Five closely related 4-chloro-6,11-dihydro-5H-benzo[b]pyrimido[5,4-f]azepines: similar molecular structures but different supramolecular assemblies

Abstract

Dibenz[b,f]azepine (DBA) is a privileged 6-7-6 tricyclic ring system of importance in both organic and medicinal chemistry. Benzo[b]pyrimido[5,4-f]azepines (BPAs), which also contain a privileged 6-7-6 ring system, are less well investigated, probably because of a lack of straightforward and versatile methods for their synthesis. A simple and versatile synthetic approach to BPAs based on intramolecular Friedel-Crafts alkylation has been developed. A group of closely-related benzo[b]pyrimido[5,4-f]azepine derivatives, namely (6RS)-4-chloro-6,11-dimethyl-6,11-dihydro-5H-benzo[b]pyrimido[5,4-f]azepine, C14H14ClN3, (I), (6RS)-4-chloro-8-hydroxy-6,11-dimethyl-6,11-dihydro-5H-benzo[b]pyrimido[5,4-f]azepine, C14H14ClN3O, (II), (6RS)-4-chloro-8-methoxy-6,11-dimethyl-6,11-dihydro-5H-benzo[b]pyrimido[5,4-f]azepine, C15H16ClN3O, (III), and (6RS)-4-chloro-8-methoxy-6,11-dimethyl-2-phenyl-6,11-dihydro-5H-benzo[b]pyrimido[5,4-f]azepine, C21H20ClN3O, (IV), has been prepared and their structures compared with the recently published structure [Acosta-Quintero et al. (2015). Eur. J. Org. Chem. pp. 5360-5369] of (6RS)-4-chloro-2,6,8,11-tetramethyl-6,11-dihydro-5H-benzo[b]pyrimido[5,4-f]azepine, (V). All five compounds crystallize as racemic mixtures and they have very similar molecular conformations, with the azepine ring adopting a boat-type conformation in each case, although the orientation of the methoxy substituent in each of (III) and (IV) is different. The supramolecular assemblies in (II) and (IV) depend upon hydrogen bonds of the O-H...N and C-H...π(arene) types, respectively, those in (I) and (V) depend upon π-π stacking interactions involving pairs of pyrimidine rings, and that in (III) depends upon a π-π stacking interaction involving pairs of phenyl rings. Short C-Cl...π(pyrimidine) contacts are present in (I), (II) and (IV) but not in (III) or (V).

Keywords: BPAs; azepines; benzo[b]pyrimido[5,4-f]azepines; crystal structure; hydrogen bonding; molecular conformation; novel synthesis; π-stacking interactions.