Substituent influence on the diastereoselectivity of the alkylation of cyclic chiral imines

The normal very high diastereoselectivity of alkylation reaction of chiral imines derived from cyclic ketones is not observed in the case of substituted chiral ketones. The influence of the substituent size was studied for imines prepared from (5R)-2,5-dimethylcyclohexanone and (5R)-carvomenthone with both enantiomers of 1-phenylethylamine (PEA). Using methyl vinyl ketone as electrophile a matched situation was observed with (S)-PEA imine derivative. However, the de is strongly dependent of the substituent in the mismatched case, starting from (R)-PEA. Enlargement of substituent size leads to lowering de in the order iPr- < CH2=C(CH3)- < Me-. The results are reasoned by the assumption of a stereoelectronic control in favour of an axial attack at enamine tautomer. The conformational equilibrium of enamine seems to be crucial to explain experimental data. In matched case only one conformer is expected while a mixture of conformers is postulated in the mismatched one. (author)