Deprotection Strategy Research Articles

Designed Synthesis of Diversely Substituted Hydantoins and Hydantoin-Based Hybrid Molecules: A Personal Account

AbstractHydantoin and its analogues such as thiohydantoin and iminohydantoin have received substantial attention from both a chemical and a biological point of view. Several compounds of this class have shown useful pharmacological activities such as anticonvulsant, antitumor, antiarrhythmic, and herbicidal properties that have led, in some cases, to clinical applications. Because of these broad-spectrum activities, intensive research efforts have been dedicated in industry and academia to the synthesis and structural modifications of hydantoin and its derivatives. Realizing the importance of hydantoin in organic and medicinal chemistry, we also initiated a research program that successfully designed and developed new routes and methods for the formation of hydantoin, thiohydantoin, and iminohydantoin substituted at various positions, particularly at the N-1 position without following a protection–deprotection strategy. Because combinations of two or more pharmacophoric groups can lead to hybrid molecules that display a mixed mechanism of action on biological targets, we extended our developed strategy to the syntheses of new types of hydantoin-based hybrid molecules by combining hydantoin with a triazole, isoxazoline, or phosphate scaffold as a second pharmacophore to exploit their diverse biological functions.1 Introduction2 Chemistry and Properties2.1 Physical Properties2.2 Chemical Properties2.3 Biological Properties3 General Synthetic Methods4 Synthesis of Diversely Substituted Hydantoins5 Synthesis of Diversely Substituted Thiohydantoins6 Synthesis of Diversely Substituted Iminohydantoins7 Fused or Bicyclic (Thio)hydantoins8 Di- or Multivalent (Thio)hydantoins9 Hydantoin-Based Hybrid Molecules9.1 Hydantoin–Isooxazoline Hybrids9.2 Hydantoin–Triazole Hybrids9.3 Hydantoin–Phosphate Hybrids: Phosphorylated Hydantoins10 Summary and Outlook

Building Addressable Libraries: Olefin Functionalization Reactions and the Development of a Site-Selective, Cleavable Linker

Microelectrode arrays are potentially powerful tools for monitoring molecular interactions between small molecules and biological targets in "real-time". Yet while the potential for the arrays is coming into focus, there are still barriers that prevent us from taking full advantage of the devices. Chief among these is the need to construct and characterize molecules on the arrays so that they are specifically located by individually addressable electrodes or groups of electrodes in the array. Currently, we are working on methods to construct larger molecular libraries directly on the arrays, an effort that seeks to avoid transferring the libraries to the array one member at a time. To do so, we need to expand the synthetic toolbox available on the arrays. In that context, olefin functionalization is one of the most powerful strategies in modern organic chemistry because it converts readily available double bonds into a wide array of functional groups. For this reason, we are initially examining the role of cis-dihydroxylation, epoxidation, and diazidation reactions which can play in a site-selective array-based synthesis. The chemistry is already paying dividends. In one application, we have shown that a site-selective cis-hydroxylation reaction can enable a new “safety-catch" linker strategy that is completely orthogonal to the typical deprotection strategies used for diversity of oriented syntheses. The result provides a foundation for future synthetic efforts because it enables full characterization of molecules synthesized by any electrode in an array. In this poster, we will discuss progress towards additional array-based olefin functionalization reactions as well as detail the safety-catch linker strategy mentioned above. Figure 1

High-yield synthesis of a tau PET radioligand and its nonradioactive ligand using an alternative protection and deprotection strategy.

Recently developed tau imaging radiopharmaceuticals show specific uptake in tau protein-rich regions in human brains without off-target binding. These radiopharmaceuticals and their nonradioactive reference ligands are generally obtained in low (radio)chemical yields. In the present study, we investigated high-yield synthesis of 18 F-RO948 ([18 F]1) and its nonradioactive ligand (1). The ligand 1 was synthesized by a Suzuki-Miyaura coupling reaction between 9-(4-methoxybenzyl)-9H-pyrrolo[2,3-b:4,5-c']dipyridin-2-yl trifluoromethanesulfonate (3) and 2-fluoro-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (4), followed by oxidative removal of the para-methoxybenzyl (PMB) group with ceric ammonium nitrate (CAN). This two-step reaction gave 1 in 55.8% yield. The precursor for [18 F]1 was synthesized from 3 and 2-nitro-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (6). The resulting PMB-protected precursor 8 was obtained in 74.5% yield. [18 F]1 was synthesized by radiofluorination of 8 (radiochemical conversion (RCC): 95.7 ± 1.7%), followed by deprotection of the PMB group with CAN. This one-pot, two-step radiochemical synthesis followed by HPLC purification gave [18 F]1 in high decay-corrected radiochemical yield (54-60%). The RCC of [18 F]fluoride to [18 F]1 in our two-step synthesis method was similar to that in a one-step radiofluorination reaction of a tert-butoxycarbonyl (BOC)-protected precursor 10 that proceeds with concomitant thermal deprotection of the BOC group. Taken together, the results of this study suggest that this high-yield synthesis method is useful for the synthesis of 18 F-labeled (NH)heteroarene compounds.

A Simple and Efficient Method for Synthesis of sn-Glycero-Phosphoethanolamine.

An efficient three-step strategy for the convenient synthesis of Sn-glycero-3-phosphoethanolamine (GroPEtn) from a commercially available 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) is reported. Direct hydrolysis of DPPE produces a complex inseparable mixture, hence a protection and deprotection strategy is employed to prepare GroPEtn. The primary amine of DPPE is protected with a highly stable acid-labile trityl group, followed by strong base hydrolysis of N-trityl-DPPE gives N-trityl-GroPEtn. Further a mild, rapid, and efficient deprotection method is established using trifluoroacetic acid to remove N-trityl moiety, affords GroPEtn as a single product. This is the first semisynthetic approach and efficient method to produce GroPEtn with a total yield of 66% in three steps. GroPEtn did not show any cytotoxicity against human kidney (HK-2) cells and reporter gene assay for activation of Keap1-Nrf2-mediated antioxidant defense mechanism showed no significant effects.

One‐pot peptide cleavage and macrocyclization through direct amidation using triazabicyclodecene

AbstractWe have developed a novel protocol for a one‐pot cleavage from solid support and subsequent cyclization to form small macrocyclic peptides. Synthesized on a 4‐hydroxymethyl benzamide (HMBA) resin, the peptides are cleaved from the solid support through a 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) assisted acyl transfer reaction. The resulting linear peptides then undergo an intramolecular cyclization, presumably upon in situ attack of the N‐terminal amine on the TBD‐activated ester. This protocol requires catalytic quantities of TBD and eliminates the need for orthogonal side chain deprotection strategies and ether trituration/work‐up steps, streamlining the synthesis of head‐to‐tail macrocyclic peptides.

Deprotection of N‐tert‐Butoxycarbonyl (Boc) Protected Functionalized Heteroarenes via Addition–Elimination with 3‐Methoxypropylamine

Continued pursuit of functionalized soft‐N‐donor complexant scaffolds with favorable solubility and kinetics profiles applicable for the separation of the trivalent minor actinides from the lanthanides has attracted significant interest over the last three decades. Recent work from this laboratory resulted in the production of various N‐Boc protected [1,2,4]triazinyl‐pyridin‐2‐yl indole Lewis basic procomplexants which necessitated the removal of the indole N‐Boc protecting group prior to evaluation of complexant efficacy in separations assays. Traditional deprotection strategies involving trifluoroacetic and other protic and Lewis acids proved unsuccessful in removal of the recalcitrant indole‐N‐Boc protecting group necessitating the development of a new strategy for deprotection of this complexant class. A serendipitous result facilitated utilization of 3‐methoxypropylamine as a mild deprotecting agent for various N‐Boc protected heteroarenes via a proposed addition–elimination mechanism. Method development, application to various heteroarenes including indoles, 1,2‐indazoles, 1,2‐pyrazoles, and related derivatives, a ten‐fold scale‐up reaction, and experimental evaluation of a preliminary mechanistic hypothesis are reported herein.

Synthesis and cytotoxicity of novel 14α-O-(andrographolide-3-subsitutedisoxazole-5-carboxylate) derivatives

Simple and efficient method was established for the synthesis of a new family of 14α-O-(andrographolide-3-subsitutedisoxazole-5-carboxylate) derivatives (10a-j) from naturally occurring andrographolide (1) by selective esterification with propiolic acid at C-14 using protection and deprotection strategy followed by metal free 1,3-dipolar cycloaddition with aryl nitrile oxides. All the synthesised derivatives were tested for their cytotoxicity against HCT-15, HeLa and DU145 cell lines. Most of the compounds exhibited improved cytotoxic activity compared to the parent molecule andrographolide (1), as the compounds 10b, 10c, 10i, 10j, 11d and 11f showed significant cytotoxicity against three cancer cell lines. Except the compound 10b and 11d, all the compounds did not inhibit the normal cell line (VERO). Based on these studies isoxazole ester derivatives at C-14 of andrographolide with various substitutions promoting anticancer activities and better safety profiles. Further studies in this direction with improved water solubility and oral bioavailability are in progress in future.

Development of Poly(1,8-octanediol-co-citrate-co-ascorbate) Elastomers with Enhanced Ascorbate Performance for Use as a Graft Coating to Prevent Neointimal Hyperplasia.

Small-diameter expanded polytetrafluoroethylene (ePTFE) graft surfaces have poor long-term patency due to limited endothelial cell (EC) coverage and anastomotic intimal hyperplasia. Multifunctional elastomers that coat the ePTFE graft surface to promote EC adhesion while simultaneously inhibiting intimal hyperplasia are highly desirable. Poly(diol-co-citrate) (PDC), a thermoset elastomer, is biodegradable, biocompatible, and mimics vascular mechanical properties. Engineering antioxidant components into PDC polymeric structures improves biocompatibility by attenuating oxidative stress yet is limited by bioavailability. Herein, we develop a new ascorbate protection and deprotection strategy (APDS) for loading bioactive ascorbic acid into the structure of PDC elastomers to improve poly(1,8-octanediol-co-citrate-co-ascorbate) (POCA) prepolymer ascorbate activity. Elastomers cured from APDS POCA prepolymers provide twice the active ascorbate sites on the elastomer surface (35.19 ± 1.64 ng mg-1 cm-2) versus unprotected POCA (Un.POCA, 18.31 ± 0.97 ng mg-1 cm-2). APDS POCA elastomers displayed suitable mechanical properties for vascular graft coating [Young's modulus (2.15-2.61 MPa), elongation (189.5-214.6%) and ultimate tensile strength (2.73-3.61 MPa)], and superior surface antioxidant performance through 1,1-diphenyl-2-picrylhydrazyl free radical scavenging and lipid peroxidation inhibition as compared to poly(1,8-octanediol-co-citrate) (POC) and Un.POCA. Hydrolytic degradation of APDS POCA occurred within 12 weeks under physiological conditions with a mass loss of 25.8 ± 3.4% and the degradation product retaining ascorbate activity. APDS POCA elastomer surfaces supported human aortic endothelial cell proliferation while inhibiting human aortic smooth muscle cell proliferation in vitro. APDS POCA elastomer surfaces displayed superior decomposition of S-nitrosothiols compared to POC and Un.POCA. Taken together, these findings indicate the potential of APDS POCA elastomers to serve as bioactive, therapeutic coatings that enhance the long-term patency of small diameter ePTFE grafts.

Synthesis of Phospho-Amino Acid Analogues as Tissue Adhesive Cement Additives.

In this paper we report the synthesis of a library of phospho-amino acid analogues, via a novel single-step allyl-phosphoester protection/Pd-mediated deprotection strategy. These phosphoserine and phosphotyrosine analogues were then applied as additives to create adhesive calcium phosphate cements, allowing us to probe the chemical origins of the increased surface binding strength. We demonstrate the importance of multiple calcium binding motifs in mediating adhesion, as well as highlighting the crucial role played by substrate hydrophobicity and orientation in controlling binding strength.

Facile deprotection of F-BODIPYs using methylboronic acid.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) are deprotected through removal of the –BF2 moiety upon treatment with methylboronic acid. The tolerance of various substitution patterns about the dipyrrinato core is demonstrated via the deprotection of thirteen F-BODIPYs and an F-aza-BODIPY. Work-up with aq. HBr affords the desired dipyrin HBr salt in quantitative yield without need for purification.

Mild deprotection of the N-tert-butyloxycarbonyl (N-Boc) group using oxalyl chloride.

We report a mild method for the selective deprotection of the N-Boc group from a structurally diverse set of compounds, encompassing aliphatic, aromatic, and heterocyclic substrates by using oxalyl chloride in methanol. The reactions take place under room temperature conditions for 1–4 h with yields up to 90%. This mild procedure was applied to a hybrid, medicinally active compound FC1, which is a novel dual inhibitor of IDO1 and DNA Pol gamma. A broader mechanism involving the electrophilic character of oxalyl chloride is postulated for this deprotection strategy.

4 + 2]-Cycloadditions of a thiazol-based tricyclic 1,4-diphosphinine and a new easy 1,4-diphosphinine protection deprotection strategy.

Diels-Alder-reactions of a thiazol-2-thione-based, tricyclic 1,4-di-phosphinine were investigated, showing that the central aromatic π-system can react with various dienophiles. The reaction with 4-phenyl-1,2,4-triazoline-3,5-dione was special as the product revealed a remarkable sensitivity towards light, thus enabling the photochemical deprotection of the tricyclic 1,4-diphosphinine.

Deprotection Strategies for Thioimidates during Fmoc Solid-Phase Peptide Synthesis: A Safe Route to Thioamides.

Thioamides are important biophysical probes of peptide folding but are prone to α-C epimerization during Fmoc solid-phase peptide synthesis. The stereochemical integrity of thioamide-containing peptides can be dramatically improved by protecting the thioamide as a thioimidate during synthesis. A drawback of this approach, however, is that once synthesis of the peptide is complete, regeneration of the thioamide requires the toxic, corrosive, and flammable gas H2S. This work examines several approaches to supplant H2S as a deprotection reagent in favor of a safer and more convenient alternative. Ultimately, a new application of the 4-azidobenzyl protecting group to thioamides was found to provide the most suitable means of both protection of α-C stereochemistry and conversion back to thioamide.

Synthesis and cytotoxicity of novel 14α-O-(1,4-disubstituted-1,2,3-triazolyl) ester derivatives of andrographolide

A series of novel 14α-O-(1,4-disubstituted-1,2,3-triazolyl) ester derivatives of andrographolide (5a-n) were synthesized from andrographolide (1). For this endeavour, selective esterification at C-14 hydroxyl group of andrographolide (1) with propiolic acid via protection, deprotection strategy followed by 1,4-regioselective [1,3]dipolar cycloaddition of alkyne, azide using Cu(I) catalyzed Click chemistry. All the synthesized derivatives were screened for their cytotoxicity on HCT-15, HeLa and K562 cell lines. Compounds 5c and 5j showed highest activity against HCT-15 and K562 cell lines whereas compound 5a displayed activity in all the three cell lines. Loss of cell viability was not observed with the non-transformed cell line MRC-5 with compounds 5j, 5k, 5h and 2 indicating cytotoxic activity of these compounds towards cancer cell lines. Further, molecular docking analysis and SAR studies of highly active compounds 5c and 5j revealed enhanced binding affinity to the target NF-κB protein.

Enhancement of Galloylation Efficacy of Stigmasterol and β-Sitosterol Followed by Evaluation of Cholesterol-Reducing Activity.

In this study, incorporation of gallic acid into typical phytosterols (β-sitosterol and stigmasterol) through Steglich esterification was optimized employing the protection and deprotection strategy. A novel mechanism leading to side esterification was discovered. Complication of the phenolic hydroxyl groups and side reactions were successfully reduced under the optimized conditions. The structural identity and purity of galloyl stigmasterol and galloyl β-sitosterol were confirmed by NMR, FT-IR, and HPLC-MS. Evaluation of galloyl β-sitosterol and galloyl stigmasterol revealed their excellent antioxidant and cholesterol-reducing activities. Significant enhancement of cholesterol-reducing activity by galloylation was unveiled especially for β-sitosterol. Galloyl β-sitosterol had slightly better antioxidant activity at ambient temperature and better cholesterol-reducing activity. Molecular modeling suggested that a subtle difference of galloyl β-sitosterol and galloyl stigmasterol in activities could be attributed to variation of molecular rigidity and conformation. The excellent properties of galloyl β-sitosterol and galloyl stigmasterol suggested their great potential application in the food industry.

Synthesis of Sulfonimidamide‐Based Amino Acid Building Blocks with Orthogonal Protecting Groups

Herein, we report the synthesis of novel sulfonimidamides (SIAs) based on amino acid building blocks using a one‐pot method from tert‐butyldiphenylsilyl‐protected (TBDPS) sulfonamides, as well as exploration of orthogonal deprotection strategies. Among the several protecting groups investigated, TBDPS showed higher conversion, allowed UV detection and simple diastereomeric separation; in particular in combination with amino acid tert‐butyl esters. Moreover, we applied the present method to synthesize cyclic five‐membered acyl sulfonimidamides in two steps. The described synthesis of SIA‐based amino acid building blocks in combination with the orthogonal protection groups provide access to unnatural amino acid building blocks useful for further incorporation into larger molecules, such as peptide‐based transition‐state analogues and peptidomimetics. The chirality of the SIA group, as well as its additional point of diversity provided by the extra NH group, creates opportunities for the development of unique compound libraries that explore new chemical space, which is of considerable importance for the pharmaceutical and agrochemical industry.

Reduction of cysteine-S-protecting groups by triisopropylsilane.

Triisopropylsilane (TIS), a hindered hydrosilane, has long been utilized as a cation scavenger for the removal of amino acid protecting groups during peptide synthesis. However, its ability to actively remove S-protecting groups by serving as a reductant has largely been mischaracterized by the peptide community. Here, we provide strong evidence that TIS can act as a reducing agent to facilitate the removal of acetamidomethyl (Acm), 4-methoxybenzyl (Mob), and tert-butyl (But ) protecting groups from cysteine (Cys) residues in the presence of trifluoroacetic acid (TFA) at 37°C. The lability of the Cys protecting groups in TFA/TIS (98/2) in this study are in the order: Cys(Mob)>Cys(Acm)>Cys(But ), with Cys(Mob) being especially labile. Unexpectedly, we found that TIS promoted disulfide formation in addition to aiding in the removal of the protecting group. Our results raise the possibility of using TIS in orthogonal deprotection strategies of Cys-protecting groups following peptide synthesis as TIS can be viewed as a potential deprotection agent instead of merely a scavenger in deprotection cocktails based on our results. We also tested other common scavengers under these reaction conditions and found that thioanisole and triethylsilane were similarly effective as TIS in enhancing deprotection and catalyzing disulfide formation. Our findings reported herein show that careful consideration should be given to the type of scavenger used when it is desirable to preserve the Cys-protecting group. Additional consideration should be given to the concentration of scavenger, temperature of the reaction, and reaction time.

P, P-Dimethylformylphosphine: The Phosphorus Analogue of N, N-Dimethylformamide.

The neutral PCO derivative Ph3GePCO undergoes addition of borohydride to generate the anion [Ph3GePC(H)OBPh3]-. A subsequent deprotection strategy provides access to the monomethyl and dimethylformyl phosphines, Ph3GeP(Me)C(H)O and Me2PC(H)O, respectively. The latter species which is the phosphorus analogue of DMF, was characterized and shown to complex via phosphorus to Ru. A computational study also highlights the similarities and differences between the P- and N-containing molecules.

In Situ Network Formation in PBT Vitrimers via Processing-Induced Deprotection Chemistry.

Although the network dynamics and mechanical properties of poly(butylene terephthalate) vitrimers can to some extent be controlled via chemical and physical approaches, it remains a challenge to be able to process PBT vitrimers with the same processing conditions via, for example, injection molding as neat PBT. Here, it is shown that the use of protected pentaerythritol as a latent cross-linker and the use of a Zn(II) transesterification catalyst allows for the in situ dynamic network formation in PBT during processing, with a delayed onset of gelation. This process can be controlled by adjusting the processing temperature, (protected) cross-linker content, and the type of protection group. This solvent-free deprotection strategy opens the way to high production rates of PBT vitrimer products via injection molding with the combination of low viscosity during processing and vitrimer characteristics in the final product.

Synthesis of Novel Oxazolyl Amino Acids and Their Use in the Parallel Synthesis of Disubstituted Oxazole Libraries

Novel chiral oxazolyl alanine and homologues are synthesized and utilized as building blocks for the solid-phase parallel synthesis of novel trifunctional oxazole small molecules in good to excellent overall yields and with high purity. The orthogonal deprotection strategy of oxazolyl amino acids, prepared from serine methyl ester and amino acids such as aspartic and glutamic acids, allows multiple sites of diversification to make a variety of pharmacologically relevant small molecules. The general nature of this approach allows the preparation of a large number of small molecules and peptidomimetics.