Organometallic chemistry provides powerful tools for the stereocontrolled synthesis of heterocycles and carbocycles. The electrophilic transition metals Pt(II) and Au(I, III) are efficient catalysts ...in these transitions and promote a variety of organic transformations of unsaturated precursors. These reactions produce functionalized cyclic and acyclic scaffolds for the synthesis of natural and non-natural products efficiently, under mild conditions, and with excellent chemoselectivity. Because these transformations are strongly substrate-dependent, they are versatile and may yield diverse molecular scaffolds. Therefore, synthetic chemists need a mechanistic interpretation to optimize this reaction process and design a new generation of catalysts. However, so far, no intermediate species has been isolated or characterized, so the formulated mechanistic hypotheses have been primarily based on labeling studies or trapping reactions. Recently, theoretical DFT studies have become a useful tool in our research, giving us insights into the key intermediates and into a variety of plausible reaction pathways. In this Account, we present a comprehensive mechanistic overview of transformations promoted by Pt and Au in a non-nucleophilic medium based on quantum-mechanical studies. The calculations are consistent with the experimental observations and provide fundamental insights into the versatility of these reaction processes. The reactivity of these metals results from their peculiar Lewis acid properties: the alkynophilic character of these soft metals and the π-acid activation of unsaturated groups promotes the intra- or intermolecular attack of a nucleophile. 1,n-Enynes (n = 3−8) are particularly important precursors, and their transformation may yield a variety of cycloadducts depending on the molecular structure. However, the calculations suggest that these different cyclizations would have closely related reaction mechanisms, and we propose a unified mechanistic picture. The intramolecular nucleophilic attack of the double bond on the activated alkyne takes place by an endo-dig or exo-dig pathway to afford a cyclopropyl-metallocarbenoid. Through divergent routes, the cyclopropyl intermediate formed by exo-cyclopropanation could yield the metathesis adduct or bicyclic compounds. The endo-cyclization may be followed by a 1,2-migration of the propargyl moiety to the internal acetylenic position to afford bicyclic n.1.0 derivatives. This reaction mechanism is applicable for functional groups ranging from H to carboxylate propargyl substituents (Rautenstrauch reaction). In intramolecular reactions in which a shorter enyne bears a propargyl ester or in intermolecular reactions of an ester with an alkene, the ester preferentially attacks the activated alkyne because of enthalpic (ring strain) and entropic effects. Our calculations can predict the correct stereochemical outcome, which may aid the rational design of further stereoselective syntheses. The alkynes activated by electrophilic species can also react with other nucleophiles, such as aromatic rings. The calculations account for the high endo-selectivity observed and suggest that this transformation takes place through a Friedel−Crafts-type alkenylation mechanism, where the endo-dig cyclization promoted by PtCl2 may involve a cyclopropylmetallacarbene as intermediate before the formation of the expected Wheland-type intermediate. These comparisons of the computational approach with experiment demonstrate the value of theory in the development of a solid mechanistic understanding of these reaction processes.
The recent advances of tetramethylpyrazine nitrones and quinolylnitrones for the treatment of stroke have been reviewed and compared with other agents, showing promising therapeutic applications. As ...a result of a functional transformation of natural product ligustrazine, (Z)-N-tert-butyl-1-(3,5,6-trimethylpyrazin-2-yl)methanimine oxide (6) is a multitarget small nitrone showing potent thrombolytic activity and free radicals scavenging power, in addition to nontoxicity and blood–brain barrier permeability. Similarly, antioxidant (Z)-N-tert-butyl-1-(2-chloro-6-methoxyquinolin-3-yl)methanimine oxide (17) is a novel agent for cerebral ischemia therapy as it is able to scavenge different types of free radical species, showing strong neuroprotection and reduced infarct size.
A new family of multitarget molecules able to interact with acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), as well as with monoamino oxidase (MAO) A and B, has been synthesized. Novel ...compounds (3–9) have been designed using a conjunctive approach that combines the benzylpiperidine moiety of the AChE inhibitor donepezil (1) and the indolyl propargylamino moiety of the MAO inhibitor N-(5-benzyloxy-1-methyl-1H-indol-2-yl)methyl-N-methylprop-2-yn-1-amine (2), connected through an oligomethylene linker. The most promising hybrid (5) is a potent inhibitor of both MAO-A (IC50 = 5.2 ± 1.1 nM) and MAO-B (IC50 = 43 ± 8.0 nM) and is a moderately potent inhibitor of AChE (IC50 = 0.35 ± 0.01 μM) and BuChE (IC50 = 0.46 ± 0.06 μM). Moreover, molecular modeling and kinetic studies support the dual binding site to AChE, which explains the inhibitory effect exerted on Aβ aggregation. Overall, the results suggest that the new compounds are promising multitarget drug candidates with potential impact for Alzheimer’s disease therapy.
The therapy of complex neurodegenerative diseases requires the development of multitarget‐directed drugs (MTDs). Novel indole derivatives with inhibitory activity towards ...acetyl/butyrylcholinesterases and monoamine oxidases A/B as well as the histamine H3 receptor (H3R) were obtained by optimization of the neuroprotectant ASS234 by incorporating generally accepted H3R pharmacophore motifs. These small‐molecule hits demonstrated balanced activities at the targets, mostly in the nanomolar concentration range. Additional in vitro studies showed antioxidative neuroprotective effects as well as the ability to penetrate the blood–brain barrier. With this promising in vitro profile, contilisant (at 1 mg kg−1 i.p.) also significantly improved lipopolysaccharide‐induced cognitive deficits.
Hitting multiple targets at once: The therapy of multifactorial disease conditions needs to address multiple targets, which is challenging particular for neurodegenerative diseases. Designing multitarget‐directed ligands that may cross the blood–brain barrier is a complex task. The small‐molecule ligands described herein affect four neurotransmitter‐catabolizing enzymes as well as the histamine H3 receptor as a procognitive G‐protein‐coupled receptor.
The recent advances in research on the use of the antioxidant and neuroprotective agent α-phenyl-
-butylnitrone (PBN) for the therapy of stroke have been reviewed. The protective effect of PBN in the ...transient occlusion of the middle cerebral artery (MCAO) has been demonstrated, although there have been significant differences in the neuronal salvaging effect between PBN-treated and untreated animals, each set of data having quite large inter-experimental variation. In the transient forebrain ischemia model of gerbil, PBN reduces the mortality after ischemia and the neuronal damage in the hippocampal cornu ammonis 1 (CA1) area of the hippocumpus caused by ischemia. However, PBN fails to prevent postischemic CA1 damage in the rat. As for focal cerebral ischemia, PBN significantly reduces cerebral infarction and decreases neurological deficit after ischemia using a rat model of persistent MCAO in rats. Similarly, the antioxidant and neuroprotective capacity of a number of PBN-derived nitrones prepared in the author's laboratory have also been summarized here, showing their high potential therapeutic power to treat stroke.
The synthesis, antioxidant capacity, and anti-inflammatory activity of four novel N-benzyl-2-4-(aryl)-1H-1,2,3-triazol-1-ylethan-1-imine oxides 10a–d are reported herein. The nitrones 10a–d were ...tested for their antioxidant properties and their ability to inhibit soybean lipoxygenase (LOX). Four diverse antioxidant tests were used for in vitro antioxidant assays, namely, interaction with the stable free radical DPPH (1,1-diphenyl-2-picrylhydrazyl radical) as well as with the water-soluble azo compound AAPH (2,2′-azobis(2-amidinopropane) dihydrochloride), competition with DMSO for hydroxyl radicals, and the scavenging of cationic radical ABTS•+ (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) radical cation). Nitrones 10b, 10c, and 10d, having the 4-fluorophenyl, 2,4-difluorophenyl, and 4-fluoro-3-methylphenyl motif, respectively, exhibited high interaction with DPPH (64.5–81% after 20 min; 79–96% after 60 min), whereas nitrone 10a with unfunctionalized phenyl group showed the lowest inhibitory potency (57% after 20 min, 78% after 60 min). Nitrones 10a and 10d, decorated with phenyl and 4-fluoro-3-methylphenyl motif, respectively, appeared the most potent inhibitors of lipid peroxidation. The results obtained from radical cation ABTS•+ were not significant, since all tested compounds 10a–d showed negligible activity (8–46%), much lower than Trolox (91%). Nitrone 10c, bearing the 2,4-difluorophenyl motif, was found to be the most potent LOX inhibitor (IC50 = 10 μM).
Alzheimer’s disease (AD) is a complex disorder characterized by impaired neurotransmission in cholinergic and monoaminergic neurons, which, in combination with the accumulation of misfolded proteins ...and increased oxidative stress, leads to the typical features of the disease at the biomolecular level. Given the limited therapeutic success of approved drugs, it is imperative to explore rationally supported therapeutic approaches to combat this disease. The search for novel scaffolds that bind to different receptors and inhibit AD disease-related enzymes could lead to new therapeutic solutions. Here, we describe N-hydroxy-N-propargylamide hybrids 1–6, which were designed by combining the structures of Contilisant—a multifunctional anti-AD ligand—and ferulic acid, a natural antioxidant with various other biological activities. Among the synthesized compounds, we identified compound 4 as a micromolar inhibitor of hAChE with a potent radical-scavenging capacity comparable to resveratrol and Trolox. In addition, compound 4 chelated copper(II) ions associated with amyloid β pathology, mitochondrial dysfunction, and oxidative stress. The promising in vitro activity combined with favorable drug-like properties and predicted blood-brain barrier permeability make compound 4 a multifunctional ligand that merits further studies at the biochemical and cellular levels.
We report the theoretical analysis of the mechanism of the intramolecular cyclization of biaryl alkynes and allenes, catalyzed by electrophilic transition metal complexes and soft Lewis acids. These ...reactions proceed through initial π-complexation of the alkyne or allene moiety, which triggers the nucleophilic attack of the arene via an endo- or exo-dig pathway in a Friedel−Crafts-type mechanism. In addition, alternative reaction mechanisms have been considered to account for some experimental observations. The computed results over a variety of substrates and catalyst systems agree with experimental evidence and suggest that both structural and electronic effects play a crucial role in the regioselectivity outcome and in the involvement of operative intermediates in the reaction pathway.
The recent advances of tetramethylpyrazine nitrones and quinolylnitrones for the treatment of stroke have been reviewed and compared with other agents, showing promising therapeutic applications. As ...a result of a functional transformation of natural product ligustrazine, (
)-
-butyl-1-(3,5,6-trimethylpyrazin-2-yl)methanimine oxide (
) is a multitarget small nitrone showing potent thrombolytic activity and free radicals scavenging power, in addition to nontoxicity and blood-brain barrier permeability. Similarly, antioxidant (
)-
-butyl-1-(2-chloro-6-methoxyquinolin-3-yl)methanimine oxide (
) is a novel agent for cerebral ischemia therapy as it is able to scavenge different types of free radical species, showing strong neuroprotection and reduced infarct size.
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