SIMPLE AND CONDENSED BETA -LACTAMS. XVI: RING TRANSFORMATION OF 4-NITROBENZYL 2-AZIDO-2-(2-CYANOMETHYL-4-OXOAZETIDIN-1-YL)ACETATE INTO 4-NITROBENZYL 4 (6)-CYANOMETHYL-6(4)-OXO-1,4,5,6-TETRAHYDROPYRIMIDINE-2-CARBOXYLATE. A NOVE L RING TRANSFORMATION IN THE BETA -LACTAM SERIES