Bromolactamization: key step in the stereoselective synthesis of enantiomerically pure, cis-configured perhydropyrroloquinoxalines.

Abstract

Compounds based on the pyrroloquinoxaline system can interact with serotonin 5-HT3 , cannabinoid CB1 , and μ-opioid receptors. Herein, a chiral pool synthesis of diastereomerically and enantiomerically pure bromolactam (S,R,R,R)-14A is presented. Introduction of the cyclohexenyl ring at the N-atom of (S)-proline derivatives 8 or methyl (S)-pyroglutamate (12) led to the N-cyclohexenyl substituted pyrrolidine derivatives 4 and 13, respectively. All attempts to cyclize the (S)-proline derivatives 4 with a basic pyrrolidine N-atom via [3 + 2] cycloaddition, aziridination, or bromolactamization failed. Fast aromatization occurred during treatment of cyclohexenamines under halolactamization conditions. In contrast, reaction of a 1:1 mixture of diastereomeric pyroglutamates (S,R)-13bA and (S,S)-13bB with LiO(t) Bu and NBS provided the tricyclic bromolactam (S,R,R,R)-14A with high diastereoselectivity from (S,R)-13bA, but did not transform the diastereomer (S,S)-13bB. The different behavior of the diastereomeric pyroglutamates (S,R)-13bA and (S,S)-13bB is explained by different energetically favored conformations.