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At least 19 recordsLinked to original sources
PubMed2004

N-h insertion reactions of Boc-amino acid amides: solution- and solid-phase synthesis of pyrazinones and pyrazines.

A series of alpha-diazo-beta-ketoesters were reacted with Boc amino acid amides in the presence of rhodium octanoate catalyst. The resulting N-H insertion products were treated with acid, providing the 1,4-azine intermediates, which were oxidized by air to form the corresponding pyrazine-6-one products. The pyrazine-6-ones were further derivatized by N-alkylation or by conversion to the arylpyrazines using sequential bromination and Suzuki coupling reactions. [reaction: see text]

Amino Acids↗
PubMed1993

Identification and suppression of decomposition during carbodiimide-mediated reactions of Boc-amino acids with phenols, hydroxylamines and amino acid ester hydrochlorides.

N-tert-Butoxycarbonylamino acids (Boc-Xaa-OH) were coupled with p-nitrophenol (HONp) in dichloromethane using N,N'-dicyclohexylcarbodiimide (DCC) and N-ethyl-N'(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), and the products were identified and quantitated by high-performance liquid chromatography. Boc-Xaa-OH with Xaa = Val was coupled also with pentafluorophenol (HOPf) and hydroxy-containing additives (HOR), and the products were similarly determined as the methylamides. EDC-mediated reactions of Boc-Xaa-OH gave 8-25% of Boc-Xaa-Xaa-OR as well as Boc-Xaa-OR for R = Ph, Np, Pf, benzotriazole (Bt) and 5-norbornene-endo-2,3-dicarboxamide; DCC-mediated couplings, 5-7% for R = Np and Bt. No dimer was formed in couplings with N-hydroxysuccinimide or 3-hydroxy-4-oxo-3,4-dihydro-1,2,3-benzotriazine. Dimerization was eliminated from DCC-mediated reactions by the addition of 1 equiv. of N-methylmorpholine, from the EDC-mediated reactions by carrying them out in pyridine. Dimerization is attributed to formation of the intermediate 2-alkoxy-5(4H)-oxazolone that undergoes fragmentation to the N-carboxyanhydride, which reacts with the alcohol giving amino acid ester. Ester produces dimer by aminolysis of the O-acylisourea. Decomposition (1.4%) was also detected by analysis for H-Val-Phe-OMe in DCC-mediated reactions of Boc-valine with H-Phe-OMe, and was demonstrated to be caused by the hydrochloride of the ester salt that had been neutralized with N-methylmorpholine. Decomposition was eliminated by the addition of 5% of pyridine, which also had the beneficial effect of suppressing N-acylurea formation.

Amino Acids↗
PubMed1997

Defensive production of formic acid (80%) by a carabid beetle (Galerita lecontei).

The carabid beetle Galerita lecontei has a pair of abdominal defensive glands that secrete a mixture of formic acid, acetic acid, and lipophilic components (long-chain hydrocarbons and esters). Formic acid, at the concentration of 80%, is the principal constituent. The beetle ejects the secretion as a spray, which it aims accurately toward parts of the body subjected to assault. At full capacity, the glands store 4.5 mg of formic acid (3% of body mass), enough for upward of six ejections. The beetle reloads the glands at a rate of 126 microg of formic acid per day. For the approximately 500 secretory cells of the glands, this means an hourly output of 10 ng of formic acid per cell, or about 5% of cell volume. Replenishing empty glands to their full formic acid load takes the beetle an estimated 37 days. Replenishing the 0.7 mg of formic acid expended in a single discharge takes 5.5 days.

Animals↗
PubMedDate not supplied

Powerful solvent systems useful for synthesis of sparingly-soluble peptides in solution.

Our maximum protection strategy for the synthesis of human parathyroid hormone(1-84) indicates that fully protected peptide segments in the form of Boc-peptide phenacyl (Pac) ester are relatively soluble in ordinary organic solvents such as DMF, NMP or DMSO, which are suitable for coupling segments. However, about 1% of such segments synthesized were found to be insoluble even in the most polar solvent, DMSO. Thus, a more powerful solvent which can be used for their peptide synthesis was pursued. Among the solvent systems tested, a mixture of trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) and trichloromethane (TCM) or dichloromethane (DCM) was found to be most powerful for dissolving such sparingly-soluble protected peptides. These solvent systems were confirmed to be useful for the removal reaction of the carboxy-terminal Pac esters from the sparingly-soluble segments. They were then tested for the coupling reactions of fully protected Boc-peptides with other sparingly-soluble peptide esters. The TFE/TCM or TFE/DCM system was extremely useful for coupling segments without danger of racemization and of trifluoroester formation, if WSCI was used as the coupling reagent in the presence of 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine (HOOBt).

Amino Acid Sequence↗
PubMed1992

Unexpected racemization of proline or hydroxy-proline phenacyl ester during coupling reactions with Boc-amino acids.

When L-proline or O-benzyl-trans-4-hydroxy-L-proline phenacyl ester was coupled with Boc-amino acids in dimethylformamide using water-soluble carbodiimide (WSCI) in the presence of anhydrous 1-hydroxybenzotriazole (HOBt) as coupling reagents, extensive racemization was observed at the C alpha of the proline or hydroxy-proline residue. The extent of racemization was measured by HPLC after the coupling with Boc-L-Leu-OH in the presence or absence of HOBt. The extent of racemization increased when HOBt was added to the reaction mixture, but greatly decreased when it was not, indicating that HOBt was needed for inducing racemization. Almost no racemization was observed when the coupling reaction was carried out by the mixed anhydride procedure in tetrahydrofuran or by the carbodiimide method in dichloromethane without using HOBt. In the case of coupling reactions with ordinary L-amino acid phenacyl esters, no racemization was observed. Examination of some model systems yielded sufficient evidence to prove that HOBt is an efficient catalyst for racemizing proline or hydroxy-proline phenacyl ester not only in the stage of cyclic intermediate formation but also in the opening of the ring structure. Thus, the racemization reaction was found to be closely related to the formation of the cyclic carbinol-amine derivative.

Acetophenones↗
PubMedDate not supplied

Branched oligoester microspheres fabricated by a rapid emulsion solvent extraction method.

Methyl formate was used as the solvent of biodegradable oligoesters for the fabrication of microspheres with encapsulated bovine serum albumin (BSA). The procedure of dispersion of the double emulsion of the w/o/w type and its dilution and solvent extraction is very rapid, taking only several minutes. A higher yield and better encapsulation efficiency were obtained with copolymers of DL-lactic acid with mannitol than with pure linear poly DL-lactic acid. The procedure was accelerated, and yields and encapsulation efficacy were enhanced by the addition of 5% methyl formate to the external water phase. The microspheres were smaller than 100 microm. No benefits were obtained from the addition of wetting agents or other additives to the intermediate (oligoesteric) phase. Further development should concentrate particularly on hydrodynamic conditions and optimization of the composition of the external phase.

Animals↗
PubMed2001

The development and application of a novel safety-catch linker for BOC-based assembly of libraries of cyclic peptides.

Cyclic peptides are appealing targets in the drug-discovery process. Unfortunately, there currently exist no robust solid-phase strategies that allow the synthesis of large arrays of discrete cyclic peptides. Existing strategies are complicated, when synthesizing large libraries, by the extensive workup that is required to extract the cyclic product from the deprotection/cleavage mixture. To overcome this, we have developed a new safety-catch linker. The safety-catch concept described here involves the use of a protected catechol derivative in which one of the hydroxyls is masked with a benzyl group during peptide synthesis, thus making the linker deactivated to aminolysis. This masked derivative of the linker allows BOC solid-phase peptide assembly of the linear precursor. Prior to cyclization, the linker is activated and the linear peptide deprotected using conditions commonly employed (TFMSA), resulting in deprotected peptide attached to the activated form of the linker. Scavengers and deprotection adducts are removed by simple washing and filtration. Upon neutralization of the N-terminal amine, cyclization with concomitant cleavage from the resin yields the cyclic peptide in DMF solution. Workup is simple solvent removal. To exemplify this strategy, several cyclic peptides were synthesized targeted toward the somatostatin and integrin receptors. From this initial study and to show the strength of this method, we were able to synthesize a cyclic-peptide library containing over 400 members. This linker technology provides a new solid-phase avenue to access large arrays of cyclic peptides.

Combinatorial Chemistry Techniques↗
PubMed1976

[Comparative analysis of the theoretical methods currently used to study the conformational properties of molecules of pharmacologic interest].

The study of a simple conformational problem, the rotation about the ester bond common to a series of R1COOR2 formula molecules, shows that CNDO/2, INDO and PCILO methods overestimate the interaction term between non bonded atoms. This leads to the prevision of very low barriers and amounts for the CNDO method to the failure in the prevision of some minima. Because of the exaggerated overestimation of this term, the EHT method seems not to be useful except when the steric interaction constitutes the dominant term in the conformation energy.

Acetates↗
PubMed1975

Oxidase-peroxidase enzymes of Datura innoxia. Oxidation of reduced nicotinamide-adenine dinucleotide in the presence of formylphenylacetic acid ethyl ester.

The oxidase-peroxidase from Datura innoxia which catalyses the oxidation of formylphenylacetic acid ethyl ester to benzoylformic acid ethyl ester and formic acid was also found to catalyse the oxidation of NADH in the presence of Mn2+ and formylphenylacetic acid ethyl ester. NADH was not oxidized in the absence of formylphenylacetic acid ethyl ester, although formylphenylacetonitrile or phenylacetaldehyde could replace it in the reaction. The reaction appeared to be complex and for every mol of NADH oxidized 3-4 g-atoms of oxygen were utilized, with a concomitant formation of approx. 0.8 mol of H2O2, the latter being identified by the starch-iodide test and decomposition by catalase. Benzoylformic acid ethyl ester was also formed in the reaction, but in a nonlinear fashion, indicating a lag phase. In the absence of Mn2+, NADH oxidation was not only very low, but itself inhibited the formation of benzoylformic acid ethyl ester from formylphenylacetic acid ethyl ester. A reaction mechanism for the oxidation of NADH in the presence of formylphenylacetic acid ethyl ester is proposed.

Catalase↗
PubMed1975

Oxidase-peroxidase enzymes of Datura innoxia. Oxidation of formylphenylacetic acid ethyl ester.

An enzyme system from Datura innoxia roots oxidizing formylphenylacetic acid ethyl ester was purified 38-fold by conventional methods such as (NH4)2SO4 fractionation, negative adsorption on alumina Cy gel and chromatography on DEAE-cellulose. The purified enzyme was shown to catalyse the stoicheiometric oxidation of formylphenylacetic acid ethyl ester to benzoylformic acid ethyl ester and formic acid, utilizing molecular O2. Substrate analogues such as phenylacetaldehyde and phenylpyruvate were oxidized at a very low rate, and formylphenylacetonitrile was an inhilating agents, cyanide, thiol compounds and ascorbic acid. This enzyme was identical with an oxidase-peroxidase isoenzyme. Another oxidase-peroxidase isoenzyme which separated on DEAE-chromatography also showed formylphenylacetic acid ethyl ester oxidase activity, albeit to a lesser extent. The properties of the two isoenzymes of the oxidase were compared and shown to differ in their oxidation and peroxidation properties. The oxidation of formylphenylacetic acid ethyl ester was also catalysed by horseradish peroxidase. The Datura isoenzymes exhibited typical haemoprotein spectra. The oxidation of formylphenylacetic acid ethyl ester was different from other peroxidase-catalysed reactions in not being activated by either Mn2+ or monophenols. The oxidation was inhibited by several mono- and poly-phenols and by catalase. A reaction mechanism for the oxidation is proposed.

Formates↗
PubMed1994

p-Chlorotetrafluorophenyl esters of N-protected amino acids.

p-Chlorotetrafluorophenyl (Tfc) esters of protected amino acids and peptides are more reactive than are the well known pentafluorophenyl (Pfp) esters. Two reagents, p-chlorotetrafluorophenyltrifluoroacetate (Tfc-OTfa) and di-(p-chlorotetrafluorophenyl)carbonate (di-Tfc-carbonate), can be used for their syntheses, thereby avoiding use of the allergic dicyclohexylcarbodiimide. This is especially important for bulk preparations. Many Fmoc- and Boc-amino acid-OTfc esters have been synthesized and characterized. The hexadecameric tandem repeat H-(AlaAlaLysPro)4-OH was synthesized using di-Tfc-carbonate for the preparation of Tfc-esters.

Alanine↗
PubMed1997

Ester synthesis by NAD(+)-dependent dehydrogenation of hemiacetal: production of methyl formate by cells of methylotrophic yeasts.

A water-soluble ester, methyl formate, was detected as a metabolite in the culture medium of methylotrophic yeasts. Methyl formate synthase, which catalyses NAD(+)-dependent dehydrogenation of the hemiacetal adduct of methanol and formaldehyde, catalyses the ester synthesis. The enzyme activity was induced on a methanol medium and was increased further by the addition of formaldehyde. In the reaction system using intact cells of Pichia methanolica AKU 4262, 135 mM (8.1 g/liter) methyl formate was produced from 2 M methanol. This is a new biological process for ester synthesis that couples spontaneous formation of hemiacetal and alcohol dehydrogenase.

Esters↗
PubMed2002

Asymmetric catalytic Mannich reactions catalyzed by urea derivatives: enantioselective synthesis of beta-aryl-beta-amino acids.

Highly enantioselective addition reactions between silyl ketene acetals and N-Boc aldimines are catalyzed by the thiourea-based catalyst 1c. Extraordinary scope is observed in this methodology with regard to the imine substrate, with aryl and heteroaromatic derivatives generally affording nearly quantitative yields of beta-amino ester product in up to 98% enantioselectivity.

Acetals↗
PubMed2001

Fast, efficient and selective deprotection of the tert-butoxycarbonyl (Boc) group using HCl/dioxane (4 m).

Fast, efficient and selective deprotection of the tert-butoxycarbonyl (Boc) group of various amino acids and peptides was achieved by using hydrogen chloride (4 m) in anhydrous dioxane solution for 30 min at room temperature. In the cases studied in our laboratory, this protocol provided superior selectivity to deprotect Nalpha-Boc groups in the presence of tert-butyl esters and tert-butyl ethers, including thio-tert-butyl ethers, but not phenolic tert-butyl ethers.

Dioxanes↗