Asymmetric hydrogenations for the synthesis of Boc-protected 4-alkylprolinols and prolines

Alcohol-alcohol cross-coupling enabled by SH2 radical sorting

Alcohols represent a functional group class with unparalleled abundance and structural diversity. In an era of chemical synthesis that prioritizes reducing time to target and maximizing exploration of chemical space, harnessing these building blocks for carbon-carbon bond-forming reactions is a key goal in organic chemistry. In particular, leveraging a single activation mode to form a new C(sp3)-C(sp3) bond from two alcohol subunits would enable access to an extraordinary level of structural diversity. In this work, we report a nickel radical sorting-mediated cross-alcohol coupling wherein two alcohol fragments are deoxygenated and coupled in one reaction vessel, open to air.

A convenient synthetic route to (2S,4S)-methylproline and its exploration for protein engineering of thioredoxin

4-Substituted prolines, especially 4-fluoroprolines, have been widely used in protein engineering and design. Here, we report a robust and stereoselective approach for the synthesis of (2S,4S)-methylproline starting from (2S)-pyroglutamic acid. Incorporation studies with both (2S,4R)- and (2S,4S)-methylproline into the Trx1P variant of the model protein thioredoxin of E. coli show that the stereochemistry of the 4-methyl group might be a key determinator for successful incorporation during ribosomal synthesis of this protein.

Triphenylphosphine Oxide Removal from Reactions: The Role of Solvent and Temperature

Here, a large-scale feasible, chromatography-free process to purge triphenylphosphine oxide (TPPO) from the crude product of Mitsunobu and Wittig reactions has been developed. Divergence in physicochemical properties like polarity and solubility of TPPO against the product was utilized to precipitate TPPO directly from the reaction mixture and eliminate by simple filtration on a kilogram scale at a pilot plant with high purity of the product.

Harnessing the biocatalytic potential of iron- and α-ketoglutarate-dependent dioxygenases in natural product total synthesis

Covering: up to the end of 2019Iron- and α-ketoglutarate-dependent dioxygenases (Fe/αKGs) represent a versatile and intriguing enzyme family by virtue of their ability to directly functionalize unactivated C-H bonds at the cost of αKG and O2. Fe/αKGs play an important role in the biosynthesis of natural products, valuable biologically active secondary metabolites frequently pursued as drug leads. The field of natural product total synthesis seeks to contruct these molecules as effeciently as possible, although natural products continue to challenge chemists due to their intricate structural complexity. Chemoenzymatic approaches seek to remedy the shortcomings of traditional synthetic methodology by combining Nature's biosynthetic machinery with traditional chemical methods to efficiently construct natural products. Although other oxygenase families have been widely employed for this purpose, Fe/αKGs remain underutilized. The following review will cover recent chemoenzymatic total syntheses involving Fe/αKG enzymes. Additionally, related information involving natural product biosynthesis, methods development, and non-chemoenzymatic total syntheses will be discussed to inform retrosynthetic logic and synthetic design.

Highly stereoselective gram scale synthesis of all the four diastereoisomers of Boc-protected 4-methylproline carboxylates

A general and practical synthetic process for all the four diastereoisomers of Boc-protected 4-methylproline carboxylates has been developed with essentially complete stereoselectivity on the gram scale, which represents the most diastereoselective preparation of 4-methylproline derivatives to date. This synthesis features an Evans asymmetric alkylation to elegantly establish the challenging stereochemistry of the 4-methyl group, providing valuable insights for the diastereoselective preparation of 4-substituted prolines.

Stereocontrol in the synthesis of cyclic amino acids: a new ligand for directed hydrogenation through hydrogen bonding

A system for the directed hydrogenation of nitrogen heterocycles is described in which hydrogen is delivered cis to a hydroxymethyl group by a rhodium catalyst with a simple phosphine ligand. The chemistry is applied to the synthesis of the hygric acid moiety of lincomycin and the pipecolic acid moiety of Argatroban. A series of control experiments indicate that the stereoselectivity is a result of a combination of both coordination and hydrogen bonding.

The logic of translating chemical knowledge into machine-processable forms: a modern playground for physical-organic chemistry

With renewed interest and significant progress in computer-assisted synthetic planning, it is essential to codify the logic that should be followed when translating organic synthetic knowledge into reaction rules understandable to the machine.

Methods for the Construction of Collagen-Based Triple-Helical Peptides Designed as Matrix Metalloproteinase Inhibitors

The triple-helical structure of collagen has been accurately reproduced in numerous chemical and recombinant model systems. Triple-helical peptides have found application for dissecting collagen-stabilizing forces, isolating receptor and protein binding sites in collagen, evaluating collagen-mediated cell signaling activities, mechanistic examination of collagenolytic proteases, and developing novel biomaterials and drug delivery vehicles. Due to their inherent stability to general proteolysis, triple-helical peptides present an opportunity as in vivo inhibitory agents. The present chapter provides methods for the construction of collagen-based triple-helical peptides designed as matrix metalloproteinase inhibitors.

4-Vinylproline

Enantiomerically pure 4-vinylproline (Vyp) was synthesized by a five-step approach from N-(Boc)iodo-alanine (2) featuring copper-catalyzed S_N2' substitution of the corresponding zincate onto ( Z)-1,4-dichlorobut-2-ene to prepare methyl 2- N-(Boc)amino-4-(chloromethyl)hexenoate (3). Intra- and intermolecular displacement of the chloride provided respectively Vyp and methyl 2- N-(Boc)amino-4-(azidomethyl)hexenoate (7) suitable for the synthesis of constrained peptide analogs.

Second Generation Triple-Helical Peptide Inhibitors of Matrix Metalloproteinases

The design of selective matrix metalloproteinase (MMP) inhibitors that also possess favorable solubility properties has proved to be especially challenging. A prior approach using collagen-model templates combined with transition state analogs produced a first generation of triple-helical peptide inhibitors (THPIs) that were effective in vitro against discrete members of the MMP family. These THPI constructs were also highly water-soluble. The present study sought improvements in the first generation THPIs by enhancing thermal stability and selectivity. A THPI selective for MMP-2 and MMP-9 was redesigned to incorporate non-native amino acids (Flp and mep), resulting in an increase of 18 °C in thermal stability. This THPI was effective in vivo in a mouse model of multiple sclerosis, reducing clinical severity and weight loss. Two other THPIs were developed to be more selective within the collagenolytic members of the MMP family. One of these THPIs was serendipitously more effective against MMP-8 than MT1-MMP and was utilized successfully in a mouse model of sepsis. The THPI targeting MMP-8 minimized lung damage, increased production of the anti-inflammatory cytokine IL-10, and vastly improved mouse survival.

Catalytic asymmetric radical aminoperfluoroalkylation and aminodifluoromethylation of alkenes to versatile enantioenriched-fluoroalkyl amines

Although great success has been achieved in asymmetric fluoroalkylation reactions via nucleophilic or electrophilic processes, the development of asymmetric radical versions of this type of reactions remains a formidable challenge because of the involvement of highly reactive radical species. Here we report a catalytic asymmetric radical aminoperfluoroalkylation and aminodifluoromethylation of alkenes with commercially available fluoroalkylsulfonyl chlorides as the radical sources, providing a versatile platform to access four types of enantioenriched α-tertiary pyrrolidines bearing β-perfluorobutanyl, trifluoromethyl, difluoroacetyl and even difluoromethyl groups in excellent yields and with excellent enantioselectivity. The key to success is not only the introduction of the CuBr/chiral phosphoric acid dual-catalytic system but also the use of silver carbonate to suppress strong background and side hydroamination reactions caused by a stoichiometric amount of the in situ generated HCl. Broad substrate scope, excellent functional group tolerance and versatile functionalization of the products make this approach very practical and attractive.

Methods for the asymmetric introduction of organofluorine groups are often limited by the lack of variability in the starting materials. Here the authors report an asymmetric radical process for the introduction of fluoroalkyl groups using readily available fluoroalkylated sulfonyl chlorides.

Matrix metalloproteinase inhibition by heterotrimeric triple-helical Peptide transition state analogues

Matrix metalloproteinases (MMPs) have been implicated in numerous pathologies. An overall lack of selectivity has rendered active-site-targeted MMP inhibitors problematic. The present study describes MMP inhibitors that function by binding both secondary binding sites (exosites) and the active site. Heterotrimeric triple-helical peptide transition-state analogue inhibitors (THPIs) were assembled utilizing click chemistry. Three different heterotrimers were constructed, allowing for the inhibitory phosphinate moiety to be present uniquely in the leading, middle, or trailing strand of the triple helix. All heterotrimeric constructs had sufficient thermally stability to warrant analysis as inhibitors. The heterotrimeric THPIs were effective against MMP-13 and MT1-MMP, with Ki values spanning 100-400 nM. Unlike homotrimeric THPIs, the heterotrimeric THPIs offered complete selectivity between MT1-MMP and MMP-1. Exosite-based approaches such as this provide inhibitors with desired MMP selectivities.

Discovery of potent and specific dihydroisoxazole inhibitors of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme that catalyzes the posttranslational modification of glutamine residues on protein or peptide substrates. A growing body of literature has implicated aberrantly regulated activity of TG2 in the pathogenesis of various human inflammatory, fibrotic, and other diseases. Taken together with the fact that TG2 knockout mice are developmentally and reproductively normal, there is growing interest in the potential use of TG2 inhibitors in the treatment of these conditions. Targeted-covalent inhibitors based on the weakly electrophilic 3-bromo-4,5-dihydroisoxazole (DHI) scaffold have been widely used to study TG2 biology and are well tolerated in vivo, but these compounds have only modest potency, and their selectivity toward other transglutaminase homologues is largely unknown. In the present work, we first profiled the selectivity of existing inhibitors against the most pertinent TG isoforms (TG1, TG3, and FXIIIa). Significant cross-reactivity of these small molecules with TG1 was observed. Structure–activity and −selectivity analyses led to the identification of modifications that improved potency and isoform selectivity. Preliminary pharmacokinetic analysis of the most promising analogues was also undertaken. Our new data provides a clear basis for the rational selection of dihydroisoxazole inhibitors as tools for in vivo biological investigation.

Sterically directed functionalization of the redox-active bis(imino)acenaphthene ligand class: an experimental and theoretical investigation

The synthesis, characterization, and theoretical study of the sterically directed functionalization of the redox-active bis(imino)acenaphthene (BIAN) ligand class has been explored. With dependence on the steric congestion encompassing the N-C-C-N fragment of the Ar-BIAN ligand, functionalization can be directed to proceed either via a radical backbone dearomatization or a nucleophilic imine C-alkylation pathway. The structures of the Ar-BIAN derivatives 14-19 were determined by means of single-crystal X-ray diffraction. The reaction pathways involved in Ar-BIAN functionalization were monitored by means of EPR spectroscopy. The experimental results and observations were examined in conjunction with DFT-D calculations in order to explain the driving forces that direct the pathways leading to Ar-BIAN functionalization.

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.

Synthesis of an MIF-1 analogue containing enantiopure (S)-α-trifluoromethyl-proline and biological evaluation on nociception

The synthesis and the effect of a novel MIF-1 analogue on nociception during acute pain in rat model are reported. The synthesis of this enantiopure trifluoromethyl group containing tripeptide was performed through a peptide coupling reaction between the HCl. Leu-Gly-NH2 and the (S)-α-Tfm-proline. The analgesic effect of the CF3-(MIF-1) 2 has been evaluated in vivo on rat model by paw pressure (PP) and hot plate (HP) tests and compared to the native peptide MIF-1. Highest analgesic effect was observed with CF3-(MIF-1) 2 only in PP test. In order to study the mechanisms of nociception induced by the studied peptides, the involvement of the opioid and the nitric oxideergic systems was investigated. The results are in favor of a participation of both system since pretreatment, 20 min before injection of the CF3-(MIF-1) 2, with the non-competitive antagonist of opiate receptors naloxone, the nitric oxide synthase (NOS) inhibitor l-N(G)-nitroarginine ester (l-NAME) or the nitric oxide (NO) donor l-arginine (l-Arg) significantly decreased the pain perception in PP and HP tests.

Stabilization of collagen-model, triple-helical peptides for in vitro and in vivo applications

The triple-helical structure of collagen has been accurately reproduced in numerous chemical and recombinant model systems. Triple-helical peptides and proteins have found application for dissecting collagen-stabilizing forces, isolating receptor- and protein-binding sites in collagen, mechanistic examination of collagenolytic proteases, and development of novel biomaterials. Introduction of native-like sequences into triple-helical constructs can reduce the thermal stability of the triple-helix to below that of the physiological environment. In turn, incorporation of nonnative amino acids and/or templates can enhance triple-helix stability. We presently describe approaches by which triple-helical structure can be modulated for use under physiological or near-physiological conditions.

Introduction of C(5/6) side chains onto 2-azabicyclo[2.1.1]hexanes via a 6-anti-bromo-5-anti-hydroxy derivative

Oxidation of the title bromoalcohol provided the strained ketone, 5-bromo-6-oxo-2-azabicyclo[2.1.1]hexane. Additions of nucleophiles to either this or the debrominated ketone have been used to introduce 5(6)-syn-alkyl and aryl groups, 5(6)-alkylidene linkages, and 5(6)-anti-alkyl and acyl substituents. Facial selectivity is for additions to the 6-bromo-5-ketone and 5-alkylidene azabicycles to occur from the face syn to the nitrogen atom. The bromine atom of the title alcohol has also been replaced by a 6-anti-(1-hydroxyethyl) substituent using a directed radical addition process. The stereoselective functionalization reactions expand the range of available methano-bridged pyrrolidines.

Synthesis of 2-trifluoromethyl-1,3-oxazolidines as hydrolytically stable pseudoprolines

Trifluoromethyl group containing oxazolidines (Fox) are conveniently synthesized by condensation of serine esters with trifluoroacetaldehyde hemiacetal or trifluoroacetone. These oxazolidines can undergo N-acylation and amidification reactions and are completely configurationally and hydrolytically stable. Therefore, they can be considered as highly valuable proline surrogates (Tfm-pseudoprolines).

Concise, stereoselective route to the four diastereoisomers of 4-methylproline

The full stereochemical characterization of 4-methylproline, a rare amino acid found in a number of peptidic secondary metabolites, has often been hindered by long reaction sequences or low stereoselectivity in the synthesis leading to reference samples. The preparation of the four diastereoisomers of 4-methylproline by a concise and stereoselective route is presented and features a six-step route with late-stage stereodivergence, good stereoselectivity for both cis- and trans-series (75% and 88% de, respectively), and good overall yields (cumulative yields of 30-40%). Additional data on the Marfey's derivatives of the stereoisomers are also presented.

Discovery of SCH446211 (SCH6): a new ketoamide inhibitor of the HCV NS3 serine protease and HCV subgenomic RNA replication

Introduction of various modified prolines at P(2) and optimization of the P(1) side chain led to the discovery of SCH6 (24, Table 2), a potent ketoamide inhibitor of the HCV NS3 serine protease. In addition to excellent enzyme potency (K(i)*= 3.8 nM), 24 was also found to be a potent inhibitor of HCV subgenomic RNA replication with IC(50) and IC(90) of 40 and 100 nM, respectively. Recently, antiviral activity of 24 was demonstrated with inhibition of the full-length genotype 2a HCV genome. In addition, 24 was found to restore the responsiveness of the interferon regulatory factor 3 (IRF-3) in cells containing HCV RNA replicons.

Reciprocity of Steric and Stereoelectronic Effects in the Collagen Triple Helix

In previous work, we demonstrated that 4-fluoroproline residues can contribute greatly to the conformational stability of the collagen triple helix, and that this stability arises from stereoelectronic effects that fix the pucker of the pyrrolidine ring and thereby preorganize the backbone properly for triple-helix formation. Here, we take a reciprocal approach, demonstrating that the steric effect of a 4-methyl group confers stability similar to that from a 4-fluoro group in the opposite configuration. Such fundamental interplay between steric and stereoelectronic effects is heretofore unknown in proteins-natural or synthetic-and provides a new means to modulate conformational stability.

Total synthesis of cyclic tetrapeptide FR235222, a potent immunosuppressant that inhibits mammalian histone deacetylases

[structure: see text] The total synthesis of FR235222, a potent immunosuppressant with in vivo activities, has been achieved. The key steps include assembling its (2S,9R)-2-amino-9-hydroxy-8-oxodecanoic acid residue via an olefin cross-metathesis of a methyl (R)-lactate-derived homoallyl ketone with protected allyl amino acid and constructing its trans-(2R,4S)-4-methylproline unit from protected (R)-pyroglutamic acid in seven steps.

5-Carboxy-2-azabicyclo[2.1.1]hexanes as Precursors of 5-Halo, Amino, Phenyl, and 2-Methoxycarbonylethyl Methanopyrrolidines

Novel 5-X-substituted-2-azabicyclo[2.1.1]hexanes (X = 5-syn-Cl, -Br, -I, -Ph, -NHCOOR (R = Me, Bn, t-Bu), -CH2CH2COOMe and X = 5-anti-Br, -I, -Ph) were synthesized from the X = 5-syn-carboxy derivative. New 5-anti-X-2-azabicyclo[2.1.1]hexanes, X = NHCOOR (R = Me, Bn), were prepared stereoselectively from the X = 5-anti-carboxy substrate.

Locked conformations for proline pyrrolidine ring: synthesis and conformational analysis of cis- and trans-4-tert-butylprolines

The motional restrictions of the proline pyrrolidine ring allow this secondary amine amino acid to act as a turn inducer in many peptides and proteins. The pyrrolidine ring is known to exhibit two predominant pucker modes (i.e., C-4 (Cgamma) exo and endo envelope conformers whose ratio can be controlled by proper substituents in the ring). In nature, the exo puckered 4(R)-hydroxy-l-proline plays a crucial role as a building block in collagen and collagen-like structures. It has been previously concluded that the electronegativity of the 4-cis-substituent increases the endo puckering while the electronegativity of the 4-trans-substituent favors the exo puckering. Here, we have introduced a sterically demanding tert-butyl group at C-4 in trans- and cis-configurations. In the case of trans-substitution, the induced puckering effect on the pyrrolidine ring was studied with X-ray crystallography and 1H NMR spectral simulations. Both cis- and trans-4-tert-butyl groups strongly favor pseudoequatorial orientation, thereby causing opposite puckering effects for the pyrrolidine ring, cis-exo and trans-endo for l-prolines, in contrast to the effects observed in the case of electronegative C-4 substituents. The syntheses and structural analysis are presented for the conformationally constrained 4-tert-butylprolines. The prolines were synthesized from 4-hydroxy-l-proline, substitution with t-BuCuSPhLi being the key transformation. This reaction gave N-Boc-trans-4-tert-butyl-l-proline tert-butyl ester in 94% ee and 57% de. Enantioselectivity was increased to 99.2% ee by crystallization of N-Boc-trans-4-tert-butyl-l-proline in the final step of the synthesis.

Utilizing the intramolecular Fukuyama–Mitsunobu reaction for a flexible synthesis of novel heterocyclic scaffolds for peptidomimetic drug design

We report the synthesis of the novel scaffolds pyrazino[1,2-b]isoquinoline and pyrrolo[1,2-a]pyrazine displaying the somatostatin pharmacophores. Both classes of compounds contain a pyrazine heterocycle, which can be prepared in a straightforward manner utilizing an intramolecular Fukuyama-Mitsunobu reaction. As both the families derive from amino acids, they can be accessed in high optical purity.

Novel Approach to 5-Substituted Proline Derivatives Using a Silver-Catalyzed Cyclization as the Key Step

A novel synthetic approach to the synthesis of enantiomerically pure 2,5-disubstituted pyrrolines is described. The methodology involves a Ag-catalyzed 5-endo-dig cyclization of enantiopure aryl-substituted acetylene-containing amino acids. It has also been shown that the obtained pyrrolines can be efficiently transformed into the corresponding saturated 5-aryl-substituted proline derivatives.

The Rearrangement Route to 3-Carboxy- and 3-Hydroxymethyl-2-azabicyclo[2.1.1]hexanes: 3,5-Methanoprolines

Improved stereocontrolled syntheses of 5-anti-hydroxy-3-exo-methoxycarbonyl-2-azabicyclo[2.1.1]hexanes have been effected from pyridine. The key step in the electrophilic addition-rearrangement of 2-azabicyclo[2.2.0]hex-5-ene precursors incorporates either a 3-endo-phenyl group, as an acid precursor, or a 3-endo-phenyldimethylsilylmethyl group, as a potential hydroxymethyl and acid precursor.