A NEW AND EFFICIENT ROUTE TO 3-AMINO-2-AZETIDINONES VIA ZINC ENOLATES OF N,N-DISUBSTITUTED GLYCINE ESTERS

This report describes novel and efficient ''one-pot'' syntheses of 1-unsubstituted-3-amino-4-substituted-2-azetidinones (8 and 9) involving the in situ preparation of lithium and particularly zinc enolates (5 and 6, respectively) of N,N-disubstituted glycine esters (4) and subsequent reactions of these enolates with (simple) imines (7). Lithium enolates 5 only react with activated imines that are N-substituted with an electron-withdrawing group (e.g. aryl, trialkylsilyl), affording cis-3-amino-2-azetidinones in excellent yields with moderate to good stereoselectivity (de 68-92%). Zinc enolates 6 are more generally applicable since they react with activated imines as well as unactivated imines (e.g. those which are N-substituted with an electron-donating group such as alkyl) to afford 3-amino-2-azetidinones in excellent yields. The trans diastereoselectivity of the zinc-mediated enolate-imine condensation can be tuned by changing the steric and electronic properties of the substituents of the reagents (i.e. both enolate and imine), as well as the solvent polarity. The observed stereoselectivities are explained in terms of two highly ordered transition states, consisting of a Z-zinc ester enolate and an E-imine. Protection of the amino function of the metal enolates as a 2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentane ring affords 2-azetidinone products that can be easily deprotected to provide a free 3-amino function. In this way, trans-1-benzyl-3-(protected amino)-4-methyl-2-azetidinone (9a) and trans-3-(protected amino)-4-[(trimethylsilyl)ethynyl]-2-azetidinone (9g), key intermediates in the synthesis of Aztreonam (and 9g for bicyclic beta-lactam antibiotics as well), have been prepared in excellent yields (98 and 93%, respectively) with a high diastereoselectivity (de 82 and 94%, respectively). Furthermore, depending on the reactivity of imines 7, our method is also applicable using a catalytic amount (10 mol %) of ZnCl2.