A trifunctional steroid-based scaffold for combinatorial chemistry

A Trifunctional Linker for Palmitoylation and Peptide and Protein Localization in Biological Membranes

Attachment of lipophilic groups is an important post-translational modification of proteins, which involves the coupling of one or more anchors such as fatty acids, isoprenoids, phospholipids, or glycosylphosphatidyl inositols. To study its impact on the membrane partitioning of hydrophobic peptides or proteins, we designed a tyrosine-based trifunctional linker. The linker allows the facile incorporation of two different functionalities at a cysteine residue in a single step. We determined the effect of the lipid modification on the membrane partitioning of the synthetic α-helical model peptide WALP with or without here and in all cases below; palmitoyl groups in giant unilamellar vesicles that contain a liquid-ordered (Lo ) and liquid-disordered (Ld ) phase. Introduction of two palmitoyl groups did not alter the localization of the membrane peptides, nor did the membrane thickness or lipid composition. In all cases, the peptide was retained in the Ld phase. These data demonstrate that the Lo domain in model membranes is highly unfavorable for a single membrane-spanning peptide.

Mimicking Enzymes: Asymmetric Induction inside a Carbamate-Based Steroidal Cleft

Cholic acid has been elaborated into a carbamate-based tripodal architecture, which is able to promote an asymmetric organic transformation inside its chiral cavity. The nature of this steroidal catalyst has been disclosed by quantum-chemical calculations. It comprises the preorganization and confinement of the reagents within the cavity of the steroid to form a supramolecular complex held together by means of cooperative H-bond contacts. This operational mode resembles that of some enzymes.

Reconstructing the discontinuous and conformational β1/β3-loop binding site on hFSH/hCG by using highly constrained multicyclic peptides

Making peptide-based molecules that mimic functional interaction sites on proteins remains a challenge in biomedical sciences. Here, we present a robust technology for the covalent assembly of highly constrained and discontinuous binding site mimics, the potential of which is exemplified for structurally complex binding sites on the "Cys-knot" proteins hFSH and hCG. Peptidic structures were assembled by Ar(CH2 Br)2-promoted peptide cyclizations, combined with oxime ligation and disulfide formation. The technology allows unprotected side chain groups and is applicable to peptides of different lengths and nature. A tetracyclic FSH mimic was constructed, showing >600-fold improved binding compared to linear or monocyclic controls. Binding of a tricyclic hCG mimic to anti-hCG mAb 8G5 was identical to hCG itself (IC50 =260 vs. 470 pM), whereas this mimic displayed an IC50 value of 149 nM for mAb 3468, an hCG-neutralizing antibody with undetectable binding to either linear or monocyclic controls.

A multiple multicomponent approach to chimeric peptide-peptoid podands

The success of multi-armed, peptide-based receptors in supramolecular chemistry traditionally is not only based on the sequence but equally on an appropriate positioning of various peptidic chains to create a multivalent array of binding elements. As a faster, more versatile and alternative access toward (pseudo)peptidic receptors, a new approach based on multiple Ugi four-component reactions (Ugi-4CR) is proposed as a means of simultaneously incorporating several binding and catalytic elements into organizing scaffolds. By employing α-amino acids either as the amino or acid components of the Ugi-4CRs, this multiple multicomponent process allows for the one-pot assembly of podands bearing chimeric peptide-peptoid chains as appended arms. Tripodal, bowl-shaped, and concave polyfunctional skeletons are employed as topologically varied platforms for positioning the multiple peptidic chains formed by Ugi-4CRs. In a similar approach, steroidal building blocks with several axially-oriented isocyano groups are synthesized and utilized to align the chimeric chains with conformational constrains, thus providing an alternative to the classical peptido-steroidal receptors. The branched and hybrid peptide-peptoid appendages allow new possibilities for both rational design and combinatorial production of synthetic receptors. The concept is also expandable to other multicomponent reactions.

Shortcut access to peptidosteroid conjugates: building blocks for solid-phase bile acid scaffold decoration by convergent ligation

We present three versatile solid-supported scaffold building blocks based on the (deoxy)cholic acid framework and decorated with handles for further derivatization by modern ligation techniques such as click chemistry, Staudinger ligation or native chemical ligation. Straightforward procedures are presented for the synthesis and analysis of the steroid constructs. These building blocks offer a new, facile and shorter access route to bile acid-peptide conjugates on solid-phase with emphasis on heterodipodal conjugates with defined spatial arrangements. As such, we provide versatile new synthons to the toolbox for bile acid decoration.

Synthesis and biological evaluation of bile acid dimers linked with 1,2,3-triazole and bis-beta-lactam

We report herein the synthesis and biological evaluation of bile acid dimers linked through 1,2,3-triazole and bis-beta-lactam. The dimers were synthesized using 1,3-dipolar cycloaddition reaction of diazido bis-beta-lactams , and terminal alkynes derived from cholic acid/deoxycholic acid in the presence of Cu(i) catalyst (click chemistry). These novel molecules were evaluated in vitro for their antifungal and antibacterial activity. Most of the compounds exhibited significant antifungal as well as antibacterial activity against all the tested fungal and bacterial strains. Moreover, their in vitro cytotoxicities towards HEK-293 and MCF-7 cells were also established.

Validation of a solid-phase-bound steroid scaffold for the synthesis of novel cyclic peptidosteroids

The current article reports on the synthesis of a new type of cyclic peptidosteroid, in which a bile-acid-based scaffold was used for the conformational restriction of a loop-like peptide. Convergent coupling of two tetrapeptides to the non-peptidic steroidal entity was carried out once in the classical C-to-N and once in the non-classical N-to-C direction. Peptide backbone cyclisation was then carried out, giving rise to a ring size equivalent to approximately 12 amino acids. This type of construct will be used in the development of a peptide vaccine against measles.

Bile acid scaffolds in supramolecular chemistry: the interplay of design and synthesis

Since early work in the 1980s, the bile acids have become well established as building blocks for supramolecular chemistry. The author's laboratory has specialised in converting cholic acid, the archetypal bile acid, into macrocyclic and acyclic receptors for anions and carbohydrates. This review highlights the synthetic aspects of this work, especially the use of modern synthetic methodology to perform less obvious structural transformations.

Synthesis of (S,S)-Isodityrosine by Dötz Benzannulation

[reaction: see text] A synthesis of (S,S)-isodityrosine 1, a naturally occurring, key structural subunit of numerous biologically active macromolecules, is described. A formal [3 + 2 + 1] cycloaddition (Dötz benzannulation) approach was utilized to simultaneously construct an aromatic ring and the diaryl ether linkage in one step. This key step was extended to the synthesis of (S,S)-isodityrosine in two separate convergent synthetic routes. This method demonstrates a novel and mild method for the synthesis of diaryl ethers.

Differentially-protected steroidal triamines; scaffolds with potential for medicinal, supramolecular, and combinatorial chemistry

Cholic acid 2a has been converted into two new orthogonally-protected triamino scaffolds, 13 and 14. The synthesis proceeds via the bis-Boc-NH-substituted azide 10, for which an improved preparation is described. After removal of the Boc groups, the two axial amines are differentiated through a novel monoprotection employing 1-(2-nitrobenzenesulfonyloxy)-benzotriazole 29. Regioselectivity of > or 50 : 1 is achieved, presumably reflecting an exceptional sensitivity to steric hindrance. Protection of the remaining amino group as Boc or Alloc gives the scaffolds in approximately 40% overall yield from cholic acid. Scaffold 13 has been sequentially deprotected and derivatised with N-carbamoyl amino acids, to give a model for tripodal peptide libraries.

Steroidal ureas as enantioselective receptors for an N-acetyl alpha-amino carboxylate

[reaction: see text] Cholic acid has been elaborated into three regioisomeric bis-carbamoylureas, which have been investigated as enantioselective receptors for N-acetyl phenylalanine. L/D selectivities, peaking at 5:1, have been determined by a sensitive and rapid MS-based extraction method that should be generalizable to related systems.

Evaluation of a two-stage screening procedure in the combinatorial search for serine protease-like activity

A series of peptidosteroid derivatives containing two independent peptide chains in which Ser and His are incorporated were synthesized by solid-phase peptide synthesis. The activity of the different compounds in the hydrolysis of the activated substrate NF31 was assessed in a stepwise fashion. First, the different resin-bound derivatives 6a-l and 6x-z were individually assayed for serine esterification in the absence of water. The use of a colored substrate allowed for a visual identification of the most active compounds. Through the inclusion of control substances, the involvement of histidine in the mechanism for serine acylation was shown. Second, the hydrolysis and methanolysis of the different acylated derivatives 8a-l and 8x were evaluated using UV spectroscopy, again indicating the involvement of histidine. The feasibility of applying the above procedures in a combinatorial context was proven via the screening of artificial libraries, created by mixing the different resin-bound peptidosteroid compounds. In this respect, the use of a photocleavable linker allowed for the unambiguous structural characterization of the selected members via application of single-bead electrospray tandem mass spectrometry.

Polymeric materials containing bile acids

Bile acids are biological compounds in the body that have interesting properties and have been used to make special chemical structures in molecular recognition. Various polymers have been synthesized from bile acids. The materials should preserve some of the properties of bile acids, such as biocompatibility, high stability of the steroid nucleus, reactivity of the side groups, optical activity and self-assembling capacity. The synthesis and applications proposed for such polymers are discussed.

Combinatorial synthesis of natural product-like molecules using a first-generation spiroketal scaffold

Recently, significant attention has been focused on the synthesis small-molecule libraries based on natural product or natural product-like structures. In this paper, we report our initial studies on the use of the 1,7-dioxaspiro[5,5]undecane (spiroketal) moiety as a rigid-core template for elaboration using parallel synthesis techniques. The synthesis of a spiroketal scaffold that is reminiscent of the spiroketal subunits found in the spiroketal macrolide antibiotics will be described. Elaboration of three independently addressable functional groups on the scaffold using solution-phase parallel synthesis techniques led to the preparation of a small library of natural product-like compounds. These studies pave the way for evaluation of highly functionalized spiroketals in phenotypic assays and as prospective antagonists of protein-protein interactions.

A pentaerythritol-based molecular scaffold for solid-phase combinatorial chemistry

A convergent synthesis has been developed for the preparation of solid-phase bound construct 1, consisting of an orthogonally protected trifunctional core structure that is attached to TentaGel via a photocleavable linker. [structure: see text]

A selectively deprotectable triazacyclophane scaffold for the construction of artificial receptors

[reaction: see text]. The synthesis of a triazacylclophane scaffold bearing a set of selectively removable protecting groups is described. This versatile scaffold, which can be linked to a solid support, allows the attachment of three different side chains and can therefore be used for the combinatorial synthesis of libraries of artificial receptor molecules of high structural diversity.

A rigid linker-scaffold for solid-phase synthesis of dimeric pharmacophores

Bifunctional linker-scaffolds (compounds 1-3) were designed to meet several criteria for solid-phase syntheses of bivalent ligands. They have two amine-functionalized arms that can be differentially protected. Elaboration of these arms could give ligand-pharmacophore dimers wherein the two active components are held reasonably rigidly at around 10 A separation. Their bifunctional design also enables reactions of libraries with libraries to amplify diversity in a truly combinatorial fashion. Molecules 1-3 are also designed so that cleavage of the linker liberates the scaffold entity into solution under conditions that create only byproducts that should not interfere with biological assays. Thus they contain 2-nitrobenzene sulfonamide components that cleave in the presence of good nucleophiles. In the event, the linker-scaffolds 1-3 were prepared (Schemes 1 and 2). The N-benzyl system 2 was shown to have good stability to the types of conditions that might be used to functionalize the scaffold arms and to be sufficiently labile to the cleavage nucleophile (vide infra). The nucleophiles generally used to cleave nitrobenzene sulfonamides either generate undesirable byproducts (thiophenol or alkane thiols) or proved to be insufficiently reactive for the required solid-phase transformations (n-propylamine). However, sodium sulfide was investigated as a new alternative and shown to be a highly reactive cleavage agent that gives only volatile byproducts and sodium hydroxide. It is suggested that sodium sulfide is a highly desirable nucleophile for cleavage of 2-nitrobenzene sulfonamides, in general. The linker-scaffolds 1-3 were used to prepare a small library of bivalent ligands targeted to a protein receptor having charged cavities separated by approximately 10 A. These systems were made from guanidine, pyridinium, carboxylic acid, and sulfonic acid constituents (Tables 1 and 2).

Solid-phase synthesis of hydroxysteroid derivatives using the diethylsilyloxy linker

Four different types of hydroxysteroids (primary alcohol, secondary alcohols, and phenol), bearing either an oxirane or an azide as a precursor of molecular diversity, were linked in good yields to solid support using the butyldiethylsilane polystyrene (PS-DES) resin. These molecules were then used as scaffolds to generate hydroxysteroid derivatives containing two levels of diversity. The proposed libraries were tested by running steroidal alcohols through a model sequence of reactions (solid-phase coupling, aminolysis of oxirane or reduction of azide, amidation, and final cleavage). As a result, two linked secondary alcohols (17beta-hydroxy-spiro-3(R)-oxirane-5alpha-androstane and 3beta-hydroxy-spiro- 17(S)-oxirane-5alpha-androstane) and a primary alcohol (spiro-17(S)-oxirane-3-(hydroxymethyl)-1,3,5(10)-estratriene) afforded good overall yields (>45%) and high HPLC purities (>90%) of hydroxysteroids derivatized as alkylamides without purification. One limitation was noted for the fourth library: the phenolic steroid linked by the diethylsilyloxy linker gave a poor overall yield of 8% of the desired model compound. Finally, the diethylsilyloxy linker was used successfully for a rapid solid-phase synthesis of a model library of twenty C19-steroid derivatives (3beta-amido-3alpha-hydroxy-5alpha-androstane-17-ones), with an average yield of 53% and average HPLC purity of 97% without purification steps.

Preparation of a protected triamino analogue of cholic acid and sequential incorporation of amino acids in solution and on a solid support

We have prepared a triamine derivative of cholic acid with protecting groups on the amines that allow sequential amide formation. The triamine was formed from 3 alpha,7 alpha, 12 alpha-trihydroxycholan-24-ol with good stereoselectivity. Sequential removal of the amine protecting groups and amide formation was achieved in high-yielding steps and was performed in solution and on a solid support.