The SelectiveO-Acylation of Hydroxyproline as a Convenient Method for the Large-Scale Preparation of Novel Proline Polymers and Amphiphiles

Effect of the acyl-group length on the chemoselectivity of the lipase-catalyzed acylation of propranolol-a computational study

The selective N-acylation of 1,2-amino alcohols has been proposed to occur through the proton shuttle mechanism. However, the O-acetylation of propranolol catalyzed by Candida antarctica lipase B is an exception. We investigated the relation between the chemoselectivity of this reaction and the acyl group length. For this purpose, we compared the acyl groups: ethanoyl, butanoyl, octanoyl, and hexadecanoyl. We studied the Michaelis complexes between serine-acylated Candida antarctica lipase B and propranolol, employing a computational approach that involved sampling Michaelis complex conformations through ensemble docking plus consensus scoring and molecular dynamics simulations. The conformations were then classified as near attack conformations for acylation of the amino or hydroxy group. The relative populations of these two classes of conformations were found to be consistent with the experimentally observed chemoselective O-acetylation. We predict that increasing the length of the hydrocarbon chain of the acyl group will cause O-acylation to be unfavorable with respect to N-acylation. The nucleophilic attack of propranolol to the acylated lipase was found to be more favorable through the classical mechanism when compared with the proton shuttle mechanism.

Micellar Organocatalysis Using Smart Polymer Supports: Influence of Thermoresponsive Self-Assembly on Catalytic Activity

Micellar catalysts with a switchable core are attractive materials in organic synthesis. However, little is known about the role of the shell forming block on the performance of the catalyst. Thermoresponsive block copolymers based on poly(N-isopropylacrylamide-co-vinyl-4,4-dimethylazlactone) attached to different permanently hydrophilic blocks, namely poly(ethylene glycol), poly(N,N-dimethylacrylamide), and poly(2,3-dihydroxypropyl acrylate), were successfully synthesized via reversible addition/fragmentation chain transfer radical polymerization (RAFT). Post-polymerization attachment of an amino-functionalized L-prolineamide using the azlactone ring-opening reaction afforded functionalized thermoresponsive block copolymers. Temperature-induced aggregation of the functionalized block copolymers was studied using dynamic light scattering. It was shown that the chemical structure of the permanently hydrophilic block significantly affected the size of the polymer self-assemblies. The functionalized block copolymers were subjected to an aldol reaction between p-nitrobenzaldehyde and cyclohexanone in water. Upon temperature-induced aggregation, an increase in conversion was observed. The enantioselectivity of the polymer-bound organocatalyst improved with an increasing hydrophilic/hydrophobic interface as a result of the different stability of the polymer aggregates.

Microbial esterases and ester prodrugs: An unlikely marriage for combating antibiotic resistance

The rise of antibiotic resistance necessitates the search for new platforms for drug development. Prodrugs are common tools for overcoming drawbacks typically associated with drug formulation and delivery, with ester prodrugs providing a classic strategy for masking polar alcohol and carboxylic acid functionalities and improving cell permeability. Ester prodrugs are normally designed to have simple ester groups, as they are expected to be cleaved and reactivated by a wide spectrum of cellular esterases. However, a number of pathogenic and commensal microbial esterases have been found to possess significant substrate specificity and can play an unexpected role in drug metabolism. Ester protection can also introduce antimicrobial properties into previously nontoxic drugs through alterations in cell permeability or solubility. Finally, mutation to microbial esterases is a novel mechanism for the development of antibiotic resistance. In this review, we highlight the important pathogenic and xenobiotic functions of microbial esterases and discuss the development and application of ester prodrugs for targeting microbial infections and combating antibiotic resistance. Esterases are often overlooked as therapeutic targets. Yet, with the growing need to develop new antibiotics, a thorough understanding of the specificity and function of microbial esterases and their combined action with ester prodrug antibiotics will support the design of future therapeutics.

Poly(ectoine) Hydrogels Resist Nonspecific Protein Adsorption

The development of nonfouling zwitterionic materials has a wide range of biomedical and engineering applications. This work delineates the design and synthesis of a new zwitterionic material based on a naturally occurring compatible solute, ectoine, which is known to possess additional protective properties that stabilize even whole cells against ultraviolet radiation or cytotoxins. These properties and applications of ectoine inspire us to design a functional monomer containing the natural zwitterion moiety of ectoine imparting nonfouling properties and the methacrylate moiety for polymerization. The synthesis route designed for the ectoine methacrylate monomer is simple with a high yield, which is characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. After monomer synthesis, we have prepared a poly(ectoine) hydrogel via thermal polymerization. The equilibrium water content, degree of cross-linking, mechanical strength, and nonfouling properties are determined for polyectoine hydrogels with different cross-linking conditions. Poly(ectoine) hydrogels are shown to have highly hydrated and excellent nonfouling properties and can be considered to be a promising biomaterial.

Ester-prodrugs of ethambutol control its antibacterial activity and provide rapid screening for mycobacterial hydrolase activity

M. tuberculosis contains an unusually high number of serine hydrolases by proteome percentage compared to other common bacteria or humans. This letter describes a method to probe the global substrate specificity of mycobacterial serine hydrolases with ester-protected prodrugs of ethambutol, a first-line antibiotic treatment for TB. These compounds were synthesized directly from ethambutol using a selective o-acylation to yield products in high yield and purity with minimal workup. A library of derivatives was screened against M. smegmatis, a non-infectious model for M. tuberculosis, which displayed significantly lowered biological activity compared to ethambutol. Incubation with a general serine hydrolase reactivated each derivative to near-ethambutol levels, demonstrating that esterification of ethambutol should provide a simple screen for mycobacterial hydrolase activity.

Nonlinear effects at the soft interface of an emulsion in the List–Lerner–Barbas aldol reaction

“Soft” yet promising: Nonlinear asymmetric induction of the List–Lerner–Barbas aldol reaction observed at the soft interface of an emulsion opens up hitherto unexplored possibilities which were previously restricted to solid–liquid phase equilibria.

Amino Acid Functional Polymers: Biomimetic Polymer Design Enabling Catalysis, Chiral Materials, and Drug Delivery

Amino acids are the natural building blocks for the world around us. Highly functional, these small molecules have unique catalytic properties, chirality, and biocompatibility. Imparting these properties to surfaces and other macromolecules is highly sought after and represents a fast-growing field. Polymers functionalized with amino acids in the side chains have tunable optical properties, pH responsiveness, biocompatibility, structure and self-assembly properties. Herein, we review the synthesis of amino acid functional polymers, discuss manipulation of available strategies to achieve the desired responsive materials, and summarize some exciting applications in catalysis, chiral particles, and drug delivery.

Synthesis of Spiroligomer-Containing Macrocycles

We demonstrate the synthesis and characterization of the solution conformations of a collection of functionalized spiroligomer-based macrocycles. These macrocycles contain 14 independently controllable stereocenters and four independently controllable functional groups on a highly preorganized scaffold. These molecules are being developed to display complex, preorganized surfaces for binding proteins and to create enzyme-like active sites. In this work, we demonstrate the convergent synthetic approach to this new class of macrocycles and demonstrate that the conformational properties of these molecules can be changed by altering the configuration stereocenters within the backbone.

Chemoselective O-acylation of hydroxyamino acids and amino alcohols under acidic reaction conditions: History, scope and applications

Amino acids, whether natural, semisynthetic or synthetic, are among the most important and useful chiral building blocks available for organic chemical synthesis. In principle, they can function as inexpensive, chiral and densely functionalized starting materials. On the other hand, the use of amino acid starting materials routinely necessitates protective group chemistry, and in reality, large-scale preparations of even the simplest side-chain derivatives of many amino acids often become annoyingly strenuous due to the necessity of employing protecting groups, on one or more of the amino acid functionalities, during the synthetic sequence. However, in the case of hydroxyamino acids such as hydroxyproline, serine, threonine, tyrosine and 3,4-dihydroxyphenylalanine (DOPA), many O-acyl side-chain derivatives are directly accessible via a particularly expedient and scalable method not commonly applied until recently. Direct acylation of unprotected hydroxyamino acids with acyl halides or carboxylic anhydrides under appropriately acidic reaction conditions renders possible chemoselective O-acylation, furnishing the corresponding side-chain esters directly, on multigram-scale, in a single step, and without chromatographic purification. Assuming a certain degree of stability under acidic reaction conditions, the method is also applicable for a number of related compounds, such as various amino alcohols and the thiol-functional amino acid cysteine. While the basic methodology underlying this approach has been known for decades, it has evolved through recent developments connected to amino acid-derived chiral organocatalysts to become a more widely recognized procedure for large-scale preparation of many useful side-chain derivatives of hydroxyamino acids and related compounds. Such derivatives are useful in peptide chemistry and drug development, as amino acid amphiphiles for asymmetric catalysis, and as amino acid acrylic precursors for preparation of catalytically active macromolecular networks in the form of soluble polymers, crosslinked polymer beads or nanoparticulate systems. The objective of the present review is to increase awareness of the existence and convenience of this methodology, assess its competitiveness compared to newer and more elaborate procedures for chemoselective O-acylation reactions, spur its further development, and finally to chronicle the informative, but poorly documented history of its development.

Recyclable l-Proline Functional Nanoreactors with Temperature-Tuned Activity Based on Core-Shell Nanogels

Recyclable core-shell (CS) nanogels based on l-proline-containing hydrophobic cores with a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) shell have been synthesized via a seeded precipitation polymerization process. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to verify the successful addition of the shell and investigate the thermoresponsive properties of the nanostructures. The catalytic activity of the nanogels was assessed in a model asymmetric aldol reaction, where an enhancement was observed with increasing temperature, attributed to the hydrophobic nature of the PNIPAM shell. However, when a nanogel was synthesized with core-shell morphology based on a gradient of cross-linking density in the corona (GS), a dramatic drop in activity was observed at elevated temperatures: the collapse of the outer, lightly cross-linked, "corona" polymer chains appears to block access to the catalytic core. High activity and enantioselectivity were maintained in a number of recovery and reuse cycles, highlighting the recycling potential of these catalytic nanostructures.

Proline editing: a general and practical approach to the synthesis of functionally and structurally diverse peptides. Analysis of steric versus stereoelectronic effects of 4-substituted prolines on conformation within peptides

Functionalized proline residues have diverse applications. Herein we describe a practical approach, proline editing, for the synthesis of peptides with stereospecifically modified proline residues. Peptides are synthesized by standard solid-phase peptide synthesis to incorporate Fmoc-hydroxyproline (4R-Hyp). In an automated manner, the Hyp hydroxyl is protected and the remainder of the peptide synthesized. After peptide synthesis, the Hyp protecting group is orthogonally removed and Hyp selectively modified to generate substituted proline amino acids, with the peptide main chain functioning to "protect" the proline amino and carboxyl groups. In a model tetrapeptide (Ac-TYPN-NH2), 4R-Hyp was stereospecifically converted to 122 different 4-substituted prolyl amino acids, with 4R or 4S stereochemistry, via Mitsunobu, oxidation, reduction, acylation, and substitution reactions. 4-Substituted prolines synthesized via proline editing include incorporated structured amino acid mimetics (Cys, Asp/Glu, Phe, Lys, Arg, pSer/pThr), recognition motifs (biotin, RGD), electron-withdrawing groups to induce stereoelectronic effects (fluoro, nitrobenzoate), handles for heteronuclear NMR ((19)F:fluoro; pentafluorophenyl or perfluoro-tert-butyl ether; 4,4-difluoro; (77)SePh) and other spectroscopies (fluorescence, IR: cyanophenyl ether), leaving groups (sulfonate, halide, NHS, bromoacetate), and other reactive handles (amine, thiol, thioester, ketone, hydroxylamine, maleimide, acrylate, azide, alkene, alkyne, aryl halide, tetrazine, 1,2-aminothiol). Proline editing provides access to these proline derivatives with no solution-phase synthesis. All peptides were analyzed by NMR to identify stereoelectronic and steric effects on conformation. Proline derivatives were synthesized to permit bioorthogonal conjugation reactions, including azide-alkyne, tetrazine-trans-cyclooctene, oxime, reductive amination, native chemical ligation, Suzuki, Sonogashira, cross-metathesis, and Diels-Alder reactions. These proline derivatives allowed three parallel bioorthogonal reactions to be conducted in one solution.

Low-loading asymmetric organocatalysis

Asymmetric organocatalysis is now recognized as the third pillar of asymmetric synthesis. Recent years have witnessed increasing interest towards the use of highly active and stereoselective organocatalysts. This critical review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Brønsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalyses (194 references).