ZK-93423

β-Carbolines are a class of drug which can interact with a high affinity with the benzodiazepine (BDZ) binding site of the GABAA receptor. The present paper, aimed at obtaining a deeper insight into the structure–properties relationships of this class of molecules, reports the crystal structures of four β-carbolines: ZK93423 (3-carboethoxy-4-methoxymethyl-6-benzyloxy-β-carboline), ZK91296 (3-carboethoxy-4-methoxymethyl-5-benzyloxy-β-carboline), FG7142 (N-methyl-3-carbamoyl-β-carboline) and the low-affinity ligand harmine hydrochloride (1-methyl-7-methoxy-β-carboline). This set of structural data is completed by the X-ray structures of other carbolines of known biological activity retrieved from the Cambridge Crystallographic Database and by the structures of β-CCE (3-carboethoxy-β-carboline), 6-PBC (3-carboethoxy-4-methoxymethyl-6-isopropoxy-β-carboline), PRCC (3-isopropoxy-β-carboline) and ZK93426 (3-carboethoxy-4-methyl-5-isopropoxy-β-carboline), which have been obtained by molecular-mechanics simulations. The structural features of all these molecules have been compared according to the stereochemical model we proposed in 1987. The structural comparison is integrated by the Free–Wilson analysis on 32 β-carbolines of known binding affinity data.

A variety of substituted beta- and gamma-carbolines have been prepared in moderate to excellent yields by the palladium-catalyzed annulation of internal and terminal acetylenes by the tert-butylimines of N-substituted 3-iodoindole-2-carboxaldehydes and 2-haloindole-3-carboxaldehydes, respectively. This annulation chemistry is effective for a wide range of alkynes, including aryl-, alkyl-, hydroxymethyl-, ethoxycarbonyl-, and trimethylsilyl-substituted alkynes. When an unsymmetrical internal alkyne is employed, this method generally gives two regioisomers. When a terminal alkyne is employed, only one regioisomer has been isolated. This palladium-catalyzed annulation chemistry has also been successfully applied to the synthesis of two biologically interesting beta-carboline alkaloids, ZK93423 and abecarnil (ZK112119).