref. 7c.