Chiral ligands derived from abrine. Part 6: Importance of a bulky N-alkyl group in indole-containing chiral β-tertiary amino alcohols for controlling enantioselectivity in addition of diethylzinc toward aldehydes

A number of chiral β-amino alcohols possessing a 3-indolylmethyl group have been synthesized from the alkaloid, (S)-abrine and elucidated for potency in the catalytic enantioselective ethylation of PhCHO with Et₂Zn. In general, the secondary amines 15a-d bearing a dialkylhydroxymethyl group induced (R)-1-phenyl-1-propanol, whereas 15e-g and 18 bearing a diarylhydroxymethyl group favored the (S)-enantiomer. In contrast, the β-tertiary amino alcohols 20b-d and 21 produced (R)-1-phenyl-1-propanol, regardless of the substituents at the carbon bearing the hydroxy group. Enantiomeric excess of 87.5% was obtained for (R)-1-phenyl-1-propanol using ligand 21 as the promoter. Eleven substituted benzaldehydes and naphthaldehydes were examined for enantioselective ethylation by using 21 and the chiral alcohols were obtained in 93-97% ee, except for o-BrC₆H₄CHO and p-Me₂NC₆H₄CHO. Excellent enantioselectivity was also observed in the ethylation of cyclohexanecarboxaldehyde (94.8% ee) and 2-thiophenecarboxaldehyde (94.9% ee) by using catalytic 21. The anti 5/4/4-fused tricyclic TS I was proposed to rationalize the asymmetric induction. The diethylhydroxymethyl and N-2-t-butylethyl groups are believed to enforce the preference for the anti-TS(R) I and it results in high enantioselectivity. Copyright (C) 2000 Elsevier Science Ltd.