AbstractThe Staudinger‐like [2+2] cycloaddition of aliphatic hydrazones derived from (2R,5R)‐1‐amino‐2,5‐dimethylpyrrolidine to N‐benzyl‐N‐(benzyloxycarbonyl)aminoketene takes place to afford the corresponding β‐lactams in good yields when iPr2EtN is used as the base. The reaction proceeds in all cases with excellent stereocontrol to afford exclusively products having the (3R) configuration. Temperature was observed to exert a strong influence on the cis/trans selectivity, allowing in most cases the obtention of single trans or cis cycloadducts simply by performing the reactions at 80 °C or room temperature, respectively. Additional experiments support the hypothesis that the observed stereochemistry is the result of C=N isomerization in the zwitterionic intermediate, which takes place by a nucleophilic addition―rotation―elimination mechanism effected by the nucleophiles present in the reaction medium. Release of the dialkylamino group by oxidative cleavage of the N–N bond affords valuable compounds such as α,β‐diamino acids and the azetidinone cores of monobactam antibiotics such as aztreonam and carumonam.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)