Synthesis of N-glyoxyl prolyl and pipecolyl amides and thioesters and evaluation of their in vitro and in vivo nerve regenerative effects

Visible-Light-Mediated Cross Dehydrogenative Coupling of Thiols with Aldehydes: Metal-Free Synthesis of Thioesters at Room Temperature

Thioesters play a crucial role in biological systems and serve as important building blocks for organic synthesis. Herein, Eosin Y and TBHP mediated photochemical cross dehydrogenative coupling (PCDC) between feedstock aldehydes and thiols has been described at room temperature to synthesize thioesters. This thioesterification protocol proceeds smoothly to give the desired products in good to excellent yields by the suitable PCDC of both alkyl/aryl- aldehydes with a variety of alkyl/aryl-thiols and generates water and ^tBuOH as green byproducts. This method is also found to be scalable with good efficiency. Mechanistic investigations reveal that under this photochemical condition, the formation of acyl radical can be achieved from aldehyde. This acyl radical was further intercepted with an intermediate disulfide, generated in situ via the dehydrogenation of thiol to give the desired thioester. Moreover, disulfides, which are relatively easier to handle, also provided good to excellent yields in the optimized reaction condition. This protocol was further extended toward the more challenging direct transformation of alcohols to thioesters.

Ligand-Controlled Regiodivergent Thiocarbonylation of Alkynes toward Linear and Branched α,β-Unsaturated Thioesters

Thiocarbonylation of alkynes offers an ideal procedure for the synthesis of unsaturated thioesters. A robust ligand-controlled regioselective thiocarbonylation of alkynes is developed. Utilizing boronic acid and 5-chlorosalicylic acid as the acid additive to in situ form 5-chloroborosalicylic acid (5-Cl-BSA), and bis(2-diphenylphosphinophenyl)ether (DPEphos) as the ligand, linear α,β-unsaturated thioesters were produced in a straightforward manner. Switching the ligand to tri(2-furyl)phosphine can turn the reaction selectivity to give branched products. Remarkably, this approach also represents the first example on thiocarbonylation of internal alkynes.

Palladium-Catalyzed Linear Hydrothiocarbonylation of Unactivated Terminal Alkenes: Synthesis of Aliphatic Thioesters

A Pd-catalyzed hydrothiocarbonylation of unactivated terminal alkenes is presented. According to this protocol, aliphatic thioesters were synthesized with exclusive linear selectivity under mild reaction conditions. Good to excellent yields (up to 91% yield), broad substrate scope, broad functional group tolerance, and utility of the method demonstrated the advantages of this protocol.

Deaminative carbonylative thioesterification of activated alkylamines with thiophenols under transition-metal-free conditions

A transition-metal-free radical carbonylation of activated alkylamines with thiophenols has been successfully developed. Various thioesters were selectively produced with moderate to good yields.

Autocatalytic Synthesis of Thioesters via Thiocarbonylation of gem-Difluoroalkenes

Herein, we report a new method for the synthesis of acyethanethioates via thiocarbonylation of gem-difluoroalkenes with thiols. This reaction provides a new pathway to prepare thioesters under mild conditions without the use of any additives. Mechanistic studies revealed that in situ generated HF facilitated the C-F bond cleavage in an autocatalytic manner.

Synthesis and Inhibitory Studies of Phosphonic Acid Analogues of Homophenylalanine and Phenylalanine towards Alanyl Aminopeptidases

A library of novel phosphonic acid analogues of homophenylalanine and phenylalanine, containing fluorine and bromine atoms in the phenyl ring, have been synthesized. Their inhibitory properties against two important alanine aminopeptidases, of human (hAPN, CD13) and porcine (pAPN) origin, were evaluated. Enzymatic studies and comparison with literature data indicated the higher inhibitory potential of the homophenylalanine over phenylalanine derivatives towards both enzymes. Their inhibition constants were in the submicromolar range for hAPN and the micromolar range for pAPN, with 1-amino-3-(3-fluorophenyl) propylphosphonic acid (compound 15c) being one of the best low-molecular inhibitors of both enzymes. To the best of our knowledge, P1 homophenylalanine analogues are the most active inhibitors of the APN among phosphonic and phosphinic derivatives described in the literature. Therefore, they constitute interesting building blocks for the further design of chemically more complex inhibitors. Based on molecular modeling simulations and SAR (structure-activity relationship) analysis, the optimal architecture of enzyme-inhibitor complexes for hAPN and pAPN were determined.

Peptidyl-Proline Isomerases (PPIases): Targets for Natural Products and Natural Product-Inspired Compounds

Peptidyl-proline isomerases (PPIases) are a chaperone superfamily comprising the FK506-binding proteins (FKBPs), cyclophilins, and parvulins. PPIases catalyze the cis/trans isomerization of proline, acting as a regulatory switch during folding, activation, and/or degradation of many proteins. These "clients" include proteins with key roles in cancer, neurodegeneration, and psychiatric disorders, suggesting that PPIase inhibitors could be important therapeutics. However, the active site of PPIases is shallow, solvent-exposed, and well conserved between family members, making selective inhibitor design challenging. Despite these hurdles, macrocyclic natural products, including FK506, rapamycin, and cyclosporin, bind PPIases with nanomolar or better affinity. De novo attempts to derive new classes of inhibitors have been somewhat less successful, often showcasing the "undruggable" features of PPIases. Interestingly, the most potent of these next-generation molecules tend to integrate features of the natural products, including macrocyclization or proline mimicry strategies. Here, we review recent developments and ongoing challenges in the inhibition of PPIases, with a focus on how natural products might inform the creation of potent and selective inhibitors.

Structure–activity relationships in Pd catalysed C–S activation of thioesters

The structure–activity relationship of thioesters promoted by Pd(0) complex has been studied using the density functional theory calculation methods.

Direct oxidative coupling of thiols and benzylic ethers via C(sp3)–H activation and C–O cleavage to lead thioesters

An unprecedented protocol for the synthesis of thioesters via C–H thioesterification and C–O cleavage was developed. This CDC protocol was found convenient and easy-to-handle involving the use of DTBP as the only green oxidant.

Functional expression of L-lysine α-oxidase from Scomber japonicus in Escherichia coli for one-pot synthesis of L-pipecolic acid from DL-lysine

L-Pipecolic acid is a key component of biologically active molecules and a pharmaceutically important chiral building block. It can be stereoselectively produced from L-lysine by a two-step bioconversion involving L-lysine α-oxidase and ∆(1)-piperideine-2-carboxylae (Pip2C) reductase. In this study, we focused on an L-lysine α-oxidase from Scomber japonicus that was originally identified as an apoptosis-inducing protein (AIP) and applied the enzyme to one-pot fermentation of L-pipecolic acid in Escherichia coli. A synthetic gene coding for an AIP was expressed in E. coli, and the recombinant enzyme was purified and characterized. The purified enzyme was determined to be a homodimer with a molecular mass of 133.9 kDa. The enzyme essentially exhibited the same substrate specificity as the native enzyme. Optimal temperature and pH for the enzymatic reaction were 70 °C and 7.4, respectively. The enzyme was stable below 60 °C and at a pH range of 5.5-7.5 but was markedly inhibited by Co(2+). To establish a one-pot fermentation system for the synthesis of optically pure L-pipecolic acid from DL-lysine, an E. coli strain carrying a plasmid encoding AIP, Pip2C reductase from Pseudomonas putida, lysine racemase from P. putida, and glucose dehydrogenase from Bacillus subtilis was constructed. The one-pot process produced 45.1 g/L of L-pipecolic acid (87.4 % yield from DL-lysine) after a 46-h reaction with high optical purity (>99.9 % enantiomeric excess).

Significance of the natural occurrence of L- versus D-pipecolic acid: a review

Pipecolic acid naturally occurs in microorganisms, plants, and animals, where it plays many roles, including the interactions between these organisms, and is a key constituent of many natural and synthetic bioactive molecules. This article provides a review of current knowledge on the natural occurrence of pipecolic acid and the known and potential significance of its L- and D-enantiomers in different scientific disciplines. Knowledge gaps with perspectives for future research identified within this article include the roles of the L- versus the D-enantiomer of pipecolic acid in plant resistance, nutrient acquisition, and decontamination of polluted soils, as well as rhizosphere ecology and medical issues.

Combinatorial techniques for enhancing neuroprotection: hypothermia and alkalinization

Brain and spinal cord (CNS) trauma typically kill a number of neurons, but even more neurons are killed by secondary causes triggered by the initial trauma. Thus, a minor insult may rapidly cause the death of a vastly larger number of neurons and complete paralysis. The best mechanism for reducing the extent of neurological deficits is to minimize the number of neurons killed by post-trauma sequelae. Neuroprotection techniques take many diverse forms with a breadth too great for a short review. Therefore, this review focuses on the neuroprotection provided by hypothermia and a number of other neuroprotective techniques, when administered singly or in combination, because it is generally found that combinations of applications lead to significantly better neuroprotection than is achieved by any one alone. The combinatorial approach to neuroprotection holds great promise for enhancing the degree of neuroprotection following trauma, leading to maximum maintenance of neurological function.

Fragment-Based Synthesis and SAR of Modified FKBP Ligands: Influence of Different Linking on Binding Affinity

The viability of the fragment-based approach for lead discovery depends on reliable fragment-screening methods combined with straightforward fragment-linking- or fragment-growing-chemistry. In the present study we sought a flexible synthetic approach that would allow efficient synthesis of a variety of linkers that can subsequently be tested for biological activity. We applied this approach to fragments known to bind to FKBP12 (FK506 binding protein), a peptidyl-prolyl isomerase involved in immunosuppression and neural functioning. In our set of linked FKBP ligands, ester and thioester linkages resulted in high-affinity ligands, whereas an amide linkage decreased affinity remarkably; oxime and triazole linkages were not tolerated by the target protein's binding pocket, rendering these ligands ineffective. By investigating corresponding derivatized non-linked fragments and docking studies of linked fragments, we were able to evaluate the effect of the linker region on ligand binding affinity.

Wittig Reactions in Water Media Employing Stabilized Ylides with Aldehydes. Synthesis of α,β-Unsaturated Esters from Mixing Aldehydes, α-Bromoesters, and Ph3P in Aqueous NaHCO3

Water is demonstrated to be an effective medium for the Wittig reaction over a wide range of stabilized ylides and aldehydes. Despite sometimes poor solubility of the reactants, good chemical yields normally ranging from 80 to 98% and high E-selectivities (up to 99%) are achieved, and the rate of the reactions in water is unexpectedly accelerated. The efficiency of water as a medium in the Wittig reaction is compared to conventional organic solvents ranging from carbon tetrachloride to methanol. The aqueous Wittig reaction works best when large hydrophobic entities are present, such as aromatic, heterocyclic aromatic carboxaldehydes, and long-chain aliphatic aldehydes with triphenylphosphoranes. The E/Z-isomeric ratio of the Wittig products appears dependent on the electron-accepting/donating capacity and the location of the substituents present in the aromatic ring. The effect of additives, such as benzoic acid, LiCl, and sodium dodecyl sulfate (SDS), on the Wittig reaction has been explored. The Wittig reaction can also be conducted in the presence of acidic entities, such as phenols and carboxylic acids. In addition, large alpha-substituents in the aliphatic aldehydes do not jeopardize the reaction. It is also demonstrated that hydrates of aldehydes can be used directly in the aqueous Wittig reaction as substrates. The scope of the aqueous Wittig reaction is extended to 24 examples of one-pot mixtures of Ph3P, alpha-bromoesters, and aldehydes in sodium bicarbonate solution (at 20 degrees C for 40 min to 3 h) to provide Wittig products of up to 99% yield and up to 98% E-selectivity. Since water is inexpensive, extremely easy to handle, and represents no environmental concerns, it should be considered a possible medium for new organic reactions.

Peptidyl-prolyl isomerase inhibitors

Designed peptidyl-prolyl isomerase (PPIase) inhibitors of Pin1, cyclophilin (CyP), and FK506 binding protein (FKBP) are reviewed. Emphasis is placed on the design, structure, and biological activity of the inhibitors. While CyP and FKBP inhibitors have been explored fairly thoroughly, inhibitors of the relatively new Pin1 cell cycle regulator are in their infancy. Ligands designed for Pin1 and CyP have primarily been ground state analogues: alkenes and bicyclic compounds. For FKBP, more of the focus has been on analogues of bonds at the reactive center, the prolyl amide, because of the idea that the alpha-ketoamide of FK506 is an analogue of the twisted amide in the transition state.

Collateral sprouting of human epidermal nerve fibers following intracutaneous axotomy

Despite the clinical need, there are no therapeutic compounds available to promote peripheral nerve regeneration. In part, this may be due to a lack of sensitive measures of nerve growth. Here, we describe a novel approach of measuring collateral sprouting of epidermal nerve fibers (ENF) in human subjects and describe the effect of the neuroimmunophilin ligand timcodar dimesylate on collateral nerve sprouting. The objective of this study was to describe a model of intracutaneous axotomy and evaluate the ability of timcodar dimesylate to accelerate human cutaneous nerve regeneration through collateral sprouting. Subjects were randomized to receive placebo, 12.5 or 50 mg/day timcodar dimesylate in a prospective, two-center, double-blind, placebo-controlled trial. A 3-mm distal thigh punch skin biopsy was performed at baseline, and a 4-mm overlapping concentric biopsy was taken after 56 days of treatment. Biopsies were processed to visualize ENF, and the collateral sprouting distance (CSD) was measured. Sixty-two subjects completed the trial, and the CSD was measurable in 52. The CSD (mean +/- SEM) was 474.5 microm +/- 38.3, 473.4 microm +/- 28.4, and 450.8 microm +/- 26.5 for the placebo, low and high dose groups, respectively (p = 0.84). The baseline ENF density was associated with the CSD (p = 0.02). Collateral sprouting was efficiently measured using an intracutaneous axotomy model and suggests a collateral sprouting rate of 8.5 microm/day in healthy subjects. The model was consistent across treatment groups and had a low coefficient of variation. Timcodar dimesylate treatment was safe over an 8-week period but did not improve collateral sprouting among healthy subjects.

Synthesis of the Benzhydryl Motif via a Suzuki−Miyaura Coupling of Arylboronic Acids and 3-Chloroacrylonitriles

[reaction: see text] A simple two-step procedure for synthesizing functionalized benzhydrylamines is described. The first step involves a Suzuki-Miyaura coupling reaction between arylboronic acids and 3-chloro-3-arylacrylonitriles at 45 degrees C. A variety of boronic acids and substituted acrylonitriles can be used for the reaction. The resulting 3,3-diaryl-substituted acrylonitriles can be converted into their corresponding Boc-protected amines by catalytic hydrogenation.

Immunosuppressants: neuroprotection and promoting neurological recovery following peripheral nerve and spinal cord lesions

No clinical techniques induce restoration of neurological losses following spinal cord trauma. Peripheral nerve damage also leads to permanent neurological deficits, but neurological recovery can be relatively good, especially if the ends of a transected nerve are anastomosed soon after the injury. The time until recovery generally depends on the distance the axons must regenerate to their targets. Neurological recovery following the destruction of a length of a peripheral nerve requires a graft to bridge the gap that is permissive to, and promotes, axon regeneration. But neurological recovery is slow and limited, especially for gaps longer than 1.5 cm, even using autologous peripheral nerve grafts. Without a reliable means of bridging long nerve gaps, such injuries commonly result in amputations. Promoting extensive neurological recovery requires techniques that simultaneously provide protection to injured neurons and increase the numbers of neurons that extend axons, while inducing more rapid and extensive axon regeneration across long nerve gaps. Although conduits filled with various materials enhance axon regeneration across short nerve gaps, pure sensory nerve graft remains the gold standard for use across long nerve gaps, even though they lead to only limited neurological recovery. Consistent results demonstrate that several immunosuppressive agents enhance the number of axons and the rate at which they regenerate. This review examines the roles played by immunosuppressants, especially FK506, with primary focus on its role as a neuroprotectant and neurotrophic agent, and its potential clinical use to promote improved neurological recovery following peripheral nerve and spinal cord injuries.

Effects of indole fatty alcohols on the differentiation of neural stem cell derived neurospheres

In a search for inducers of neuronal differentiation to treat neurodegenerative diseases such as Alzheimer's disease, a series of indole fatty alcohols (IFAs) were prepared. 13c (n = 18) was able to promote the differentiation of neural stem cell derived neurospheres into neurons at a concentration of 10 nM. Analysis of the expression of the Notch pathway genes in neurospheres treated during the differentiation phase with 13c (n = 18) revealed a significant decrease in the transcription of the Notch 4 receptor.

Synthesis and evaluation of chiral bicyclic proline FKBP12 ligands

As part of our ongoing program to explore novel structural classes of FKBP12 ligands, we herein wish to report a new class of FKBP12 ligands containing chiral bicyclic proline analogues. Details of the synthetic routes, together with preliminary biological activity, will be presented.

Synthesis, molecular modeling and biological evaluation of aza-proline and aza-pipecolic derivatives as FKBP12 ligands and their in vivo neuroprotective effects

Nonimmunosuppressant ligands, exemplified by GPI 1046 (1), for the peptidyl-prolyl isomerase FKBP12 have been found to unexpectedly possess powerful neuroprotective and neuroregenerative effects in vitro and in vivo. We have extensively explored the therapeutic utility of FKBP12 ligands based on analogues of proline and pipecolic acid. As part of our ongoing program to explore novel structural classes of FKBP12 ligands, we herein wish to report a new class of FKBP12 ligands containing aza-proline and aza-pipecolic acid analogues. Details of the synthetic studies, together with biological activity will be presented.

Synthesis and biological evaluation of non-peptidic cyclophilin ligands

Peptidylprolyl isomerase cyclophilins play critical roles in a variety of biological processes. Recent findings that cyclophilins are present at high levels in the CNS and that cyclosporin A may possess neuroprotective/neurotrophic effects have prompted us to search for nonimmunosuppressant small molecule cyclophilin ligands. To this end, we report the lead identification through "virtual screening" and the synthesis of our first series of non-peptidic cyclophilin ligands, along with the preliminary biological results.