No abstract

Carbacephems (1) are known to exhibit antibiotic activity1-3 and have increased chemical stability compared to cephalosporins.3 They are generally synthesized by first building the /3-lactam ring (according to the Staudinger reaction4,5) followed by the formation of the six-membered ring.6We describe herein a new approach to asymmetric synthesis of the carbacepham skeleton 2 (which can be conveniently used to prepare carbacephems 1 through known procedures in the field of carbapenem chemistry).7In a recent paper,8 we described a novel access to enantiomerically pure 6-alkylated pipecolic acids 5 from the chiral synthon 6 (Scheme 1). The key step for this preparation was the diastereoselective alkylation of oxazolopipecolic ester 4 by a Grignard reagent to give the pipecolic acid 5. We envisaged that this reaction could be extended to the addition of enolates in place of Grignard reagents to give the /3-amino acid 3 which could then be cyclized to carbacepham 2.This route was first tested on a simple oxazolidine, namely 6-propyl-2-oxazolopiperidine 7 (Scheme 2), obtained in good yield in a two-step sequence from 6.9

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