The intramolecular aromatic nucleophilic substitution as a route to tricyclic beta-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 3 and 7 gave quantitatively the stereoisomeric carbinols (4R*,5S*)-4 and (4R*,5R*)-5. Treatment of the latter with sodium hydride gave the title compounds 8 and 9, respectively, with good overall yield. The rationale of the stereochemical relationships outlined in the sequences 3 (or 7)-->4-->8 and 3 (or 7)-->5-->9 is given according to the conformational and keto-enol equilibria of the reactant(s). (C) 2003 Elsevier Science Ltd. All rights reserved.