Design and Synthesis of 2,3,5-Substituted Imidazolidin-4-one Inhibitors of BACE-1

Isocyanide-based Multicomponent Reactions (IMCRs) in Water or Aqueous Biphasic Systems

BACKGROUND

Isocyanide is an intriguing one-carbon synthon that is frequently employed in a variety of carbon-carbon and carbon-heteroatom bond-forming reactions. Isocyanide-based multicomponent reactions (IMCRs) are effective synthetic tools in organic synthesis for the preparation of complex heterocyclic molecules. The IMCRs in water have become an attractive research direction, enabling simultaneous growth of both IMCRs and green solvents towards ideal organic synthesis.

OBJECTIVE

The goal of this review is to provide a general overview of IMCRs in water or biphasic aqueous systems for accessing various organic molecules, as well as an examination of their benefits and mechanistic insights.

CONCLUSION

High atom economies, mild reaction conditions, high yields, and catalyst-free processes are crucial features of these IMCRs in water or biphasic aqueous systems.

Reaction of Picolinamides with Ketones Producing a New Type of Heterocyclic Salts with an Imidazolidin-4-One Ring

Reactions of picolinamides with 1,3-propanesultone in methanol followed by the treatment with ketones led to a series of previously unknown chemical transformations, yielding first pyridinium salts (2a-f), with a protonated endocyclic nitrogen atom, and then heterocyclic salts (3a-j) containing an imidazolidin-4-one ring. The structures of intermediate and final products were determined by IR and 1H, 13C NMR spectroscopy, and X-ray study. The effects of the ketone and alcohol structures on the product yield were studied by quantum-chemical calculations. The stability of salts 3a-j towards hydrolysis and alcoholysis makes them excellent candidates for the search for new types of biologically active compounds.

Synthesis of Polycyclic Imidazolidinones via Cascade [3 + 2]-Annulation of β-Oxo-acrylamides with Cyclic N-Sulfonyl Imines

An Et3N-catalyzed cascade [3 + 2]-annulation of β-oxo-acrylamides with cyclic N-sulfonyl ketimines or sulfamate-derived imines is developed under mild reaction conditions, which provides a concise and efficient route to access valuable sultam- or sulfamidate-fused imidazolidinone derivatives in good to excellent yields (80-95% yields) with excellent diastereoselectivities (>20:1 drs). The current protocol features atom economy, a transition-metal-free process, and broad functional group tolerance. Moreover, the asymmetric variant of the [3 + 2]-cycloaddition reaction was achieved in the presence of diphenylethanediamine or quinine-based bifunctional squaramide organocatalysts C-1 and C-11, giving the corresponding chiral polycyclic imidazolidinones in 68-90% yields with 25-94% ees and >20:1 drs in all cases.

Crystal structure, Hirshfeld surface analysis and PIXEL calculations of a 1:1 epimeric mixture of 3-[(4-nitro-benzyl-idene)amino]-2(R,S)-(4-nitro-phenyl)-5(S)-(propan-2-yl)imidazolidin-4-one

A 1:1 epimeric mixture of 3-[(4-nitro-benzyl-idene)amino]-2(R,S)-(4-nitro-phen-yl)-5(S)-(propan-2-yl)imidazolidin-4-one, C19H19N5O5, was isolated from a reaction mixture of 2(S)-amino-3-methyl-1-oxo-butane-hydrazine and 4-nitro-benz-alde-hyde in ethanol. The product was derived from an initial reaction of 2(S)-amino-3-methyl-1-oxo-butane-hydrazine at its hydrazine group to provide a 4-nitro-benzyl-idene derivative, followed by a cyclization reaction with another mol-ecule of 4-nitro-benzaldehyde to form the chiral five-membered imidazolidin-4-one ring. The formation of the five-membered imidazolidin-4-one ring occurred with retention of the configuration at the 5-position, but with racemization at the 2-position. In the crystal, N-H⋯O(nitro) hydrogen bonds, weak C-H⋯O(carbon-yl) and C-H⋯O(nitro) hydrogen bonds, as well as C-H⋯π, N-H⋯π and π-π inter-actions, are present. These combine to generate a three-dimensional array. Hirshfeld surface analysis and PIXEL calculations are also reported.

One-step synthesis of N,N'-substituted 4-imidazolidinones by an isocyanide-based pseudo-five-multicomponent reaction

A pseudo-five-multicomponent reaction involving an isocyanide, a primary amine, two molecules of formaldehyde and water is reported, which gives N,N'-substituted 4-imidazolidinones when trifluoroethanol is used as the solvent. The reaction proceeds with good yields and with a wide variety of amines and isocyanides, providing an efficient new entry to these heterocycles. A preliminary study of the reaction mechanism suggests that trifluoroethanol, although acting as the solvent, is directly involved as a reagent in the reaction pathway.

BACE1 (β-secretase) inhibitors for the treatment of Alzheimer's disease

BACE1 (β-secretase, memapsin 2, Asp2) has emerged as a promising target for the treatment of Alzheimer's disease. BACE1 is an aspartic protease which functions in the first step of the pathway leading to the production and deposition of amyloid-β peptide (Aβ). Its gene deletion showed only mild phenotypes. BACE1 inhibition has direct implications in the Alzheimer's disease pathology without largely affecting viability. However, inhibiting BACE1 selectively in vivo has presented many challenges to medicinal chemists. Since its identification in 2000, inhibitors covering many different structural classes have been designed and developed. These inhibitors can be largely classified as either peptidomimetic or non-peptidic inhibitors. Progress in these fields resulted in inhibitors that contain many targeted drug-like characteristics. In this review, we describe structure-based design strategies and evolution of a wide range of BACE1 inhibitors including compounds that have been shown to reduce brain Aβ, rescue the cognitive decline in transgenic AD mice and inhibitor drug candidates that are currently in clinical trials.

Virtual screening and structure-based discovery of indole acylguanidines as potent β-secretase (BACE1) inhibitors

Proteolytic cleavage of amyloid precursor protein by β-secretase (BACE1) is a key step in generating the N-terminal of β-amyloid (Aβ), which further forms into amyloid plaques that are considered as the hallmark of Alzheimer’s disease. Inhibitors of BACE1 can reduce the levels of Aβ and thus have a therapeutic potential for treating the disease. We report here the identification of a series of small molecules bearing an indole acylguanidine core structure as potent BACE1 inhibitors. The initial weak fragment was discovered by virtual screening, and followed with a hit-to-lead optimization. With the aid of co-crystal structures of two discovered inhibitors (compounds 19 and 25) with BACE1, we explored the SAR around the indole and aryl groups, and obtained several BACE1 inhibitors about 1,000-fold more potent than the initial fragment hit. Accompanying the lead optimization, a previously under-explored sub-site opposite the flap loop was redefined as a potential binding site for later BACE1 inhibitor design.

Structure-based design of β-site APP cleaving enzyme 1 (BACE1) inhibitors for the treatment of Alzheimer's disease

The amyloid hypothesis asserts that excess production or reduced clearance of the amyloid-β (Aβ) peptides in the brain initiates a sequence of events that ultimately lead to Alzheimer's disease and dementia. The Aβ hypothesis has identified BACE1 as a therapeutic target to treat Alzheimer's and led to medicinal chemistry efforts to design its inhibitors both in the pharmaceutical industry and in academia. This review summarizes two distinct categories of inhibitors designed based on conformational states of "closed" and "open" forms of the enzyme. In each category the inhibitors are classified based on the core catalytic interaction group or the aspartyl binding motif (ABM). This review covers the description of inhibitors in each ABM class with X-ray crystal structures of key compounds, their binding modes, related structure-activity data highlighting potency advances, and additional properties such as selectivity profile, P-gp efflux, pharmacokinetic, and pharmacodynamic data.

Synthesis and structure-activity relationship studies of 1,3-disubstituted 2-propanols as BACE-1 inhibitors

A library of 1,3-disubstituted 2-propanols was synthesized and evaluated as low molecular weight probes for β-secretase inhibition. By screening a library of 121 1,3-disubstituted 2-propanol derivatives, we identified few compounds inhibiting the enzyme at low micromolar concentrations. The initial hits were optimized to yield a potent BACE-1 inhibitor exhibiting an IC(50) constant in the nanomolar range. Exploration of the pharmacological properties revealed that these small molecular inhibitors possessed a high selectivity over cathepsin D and desirable physicochemical properties beneficial to cross the blood-brain barrier.

Small-molecule BACE1 inhibitors: a patent literature review (2006 - 2011)

INTRODUCTION

Alzheimer's disease is a devastating neurodegenerative disorder for which no disease-modifying therapy exists. The amyloid hypothesis, which implicates Aβ as the toxin initiating a biological cascade leading to neurodegeneration, is the most prominent theory concerning the underlying cause of the disease. BACE1 is one of two aspartyl proteinases that generate Aβ, thus inhibition of BACE1 has the potential to ameliorate the progression of Alzheimer's disease by abating the production of Aβ.

AREAS COVERED

This review chronicles small-molecule BACE1 inhibitors as described in the patent literature between 2006 and 2011 and their potential use as disease-modifying treatments for Alzheimer's disease. Over the past half a dozen years, numerous BACE1 inhibitors have been published in the patent applications, but often these contain a paltry amount of pertinent biological data (e.g. potency, selectivity, and efficacy). Fortunately, numerous relevant publications containing important data have appeared in the journal literature during this period. The goal in this effort was to create an amalgam of the two records to add value to this review.

EXPERT OPINION

The pharmaceutical industry has made tremendous progress in the development of small-molecule BACE1 inhibitors that lower Aβ in the central nervous system. Assuming the amyloid hypothesis is veracious, we anticipate a disease-modifying therapy to combat Alzheimer's disease is near.

Carbazole-containing arylcarboxamides as BACE1 inhibitors

β-Secretase (BACE1) is widely recognized as a prime drug target for the treatment of Alzheimer's disease (AD). In this Letter, we report the synthesis and the BACE1 inhibitory activity of novel, variously substituted N-[3-(9H-carbazol-9-yl)-2-hydroxypropyl]-arylcarboxamides. The best results have been obtained with the introduction of a 4-OMe substituent (IC(50)=3.8 μM) or a 3,4-dichloro substituent (IC(50)=2.5 μM) in the amidic aromatic ring. The blood-brain barrier penetration predictions resulted to be promising for this type of compounds. To better understand the structure-activity relationships (SAR) of the new derivatives, a docking study procedure has been applied exploiting different conformational and ionic states of BACE1.

Progress in the development of beta-secretase inhibitors for Alzheimer's disease

Since the original identification of BACE in 1999 and until quite recently, BACE was often regarded as a "difficult" drug target, much as renin has proven to be. The reasons for this include the following. First, the long and shallow nature of the substrate binding pocket suggested that it would not be possible to identify small molecule drugs that could have adequate binding affinity. Second, functional groups that typically interact with the active site aspartates are usually highly polarized and, therefore, contribute to reduced CNS localization. Early BACE inhibitors were all designed using knowledge of the peptide substrates and usually contained some variation of a few well-known transition-state isosteres. While these had great impact on fundamental understanding of the enzyme structure and key interaction regions, they were very large, very polar, and had essentially no CNS availability. Continued progress by reducing the peptidic nature of these compounds resulted in incremental advances and has provided compounds that meet, or nearly meet, typical CNS drug-like criteria. The challenges associated with peptidic starting points inspired innovative new approaches to search for different starting points. Several groups employed high concentration screening (ligand concentration 100 microM and higher) to find weak hits after conventional screening (typically at 10 microM) failed to find more potent ones. Fragment-based methods have also been developed to identify even weaker hits (IC50 1 mM and greater). This was accomplished through the evolution and refinement of several detection methodologies including calorimetry, surface plasmon resonance, NMR, and crystallography. Coupled with detailed structural understanding of ligand-enzyme interactions and focus on maintaining ligand efficiency, these developments have resulted in several examples where potency was improved by 10,000-fold to afford compounds with IC50 values < 10 nM and promising drug-like characteristics. Together, all these efforts have afforded a diverse array of chemotypes as BACE inhibitors. Early work focused on improving BACE potency in isolated enzyme assays. However, most of these compounds showed potency reductions in cellular assays. Continued improvements in drug properties and in understanding of the physiologically relevant conditions have resulted in many compounds that show strong potency in both isolated and cellular assays. Several compounds have shown reduction of Abeta using rodent in-vivo models both peripherally and in the brain. Recently, one compound has demonstrated reduction of brain Abeta levels in a non-human primate. Phase I clinical trials were initiated on BACE inhibitor CTS-21166 from CoMentis in July of 2007. This compound derives from the earliest described peptidic inhibitors such as OM99-2 [58] but no details have been reported. In addition to strategies involving small molecule inhibitors of BACE and gamma-secretase to reduce Abeta levels, the application of biological agents has been under investigation since the identification of Abeta. The earliest efforts in this area failed. Despite encouraging results in preclinical models, immunization against Abeta by administration of AN-1792 from Elan led to development of aseptic meningoencephalitis in 6% of the patients receiving the drug. Nevertheless, continued efforts with other biological approaches appear encouraging. Most advanced in clinical trials is bapineuzumab from Elan, which is in Phase III clinical trials. This is a humanized monoclonal antibody against Abeta plaques. A recent monograph is devoted to progress in these areas. Taken together, considerable progress has been made in developing CNS-penetrant agents that reduce AP levels and in providing validation that such agents will be therapeutically beneficial for the treatment of Alzheimer's disease.