ASYMMETRIC ALKYLATION OF CHIRAL 2-AZAPENTADIENYL METAL-COMPOUNDS - DIASTEREOSELECTIVE SYNTHESIS OF ALKYL-SUBSTITUTED N-ALLYLIMINES (2-AZA-1,4-PENTADIENES)

N-Allylimines (2-aza-1,4-pentadienes) 3 bearing a chiral auxiliary gro up at C-1 were prepared by a three-step sequence involving condensatio n of pivaloyl chloride with allylamine, chlorination and reaction with a chiral amine (SMP or SMEMP) as an auxiliary group. Deprotonation of 3 by treatment with butyllithium in tert-butyl methyl ether as solven t at -20 degrees C afforded the chiral 2-azapentadienyllithium compoun ds 4. Transmetalation of 4a with MgBr2 . Et(2)O gave the corresponding magnesium bromide 5. Alkylation with primary alkyl halides is predomi nantly kinetically controlled and leads to the 3-substituted N-allylim ines 6. Secondary and tertiary alkyl halides, however, preferentially give 5-substituted 2-aza-1,3-pentadienes 7. The diastereoselectivity o f the alkylation reaction may be increased by transmetalation from the Lithium compound to the corresponding magnesium bromide or by exchang e of the SMP group for the stronger chelating SMEMP group. The halide ion of the alkylating agent strongly influences the configuration of t he newly formed stereogenic center. Thus, the chlorides lead to the op posite stereochemistry compared to the bromides. The effect of differe nt solvents on the regio- and diastereoselectivity of the alkylation r eactions was studied. Semiempirical calculations (PM3, MNDO) were used to design a stereochemical model for the metalated intermediates in o rder to rationalize the stereochemistry observed in the experiments.