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■ Lewis Acid Promoted Diastereoselective Mannich Reaction of β-Lactam-Tethered Aldimines with 1-Methoxy-1-trimethylsilyloxy-2,2-dimethylethene

Katarina Vazdar, Davor Margetić, and Ivan Habuš*

*Ruđer Bošković Institute, Bijenička c. 54, P.O.B. 180, HR-10002 Zagreb, Croatia

Abstract

Lewis acid (ZnI2) – promoted Mannich reaction of azetidin-2-one – tethered aldimines 3a-r with silylenol ether 4 in toluene at –20 °C afforded a diastereomeric mixture of β-amino acid esters 5a-r/6a-r. The effect of the amount of zinc(II) iodide, equimolar (100 mol%) and catalytic (20 mol%), on the chemical yield and the product stereoisomeric ratio was studied. The diastereoselective Mannich reaction of imine 3a with silylenol ether 4 at low temperature (–20 °C) in toluene under equimolar zinc(II) iodide catalysis provided the best chemical yield – 99% combined with the highest diastereoselectivity of β-amino acid esters 5a/6a – 85:15%. Furthermore, the influence of various groups on azetidin-2-one – tethered imines 3a-r (R1 = phenyl, ferrocenyl; R2 = alkyl, aryl, ferrocenyl) has been applied in the Mannich reaction leading to the formation of two stereoisomers of β-amino acid esters 5a-r/6a-r with diastereomeric ratio varying from 92:8 to 59:41%.