[chemical substances]
   reactive intermediates
     Class dipolar compounds
       Class ylides [[RmX+-YRn [<-> RmX=YRn] (X=N,O,P,S,etc.; Y=C, etc.)] or [Z=X+-YRn <-> Z-+X=YRn] or [RmX+-Y <-> RmX=Y]]
       1 Class 1,2-dipolar compounds
       2 Class 1,3-dipolar compounds