Lactam ring closure elimination

The reaction of Dewar pyrimidinone 220 with water in 1 9 mixture of water and acetone at 20°C for 1 hour afforded 4//-pyrido[ 1,2-a]pyrimidin-4-one 108 and enamine 107 in 34% and 64% yields, respectively [89JCS(P1)1231]. Enamine 107 was formed by the cleavage of bond a of 4-hydroxyazetidin-2-one 221, and ring closure of enamine 107 by elimination of water gave 4//-pyrido[l, 2-a]pyrimidin-4-one 108 (Scheme 15). When the solvent polarity was increased, the cleavage of bond b of 4-hydroxyazetidin-2-one 221 also occurred. The reaction in aqueous acetoni-trile-d3 at 35°C for 37 hours gave 4//-pyrido[ 1,2-a]pyrimidin-4-one 109 and lactam 222 in 85% and 10% yields, respectively. The reaction of Dewar... 

Ring closure across substituents of 1,4-disubstituted p-lactams is a convenient route to both bicyclic and tricyclic systems. For instance, cyclization of 81 gives the carbacephem 82 in high yield in the presence of benzylidenebis(tricyclohexylphosphine)ruthenium dichloride through an enyne metathesis process <97CCI375>. The 4-formyloxy-P-lactam 83 gives the oxacephem 84 by the action of Lewis acids but here the 4-substituent is eliminated to form the intermediate cation 85 <97TA2553>. 

Reduction (s. a. Cleavage, reductive. Elimination, -, Hydrogenation, Lactam reduction, Redox. .., Photoreduction, Ring closure, reductive, Substitution, nucleophilic, -)... 

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