Chiral ferrocenylalkyl amines auxiliaries

In addition to four component condensation, several other applications of chiral primary ferrocenylalkyl amines have been published. Thus, an asymmetric synthesis of alanine was developed (Fig. 4-3la), which forms an imine from 1-ferrocenylethyl amine and pyruvic acid, followed by catalytic reduction (Pd/C) to the amine. Cleavage of the auxiliary occurs readily by 2-mercaptoacetic acid, giving alanine in 61% ee and allowing for recycling of the chiral auxiliary from the sulfur derivative by the HgClj technique [165]. Enantioselective reduction of imines is not limited to pyruvic acid, but has recently also been applied to the imine with acetophenone, although the diastereoisomeric ferrocenylalkyl derivatives of phenylethylamine were obtained only in a ratio of about 2 1 (Fig. 4-31 b). The enantioselective addition of methyl lithium to the imine with benzaldehyde was of the same low selectivity [57]. Recycling of the chiral auxiliary was possible by treatment of the secondary amines with acetic acid/formaldehyde mixture that cleaved the phenylethylamine from the cation and substituted it for acetate. 

Some kinds of optically active ferrocenyl compounds have been known as useful chiral auxiliaries in asymmetrically induced synthesis. Chiral a-ferrocenylalkyl-amines have been prepared [1 — 5] and used as chiral auxiliaries for steroselective peptide synthesis by four-component condensation [6], asymmetric transamination... 

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