Background:
In modern chemistry, the asymmetric synthesis for the preparation of high purity
chiral compounds to be used as pharmaceuticals or additives in foods have been of capital importance. Chiral
auxiliary reagents are used to control the stereochemistry of the reaction in the generation of new chiral compounds,
in this context, Ephedra compounds (ephedrines and pseudoephedrines) and some of their derivatives
have been broadly used as chiral ligands in catalysis or chiral inductors in asymmetric synthesis.
Objective:
This review focuses on recent progress in the use of ephedra compounds and their N-substituted derivatives
as chiral auxiliaries in the area of asymmetric synthesis, via the alkylation reaction of the enolates derived
from their corresponding N-Acyl or O-Acyl derivatives, in the C-C bond formation.
Conclusion:
A vast amount of work has been done about the use of ephedra compounds in asymmetric synthesis
area, in general, it was found that pseudoephedrines are much more effective than ephedrines and are preferred
as chiral auxiliaries in the asymmetric alkylation of the corresponding N-acyl amides or O-Acyl esters.
Alkylation with alkyl halides requires the use of more than 4 equivalents of LiCl to accelerate the alkylation
rate and to complete the reaction without effecting the diastereoselectivity of the process. In contrast, the use of
secondary alkyl halides was found to make the reaction very slow. Furthermore, a lot of work about the alkylation
reaction in the opening of epoxides and aziridines, aldolic condensation, Manich reaction, addition of nucleophiles
to α,β-unsaturated ephedrine amides and Michael additions have been demonstrated to be effective
in the C-C bond formation. The aldol reaction of chiral enolates, proceeds with decreasing yields and enantioselectivities
as the steric demand of the α-R of ephedrine amides and the size of carbonyl compound increase.
In addition, the use of branched groups on N,N-disubstituted norephedrine esters is highly recommended in the
aldol reactions of aromatic and aliphatic aldehydes.In the case of N-Acyl or O-Acyl ephedrines supported on
polymers, the reaction proceed with good enantioselectivities but low yields, the enantioselectivities are goods
but the yields are low. In general, the removal step of the auxiliary proceeds with low to high yields but without
epimerization.
Convenient Asymmetric Synthesis of Fmoc-(S)-6,6,6-Trifluoro-Norleucine
