Synthesis and pKa determination of new enantiopure dimethyl‐substituted acridino‐crown ethers containing a carboxyl group: Useful candidates for enantiomeric recognition studies

New enantiopure dimethyl‐substituted acridino‐18‐crown‐6 and acridino‐21‐crown‐7 ethers containing a carboxyl group at position 9 of the acridine ring [(S,S )‐ 8 , (S,S )‐ 9 , (R,R )‐ 10 ] were synthesized. The pK _a values of the new crown ethers [(S,S )‐ 8 , (S,S )‐ 9 , (R,R )‐ 10 ] and of an earlier reported macrocycle [(R,R )‐ 2 ] were determined by UV‐pH titrations. Crown ether (S,S )‐ 8 was attached to silica gel by covalent bonds and the enantiomeric separation ability of the newly prepared chiral stationary phase [(S,S )‐CSP‐ 12 ] was studied by high‐performance liquid chromatography (HPLC). Homochiral preference was observed and the best separation was achieved for the enantiomers of 1‐NEA. Ligands (S,S )‐ 9 and (R,R )‐ 10 are precursors of enantioselective sensor and selector molecules for the enantiomers of protonated primary amines, amino acids, and their derivatives.     

Axially chiral N,N′‐dioxides ethers for catalysis in enantioselective allylation of aldehydes

A series of axially chiral ethers synthesized from biscarboline N,N′‐dioxides, (S)‐ 1a to (S)‐ 1n , was investigated in enantioselectivity addition reactions of allyltrichlorosilane with a series of substituted aldehydes, including bulky substituted aldehydes. High enantioselectivities (up to 96%ee) were achieved using the catalyst (S)‐ 1k at 1 mol % loading.     

Synthesis and Enantioseparation Ability of Xylan Bisphenylcarbamate Derivatives as Chiral Stationary Phases in HPLC

Ten novel xylan bisphenylcarbamate derivatives bearing meta‐ and para‐substituents on their phenyl groups were synthesized and their chiral recognition abilities were evaluated as the chiral stationary phases (CSPs) for high‐performance liquid chromatography (HPLC) after coating them on macroporous silica. The chiral recognition abilities of these CSPs depended on the nature, position, and number of the substituents on the phenyl moieties. The introduction of an electron‐donating group was more attractive than an electron‐withdrawing group to improve the chiral recognition ability of the xylan phenylcarbamate derivatives. Among the CSPs discussed in this study, xylan bis(3,5‐dimethylphenylcarbamate)‐based CSP seems to possess the highest resolving power for many racemates, and the meta‐substituted CSPs showed relatively better chiral recognition than the para‐substituted ones. For some racemates, the xylan bis(3,5‐dimethylphenylcarbamate) derivative exhibited higher enantioselectivity than the CSP based on cellulose tris(3,5‐dimethylphenylcarbamate). Chirality 27:518–522, 2015 © 2015 Wiley Periodicals, Inc.     

Enantiopure 4‐oxazolin‐2‐ones and 4‐methylene‐2‐oxazolidinones as chiral building blocks in a divergent asymmetric synthesis of heterocycles

Enantiopure 3‐((R)‐ and 3‐((S)‐1‐phenylethyl)‐4‐oxazoline‐2‐ones were evaluated as chiral building blocks for the divergent construction of heterocycles with stereogenic quaternary centers. The N‐(R)‐ or N‐(S)‐1‐phenylethyl group of these compounds proved to be an efficient chiral auxiliary for the asymmetric induction of the 4‐ and 5‐positions of the 4‐oxazolin‐2‐one ring through thermal and MW‐promoted nucleophilic conjugated addition to Michael acceptors and alkyl halides. The resulting adducts were transformed via a cascade process into fused six‐membered carbo‐ and heterocycles. The structure of the reaction products depended on the electrophiles and reaction conditions used. Alternative isomeric 4‐methylene‐2‐oxazolidinones served as chiral precursors for a versatile and divergent approach to highly substituted cyclic carbamates. DFT quantum calculations showed that the formation of bicyclic pyranyl compounds was generated by a diastereoselective concerted hetero‐Diels‐Alder cycloaddition.     

Synthesis and Antiviral Activity of Novel Fluorinated 2′,3′‐Dideoxynucleosides

A series of 5‐(trifluoroethoxymethyl)‐2′,3′‐dideoxyuridines and 5‐[bis(trifluoroethoxy)‐methyl]‐2′,3′‐dideoxyuridines have been prepared and screened for antiviral activity. The conformations of these compounds are discussed on the bases of NOE studies and the MO calculations. Modelling and NOE studies suggest both syn‐ and anti conformations for these 5‐(2,2,2‐trifluoroethoxymethyl)‐ and 5‐[bis(2,2,2‐trifluoroethoxy)‐methyl]‐ derivatives. The NOE parameters are also suggested to be more attributable to the nature of the fluorine atom than to structural or conformational changes. Compounds 17, 26 and 30 showed some activity in anti‐HIV‐1 and anti‐HIV‐2 assays, but the compounds were devoid of activity against HSV and human rhinovirus. The compounds tested exhibited low cytotoxicity and were inactive against a bank of cancer cells in vitro.     

Easy Access to Enantiomerically Pure Nonproteinogenic Amino Acids

The protease from Bacillus licheniformis (alcalase) shows a remarkable broad substrate tolerance and high enantioselectivity against nonproteinogenic racemic amino acid derivatives. N‐acetyl protected amino acid esters of mono‐, di‐ or tri‐substituted phenyl alanines and even tert.‐leucine were hydrolyzed with high enantioselectivity. The obtained mixtures of (S)‐N‐acetyl amino acid and (R)‐N‐acetyl amino acid ester can easily be separated. The R‐ or S‐amino acids were obtained by acidic cleavage of the optically pure derivatives or the (R)‐ester was racemized by treatment with potassium t‐butylate.     

Simultaneous determination of enantiomerization and hydrolysis kinetic parameters of chiral N‐alkylbenzothiadiazine derivatives

On‐column stopped flow multidimensional HPLC (sfMDHPLC) and dynamic high‐performance liquid chromatography were applied to investigate the influence of alkyl substituents at the sulfonamidic and amino moieties of benzothiadiazine 1,1‐dioxide derivatives on hydrolysis and enantiomerization rate constants. The data obtained indicate the presence of pyrrolo substituent at the 3,4 positions on benzothiadiazine rings inhibits the hydrolysis, whereas the enantiomerization occurs in acidic medium. Hydrolysis rates are quite similar for the two benzothiadiazines methyl substituted to nitrogen at 2‐ and 4‐positions. Conversely, enantiomerization rate of 4‐N‐methyl substituted is significantly higher than 2‐N‐methyl substituted. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Immobilization and chromatographic evaluation of novel regioselectively substituted amylose‐based chiral packing materials for HPLC

The regioselectively substituted amylose derivatives bearing a 4‐tert‐butylbenzoate or 4‐chlorobenzoate group at 2‐position, and 3,5‐dichlorophenylcarbamate and a small amount of 3‐(triethoxysilyl)propylcarbamate groups at 3‐ and 6‐positions were synthesized by a two‐step process based on the esterification of 2‐position of a glucose unit. The obtained derivatives were effectively immobilized onto macroporous silica gel by intermolecular polycondensation of triethoxysilyl groups. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high‐performance liquid chromatography. These CPMs showed high chiral recognition as well as the conventional coated‐type CPM, and can be used with the eluents‐containing chloroform and tetrahydrofuran. With the extended use of these eluents, improvement of chiral recognition and reversed elution orders were realized. For some racemates, the immobilized CPM exhibited ability comparable or better to the commercial immobilized amylose‐ or cellulose‐based columns, Chiralpak IA, IB, and IC. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

First Synthesis of Double Headed 1,3,4‐Oxadiazino[6,5‐b]indole Acyclo C‐Nucleosides

Condensation of 1,3‐dihydro‐2,3‐dioxo‐2H‐indoles (1a–c) with galactaric acid bis hydrazide (2) gave the corresponding galactaric acid bis[2‐(1,2‐dihydro‐2‐oxo‐3H‐indol‐3‐ylidene)hydrazides] (3a–c). Acetylation of the latter compounds with acetic anhydride in the presence of pyridine at ambient temperature gave the 2,3,4,5‐tetra‐O‐acetylgalactaric acid bis[2‐(1,2‐dihydro‐2‐oxo‐1‐substituted‐3H‐indol‐3‐ylidene)hydrazides] (4b–d). Heterocyclization of the tetra‐O‐acetates 4b–d by heating with thionyl chloride afforded the double headed acyclo C‐nucleosides: 1,2,3,4‐tetra‐O‐acetyl‐1,4‐bis{9‐substituted‐1,3,4‐oxadiazino[6,5‐b]indol‐2‐yl‐1‐ium}‐galacto‐tetritol dichlorides (5b–d). Structures of the prepared compounds were elucidated from their spectral properties.     

Theoretical Electronic Circular Dichroism Study of 1,3‐Diene Derivatives for the Elucidation of ECD Spectra of 1,3‐Cyclohexadiene and Its Derivatives

The origin of P‐ or M‐chirality of methyl substituted 1,3‐cyclohexadienes are elucidated by time‐dependent density functional theory (TD‐DFT) calculation of 1,3‐cyclohexadiene derivatives and acyclic 1,3‐dienes. The sign‐inversion of the rotatory strength of the lowest excited state between 1,3‐cyclohexadiene and (5R)‐axial‐methyl‐1,3‐cyclohexadiene is caused by the conformation around the (C=)C‐C(‐Me) dihedral angle. The correlation between the sign of the rotatory strength and conformation has been found not only in methyl substituted derivatives but also fluoro substituted compounds. Chirality 27:476–478, 2015. © 2015 Wiley Periodicals, Inc.     

Substituent effects on chiral resolutions of derivatized 1‐phenylalkylamines by heptakis(2,3‐di‐O‐methyl‐6‐O‐tert‐butyldimethylsilyl)‐β‐cyclodextrin GC stationary phase

Chiral resolutions of trifluoroacetyl‐derivatized 1‐phenylalkylamines with different type and position of substituent were investigated by capillary gas chromatography by using heptakis(2,3‐di‐O‐methyl‐6‐O‐tert‐butyldimethylsilyl)‐β‐cyclodextrin diluted in OV‐1701 as a chiral stationary phase. The influence of column temperature on retention and enantioselectivity was examined. All enantiomers of meta‐substituted analytes as well as fluoro‐substituted analytes could be resolved. Temperature had a favorable influence on enantioselectivity for small amines with substituents at the ortho‐position. The type of substituent at the stereogenic center of amines also had a crucial effect as the ethyl group led to poor enantioseparation. Among all analytes studied, trifluoroacetyl‐derivatized 1‐(2′‐fluorophenyl)ethylamine exhibited baseline resolution with the shortest analysis time.     

Synthesis of enantiopure planar chiral bis‐(para)‐pseudo‐meta‐type [2.2]paracyclophanes

A new type of planar chiral (R_p)‐ and (S_p)‐4,7,12,15‐tetrasubstituted [2.2]paracyclophanes was prepared from racemic 4,7,12,15‐tetrabromo[2.2]paracyclophane as the starting substrate. Regioselective lithiation and transformations afforded racemic bis‐(para)‐pseudo‐meta‐type [2.2]paracyclophane (4,15‐dibromo‐7,12‐dihydroxy[2.2]paracyclophane). Its optical resolution was performed by the diastereomer method using a chiral camphanoyl group as the chiral auxiliary. The diastereoisomers were readily isolated by simple silica gel column chromatography, and the successive hydrolysis afforded (R_p)‐ and (S_p)‐bis‐(para)‐pseudo‐meta‐type [2.2]paracyclophanes ((R_p)‐ and (S_p)‐4,15‐dibromo‐7,12‐dihydroxy[2.2]paracyclophanes). They can be used as pseudo‐meta‐substituted chiral building blocks.     

Bromolactamization: Key Step in the Stereoselective Synthesis of Enantiomerically Pure,cis‐Configured Perhydropyrroloquinoxalines

Compounds based on the pyrroloquinoxaline system can interact with serotonin 5‐HT₃, cannabinoid CB₁, and μ‐opioid receptors. Herein, a chiral pool synthesis of diastereomerically and enantiomerically pure bromolactam (S,R,R,R)‐ 14A is presented. Introduction of the cyclohexenyl ring at the N‐atom of (S)‐proline derivatives 8 or methyl (S)‐pyroglutamate ( 12 ) led to the N‐cyclohexenyl substituted pyrrolidine derivatives 4 and 13 , respectively. All attempts to cyclize the (S)‐proline derivatives 4 with a basic pyrrolidine N‐atom via [3 + 2] cycloaddition, aziridination, or bromolactamization failed. Fast aromatization occurred during treatment of cyclohexenamines under halolactamization conditions. In contrast, reaction of a 1:1 mixture of diastereomeric pyroglutamates (S,R)‐ 13bA and (S,S)‐ 13bB with LiO^tBu and NBS provided the tricyclic bromolactam (S,R,R,R)‐ 14A with high diastereoselectivity from (S,R)‐ 13bA , but did not transform the diastereomer (S,S)‐ 13bB . The different behavior of the diastereomeric pyroglutamates (S,R)‐ 13bA and (S,S)‐ 13bB is explained by different energetically favored conformations. Chirality 26:793–800, 2014. © 2014 Wiley Periodicals, Inc.     

Photoanalogues of the Initiation Substrates of the RNA Polymerase II, 5‐Azido‐2‐Nitrobenzoyl Derivatives of the ATP γ‐Amidophosphate: The Possible Photoinduced Degradation of the Functional Group to an N‐Arylhydroxylamine

Photoanlogues of the initiation substrates of the RNA polymerase II, N₃Ar‐ NH(CH₂)_nNHpppA where N₃Ar is 5‐azido‐2‐nitrobenzoyl group (n = 2 or 4) were synthesized, allowing the preparation of photoreactive oligonucleotides in situ by RNA polymerase II for application as photolabels. Photolysis of p‐nitro‐substituted aromatic azide in aqueous medium was investigated. Using the azoxy‐coupling reaction it was possible to determine whether a nitrene or p‐nitrophenyl hydroxylamine azoxy compound is the trappable intermediate that is generated at ambient temperature in aqueous solution.     

[D‐(p‐Benzoylphenylalanine)13, tyrosine19]‐melanin‐concentrating hormone,a potent analogue for MCH receptor crosslinking

A photoreactive analogue of human melanin‐concentrating hormone was designed, [d‐ Bpa¹³,Tyr¹⁹]‐MCH, containing the d‐ enantiomer of photolabile p‐benzoylphenylalanine (Bpa) in position 13 and tyrosine for radioiodination in position 19. The linear peptide was synthesized by the continuous‐flow solid‐ phase methodology using Fmoc‐strategy and PEG‐PS resins, purified to homogeneity and cyclized by iodine oxidation. Radioiodination of [d ‐Bpa¹³,Tyr¹⁹]‐MCH at its Tyr¹⁹ residue was carried out enzymatically using solid‐ phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed‐ phase mini‐column and HPLC. Saturation binding analysis of [¹²⁵I]‐[d‐ Bpa¹³,Tyr¹⁹]‐MCH with G4F‐7 mouse melanoma cells gave a K_D of 2.2±0.2×10⁻¹⁰ mol/l and a B_max of 1047±50 receptors/cell. Competition binding analysis showed that MCH and rANF(1–28) displace [¹²⁵I]‐[d‐ Bpa¹³,Tyr¹⁹]‐MCH from the MCH binding sites on G4F‐7 cells whereas α‐MSH has no effect. Receptor crosslinking by UV‐irradiation of G4F‐7 cells in the presence of [¹²⁵I]‐[d‐ Bpa¹³,Tyr¹⁹]‐MCH followed by SDS‐polyacrylamide gel electrophoresis and autoradiography yielded a band of 45–50 kDa. Identical crosslinked bands were also detected in B16‐F1 and G4F mouse melanoma cells, in RE and D10 human melanoma cells as well as in COS‐7 cells. Weak staining was found in rat PC12 phaeochromocytoma and Chinese hamster ovary cells. No crosslinking was detected in human MP fibroblasts. These data demonstrate that [¹²⁵I]‐[d‐ Bpa¹³,Tyr¹⁹]‐MCH is a versatile photocrosslinking analogue of MCH suitable to identify MCH receptors in different cells and tissues; the MCH receptor in these cells appears to have the size of a G protein‐coupled receptor, most likely with a varying degree of glycosylation. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.     

Oleanane‐Type Glycosides from Tremastelma palaestinum (L.) Janchen

Three new oleanane‐type glycosides, 1 – 3 , were isolated from the whole plant of Tremastelma palaestinum (L.) Janchen, along with eight known triterpene glycosides. The structures of the new compounds were established as 3‐O‐[β‐d‐ glucopyranosyl‐(1→3)‐α‐l‐ rhamnopyranosyl‐(1→3)‐β‐d‐ glucopyranosyl‐(1→3)‐α‐l‐ rhamnopyranosyl‐(1→2)‐α‐l‐ arabinopyranosyl]hederagenin ( 1 ), 3‐O‐[β‐d‐ glucopyranosyl‐(1→3)‐α‐l‐ rhamnopyranosyl‐(1→3)‐β‐d‐ glucopyranosyl‐(1→3)‐α‐l‐ rhamnopyranosyl‐(1→2)‐α‐l‐ arabinopyranosyl]hederagenin 28‐O‐β‐d‐ glucopyranosyl‐(1→6)‐β‐d‐ glucopyranosyl ester ( 2 ), and 3‐O‐[α‐l‐ rhamnopyranosyl‐(1→3)‐β‐d‐ glucopyranosyl‐(1→3)‐α‐l‐ rhamnopyranosyl‐(1→2)‐α‐l‐ arabinopyranosyl]oleanolic acid 28‐O‐β‐d‐ glucopyranosyl‐(1→6)‐β‐d‐ glucopyranosyl ester ( 3 ) by using 1D‐ and 2D‐NMR techniques and mass spectrometry. This is the first report on the phytochemical investigation of a species belonging to Tremastelma genus.     

Microwave-Assisted Synthesis of 2(1H)-Pyridones and Their Glucosides as Cell Proliferation Inhibitors

A new series of substituted 2(1H)-pyridones (4a–i) and their glucosides (5, 6a–e) were prepared as potential agents against leukemia (HL-60) cells. Glucosides (5,6a–e) were synthesized using three independent methods. Microwave protocol as an ecologically new method was used to synthesize the target compounds. Structures of the new products were confirmed using one- and two-dimensional NMR spectroscopy. In vitro exposure of pyridones substituted at position 4 with a 2-thienyl or 2-(trifluoromethyl)phenyl were found to exhibit high antiproliferation activities; in particular, 3-cyano-4-(thien-2′-yl)-6-(4″-chlorophenyl)-2(1H)-pyridone (4c) and its glucoside analogue (6c) had the highest activity.     

Design,synthesis and application of chiral tetraoxacalix[2]arene[2]triazine‐based organocatalysts in asymmetric Michael addition reactions

A novel type of oxacalix[2]arene[2]triazine‐based organocatalysts for asymmetric Michael reactions are reported for the first time. Chiral subunits were attached to the heteroatom‐bridged calixaromatic platform by a reaction of (R)‐ and (S)‐1‐aminotetraline with tetraoxacalix[2]arene[2]triazine in both enantiomeric forms. To evaluate the catalytic efficiency of the novel organocatalysts, isobutyraldehyde reacted with various substituted and unsubstituted aromatic trans‐β‐nitrostyrenes in tetrahydrofuran (THF), leading to Michael adducts in excellent yields and enantioselectivites (up to 97% yield and 99% ee).     

Synthesis and Spectral Characterization of Asymmetric Azines Containing a Coumarin Moiety: The Discovery of New Antimicrobial and Antioxidant Agents

Nine unsymmetrical azines containing a coumarin moiety were prepared by the reaction of the hydrazone of 4‐hydroxy‐3‐acetylcoumarin with differently substituted aromatic aldehydes. The azines were fully spectrally characterized, including a complete assignment of ¹H‐ and ¹³C‐NMR resonances, and were assessed for their acute toxicities in the Artemia salina model. Their free radical scavenging activities were tested in the DPPH assay, and in vitro antimicrobial activities were determined against seven bacterial and two fungal strains. The azines containing a p‐hydroxyphenyl group were shown to be the most effective antimicrobial agents, and in the case of resistant strains of Staphylococcus aureus and Acinetobacter baumannii, the activity was comparable to that of chloramphenicol. The derivative having a 3,5‐dimethoxy‐4‐hydroxyphenyl group exhibited pronounced antioxidant power reacting rapidly and in 1 : 1 mol ratio with the DPPH radical.     

Enantiomeric resolution of asymmetric glycol by a lipase‐catalyzed transesterification reaction

The lipase catalyzed enantiomeric resolution of syn‐glycol was carried out to confirm the sector method, which can determine the absolute configuration of anti‐ and syn‐glycol from the ¹H‐NMR spectra of bis‐2‐methoxy‐2‐trifluoromethyl‐2‐phenylacetic acid (MTPA) esters. The lipase catalyzed transesterification reaction was most reactive at the C2 position (C2–OH) of (2R;3R)‐2,3‐octanediol. Both (2S;3S)‐ and (2R;3R)‐2,3‐octanediol were prepared using lipase. The ¹H‐NMR spectra of their bis‐(R)‐MTPA esters agreed well with those prepared previously via mono‐(R)‐MTPA esters. The result suggests the retention of the Mosher plane in MTPA esters possessing a hydroxyl group at the β position. The reaction rate and the stereoselectivity decreased at C2–OH with the addition of 18‐crown‐6. Chirality 11:338–342, 1999. © 1999 Wiley‐Liss, Inc.