Allylboration of Ketones Catalyzed by BINOL Derivatives: Which Species Are Involved Depending on Substrate Reactivity?

Allylboration reactions of ketones catalyzed by BINOL derivatives can exhibit highly variable stereochemical courses depending on the nature and reactivity of the ketone substrate. In this Article, we put into perspective the relationship between the nature of the starting material and the active species involved in the asymmetric allyboration catalyzed by BINOL derivatives. This work, aimed at comparing different plausible mechanisms by density functional theory (DFT) at the M06-2X/6-311+G(d,p) level involving different types of allylboronates in the presence of the organocatalyst, leads to the confirmation of the hitherto accepted hypothesis of a reaction promoted by the transient cyclic allyl-1,3,2-dioxaborolane derived from BINOLs in the case of unactivated or weakly activated ketones such as indanone. A hypothetical scenario involving dimeric boronate species as chiral catalysts was also investigated.

Catalytic Enantioselective Allylboration and Related Reactions of Isatins Promoted by Chiral BINOLs: Scope and Mechanistic Studies.

An improvement in the catalytic enantioselective allylboration of isatins with 2-allyl-1,3,2-dioxaborolane in the presence of chiral BINOL derivatives is reported, offering an efficient one-step access to enantioenriched N-unprotected 3-allyl-3-hydroxy-2-oxindoles. This catalytic process is also effective for the crotylboration reaction with enantiomeric ratios (er) up to 97:3, as well as for the asymmetric synthesis of homopropargylic alcohols via an allenyl addition to indoline-2,3-diones. Origins of the high enantioselectivity in chiral BINOL-catalyzed allylboration of isatins were examined by DFT calculations. A hypothetical scenario suggested a crucial internal hydrogen bonding between the amide group (C═O···H-O) and the ethylene hydroxyl of the transient chiral mixed boronate ester, generating a rigid and stabilized system that favors the addition of the allylboron species to the Re face of the ketone function. The key role of the alcohol additive (t-BuOH or t-AmOH) in the enantioselective allylboration reaction of isatins has also been shown on the basis of a kinetics study and computational calculations by favoring the transesterification of the 2-allyl-1,3,2-dioxaborolane with BINOL via proton transfer processes.

BINOL derivatives-catalysed enantioselective allylboration of isatins: application to the synthesis of (R)-chimonamidine.

The asymmetric synthesis of the 3-allyl-3-hydroxyoxindole skeleton was accomplished in yields up to 99% via a metal-free and enantioselective allylation of isatins (90-96% ee) using BINOL derivatives as catalysts and an optimized allylboronate. This methodology was applied at a gram-scale to the synthesis of the natural product (R)-chimonamidine.

Synthesis of lactams by isomerization of oxindoles substituted at C-3 by an ω-amino chain.

Oxindoles substituted at N-1 by electron-withdrawing groups and at C-3 by ω-amino chains of various lengths undergo mild and easy isomerization to new 5- to 12-membered lactams in good yields (30-96%). As efficient asymmetric syntheses of diversely 3,3-disubstituted oxindoles are currently developed, this isomerization provides a new and valuable access to medium-sized lactams α-substituted with a quaternary asymmetric carbon bearing a 2-aminophenyl residue.

N-silyloxaziridines: synthesis and use for electrophilic amination.

N-Silyloxaziridines were synthesized for the first time. Their tert-butyldiphenylsilyl (TBDPS) derivatives were stable reagents that were prepared on a multigram scale in three steps and in 44% overall yield from the corresponding benzylamines. They were mild electrophilic aminating reagents that reacted at room temperature with diversely substituted primary and secondary amines to produce N-monoalkyl or N,N-dialkyl benzaldehyde hydrazones in 44-87% yield.

Structure-based design of human immuno- and constitutive proteasomes inhibitors.

Starting from the X-ray structure of our previous tripeptidic linear mimics of TMC-95A in complex with yeast 20S proteasome, we introduced new structural features to induce a differential inhibition between human constitutive and immunoproteasome 20S particles. Libraries of 24 tripeptidic and 6 dipeptidic derivatives were synthesized. The optimized preparation of 3-hydroxyoxindolyl alanine residues from tryptophan and their incorporation in peptides were described. Several potent inhibitors of human constitutive proteasome and immunoproteasome acting at the nanomolar level (IC50 = 7.1 nM against the chymotrypsin-like activity for the best inhibitor) were obtained. A cytotoxic effect at the submicromolar level was observed against 6 human cancer cell lines.

Linear TMC-95-based proteasome inhibitors.

We have designed and evaluated 45 linear analogues of the natural constrained cyclopeptide TMC-95A. These synthetically less demanding molecules are based on the tripeptide sequence Y-N-W of TMC-95A. Structural variations in the amino acid side chains and termini greatly influenced both the efficiency and selectivity of action on a given type of active site. Inhibition constants were submicromolar (Ki approximately 300 nM) despite the absence of the entropically favorable constrained conformation that is characteristic of TMC-95A and its cyclic analogues. These linear compounds were readily prepared and reasonably stable in culture medium and could be optimized to inhibit one, two, or all three proteasome catalytic sites. Cytotoxicity assays performed on a series of human tumor cell lines identified the most potent inhibitors in cells.

Blockade of the malignant phenotype by ß-subunit selective noncovalent inhibition of immuno- and constitutive proteasomes.

A structure-based virtual screening of over 400,000 small molecules against the constitutive proteasome activity followed by in vitro assays led to the discovery of a family of proteasome inhibitors with a sulfonyl piperazine scaffold. Some members of this family of small non-peptidic inhibitors were found to act selectively on the ß2 trypsin-like catalytic site with a preference for the immunoproteasome ß2i over the constitutive proteasome ß2c, while some act on the ß5 site and post-acid site ß1 of both, the immunoproteasome and the constitutive proteasome. Anti-proliferative and anti-invasive effects on tumor cells were investigated and observed for two compounds. We report novel chemical inhibitors able to interfere with the three types of active centers of both, the immuno- and constitutive proteasomes. Identifying and analyzing a novel scaffold with decorations able to shift the binders' active site selectivity is essential to design a future generation of proteasome inhibitors able to distinguish the immunoproteasome from the constitutive proteasome.

Noncovalent fluorescent probes of human immuno- and constitutive proteasomes.

We report here the synthesis and biological evaluation of fluorescent probes functioning as inhibitors that noncovalently block human immuno- and constitutive proteasomes. These cell-penetrating linear analogues of the natural cyclopeptide TMC-95A were efficient on cells at the nanomolar level and assessed by confocal microscopy and flow cytometry. They may constitute an alternative to previously reported fluorescent probes that all bind covalently to proteasomes.

Dimerized linear mimics of a natural cyclopeptide (TMC-95A) are potent noncovalent inhibitors of the eukaryotic 20S proteasome.

Noncovalent proteasome inhibitors introduce an alternative mechanism of inhibition to that of covalent inhibitors used in cancer therapy. Starting from a noncovalent linear mimic of TMC-95A, a series of dimerized inhibitors using polyaminohexanoic acid spacers has been designed and optimized to target simultaneously two of the six active sites of the eukaryotic 20S proteasome. The homodimerized compounds actively inhibited chymotrypsin-like (Ki = 6-11 nM) and trypsin-like activities, whereas postacid activity was poorly modified. The noncovalent binding mode was ascertained by X-ray crystallography of the inhibitors complexed with the yeast 20S proteasome. The inhibition of proteasomal activities in human cells was evaluated. The use of the multivalency inhibitor concept has produced highly efficient and selective noncovalent compounds (no inhibition of calpain and cathepsin) that have potential therapeutic advantages compared to covalent binders such as bortezomib and carfilzomib.

Noncovalent inhibition of 20S proteasome by pegylated dimerized inhibitors.

We exploited the concept of polyvalent interactions to produce highly selective and efficient inhibitors of eukaryotic proteasome. This multicatalytic protease with the unique topography of its 6 active sites has emerged as a promising target to treat cancer with the use of the covalent inhibitor bortezomib. We used our reference noncovalent inhibitor, a selective TMC-95A tripeptide linear mimic, to design dimeric noncovalent proteasome inhibitors that target two active sites simultaneously. We synthesized pegylated monomer and dimers of the reference inhibitor and evaluated their capacity to inhibit a mammalian 20S proteasome. The inhibitory power of the dimers depended on the average length of their spacer. Lineweaver-Burk double-reciprocal plots indicated competitive inhibition. The best dimer inhibited CT-L activity 800-times more efficiently than the reference inhibitor.

Novel organic proteasome inhibitors identified by virtual and in vitro screening.

Proteasome inhibition is a promising strategy for treating cancers. Herein, we report the discovery of novel drug-like inhibitors of mammalian proteasome 20S using a multistep structure-based virtual ligand screening strategy. Sulfone- or piperazine-containing hits essentially belong to the under-represented class of noncovalent and nonpeptidic proteasome inhibitors. Several of our compounds act in the micromolar range and are cytotoxic on human tumoral cell lines. Optimization of these molecules could lead to better anticancer therapy.

Synthesis of macrocyclic peptide analogues of proteasome inhibitor TMC-95A.

The synthesis of three constrained macrocyclic peptide analogues 1 of TMC-95A as potential proteasome inhibitors is described. The key step involves a Ni(0)-mediated macrocyclization of tripeptides 2 bearing halogenated aromatic side chains for the formation of the biaryl junction. In addition, an enantioselective preparation of l-7-bromotryptophan methyl ester 3 using a Corey-O'Donnell alkylation of the glycine benzophenone imine was achieved in good overall yield with very high ee (>85%) on a multigram scale.

Electrophilic amination of amino acids with N-Boc-oxaziridines: efficient preparation of N-orthogonally diprotected hydrazino acids and piperazic acid derivatives.

A general two-step preparation of enantiopure N(alpha),N(beta)-orthogonally diprotected alpha-hydrazino acids 1 is developed on a multigram scale. The key reaction is the efficient electrophilic amination of N-benzyl amino acids 6 with N-Boc-oxaziridine 7 and accommodates various functional groups encountered in side chains of amino acids. The cyclic 2,3,4,5-tetrahydro-3-pyridazine carboxylic acid (piperazic acid) derivatives 2 and 3 or the cyclic 3,4-dihydro-3-pyrazolecarboxylate 4 are conveniently prepared from glutamic acid or aspartic acid via orthogonally diprotected alpha-hydrazino acids 1m and 1n.