Enantioresolution, stereochemical characterization and biological activity of a chiral large-conductance calcium-activated potassium channel opener

• Published in: Journal of Chromatography A 2014-10, Vol.1363, p.162-168
• Main Authors: Sardella, Roccaldo, Carotti, Andrea, Manfroni, Giuseppe, Tedesco, Daniele, Martelli, Alma, Bertucci, Carlo, Cecchetti, Violetta, Natalini, Benedetto
• Format: Article
• Language: English
• Subjects: Absolute configuration; BK channel opener; Electronic circular dichroism; Polysaccharide-based stationary phases; Preparative enantioresolution; Vasorelaxing potency
• ISSN: 0021-9673
• DOI: 10.1016/j.chroma.2014.06.020

•An enantioselective HPLC method was developed for chiral BK channel opener 1.•The Sepapak-4 column allowed a preparative resolution of the enantiomers of 1.•The configuration was characterized by ECD spectroscopy and TD-DFT calculations.•No stereoselectivity was observed for the vasorelaxing activity of compound 1. A number of large-conductance calcium-activated potassium (BK) channel openers based on the 2-aryl-1,4-benzothiazine scaffold were previously synthesized, and 2-(5-bromo-2-methoxyphenyl)-6-trifluoromethyl-2H-1,4-benzothiazin-3(4H)-one (1) was identified as the most active compound. Since a stereoselective activation of BK channels was demonstrated for arylindolone derivatives, the effect of the absolute configuration at the C-2 position on the vasorelaxing potency of 2-aryl-1,4-benzothiazines is investigated in this article. Compound 1 was initially evaluated as a racemate: subsequently, the “racemic approach” was used to isolate its enantiomers. The excellent enantioresolution obtained using the Sepapak-4 column (CSP 4, cellulose tris(4-chloro-3-metylphenylcarbamate); RS=8.36; α=2.03) allowed to collect highly pure enantiomeric fractions, with enantiomeric excess (e.e.) values higher than 97% and 98% for the first- and second-eluted enantiomer, respectively. Electronic circular dichroism (ECD) studies on the two isolated enantiomers, combined with time-dependent density functional theory (TD-DFT) calculations allowed to characterize the configuration of the enantiomers and determine a (R), (S) elution order. Results from biological assays indicated that the racemate and the isolated enantiomers are endowed with comparable vasorelaxing potency.