[chemical substances]
   reactive intermediates
     Class dipolar compounds
       Class 1,3-dipolar compounds
         Class Allyl type 1,3-dipolar compounds [X=Y+-Z <-> X-+Y=Z <-> X+YZ- <-> X-YZ+ (X,Z=C,N,O; Y=N,O)]
           Class oxonium ylides [R2C=O+-Y]
             Class carbonyl ylides [R2C=O+-CR2 <-> R2C+OC-R2]