Oxazolidines cycloreversion

Generation of azomethine ylides by starting from oxazoline heterocycles consists of the following steps (1) valence isomerization of 4-isoxazolines, (2) valence isomerization of 4-oxazolines, and (3) cycloreversion of 5-oxazo-lidinones or oxazolidines. These three generation methods as variations of the oxazoline route are reviewed in this section. 

The last variation of the oxazoline route is a cycloreversion of 5-oxazoli-dinones or oxazolidines. Thermal decarboxylation of 5-oxazolidinones is a convenient method of generating nonstabilized azomethine ylides, since the heterocyclic precursors are accessible, and sometimes isolable, by simply heating a-amino acids and carbonyl compounds. This has been discussed in Section II,E. 

Phenylperhydro-l,3,4-oxadiazin-2-ones 151 react with aliphatic or aromatic aldehydes or ethyl 2-oxoacetate to give the intermediates 152. The azomethine imine ylides 152 yield primary oxazolidines 153 <200081170, 2002S1885, 2004TL3127>. These compounds can be synthesized also by treatment of 151 with ethyl oxoacetate or aldehydes in the presence of magnesium bromide etherate. The tandem cycloreversion-cycloaddition of 153 with various electron-poor dipolarophiles then leads to pyrazolidines 154-156 (Scheme 22). 

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