MONOCYCLIC β‐LACTAM TRIPEPTIDE, 1‐(D‐CARBOXY‐2‐METHYLPROPYL)‐3‐L‐(Δ‐L‐2‐AMINOADIPAMIDO)‐4‐L‐MERCAPTOAZETI# DIN‐2‐ONE, A PUTATIVE INTERMEDIATE IN PENICILLIN BIOSYNTHESIS