X=Y-ZH SYSTEMS AS POTENTIAL 1,3-DIPOLES .8. PYRROLIDINES AND DELTA-5-PYRROLINES (3,7-DIAZABICYCLO[3.3.0]OCTENES) FROM THE REACTION OF IMINES OF ALPHA-AMINO-ACIDS AND THEIR ESTERS WITH CYCLIC DIPOLAROPHILES - MECHANISM OF RACEMIZATION OF ALPHA-AMINO-ACIDS AND THEIR ESTERS IN THE PRESENCE OF ALDEHYDESN