Tetrahydrofuran as a Scaffold for Peptidomimetics. Application to the Design and Synthesis of Conformationally Constrained Metalloproteinase Inhibitors

Design of Pseudodiproline Dimers as Mimetics of Pro-Pro Units: Stereocontrolled Synthesis, Configurational Relevance, and Structural Properties

Stereocontrolled methods are described for the synthesis of hitherto unreported pseudodiproline dimers in which a cyclopentane carboxylic acid is linked to a pyrrolidine residue by a stereochemically defined hydroxymethylene tether. These proline-cyclopentane (Pro-Cyp) dimers have interesting structural characteristics as seen in their X-ray crystal structures as well as their nuclear magnetic resonance (NMR) spectra in CDCl₃. They can be considered to be novel Pro-Pro mimetics, which can be used to replace natural diproline sequences with potential applications in medicinal chemistry. They also represent a new concept in the peptidomimetic design of chimeric proline-based amino acids as carbocyclic hydroxyethylene isosteres of inhibitor molecules, in which the stereodefined bridging hydroxyl group can simulate a tetrahedral intermediate in an enzyme complex.

Privileged scaffolds targeting reverse-turn and helix recognition

BACKGROUND

Protein-protein interactions dominate molecular recognition in biologic systems. One major challenge for drug discovery arises from the very large surfaces that are characteristic of many protein-protein interactions.

OBJECTIVES

To identify 'drug-like' small molecule leads capable of modulating protein-protein interactions based on common protein-recognition motifs, such as alpha-helices, beta-strands, reverse-turns and polyproline motifs for example.

OVERVIEW

Many proteins/peptides are unstructured under physiologic conditions and only fold into ordered structures on binding to their cellular targets. Therefore, preorganization of an inhibitor into its protein-bound conformation reduces the entropy of binding and enhances the relative affinity of the inhibitor. Accordingly, this review describes a general strategy to address the challenge based on the 'privileged structure hypothesis' [Che, PhD thesis, Washington University, 2003] that chemical templates capable of mimicking surfaces of protein-recognition motifs are potential privileged scaffolds as small-molecule inhibitors of protein-protein interactions. The authors highlight recent advances in the design of privileged scaffolds targeting reverse-turn and helical recognition.

CONCLUSIONS

Privileged scaffolds targeting common protein-recognition motifs are useful to help elucidate the receptor-bound conformation and to provide non-peptidic, bioavailable substructures suitable for optimization to modulate protein-protein interactions.

Mild and efficient Lewis acid-promoted detritylation in the synthesis of N-hydroxy amides: a concise synthesis of (-)-Cobactin T

An efficient, high-yielding Lewis acid promoted deprotection of O-trityl hydroxylamine derivatives is described. A range of acid-labile protecting groups, such as N-Boc and O-TBS, were tolerated under these mild conditions. The present method is applicable to the synthesis of a broad range of hydroxylamine derivatives, including N-hydroxy amides (hydroxamic acids), N-hydroxy sulfonamides, and N-hydroxy ureas, which often exhibit significant biological activities. An application of this methodology for a concise synthesis of (-)-Cobactin T (18) is also demonstrated.

Stereoselective Synthesis of Constrained Oxacyclic Hydroxyethylene Isosteres of Aspartic Protease Inhibitors: Aldol and Mukaiyama Aldol Methodologies for Branched Tetrahydrofuran 2-Carboxylic Acids

The synthesis of diastereomeric 3-substituted-tetrahydrofuran 2-carboxylic acids in enantiopure form was achieved relying on aldol condensations of N-substituted alpha-amino aldehydes with enolates and enol silyl ethers of gamma-butyrolactone. Catalytic YbFOD leads to a high yield of a syn/syn-alpha-amino alcohol isomer. This was used as a constrained THF subunit in the synthesis of a peptidomimetic intended as an inhibitor of the enzyme BACE1, which is implicated in the cascade of events leading to plaque formation in Alzheimer's disease.

Tetrahydrofuran-Based Amino Acids as Library Scaffolds

A furanose sugar amino acid (SAA) has been utilized as a library scaffold for the first time. Two furanose SAA scaffolds were examined to illustrate their potential for derivatization. The resulting 99-member library contained three orthogonal points of diversification that allowed easy access to ethers and carbamates from a hydroxyl moiety, a range of ureas from an azide (via an amine), and a range of amides from a methyl ester. The novel amide formation (by displacement of the methoxide from the methyl ester moiety) was achieved in good yield and purity with high structural confidence. Full characterization of several library intermediates (including a crystal structure) was obtained. The library was submitted for antibacterial screening.

Synthesis of γ,δ-Unsaturated and δ,ε-Unsaturated α-Amino Acids from Fragmentation of γ- and δ-Lactones

A noncoded amino acid of cyclomarin A (1) was synthesized in a racemic fashion. The method employs a six-membered ring template to control the relative stereochemistry and introduction of the functional groups. Ultimately, Pd-catalyzed fragmentation of the lactone provided gamma,delta-unsaturated and delta,epsilon-unsaturated alpha-amino acids. A Pd-catalyzed ring opening of a gamma-lactone is also reported.

Molecular diversity through sugar scaffolds

Monosaccharides provide an excellent platform to tailor molecular diversity by appending desired substituents at selected positions around the sugar scaffold. The presence of five functionalized and stereo-controlled centres on the sugar scaffolds gives the chemist plenty of scope to custom design molecules to a pharmacophore model. This review focuses on the peptidomimetic developments in this area, as well as the concept of tailoring structural and functional diversity in a library using carbohydrate scaffolds and how this can lead to increased hit rates and rapid identification of leads, which has promising prospects for drug development.

N-Aryl sulfonyl homocysteine hydroxamate inhibitors of matrix metalloproteinases: further probing of the S(1), S(1)', and S(2)' pockets

A series of N-arylsulfonyl S-alkyl homocysteine hydroxamic acids were synthesized with variations in three subsites corresponding to P(1), P(1)', and P(2)'. Enzyme assays with a variety of MMPs revealed activity at the low nanomolar level.