The asymmetric Michael-type alkylation of chiral β-enamino esters: critical role of a benzyl ester group in the racemization of adducts

In contrast with keto diester (R)-10a bearing a methyl ester group at the quaternary carbon center, the benzyl ester analogue (R)-10a partially racemized during workup, thereby revealing that the nature of the ester group at the quaternary carbon center has a critical role in the rate of racemization of such Michael adducts.

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Methyl (S)-2-(2-benzenesulfonylethyl)-3-methyl-3-oxobutyrateC14H18O5SEe = 98% (by chiral HPLC)[α]D20=-26.0 (c 0.88, CHCl3)Source of chirality: asymmetric Michael addition using (S)-1-phenylethylamine (99% ee)Absolute configuration: (S)

Methyl (R)-3-(1-carbobenyloxy-2-oxocyclopentyl)-propionateC17H20O5Ee = 55% (by chiral HPLC)[α]D20=-1.4 (c 2.82, AcOEt)Source of chirality: asymmetric Michael addition using (R)-1-phenylethylamine (99% ee)Absolute configuration: (R)

Benzyl (S)-2-(2-benzenesulfonylethyl)-3-methyl-3-oxobutyrateC20H22O5SEe = 94% (by chiral HPLC)[α]D20=-16.4 (c 1.64, CHCl3)Source of chirality: asymmetric Michael addition using (S)-1-phenylethylamine (99% ee)Absolute configuration: (S)