Peptide thioester preparation based on an N-S acyl shift reaction mediated by a thiol ligation auxiliary

Formation of peptide thioesters, based on an N to S acyl shift mediated by an auxiliary, N-4,5-dimethoxy-2-mercaptobenzyl (Dmmb) group, under acidic conditions, is described. The protected peptide was assembled on a hydroxymethylphenylacetamidomethyl resin via an N-Dmmb-amino acid residue according to standard Fmoc solid-phase peptide synthesis following treatment with trifluoroacetic acid. The peptide α-thioester was released from the resin by reaction with 2-mercaptoethanesulfonic acid in the presence of N,N-diisopropylethylamine.     

Pyruvoyl, a novel amino protecting group on the solid phase peptide synthesis and the peptide condensation reaction

In one of the peptide condensation methods termed thioester method, an amino protecting group is required in the lysine side chain. In this study, to investigate the efficiency of the pyruvoyl group as an amino protecting group, we synthesized N^α-fluorenylmethoxycarbonyl (Fmoc)-N^ε-pyruvoyl-lysine and introduced it into peptides and glycopeptides by the ordinary Fmoc-based solid phase peptide synthesis. The pyruvoyl peptide could be condensed with a peptide thioester by the thioester method, and this protecting group was easily removed by o-phenylenediamine treatment without significant side reactions.     

A useful ring transformation route to novel thiazepino[7,6-b]indoles from monochloro-β-lactam-fused 1,3-thiazino[6,5-b]indoles,analogues of cyclobrassinin

The Staudinger ketene-imine cycloaddition reactions of cyclobrassinin phytoalexin analogues 2-aryl-4,9-dihydro-1,3-thiazino[6,5-b]indoles with chloroacetyl chloride as a ketene source were investigated under different conditions. Both β-lactam ring formation and the N-chloroacetylation of the indole moiety took place. The indole N-chloroacetyl group can be easily removed by treatment in the presence of silica gel in methanol at reflux temperature. The selective β-lactam formation can be also achieved in certain cases under milder Staudinger conditions. The treatment of azeto[2,1-b]thiazino[6,5-b]indole-1-one derivatives with sodium ethoxide in ethanol provided the novel thiazepino[7,6-b]indole ring systems in a one-step ring transformation. The structures of the new ring systems were determined by means of IR and NMR spectroscopy.     

Fluorinated N-[2-(haloalkyl)phenyl]imidoyl chloride, a key intermediate for the synthesis of 2-fluoroalkyl substituted indole derivatives via Grignard cyclization process

Fluorinated N-[2-(haloalkyl)phenyl]imidoyl chloride, which was readily available from the corresponding anilines by using Uneyama's one-pot synthesis of fluorinated imidoyl chloride, was found to be a key intermediate for the facile synthesis of 2-fluoroalkyl substituted indole derivatives via the Grignard cyclization process. The bromination of 3-methyl group of 3-methyl-2-trifluoromethyl indole with NBS/CCl₄ led to the formation of 3-bromomethyl substituted indole which can be further utilized to synthesize some new and biologically interested indole derivatives.     

A carbamoyl-protective group for tyrosine that facilitates purification of hydrophobic synthetic peptides

The Boc-N-methyl-N-[2-(methylamino)ethyl]carbamoyl group (Boc-Nmec) is reported as a new side chain-protective group for tyrosine in Fmoc solid-phase peptide synthesis. Tyrosine is incorporated into the peptide as Fmoc-Tyr(Boc-Nmec)-OH by standard coupling methods. During the cleavage of the peptide from the resin with TFA the Boc group is simultaneously cleaved while the cationic N-methyl-N-[2-(methylamino)ethyl]carbamoyl group remains attached to the tyrosine residue, thereby increasing the solubility of the peptide. After purification of the peptide, the Nmec protective group can be cleaved under neutral or mild alkaline conditions via an intramolecular cyclization reaction.     

Enhancement in the rate of conversion of peptide Cys-Pro esters to peptide thioesters by structural modification

We previously reported that the peptide containing a Cys-Pro ester (CPE) moiety is spontaneously transformed into a peptide thioester via an N to S acyl shift followed by diketopiperazine formation. In an attempt to identify more reactive structures for the formation of a peptide thioester, we modified the CPE structure, in which the Pro residue in the CPE moiety was replaced with N-substituted glycine derivatives. These peptides were transformed into a peptide thioester more rapidly. Alternatively, the addition of an amino acid residue at the C-terminus of the CPE moiety also accelerated thioester formation.     

4-Methoxybenzyloxymethyl group as an N-protecting group for histidine to eliminate side-chain-induced racemization in the Fmoc strategy

The 4-methoxybenzyloxymethyl (MBom) group was introduced at the N^π-position of histidine, and its utility was examined under the conditions for peptide synthesis by Fmoc strategy. The N^π-MBom group proved to prevent the risk of racemization during incorporation of the His residue and to possess all of the chemical properties required for Fmoc chemistry. The side reaction associated with formaldehyde generated from the N^π-MBom group upon acidolysis could be effectively prevented by performing the standard TFA treatment in the presence of methoxyamine·hydrochloride (MeONH₂·HCl).     

Novel asymmetric dihetarylethenes derived from N-isopropylindole and thiophene: synthesis and photochromic properties

Novel asymmetric dihetarylethenes, viz., 3-(1-isopropyl-5-methoxy-2-methyl-1H-indol-3-yl)-4-(3-thienyl)furan-2,5-dione and 1-alkyl/aryl-3-(1-isopropyl-5-methoxy-2-methyl-1H-indol-3-yl)-4-(3-thienyl)-1H-pyrrole-2,5-diones, were obtained. These dihetarylethenes exhibit photochromism in solutions. Replacement of the N-methyl group in the indole fragment by the N-isopropyl group imparts photochromic properties to the resulting furan-2,5-dione derivative. The open forms of (N-isopropylindol-3-yl)pyrrole-2,5-diones are characterized by lower quantum yields of fluorescence, and their cyclic forms are thermally more stable.     

Enrichment of N-terminal sulfonated peptides by a water-soluble fullerene derivative and its applications to highly efficient proteomics

Recent studies have shown that N-terminal sulfonation of tryptic peptides by various sulfonating molecules greatly improves their post-source decay processes (e.g., in matrix-assisted laser desorption ionization) or the gas phase fragmentation processes (e.g., in tandem mass spectrometer), enhancing the ability to identify their sequences de novo. In the present work, we have demonstrated that incorporation of water-soluble C₆₀-N,N-dimethylpyrrolidinium iodide selectively precipitates the 4-sulfophenyl isothiocyanate-modified peptide (SPITC-GGYR, SPITC-ASHLGLAR) by forming a noncovalent ion pair to the SO₃⁻ group of the SPITC, and thereby the C₆₀ derivative can be utilized to enrich the modified peptide. Electrospray ionization (ESI) mass analyses show that the cationic SPITC-GGYR and SPITC-ASHLGLAR species are well separated from unmodified peptides and the modified peptides are subsequently detached from the C₆₀ derivative upon using an acidic solution.     

Furan ring opening–indole ring closure: recyclization of 2-(2-aminophenyl)furans into 2-(2-oxoalkyl)indoles

The acid-catalyzed rearrangement of 5-alkyl-2-[2-(sulfonylamino)phenyl]furans into 2-(2-oxoalkyl)indoles is described. When the N-sulfonyl group in the starting compounds was displaced by an N-acyl group, the corresponding indoles were not formed under the same reaction conditions due to the in situ indole deacylation and decomposition. The presence of an alkyl group at the C5 position of the furan ring is also crucial for the efficiency of the process. The discussed rearrangement provides a simple and efficient approach to 2-(2-oxoalkyl)indoles.     

Synthesis, structure, and heck cyclization of the syn- and anti-atropisomers of N-acyl-N-(4-methyl-3,6-dihydro-2H-pyran-3-yl)-2-iodo-4,6-dimethylaniline

The reaction of 2-iodo-2,4-dimethylaniline with 3,4-dibromo-4-methyltetrahydro-2H-pyran, followed by treatment with acetyl bromide or 4-nitrobenzoyl chloride, gave syn- and anti-atropisomers of N-(2-iodo-4,6-dimethylphenyl)-N-(4-methyl-3,6-dihydro-2H-pyran-3-yl)acetamide and N-(2-iodo-4,6-dimethylphenyl)-N-(4-methyl-3,6-dihydro-2H-pyran-3-yl)-4-nitrobenzamide. Heating of the acetamide derivative with palladium(II) acetate in the presence of copper(II) acetate and N,N,N′,N′-tetramethylethane-1,2-diamine resulted in heterocyclization to N-acetyl-4a,6,8-trimethyl-1,4a,9,9a-tetrahydropyrano[3,4-b]indole.     

Synthesis of pyrrole- and indole-containing nitropropanoates on the basis of β-heteryl-α-nitroacrylates

Synthetic approach to β-furyl- and β-thienyl-α-nitroacrylates was modified. It was established that these nitroethenes react with pyrrole, N-methylpyrrole, and substituted indole derivatives in the absence of catalyst to afford products of the alkylation of pyrrole C² and indole C³ reaction sites. A possibility of this reaction to proceed under “dry reaction” condition on a silicagel surface was shown.     

Synthesis of N‐Glycoside Analogs via Thionolactones

Indolyl N‐glycoside analogs were obtained by a two‐step sequence via indole N‐thioamides. Treatment of thionobutyrolactone with indolylmagnesium bromide provides the corresponding indole N‐thioamide. The use of 10:1 toluene:THF as solvent is important in favoring N‐ over C3‐acylation. Treatment of the ω‐hydroxythioamide with 2 equiv of Meerwein's reagent followed by sodium borohydride gives the corresponding N‐(tetrahydrofuranyl)indole. Addition of carbon nucleophiles gives access to ketose nucleoside analogs, while activation of the ω‐hydroxyl group can give access to tetrahydrothiophene N‐glycosides.     

Synthesis of substituted 4-(1H-indol-6-yl)-1H-indazoles as potential PDK1 inhibitors

The development of a preparative route to a series of novel 4-(1H-indol-6-yl)-1H-indazole compounds as potential PDK1 inhibitors is described. The synthetic strategy centres on the late-stage Suzuki cross-coupling of N-unprotected indazole and indole fragments. The use of a monoligated palladium catalyst system was found to be highly beneficial in the cross-coupling reaction. The indazole and indole fragments were constructed by diazotisation/cyclisation and S_NAr/reductive cyclisation sequences, respectively.     

Preparation of RCM substrates for azepinoindole synthesis: reductive amination versus tetrahydro-γ-carboline formation

Treatment of N-(phenylsulfonyl)-2-vinyl-3-indolecarbaldehydes with primary aliphatic amines under mild reductive amination conditions leads to tetrahydro-γ-carbolines in high yield. The process can be suppressed by changing the protecting group at the indole nitrogen for a methoxymethyl group, thus allowing the preparation of RCM substrates for azepinoindole synthesis.     

Halophilic reaction of N-sodium-substituted azoles with polyhaloperfluoroethanes containing different vicinal halogen atoms

The reactions of N-sodium-substituted azoles with 2-chloro-1-iodo- tetrafluoroethane, 1,2-dichloro-1-iodotrifluoroethane, and 1,2-dibromo-1-chlorotrifluoroethane have been investigated. As shown for iodo derivatives, it is the chlorine rather than the iodine atom that is substituted by the heterocyclic residue, which is consistent with the halophilic reaction mechanism. In the case of indole, the products of simultaneous N-iodopolyfluoroalkylation and ring-iodination have been isolated. The reaction with 1,2-dibromo-1-chlorotrifluoroetane yields N-(2-bromo-2-chlorotrifluoroethyl)azoles accompanied by minor amounts of N-(2,2-dibromotrifluoroethyl) derivatives as by-products.     

Ozonolysis of ortho-alkenylanilines

Ozonolysis of N-acyl-2-(1-methylbut-2-enyl)- and N-acyl-2-(cyclopent-2-enyl)anilines followed by treatment with NaBH₄ afforded the corresponding 2-(2-hydroxyethyl-1-methyl) and 2-(1,5-dihydroxypent-2-yl) derivatives. The reaction can be directed to indole derivatives by varying the nature of both the acyl group and reducing reagent.     

An amide orthoesterification route to N-(1′-alkylthioglucopyranosyl)indoles

The addition of 3-methylindolylmagnesium bromide to tetra-O-benzyl-α-d-gluconothionolactone yields the expected indole N-gluconothioamide as its hemiorthothioamide tautomer. The thiol function is alkylated to yield the corresponding orthothioamide, a 1′-alkylthio-substituted N-glycoside. Alternatively, the 1′-alkylthio-N-glycoside can be accessed from the corresponding indole N-gluconamide via a boron trifluoride-etherate mediated orthoesterification with ethanethiol. Radical reduction of the orthothioamide yields the N-glycosides in 2:1 stereoselectivity in favor of the β-N-glycoside, while reduction via the oxonium ion leads to an improved 6:1 selectivity.     

Reactivity and regioselectivity in the synthesis of spiroindoles via indole o-quinodimethanes. An experimental and computational study

The 1,3-dipolar cycloaddition reactions of stable nitrile oxides with indole o-quinodimethanes have been examined. In all cases the ‘exo-anti’ addition products, dispiroisoxazolines, were isolated in moderate to good yields (25-47%). In addition, from the reaction of one of the indole quinodimethanes with mesitonitrile oxide the ‘exo-syn’ addition product was isolated in 7% yield along with the remarkable indole quinodimethane dimerization and cycloaddition product, which was isolated in 13% yield. An analogous dimerization and cycloaddition product was isolated in 18% yield from the reaction of the N-acetyl-indole quinodimethane with mesitonitrile oxide. In the case of the reaction of the N-benzoylindole quinodimethane with the 2,6-dichlorobenzonitrile oxide an oxime was also isolated in 13% yield. The proposed reaction mechanism is supported by semiempirical (AM1) MO calculations via FMO interactions. The observed selectivity was explained by an investigation of the transition states carried out also for analogous dispiroisoxazolines.     

An N-protection free ligation of the peptide thioester and the peptide with an N-alkoxy- or N-aryloxyamino group at its N-terminus

Peptides with an N-alkoxy or N-aryloxy amino acid at their N-terminus were synthesized and successfully ligated with a peptide thioester by silver ion activation under a slightly acidic condition without requiring protection of the side chain amino groups. The N-methoxy group was easily cleaved by the SmI₂ reduction in CH₃OH aq. to obtain the desired peptide with a native peptide bond. This method was successfully applied to the synthesis of the human atrial natriuretic peptide showing the efficiency of the novel ligation.