THE SYNTHESIS AND REACTIONS OF A MONOCYCLIC β-LACTAM TRIPEPTIDE, 1-((1R)-CARBOXY-2-METHYLPROPYL)-(3R)-((5S)-5-AMINO-5-CARBOXYPENTANAMIDO)-(4R)-MERCAPTOAZETIDIN-2-ONE, A PUTATIVE INTERMEDIATE IN PENICILLIN BIOSYNTHESIS