Hydroquinones with Conformationally Constrained Substituents: Synthesis, Characterization, and Evaluation as Calcium–ATPase Inhibitors

Abstract

Abstract-Derivatives of the compound 2,5-di-tert-butylhydroquinone (BHQ) are inhibitors of the sarco/endoplasmic reticulum calcium ATPase (SERCA). Previous work identified BHQ analogs with two alkyl substituents composed of four to six carbon atoms as the most potent representatives from this inhibitor class. In this study, we explored the effects of introducing two cycloalkyl substituents on the hydroquinone scaffold. Since these substituents have limited conformational flexibility, the entropy penalty upon binding of these compounds to SERCA was expected to be smaller than for non-cyclic BHQ analogs, potentially conveying higher inhibitory potency. We synthesized a congeneric series of four 2,5-disubstituted BHQ analogs and determined their inhibitory potencies against SERCA in bioassays. Potencies were found in the low micromolar and submicromolar concentration ranges, making the new compounds some of the most potent hydroquinone-based inhibitors to date. Computational docking facilitated a detailed analysis of enzyme/inhibitor interactions at the molecular level and showed that the entropic effect was noticeable but too small to increase potency in a substantial manner.Index Terms-Calcium homeostasis, enzyme inhibition, P-type ATPase, medicinal chemistry.

Sarco/Endoplasmic Reticulum Calcium ATPase Inhibitors: Beyond Anticancer Perspective

“…154,155,159 Numerous attempts to prepare compound 2 derivatives by modification of hydroquinone side chains were made in the past years. 160,161 It was disproved that two substituents are necessary for the SERCAinhibiting activity of the hydroquinone, and mono-and disubstituted compounds with IC 50 values in the low micromolar range were synthesized. Most recently, derivatives of compound 2 with altered side chains have been synthesized following a similar rationale as the preparation of the peptidemasked compound 1 derivatives and yielding one promising c om p ou n d ter t -b u t y l ( 1 -( ( 6 -( 4 -(t e r t-b u t y l ) -2 , 5dihydroxyphenyl)hexyl)amino)-4-methyl-1-oxopentan-2-yl)carbamate 162 (32, Figure 14).…”

“…Increasing the size of the alkyls on the hydroquinone structure significantly decreases the IC 50 value of such derivatives in comparison to compound 2 likely due to the restricted size of the interaction cavity. Decreasing the size of the alkyl in compound 2 also diminishes the SERCA inhibitory activity, causing insufficient interaction and failing to provide the translocation of Glu-309 which then causes a steric hindrance to Ca 2+ ion binding. ,, Numerous attempts to prepare compound 2 derivatives by modification of hydroquinone side chains were made in the past years. , It was disproved that two substituents are necessary for the SERCA-inhibiting activity of the hydroquinone, and mono- and disubstituted compounds with IC 50 values in the low micromolar range were synthesized. Most recently, derivatives of compound 2 with altered side chains have been synthesized following a similar rationale as the preparation of the peptide-masked compound 1 derivatives and yielding one promising compound tert -butyl (1-((6-(4-( tert -butyl)-2,5-dihydroxyphenyl)­hexyl)­amino)-4-methyl-1-oxopentan-2-yl)­carbamate ( 32 , Figure ).…”

Heck- and Suzuki-coupling approaches to novel hydroquinone inhibitors of calcium ATPase

“…1) has a somewhat lower inhibitory potency than TG (400 nM versus low nanomolar range), but offers advantages such as ease of synthesis and greatly reduced cost. Recent synthetic work from our group has shown that the tert -butyl groups of BHQ can be replaced by a variety of alkyl and cycloalkyl groups with minimal reduction in potency [7–9]. In fact, the 2,5-bis(1-methylcyclopentyl) analogue of BHQ 3 has an IC 50 value of 500 nM, which is comparable to BHQ’s value of 400 nM.…”