The intramolecular aromatic nucleophilic substitution as a route to tricyclic β-lactams. Synthesis of the novel 4-oxa-7-azabicyclo[4.2.0]octane skeleton

Sodium borohydride-carbonyl reduction of the novel 3-(2-fluoro-5-nitro) phenyl-4-benzoyl-2-azetidinones and gave quantitatively the stereoisomeric carbinols and . Treatment of the latter with sodium hydride gave the title compounds and , respectively, with good overall yield. The rationale of the stereochemical relationships outlined in the sequences (or )→→ and (or )→→ is given according to the conformational and keto-enol equilibria of the reactant(s).