Use of functionalized β-lactams as building blocks in heterocyclic chemistry

beta-Lactams represent flexible building blocks suitable for the preparation of a large variety of nitrogen-containing target compounds. In the present study, the formerly neglected synthetic potential of 4-haloalkyl-beta-lactams has been elaborated in detail with a focus on the preparation of different mono- and bicyclic heterocycles. A first approach involved ring transformations of these halogenated building blocks toward stereodefined aziridines, azetidines, pyrrolidines, and piperidines via intermediate aziridinium or azetidinium ions. In a second part, novel and stereoselective entries into 1,4- and 3,4-fused bicyclic beta-lactams were developed through either a radical or an ionic cyclization protocol. Furthermore, the ring enlargement of halogenated beta-lactams into functionalized mono- and bicyclic pyrrolidin-2-ones was established as the aza-analog of the cyclobutylmethylcarbenium ion to cyclopentylcarbenium ion rearrangement. Finally, chiral versions toward azetidin-2-ones and ring transformation products were elaborated, involving the synthesis of 3(S)-alkoxy-4(S)-[1(S)-chloroethyl]azetidin-2-ones and the preparation of bicyclic beta-lactams annelated to piperazines, morpholines, and diazepanes.