NMR of some benzodiazepine drugs: Structure elucidation with lanthanide‐induced shifts of N‐1 substituted benzodiazepinones

Abstract

Two 1,4-benzodiazepinones of psychopharmacological interest, pinazepam (2) andprazepam (3),were studiedin solution by OC and 'H NMR. 'H variable-temperature experiments showed that the seven-membered ring exists in a simple pseudo-boat form at room temperature, but the energy barrier is higher in 3 (AG405sC = 82.8 kJ mol-') than in diazepam (1). Moreover, upon addition of LSR, the boat conformation is retained in the lanthanide ion-substrate complex, as with 1 in the solid state. Using a concentrationdependent shift technique and a computer-assisted LIS method, the conformations of the N-branched side-chain were investigated. Two preferred orientations were found for the propargyl group of 2, while three unequally populated conformations of the cyclopropyl moiety of 3 are distributed preferentially in the s t e r i d y unhindered space around the complex.