1,3,4-Oxadiazines, dihydro-, synthesis

Dihydro-4 f-l,2,5-oxadiazines (21) (see Section 6.15.6.2.3.1) have been used as masked acyl groups for the stereoselective synthesis of 3(a)-(l-hydroxyethyl)-4( )-benzoyl- -lactams (24) (Scheme 2) <88EUP265169>. Condensation of the oxadiazine (21) with (2R,3R)-epoxy-butanoic acid in the presence of dicyclohexylcarbodiimide yields the A(-acyl derivative (22), which on treatment with lithium bis(trimethylsilyl)amide in THE undergoes stereoselective ring closure to the fused azetidinone (23). Hydrolysis with 1 M sulfuric acid in acetone ruptures the oxadiazine ring and forms the -lactam (24). 

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