Compounds were synthesized as illustrated in tetrazole
Compounds – were synthesized as illustrated in , , , , . 3-(1-tetrazole-5-yl)benzaldehyde was obtained in 95% yield via [2+3] cycloaddition of sodium azide to commercially available 3-cyanobenzaldehyde in presence of triethylamine as reported in the literature. Nitration of 3-hydroxyacetophenone with nitric Tenovin-1 in presence of acetic acid resulted in the desired regioisomer 1-(5-hydroxy-2-nitrophenyl)ethan-1-one in 14% yield. Aldol condensation of 1-(5-hydroxy-2-nitrophenyl)ethan-1-one with 3-carboxybenzaldehyde followed by esterification resulted in chalcone while aldol condensation of with 3-(1-tetrazole-5-yl)benzaldehyde resulted in chalcone (). Protection of the phenolic hydroxyl group of and provided TBDMS-protected chalcones derivatives and . Hydrogenation of compounds and with PtO as a catalyst reduced both the nitro group and the olefinic double bond of the TBDMS-protected chalcone derivatives to yield compounds and (). Palladium catalyzed reduction of 3,4-methylenedioxy acetophenone followed by formylation with α,α-dichloromethyl methyl ether resulted in the key intermediate (). Palladium catalyzed reduction of acetophenone was more efficient than the previously reported method using Wolff-Kishner reduction. Reductive amination of with chalcones and using sodium triacetoxyborohydride yielded the corresponding secondary amines and , respectively (). -Acylation of and with ethyl oxalyl chloride led to compounds and in excellent yields. Cyclization of compounds and in presence of the hindered base DBU yielded TBDMS-protected-4-quinolone ester derivatives and , respectively. Deprotection of the TBDMS group under acidic conditions resulted in free phenolic derivatives and . As shown in , alkylation of compounds and with various alkyl halides led to the corresponding O-alkylated derivatives – and –, respectively. Saponification of the ester group at position 2 of the quinolone ring led to the target compounds , – in the carboxylic acid series and , – in the tetrazole series. Endothelin receptor antagonist activity of compounds – was determined by Förster Resonance Energy Transfer (FRET) using GeneBLAzer® assay technology (). Besides the target compounds, BQ-123, a selective ET receptor antagonist and BQ-788, a selective ET receptor antagonist were used as positive controls for ET and ET receptors, respectively. The structure activity relationship deduced for the carboxylic acid series was found to parallel that of the tetrazole series very closely. Among the compounds tested, -propoxy analogue in the tetrazole series was found to be the most potent ET receptor antagonist with an IC value of 0.8nM. Compound was also the most selective ET receptor antagonist with an ET-selectivity of 1004. In the carboxylic acid series, the lead molecule with -propoxy substituent was found to be the most potent ET receptor antagonist with an IC value of 4.2nM. As in case of the corresponding tetrazole analogue , it was also the most selective member of its series with an ET-selectivity of 683. Increasing the chain length to obtain -butoxy analogue (IC=1.88 nM) in the carboxylic acid series led to a slight improvement in ET receptor antagonist activity but was accompanied by reduction in ET-selectivity by a factor of 2. A similar reduction in ET-selectivity for -butoxy analogue (IC=1.2nM) in the tetrazole series was observed although the ET receptor antagonist activity was slightly reduced in this series. Truncating the alkyl chain to two carbon atoms at position 6 of the quinolone ring resulted in the ethoxy analogue (IC=6.6nM) in the carboxylic acid series. This led to 1.5-fold reduction in activity and selectivity against the ET receptor compared to the lead molecule . Similar modification in the tetrazole series led to the ethoxy analogue (IC=7.1nM) which showed 10-fold decrease in the ET receptor antagonist activity and the ET-selectivity also reduced to 581 compared to the most potent compound . Substitution of a branched alkyl chain to obtain -butoxy analogue (IC=9.9 nM) of the carboxylic acid series led to a 5-fold decrease in the ET receptor antagonist activity and was accompanied by 4-fold reduction in ET-selectivity compared to the straight chain analogue . Branched alkyl, -butoxy analogue (IC=6.2nM) of the tetrazole series had 5-fold lower ET receptor antagonist activity and selectivity in comparison with straight chain analogue . Elimination of the alkyl chain entirely to obtain 6-OH analogue (IC=37.01nM) in the carboxylic acid series led to 10-fold decrease in ET receptor antagonism and was accompanied by 1.6 times decrease in ET-selectivity compared to the most potent analogue in this series. Removal of the alkyl chain in the tetrazole series to obtain analogue caused the reduction of ET receptor antagonist activity by 40-fold and selectivity by 1.8 times compared to the most potent compound in this series.