C4,C4'-bis-beta-lactam to fused bis-gamma-lactam rearrangement

Optically pure cis,cis-C4,C4'-bis-beta-lactams 1a-d are obtained in good to excellent yields, in a single step, following two different approaches. Staudinger reaction of (S)-(4-phenyl-2-oxooxazolidinyl)acetyl chloride (2a) and p-anisyldiimine gave the corresponding bis-beta-lactam la as a single enantiomer. The reaction of glyoxal diimine derived from (S)-alpha-phenylethylamine and different alkoxy-substituted acid chlorides gave diastereomeric mixtures of cis,cis-bis-beta-lactams 1b-d, enantiomers at the bicyclic skeleton. The configuration of all compounds has been determined by X-ray diffraction analysis of enantiomerically pure aldehyde 4a and bis-beta-lactam Ib-a. The remaining bicyclic lactams have been chemically correlated to compound 1b-alpha and their configurations assigned. Starting from enantiomerically pure 4-formyl-2-azetidinone 4b, sequential imine formation and ketene cycloaddition allowed the synthesis of differently substituted, optically pure cis,cis-C4,C4'-bis-beta-lactams If-i in good overall yields. C4,C4'-Bis-beta-lactams smoothly rearranged to fused trans,trans-bis-gamma-lactams 7 upon basic treatment (NaOMe/MeOH) in a totally stereoselective process. The presence of alkyl groups attached to the lactam nitrogen inhibits the rearrangement. Differently substituted (aryl and alkyl substituents in either rings) bicyclic p-lactam systems gave the selective opening of the ring with the aromatic substituent attached to the lactam nitrogen. Monocyclic 2-azetidinones 8 with an amino ester side chain at C4 are then obtained. The synthesis of trans,cis-C4,C4'-bis-beta-lactam Ij and trans,trans-C4,C4'-bis-beta-lactam 11 has also been effected in the racemic form. Compound Ij with a trans,cis stereochemistry rearranged to cis,trans-bis-gamma-lactam 7d in the presence of base while the trans,trans-bicycle 11 gave monocyclic 2-azetidinone 8c with an amino ester side chain. Finally, trans,trans-bis-gamma-lactam 11 can be synthesized in a single step from glyoxal diimine 3a employing an excess of lithium isovalerate. A reaction pathway to account for all the observed data is proposed.