Electrophilic amination of amino acids with N-Boc-oxaziridines: efficient preparation of N-orthogonally diprotected hydrazino acids and piperazic acid derivatives

A general two-step preparation of enantiopure N(alpha),N(beta)-orthogonally diprotected alpha-hydrazino acids 1 is developed on a multigram scale. The key reaction is the efficient electrophilic amination of N-benzyl amino acids 6 with N-Boc-oxaziridine 7 and accommodates various functional groups encountered in side chains of amino acids. The cyclic 2,3,4,5-tetrahydro-3-pyridazine carboxylic acid (piperazic acid) derivatives 2 and 3 or the cyclic 3,4-dihydro-3-pyrazolecarboxylate 4 are conveniently prepared from glutamic acid or aspartic acid via orthogonally diprotected alpha-hydrazino acids 1m and 1n.     

Oxathiaphospholane approach to N- and O-phosphorothioylation of amino acids

A method of highly efficient synthesis of N- and O-phosphorothioylated amino acids was developed. N- and O-(2-Thiono-1,3,2-oxathiaphospholanyl)amino acid methyl esters (3) were prepared in high yields in reaction of amino acid methyl esters with 2-chloro-1,3,2-oxathiaphospholane in pyridine in the presence of elemental sulfur. Compounds 3 were converted in high yield into the corresponding methyl or benzyl phosphorothioamides 6 and 7 by DBU-assisted treatment with methanol or benzyl alcohol. When 3-hydroxypropionitrile was used instead of methanol or benzyl alcohol, the corresponding 2-cyanoethylphosphorothioamidates 4 were obtained in high yield, from which the 2-cyanoethyl group was removed with concentrated ammonium hydroxide. The oxathiaphospholane methodology was also applied for the phosphorylation of amino acids. Thus, 2-oxo-1,3,2-oxathiaphospholane derivatives 10 were prepared by oxidation of compounds 3 with SeO(2.) Compounds 10 were transformed into the corresponding phosphate diesters or amidoesters upon treatment with 3-hydroxypropionitrile in the presence of DBU. The DBU-assisted oxathiaphospholane ring-opening process in 3 and 10 did not cause any measurable C-racemization of phosphorothioylated/phosphorylated amino acids.     

Characterization and application of a new ultraviolet derivatization reagent for amino acids analysis in capillary electrophoresis

A new ultraviolet (UV) labeling reagent, p-acetamidobenzenesulfonyl fluoride (PAABS-F), was designed and synthesized to label and determine the amino acids by capillary electrophoresis (CE) with diode-array detector (DAD). PAABS-F is very stable and easy to synthesize. It reacted with primary or secondary amino acids very quickly under facile conditions to give corresponding derivatives in high yield with excellent sensitivity and stability. No by-products were observed in amino acid derivatives when stored at room temperature under natural daylight for at least 7 days. Both amino acids standard solution and real samples reacted with this new UV labeling reagent smoothly to form high UV-absorption derivatives. The labeled 20 standard amino acids were efficiently separated by CE and the mass detection limits (S/N = 3) were ranged from 59.3 fmol for l-tryptophan to 1.70 pmol for l-histidine.     

Adsorption Behavior of Amino Acids on a Stainless Steel Surface

The adsorption behavior of various amino acids on a stainless steel surface was investigated at 30 degrees C and over a pH range of 3-10. Acidic and basic amino acids except histidine adsorbed remarkably at pH 3-4 and 7-10, respectively, and showed Langmuir-type adsorption isotherms. The effects of pH and ionic strength on the adsorption isotherms were investigated to analyze the interactions between amino acids and adsorption sites on the stainless steel. Hydrophobic amino acids and glycine showed only small adsorbed amounts at all pHs tested. For the acidic and basic amino acids, reversibility of the absorption and the influence of the ionic strength on the adsorption behavior were examined. The adsorption isotherms of the derivatives of aspartic acid were also measured in order to examine the contribution of the carboxylic groups of acidic amino acids to the adsorption. Furthermore, a Fourier-transform infrared spectroscopic analysis and semiempirical molecular orbital calculation were carried out to analyze the ionization states and the configuration of the amino acids adsorbed on a stainless steel surface. These investigations suggest that the acidic and basic amino acids adsorb through two electrostatic interactions of two ionized groups in the amino acid with a stainless steel surface. Copyright 2000 Academic Press.     

Efficient synthesis of enantiopure pyrrolizidinone amino acid

Enantiopure (3S,5R,8S)-3-[N-(Boc)amino]-1-azabicyclo[3.3.0]octan-2-one 8-carboxylic acid (1) was synthesized in nine steps and 16% overall yield from aspartate beta-aldehyde 7. Carbene-catalyzed acyloin condensation of 7, followed by acetylation and samarium iodide reduction, gave linear precursor (2S,7S)-alpha,omega-diamino-4-oxosuberate 11, which was converted to N-(Boc)aminopyrrolizidin-2-one carboxylic acid 1 by a reductive amination/lactam cyclization sequence. X-ray analysis of (3S,5R,8S)-methyl N-(Boc)aminopyrrolizidin-2-one carboxylate 21 showed that its internal backbone dihedral angles (psi = -149 degrees, phi = -49 degrees ) were in good agreement with the ideal values for a type II' beta-turn. Proton NMR experiments on N'-methyl-N-(Boc)aminopyrrolizidin-2-one carboxamide 23 demonstrated significantly different NH chemical displacements and temperature coefficients suggestive of solvent shielded and exposed hydrogens indicative of a turn conformation. Because pyrrolizidinone amino acids can serve as conformationally rigid dipeptide surrogates, this synthesis should facilitate their application in the exploration of conformation-activity relationships of various biologically active peptides.     

Relative stability of major types of beta-turns as a function of amino acid composition: a study based on Ab initio energetic and natural abundance data

Folding properties of small globular proteins are determined by their amino acid sequence (primary structure). This holds both for local (secondary structure) and for global conformational features of linear polypeptides and proteins composed from natural amino acid derivatives. It thus provides the rational basis of structure prediction algorithms. The shortest secondary structure element, the beta-turn, most typically adopts either a type I or a type II form, depending on the amino acid composition. Herein we investigate the sequence-dependent folding stability of both major types of beta-turns using simple dipeptide models (-Xxx-Yyy-). Gas-phase ab initio properties of 16 carefully selected and suitably protected dipeptide models (for example Val-Ser, Ala-Gly, Ser-Ser) were studied. For each backbone fold most probable side-chain conformers were considered. Fully optimized 321G RHF molecular structures were employed in medium level [B3LYP/6-311++G(d,p)//RHF/3-21G] energy calculations to estimate relative populations of the different backbone conformers. Our results show that the preference for beta-turn forms as calculated by quantum mechanics and observed in Xray determined proteins correlates significantly.     

Addition of a Reformatsky reagent to N-anthracene-9-sulfonyl and related imines: Synthesis of protected β-amino acids

The Reformatsky reagent tert-butoxycarbonylmethylzinc bromide adds in high yields to N-sulfonylimines, e.g. 1a–1d, derived by condensation of benzaldehyde dimethyl acetal with methanesulfonamide, toluene-4-sulfonamide, 4-(methoxycarbonyl)benzenesulfonamide and sulfamide: the products are protected β-amino acids 2a–2d. N-Deprotection occurs reductively (Na-naphthalene; low yields) for 2b and 2c or hydrolytically (refluxing aq. pyridine; 76% yield of amino acid 3a after acid hydrolysis of the t-butyl ester) for the sulfamide derivatives 2d. Anthracene-9-sulfonamide (6) is readily available by sulfonation and chlorination of anthracene, and condenses with aldehydes [RCHO; R = Ph, 4-FC₆H₄, 4-MeOC₆H₄, 4-NCC₆H₄, 2-furyl, (E)-styryl], e.g. in the presence of TiCl₄/Et₃N, to yield imines 7a–7f, which after addition of tert-butoxycarbonylmethylzinc bromide give protected amino acids 8a–8f; however, 8f cyclizes to the sultam 9 via a spontaneous intramolecular Diels-Alder reaction. Reductive cleavage of the N-anthracene-9-sulfonyl group is much easier than for traditional N-sulfonyl protecting groups, as demonstrated by the deprotection of 8a and 8c using aluminium amalgam.     

Synthesis of 5/7-, 5/8- and 5/9-bicyclic lactam templates as constraints for external beta-turns

The 5/7-, 5/8- and 5/9-bicyclic lactams 3, 17, 5 and 6 have been synthesised as single diastereoisomers by a route involving ring closing olefin metathesis. The X-ray crystal structure of the amino acid hydrochloride has been carried out and compared to that of the saturated external beta-turn constraint 18.     

Amino-Acid and Dipeptide Derivatives of 2-(6-ethyl-4-oxo-3-(4-phenyl-4<Emphasis Type="Italic">H</Emphasis>-1,2,4-triazol-3-yl)-4<Emphasis Type="Italic">H</Emphasis>-chromen-7-yloxy)acetic Acid

Chromones modified by triazole, amino acids, and dipeptides were prepared by condensation of the N-hydroxysuccinimide ester of 2-(6-ethyl-4-oxo-3-(4-phenyl-4H-1,2,4-triazol-3-yl)-4H-chromen-7-yloxy)acetic acid with salts of amino acids or dipeptides. The dipeptide derivatives were also synthesized by extending the chain of amino acids. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 435–439, September–October, 2005.     

Stereoselective synthesis of individual isomers of Leu-enkephalin analogues containing substituted beta-turn bicyclic dipeptide mimetics

Novel constrained beta-turn dipeptide mimetics, 8-phenyl thiaindolizidinone amino acids 3, have been synthesized stereoselectively and incorporated into Leu-enkephalin peptides as a replacement of dipeptide Gly3-Phe4 to afford four individual isomers of Leu-enkephalin analogues 6.     

A versatile strategy for the synthesis of N-linked glycoamino acids from glycals

A versatile route for the synthesis of N-linked glycoamino acids from readily available glycals is reported. A variety of glycals possessing different carbohydrate templates (mono-, di- and trisaccharide glycals) were shown to undergo a novel iodine catalyzed stereoselective diamination reaction with chloramine-T. Taking advantage of the difference in the reactivity between the anomeric and C2 sulfonamido groups of these diamines 7, 13, 15, 17 and 19, they could be protected differentially at the C2 and anomeric nitrogen atoms. Thus, chemoselective acetylation of these diamines installed the C2 acetamido group, an essential functionality that plays a crucial role in inducing a beta-turn in N-linked glycoproteins. Subsequent protection of the anomeric nitrogens of 20a,b,e as their Alloc (allyloxycarbonyl) derivatives followed by SmI(2) mediated facile didetosylation afforded 24a-c. Deprotection of the Alloc group of 24a and 24c and coupling of the liberated free amine with a variety of protected amino acids provided N-linked glycoamino acids 25 and 27 in high yields. An illustrative synthesis of an N-linked glycopeptide 29 is also reported.     

Sensitive derivatization reagents for hydroxyl and amino compounds for thin-layer or high-performance liquid chromatography with fluorescence detection

Three fluorescent derivatization reagents for compounds having hydroxyl and/or amino groups are described. 4-(2-Phthalimidyl)benzoyl chloride, 3-(2-phthalimidyl)benzoyl chloride and 3-(2-phthalimidyl)-4-methoxybenzoyl chloride, prepared from the corresponding phthalimidylbenzoic acid, were stable at room temperature and condensed quantitatively with alcohols, amines and amino acids in the presence of alkali under mild conditions to give strongly fluorescent derivatives. The derivatives were separated by thin-layer and high-performance liquid chromatography.     

Palladium(0)-catalyzed regioselective synthesis of alpha-dehydro-beta-amino esters from amines and allyl acetates: synthesis of a alpha-dehydro-beta-amino acid derived cyclic peptide as a constrained beta-turn mimic

Acetates derived from the adducts of the Baylis-Hillman reaction can be reacted in a regioselective manner with amines in the presence of palladium(0) catalyst to afford alpha-dehydro-beta-amino esters (2 and 3) in good yields. The regioselectivity of the reaction can be controlled by temperature and reaction medium leading to the synthesis of regioisomers 2 or 3. The alpha-dehydro-beta-amino acid 3 is a turn inducer, and the dipeptides 6 derived from it show the presence of an eight-membered intramolecular hydrogen bond. Also, cobalt(II) chloride catalyzes the cleavage of epoxy peptides with alpha-dehydro-beta-amino acid derivative 3b to afford the corresponding dipeptide derivatives 8, which exhibit an intramolecular hydrogen bond and thus mimic a beta-turn. This intramolecular hydrogen bonding preorganizes the corresponding diallylated peptide 8c for cyclization via ring-closing metathesis to afford the cyclic peptide 9 as a constrained mimic of a beta-turn.     

Anomalous nucleosides and related compounds

Amino acid and peptide derivatives of phenazinecarboxylic acids (phenazinylpeptides) have been obtained by the reaction of hydrochlorides of esters of amino acids, phosphorus trichloride, and phenazinecarboxylic acids with the subsequent splitting off of the protective ester groups in an acid or an alkaline medium.For Communication XV, see [5].Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 6, No. 7, pp. 989–990, July, 1970.     

Synthesis of Optically Active,Ring-Substituted N-Benzyloxycarbonylphenylalanines via 2-Benzyloxycarbonylamino-2-arylalkylmalonates

Diethyl or dimethyl benzyloxycarbonylaminomalonate was reacted with ring substituted benzyl halides and the resulting arylalkyl derivatives ( 3 to 6 ) half-saponified to the DL -monoacid-monoesters ( 7 to 10 ). Decarboxylation by refluxing in dioxane afforded the N-benzyl oxycarbonyl-DL -amino acid esters ( 11 to 14 ), which were resolved into their optical antipodes by enzymic hydrolysis of the ester group with Subtilisin, type Carlsberg. Enzymic hydrolysis led to the N-benzyloxycarbonyl-L -amino acids ( 15 to 18 ) and to the corresponding D -amino acid esters. The latter were converted to the N-benzyloxycarbonyl-D -amino acids ( 19 and 20 ) by alkaline hydrolysis of the ester groups. These derivatives could be used directly for further peptide synthesis. The following compounds were prepared: N-benzyloxycarbonyl derivatives of p-methyl-L -phenylalanine ( 15 ), p-methyl-D -phenylalanine ( 19 ), p-fluoro-L -phenylalanine ( 16 ), m-fluoro-L -phenylalanine ( 17 ), m-fluoro-D -phenylalanine ( 20 ) and penta-fluoro-L -phenylalanine ( 18 ). The free amino acids were obtained by removal of the benzyloxycarbonyl group with HBr in acetic acid.     

New 7-substituted quinolone antibacterial agents. II. The synthesis of 1-ethyl-1,4-dihydro-4-oxo-7-(pyrazolyl,isoxazolyl, and pyrimidinyl)-1,8-naphthyridine and quinolone-3-carboxylic acids

Synthetic methods for the construction of certain aromatic heterocyclic side chains for the quinolone anti-bacterials have been provided. In particular a series of 7-(pyrazol-3 or 4-yl, 4- or 5-isoxazolyl and 4- or 5-pyrimidinyl)-1-ethyl-1,4-dihydro-4-oxo-1,8-naphthyridine and quinoline-3-carboxylic acids have been prepared. All of the heterocycles were prepared from masked 1,3-dicarbonyl derivatives of nalidixic acid ( 9,17 ) or 7-acetyl-1-ethyl-1,4-dihydro-4-oxo-3-quinoline carboxylic acids ( 8 ). These masked 1,3-dicarbonyl derivatives were prepared by the use of t-butoxy-bis-dimethylaminomethane on the activated methyls of 9,19 and 8 . The pyrimidinyl analogs, substituted with a 2-amino or a 2-aminomethyl moiety, were the only derivatives with substantial antibacterial activity.     

Hydroxycarboxylic and oxocarboxylic acids in urine: products from branched-chain amino acid degradation and from ketogenesis

Hydroxy- and oxomonocarboxylic acids in urine of healthy individuals and of patients with diabetic ketoacidosis are analysed as methyl esters and methyl esters/O-methyloximes, respectively, by gas chromatography and gas chromatography-mass spectrometry. The derivatives are pre-fractionated by thin-layer chromatography. The acids originate mainly from ketogenesis and from the metabolism of valine, leucine and isoleucine. The amino acid metabolites fall into three groups: the 2-oxocarboxylic acids (2-oxoisovaleric acid, 2-oxoisocaproic acid and 2-oxo-3-methylvaleric acid); the 2-hydroxycarboxylic acids (2-hydroxyisovaleric acid, 2-hydroxyisocaproic acid and 2-hydroxy-3-methylvaleric acid); and the 3-hydroxycarboxylic acids (3-hydroxyisobutyric acid, 3-hydroxyisovaleric acid, 3-hydroxy-2-ethylpropionic acid, threo-3-hydroxy-2-methylbutyric acid and erythro-3-hydroxy-2-methylbutyric acid). The threo form of 3-hydroxy-2-methylbutyric acid is the major constituent within the diastereomeric pair. Of the three groups of amino acid metabolites, the 3-hydroxycarboxylic acids in particular are elevated during ketoacidosis. The characteristic general features of the mass spectrometric fragmentation of the derivatives of the identified components are systematically described. The discussion of the fragmentation includes constituents of low concentrations, such as 3-oxocaproic acid, 4-oxobutyric acid and 5-oxocaproic acid, which can be detected only when the pre-fractionation technique is applied.     

Laser-induced fluorescence detection in liquid chromatography after preliminary derivatization of carboxylic acid and primary amino groups

The development of selective derivatization for the determination of carboxylic acids, amino acids and peptides in aqueous solutions is described as a preliminary study for the determination of these compounds in biological materials. The derivatization reactions are completed before the liquid chromatographic separation and laser-induced fluorescence detection for which a continuous-wave argon-ion gas laser is used in the ultraviolet or visble mode. Carboxylic acid groups arre derivatized with 9-hydroxymethylathracene and primary amino groups are derivatized with fluorescein isothiocyanate. Detection limits, in aqueous solutions, for the carboxylic acid derivatives are ca. 190 fg (ultraviolet mode). In the visible mode, the detection limits are ca. 1 fg for the primary amino derivatives of amino acids and peptides. In al the chromatographic analyses, the derivatization mixtures are injected onto a standard reversed-phase or reversed- phase ion-pair system and conventional flow cells are used without expensive photon counting or optical systems.     

Synthesis and antibacterial properties of new 8-nitrofluoroquinolone derivatives

The objective of this research was the preparation of new 8-nitrofluoroquinolone models and investigation of their antibacterial properties. The work initially involved large scale preparation of the synthon 7-chloro-1-cyclopropyl-6-fluoro-8-nitro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (3), followed by introduction of substituted primary amine appendages at the C-7 position to give derivatives 9a-g, in which the amino group is appended to substituted benzenes or aromatic heterocycles, is part of a primary alpha-amino acid or just a simple primary aliphatic amine. This nucleophilic aromatic substitution step was a very simple procedure since the 8-nitro group of the above synthon facilitated the addition of weak nucleophiles at C-7. All compounds prepared were fully identified and characterized using NMR, IR, EA and MS, and were consistent with expected structures. The prepared targets and the intermediates have shown interesting antibacterial activity against gram positive and/or gram negative strains. In particular, the p-toluidine, p-chloroaniline and aniline derivatives showed good activity against S. aureus with MIC range approximately 2-5 microg/mL. In conclusion, more lipophilic groups seem to enhance activity against gram positive strains.     

Tetrahydrofuran Calpha-tetrasubstituted amino acids: two consecutive beta-turns in a crystalline linear tripeptide

The synthesis of tetrahydrofuran Calpha-tetrasubstituted amino acids (TAAs) and their effect on the conformation in small peptides are reported. The synthesis starts from the protein amino acid methionine, which is protected at the C and N terminus and converted into the corresponding sulfonium salt by alkylation. Simple base treatment in the presence of an aryl aldehyde leads to the formation of tetrahydrofuran tetrasubstituted Calpha-amino acids in a highly diastereoselective (trans/cis ratio up to 97:3) reaction with moderate to good yields (35-78%) depending on the aldehyde used. Palladium-catalyzed coupling reactions allow a subsequent further functionalization of the TAA. The R,S,S-TAA-Ala dipeptide amide adopts a beta-turn type I conformation, whereas its S,R,S isomer does not. The R,S,S-Gly-TAA-Ala tripeptide amide shows in the solid state and in solution a conformation of two consecutive beta-turn type III structures, stabilized by i+3-->i intramolecular hydrogen bonds.