Asymmetric synthesis of β-lactams via the zinc-mediated glycine ester enolate - Imine condensation reaction using α-amino esters as the chiral auxiliary

Imine esters of glycine, (R)-phenylglycine, (R)-(1,4-cyclohexadienyl)glycine and (S)-valine have been employed in the ester enolate-imine condensation via double activation with ZnCl2. The reaction of the chlorozinc enolate (1b) of ethyl (2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentyl)acetate with (R)-methyl N-benzylidene-2-phenylglycinate (2a) afforded the trans-β-lactam 3a in 79% yield with excellent asymmetric induction (de > 97%). Other imine esters also afforded trans-β-lactams diastereoselectively, albeit in lower conversions (37-70%). Methyl (S)-2-[N-[3-(trimethylsilyl)-2-propyn-1-ylidene]amino]-3-methylbutanoa te (2e) afforded a mixture of four diastereoisomers (68% conversion). The products were isolated as 3-phthalimido β-lactams 5b-e and as 3-[(methoxycarbonyl)amino]-β-lactam 6b. The reactivity of the zinc enolates and the diastereoselectivity of the reactions are discussed in terms of the coordination of the imine esters to ZnCl2(template effect) and the stability and aggregation equilibria of the zinc enolates.