Conformational preferences of guanine-containing threose nucleic acid building blocks in B3LYP studies

In this paper, detailed and systematic gas-phase B3LYP conformational studies of four monomers of threose nucleic acid (TNA) with guanine attached at the C1' atom and bearing different substituents (OH, OP(=O)OH2 and OCH3) in the C2' and C3' positions of the α-l-threofuranose moiety are presented. All exocyclic single-bond (χ, ε and γ) rotations, as well as the ν0-ν4 endocyclic torsion angles, were taken into consideration. Three (threoguanosines TG1-TG3) or two (TG4) energy minima were found for the rotation about the χ torsion angle. The syn orientation (the A rotamer family) is strongly privileged in geometries TG1 and TG2, whereas the anti orientation (the C rotamer family) and the syn orientation are observed to be in equilibrium (with populations of 56% and 44%, respectively) for TG3. In the case of TG4, the high-anti orientation (the B rotamer family) turned out to be by far the most favourable, with the contribution exceeding 90% in equilibrium. Such a preference can be attributed to the inability of H-bonding between sugar and nucleobase and possibly because of the steric strains. The low-energy conformers of TG1-TG4 occupy the northeastern (P ∼ 40°) and/or southern (P ∼ 210°) parts of the pseudorotational wheel, which fits the A- and B-type DNA helices quite well. Additionally, in the case of TG4, some relatively stable geometries have the furanoid ring in conformation lying on the northwestern part of the pseudorotational wheel (P ∼ 288°).

Synthesis, Antimicrobial and Mutagenic Activity of a New Class of d-Xylopyranosides

Eight N-[2-(2',3',4'-tri-O-acetyl-α/β-d-xylopyranosyloxy)ethyl]ammonium bromides, a new class of d-xylopyranosides containing a quaternary ammonium aglycone, were obtained. Their complete structure was confirmed using NMR spectroscopy (¹H, ¹³C, COSY and HSQC) and high-resolution mass spectrometry (HRMS). An antimicrobial activity against fungi (Candida albicans, Candida glabrata) and bacteria (Staphylococcus aureus, Escherichia coli) and a mutagenic Ames test with Salmonella typhimurium TA 98 strain were performed for the obtained compounds. The greatest activity against the tested microorganisms was shown by glycosides with the longest (octyl) hydrocarbon chain in ammonium salt. None of the tested compounds exhibited mutagenic activity in the Ames test.

Identification of the furanose ring conformations and the factors driving their adoption

Three groups of furanoses with restricted freedom of rotation on the C3-C4, C2-C3, and C1-C2 bonds, respectively, are presented. Conformational analysis of these furanoses is conducted based on the proton nuclear magnetic resonance (¹H NMR) spectroscopy, density functional theory (DFT) calculations, and X-ray analysis. It is shown that the particular group of the presented furanoses is locked in the specific conformation. These are the ¹T₂-like, the ⁰E-like, and the ³T₄-like conformation, respectively. Characteristic ¹H NMR spectra of these three conformations are presented and the factors influencing the conformational preferences of the analyzed furanoses are discussed.

Characteristic 1H NMR spectra of β-d-ribofuranosides and ribonucleosides: factors driving furanose ring conformations

A series of β-d-ribofuranosides and ribonucleosides fused with 2,3-O-isopropylidene ring was synthesized and studied in terms of their conformational preferences. Based on the ¹H NMR spectra, DFT calculations, and X-ray analysis the E₀-like and E₄-like conformations adopted by these furanosides are identified. The ³E-like and ²E-like conformations are assigned to ribonucleosides without the 2,3-O-isopropylidene group. The studies are supported by analysis of the structural data of β-d-ribofuranosides and ribonucleosides deposited in the Cambridge Crystallographic Data Center (CCDC) database.† Finally, the factors influencing the conformational preferences of the furanose ring with the β-d-ribo configuration are indicated. These are the unfavorable ecliptic orientation of the 2-OH and 3-OH groups, the 1,3-pseudodiaxial interaction of the aglycone and terminal hydroxymethyl group and the endo-anomeric effect. It is also proved that the exo-anomeric effect acts in β-d-ribofuranosides.

The 2,3-O-isopropylidene group locks β-d-ribofuranosides in the E₀-like conformation whereas ribonucleosides in the E₄-like conformation. The conformational preferences of β-d-ribofuranosides are determined by the anomeric effect.

Vibrationally Assisted Direct Intersystem Crossing between the Same Charge-Transfer States for Thermally Activated Delayed Fluorescence: Analysis by Marcus-Hush Theory Including Reorganization Energy

Thermally activated delayed fluorescence (TADF) has recently become an extensively investigated phenomenon due to its high potential for application in organic optoelectronics. Currently, there is still lack of a model describing correctly basic photophysical parameters of organic TADF emitters. This article presents such a photophysical model describing the rates of intersystem crossing (ISC), reverse ISC (rISC), and radiative deactivation in various media and emphasizing key importance of molecular vibrations on the example of a popular TADF dye 9,10-dihydro-9,9-dimethyl-10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-acridine (DMAC-TRZ). The presented experimental and theoretical investigations prove that ISC and rISC can occur efficiently between the singlet and triplet states of the same charge-transfer nature (¹CT and ³CT, respectively). In emitters with the orthogonal donor and acceptor fragments, such spin-forbidden ¹CT ↔ ³CT transitions are activated by molecular vibrations. Namely, the change of dihedral angle between the donor and the acceptor affords reasonable spin–orbit coupling, which together with a small energy gap and reorganization energy enable ¹CT ↔ ³CT transition rates reaching 1 × 10⁷ s^–1. Evidence of direct ¹CT ↔ ³CT spin-flip and negligible role of a second triplet state, widely believed as a key parameter in the design of (r)ISC materials, change significantly the current understanding of TADF mechanism. In authors’ opinion, photophysics, and molecular design principles of TADF emitters should be revised considering the importance of vibrationally enhanced ¹CT ↔ ³CT transitions.

Influence of a 4'-substituent on the efficiency of flavonol-based fluorescent indicators of β-glycosidase activity

This article presents novel fluorescent probes, based on the excited-state intramolecular proton transfer (ESIPT) phenomenon and flavonols, sensitive to the action of specific glycosidases. 4'-Substituted flavonols were synthesized, using various approaches, and glycosylated with d-glucose, N-acetyl-d-glucosamine and d-glucuronic acid. Evaluation of the β-glycosidase activities was performed in neutral and acidic pH. In all the cases examined, an acidic environment accelerated enzymatic hydrolysis. It was demonstrated that the 4'-chloroflavonyl glycosides of all sugars tested, both in neutral and acidic pH, are the ones most sensitive to the presence of hydrolase. In turn, 4'-dimethylaminoflavonyl glucoside is not sensitive to glucosidase action at all. Generally, the rate of enzymatic hydrolysis increases as the electron-withdrawing nature of the 4'-substituent increases. An exception is the trifluoromethyl group which, in spite of having the most favourable Hammett constant, does not contribute enough to increase the rate of hydrolysis of its glucoside. The presented experimental results are supported by the electrostatic potential (ESP) analysis and related to the mechanisms of glycoside bond enzymatic hydrolysis.

5-Fluorouracil-Complete Insight into Its Neutral and Ionised Forms

5-Fluorouracil (5FU), a common anti-cancer drug, occurs in four tautomeric forms and possesses two potential sites of both protonation and deprotonation. Tautomeric and resonance structures of the ionized forms of 5FU create the systems of connected equilibriums. Since there are contradictory reports on the ionized forms of 5FU in the literature, complex theoretical studies on neutral, protonated and deprotonated forms of 5FU, based on the broad spectrum of DFT methods, are presented. These indicate that the O4 oxygen is more willingly protonated than the O2 oxygen and the N1 nitrogen is more willingly deprotonated than the N3 nitrogen in a gas phase. Such preferences are due to advantageous charge delocalization of the respective ions, which is demonstrated by the NBO and ESP analyses. In an aqueous phase, stability differences between respective protonated and deprotonated forms of 5FU are significantly diminished due to the competition between the mesomeric effect and solvation. The calculated pK_a values of the protonated, neutral and singly deprotonated 5FU indicate that 5FU does not exist in the protonated and double-deprotonated forms in the pH range of 0–14. The neutral form dominates below pH 8 and the N1 deprotonated form dominates above pH 8.

N-Aminoacyl and N-hydroxyacyl derivatives of diosgenyl 2-amino-2-deoxy-β-d-glucopyranoside: Synthesis, antimicrobial and hemolytic activities

Diosgenyl 2-amino-2-deoxy-β-d-glucopyranoside is a semisynthetic saponin with antimicrobial and antitumor activities. To search for more effective analogues, N-aminoacyl and N-hydroxyacyl derivatives of this saponin were synthesized conventionally and with microwave assistance, and tested against the human pathogenic fungi and Gram-positive and Gram-negative bacteria. None of the tested compounds exhibit activity against Gram-negative bacteria. Almost all of the synthesized N-aminoacyl saponins exhibit antifungal activity and act effectively against Gram-positive bacteria, some better than the parent compound. The best acting saponins are the same size and possess sarcosine or l- or d-alanine attached to the parent glucosaminoside. Shorter and longer aminoacyl residues are less advantageous. d-Alanine derivative is the most effective against Gram positive bacteria. Structure-activity relationship (SAR) analysis indicates that the free α-amino group in aminoacyl residue is necessary for antimicrobial activities of the tested saponins. (N-Acetyl)aminoacyl and N-hydroxyacyl analogs are inactive. Measurements of the hemolytic activities demonstrate that the best acting saponins are not toxic towards human red blood cells.

Activity of Diosgenyl 2-amino-2-deoxy-β-D-glucopyranoside, its Hydrochloride, and N,N-dialkyl Derivatives Against Non-albicans Candida Isolates

BACKGROUND

Candida albicans belongs to the most common fungal pathogens in humans, but recently an increased proliferation of strains called non-albicans Candida has been reported. Species belonging to this group are often characterised by a reduced susceptibility to antifungal agents.

OBJECTIVE

In view of the emergence of non-albicans Candida and their resistance to available antifungals, an attempt has been made to develop novel effective agents. Biological activities of the N,N-dialkyl diosgenyl glycosides, which were previously synthesized, were determined.

METHOD

Minimum inhibitory concentration (MIC) was determined for group of clinical nonalbicans Candida isolates by serial dilution method in Sabouraud liquid medium. In order to assess the toxicity towards human cells the minimum haemolytic concentration (MHC) was determined on human erythrocytes by serial dilution method in phosphoric buffer.

RESULTS

The saponins exhibited a strong activity towards clinical isolates of C. glabrata and C. parapsilosis comparable or even stronger than that of conventional antimicrobials. A high rate of resistance to fluconazole was shown among C. glabrata isolates. Among clinical strains of C. krusei and C. tropicalis, isolates with a decreased susceptibility to saponins were identified. All the tested C. krusei isolates showed resistance to fluconazole, while among C. tropicalis numerous strains were resistant to all tested azoles. The saponins did not show haemolytic activities at their microbiologically active concentrations.

CONCLUSION

Results of the present work encourage to continue the study on steroidal saponins and their potential application for the treatment of candidemia.

Threocytidines: Insight into the Conformational Preferences of Artificial Threose Nucleic Acid (TNA) Building Blocks in B3LYP Studies

A systematic DFT conformational studies of four building blocks of TNA with cytosine attached to the C1' atom of the α-L-threofuranose moiety are presented. Structures bearing 2'-OR and 3'-OR substituents, where R represents H, CH₃ and phosphate groups, were used in the studies using a B3LYP functional in the gas phase. The χ angle (C2-N1-C1'-O4'), the ν₀-ν₄ endocyclic torsion angles and the exocyclic torsion angles ε (X-O2'-C2'-C1') and γ (X-O3'-C3'-C2') geometry parameter variations were taken into consideration. Three energy minima, high-anti, anti and syn, were found for the rotation about the C1'-N1 bond. The high-anti orientation of the base with respect to the sugar moiety, turned out to be preferred, regardless of the substituents at the C2' and C3' positions. Other orientations are at least 1.65 kcal/mol higher in Gibbs free energy than the high-anti one. It has been shown that intramolecular H-bonds and the anomeric effect of phosphate groups strongly affect the conformational preferences of the studied compounds. Further, the structure of substituents attached to the sugar moiety influence the pucker of the furanoid ring. The furanoid ring in the global minima of the compound with two OH groups (TC1) in the 2' and 3' positions, and the compound having a 3'-phosphate group (TC2), adopt roughly the same conformation located at the southern range of the pseudorotation wheel, and thus are close to those found in the B type DNA helix. The low-energy high-anti rotamers of the geometry with the phosphate group attached to the sugar ring in the 2' position (TC3) and the geometry with two methoxyl groups (TC4) have their furanoid rings in conformations resembling those found in A DNA and RNA helices (the northern range of the pseudorotation wheel).

Diosgenyl 2-amino-2-deoxy-β-D-galactopyranoside: synthesis, derivatives and antimicrobial activity

The synthesis of diosgenyl 2-amino-2-deoxy-β-D-galactopyranoside is presented for the first time. This synthetic saponin was transformed into its hydrochloride as well as N-acyl, 2-ureido, N-alkyl, and N,N-dialkyl derivatives. Antifungal and antibacterial studies show that some of the obtained compounds are active against Gram-positive bacteria and Candida type fungi.

Conformational studies of N-(α-d-glucofuranurono-6,3-lactone)- and N-(methyl β-d-glucopyranuronate)-p-nitroanilines

N-(α-d-Glucofuranurono-6,3-lactone)-p-nitroaniline and N-(methyl β-d-glucopyranuronate)-p-nitroaniline were obtained as crystalline solids. The single-crystal X-ray diffraction, NMR data and DFT calculations for N-(α-d-glucofuranurono-6,3-lactone)-p-nitroaniline indicate that this N-furanoside adopts a ³T₂/³E-like conformation in the crystal lattice, solution and gas phase. Thus, the structure of recorded for N-furanoside ¹H NMR spectrum is indicative of the ³T₂/³E region of the pseudorotational itinerary for furanose derivatives with α-d-gluco, β-L-ido and α-d-xylo configurations. Moreover, it is concluded that the ¹T₂/E₂/³T₂/³E region of the pseudorotational itinerary for furanose derivatives with d-gluco, L-ido and d-xylo configurations should be characterised by the lack of coupling between H2 and H3 protons, irrespective of the anomeric configuration. Such a lack of vicinal coupling is characteristic for some of the trans-oriented furanose ring protons. The single-crystal X-ray diffraction and NMR data for N-(methyl β-d-glucopyranuronate)-p-nitroaniline indicate that this N-glucuronide adopts the ⁴C₁ conformation, both in the crystal lattice and solution. The occurrence of anomeric effects in the presented N-glycosides is discussed. The crystal structure analysis of both N-glycosides gives evidence that the amine group in p-nitroaniline is planar due to the nitrogen sp² hybridisation.

The leaching behavior of cyclophosphamide and ifosfamide from soil in the presence of co-contaminant--Mixture sorption approach

Anticancer drugs (ACDs) exhibit high biological activity, they are cytotoxic, genotoxic, and are constantly released into the environment as a result of incomplete metabolism. Consequently they pose a serious threat to the environment and human health due to their carcinogenic, mutagenic and/or reproductive toxicity properties. Knowledge of their bioavailability, including their sorption to soils and their impact on the soil-groundwater pathway, is crucial for their risk assessment. Laboratory batch and column leaching tests are important tools for determining the release potential of contaminants from soil or waste material. Batch and column tests were carried out with soils differing in physicochemical properties, each spiked with cyclophosphamide (CK) or ifosfamide (IF). Moreover, due to the fact that environmental pollutants may occur as coexisting compounds in the soil the mobility evaluation for ACDs in the mixture with metoprolol (MET; β-blocker) as a co-contaminant was performed. In order to assess appropriateness, the batch and column tests were compared. The release depended on the properties of both the soil and the presence of co-contaminants. The faster release was observed for coarse-grained soil with the smallest organic matter content (MS soil: 90% decrease in concentration until liquid-to-solid ratio (L/S) of 0.3 L kg(-1) for all tests' layout) than for loamy sand (LS soil: 90% decrease in concentration until ratio L/S of 0.75 L kg(-1)). ACDs are highly mobile in soil systems. Furthermore, the decrease of mobility of ifosfamide was observed with the presence of a co-contaminant (metoprolol) in both of the soils (in MS soil a decrease of 29%; in LS soil a decrease of 26%). The mobility of cyclophosphamide does not depend on the presence of a contaminant for MS soil, but also exhibits a decrease of 21% in LS soil.

Flavonol-based fluorescent indicator for determination of β-glucosidase activity

A flavonol-based ESIPT fluorescence probe for evaluation of β-glucosidase activity was synthesized and tested for sensitivity to enzymatic cleavage at different conditions.

Theoretical studies on the reaction of mono- and ditriflate derivatives of 1,4:3,6-dianhydro-D-mannitol with trimethylamine--Can a quaternary ammonium salt be a source of the methyl group?

DFT studies on the mechanism of the formation of "gemini" quaternary ammonium salts in the reaction of 1,4:3,6-dianhydro-D-mannitol ditriflate derivative with trimethylamine and its subsequent conversion to tertiary amine through the methyl-transfer reaction are discussed. Two alternative reaction pathways are presented in the gas phase and in ethanol. Additionally, the transformation of the monotriflate derivative of 1,4:3,6-dianhydro-D-mannitol into the single quaternary ammonium salt is presented. Two functionals (B3LYP, M062X) and two basis sets (6-31+G** and 6-311++G**) were used for the calculations. The effect of the substituent attached to the five-membered rings at the C2 (and/or C5) carbon atom on the activation barrier is described. The trimethylammonium group bond to the five-membered ring greatly reduces the activation barrier height. The preferred reaction pathway for the conversions was established. Including the London dispersion in the calculations increases the stabilization of all the points on the potential energy surface in relation to individual reactants.

N-Alkyl derivatives of diosgenyl 2-amino-2-deoxy-β-D-glucopyranoside; synthesis and antimicrobial activity

Diosgenyl 2-amino-2-deoxy-β-D-glucopyranoside is a synthetic saponin exhibiting attractive pharmacological properties. Different pathways tested by us to obtain this glycoside are summarized here. Moreover, the synthesis of N-alkyl and N,N-dialkyl derivatives of the glucopyranoside is presented. Evaluation of antibacterial and antifungal activities of these derivatives indicates that they have no inhibitory activity against Gram-negative bacteria, whereas many of the tested N-alkyl saponins were found to inhibit the growth of Gram-positive bacteria and human pathogenic fungi.

DFT studies of the formation of furanoid derivatives of ammonium chlorides

B3LYP/6-31+G** level computations were performed on the formation of ammonium salts during the reaction of (S)-1,4-anhydro-5-chloro-2,3,5-trideoxypentitol (1) (2S,5S)-2,5-anhydro-6-chloro-1,3,4,6-tetradeoxyhexitol (2) and methyl 5-chloro-2,3,5-trideoxy-β-D-pentofuranoside (3) with ammonia in order to describe the reaction pathway in detail. All the structures were fully optimized in the gas phase, in chloroform and water. In addition, the gas phase activation barrier heights were estimated at B3LYP/6-311++G**, MPWIK/6-31+G**, MPWIK/6-311++G** and MP2/6-311++G(2d,2p)//MPWIK/6-31+G** levels of theory. All the calculations in solvents were performed the using polarizable continuum model (PCM) and the B3LYP functional with the 6-31+G** basis set. A detailed description of all the stationary points is presented, and the conformational behavior of the five-membered ring is discussed in the gas phase and in the solvents. The conversion of the reactant complexes into ion pairs is accompanied by a strong energy decrease in the gas phase and in all the solvents. The overall process is strongly unfavorable in the gas phase, but takes place readily in high-polarity solvents.

The conformational behavior, geometry and energy parameters of Menshutkin-like reaction of O-isopropylidene-protected glycofuranoid mesylates in view of DFT calculations

The formation of pyridinium salts in the transformation of three O-isopropylidene-protected mesylates of furanoid sugar derivatives under pyridine action is considered at the B3LYP/6-31+G** computation level. All the structures were optimized in the gas phase, in chloroform and water. Activation barrier heights in the gas phase were also estimated at the B3LYP/6-311++G**, MPW1K/6-31+G** and MPW1K/6-311++G** levels. The conducted calculations, both in the gas phase (regardless of the computation level) and in solvents, revealed the barrier height increasing order as follows: 1>2>3 for the three reactions studied. The conformational behavior of the five-membered ring is discussed in the gas phase and in solvents. The fused dioxolane ring makes the furanoid ring less likely to undergo conformational changes. In the case of reaction 3, the furanoid ring shape does not change either in the gas phase or in solvents. All conformers are close to E0 or (0)E.

Hydrolysis of sulphonamides in aqueous solutions

Hydrolysis is one of the most common reactions controlling abiotic degradation and is one of the main paths by which substances are degraded in the environment. Nevertheless, the available information on this process for many compounds, including sulphonamides (a group of antibiotic drugs widely used in veterinary medicine), is very limited. This is the first study investigating the hydrolytic stabilities of 12 sulphonamides, which were determined according to OECD guideline 111 (1st category reliability data on the basis of regulatory demands on data quality for the environmental risk assessment of pharmaceuticals). Hydrolysis behaviour was examined at pH values normally found in the environment. This was prefaced by a discussion of the acid-base properties of sulphonamides. All the sulphonamides tested were hydrolytically stable at pH 9.0, nine (apart from sulphadiazine, sulphachloropyridazine and sulphamethoxypyridazine) were stable in this respect at pH 7.0 and two (sulphadiazine and sulphaguanidine) at pH 4.0 (hydrolysis rate≤10%; t(0.5 (25°C))>1 year). The degradation products were identified, indicating two independent mechanisms of this process. Our results show that under typical environmental conditions (pH and temperature) sulphonamides are hydrolytically stable with a long half-life; they thus contribute to the on-going assessment of their environmental fate.

X-ray diffraction and high-resolution NMR spectroscopy of methyl 3,4-di-O-acetyl-1,5-anhydro-2-deoxy-d-arabino-hex-1-enopyranuronate

Single-crystal X-ray diffraction and high-resolution (1)H and (13)C NMR spectral data for methyl 3,4-di-O-acetyl-1,5-anhydro-2-deoxy-D-arabino-hex-1-enopyranuronate are reported. The (5)H(4) conformation was found to be the preferred form for this glycal, both in the crystal lattice and in solution. The factors determining the (4)H(5)<==>(5)H(4) conformational equilibrium for acetylated glycals are discussed.

Methyl 4-O-Acetyl-3-azido- and 3-Azido-4-O-methylsulfonyl-2,3,6,-trideoxyhex-5- enopyranosides in DFT-Level Conformational Studies

Geometry optimizations at the B3LYP level of density functional theory (DFT) are reported for methyl 4-O-acetyl-3-azido- and 3-azido-4-O-methylsulfonyl-2,3,6-trideoxy-alpha,beta-d-threo- and -beta-d-erythro-hex-5-enopyranosides. The most stable conformers for each compound are presented, along with the corresponding enthalpies and Gibbs free energies. The influence of the exocyclic double bond on the chair conformation is discussed. Conformations of the 1-OMe, 3-N3, and 4-OAc groups were examined, and delocalization in the OAc and N3 groups was demonstrated. The contributions of particular conformers to the total number of structures found for each hex-5-enopyranoside were calculated. The theoretical results are compared with assignments based on 1H NMR studies.

Conformations of methyl 2,5-di-O-acetyl-beta-D-glucofuranosidurono-6,3-lactone and 1,2,5-tri-O-acetyl-beta-D-glucofuranurono-6,3-lactone in the crystal structure and in solution

The single-crystal X-ray diffraction and high-resolution 1H and 13C NMR spectral data for methyl 2,5-di-O-acetyl-beta-D-glucofuranosidurono-6,3-lactone and 1,2,5-tri-O-acetyl-beta-D-glucofuranurono-6,3-lactone are reported. The lactones were synthesized as byproducts of reactions carried out to obtain methyl 1,2,3,4-tetra-O-acetyl-D-glucopyranuronate. The conformations of these lactones in the crystal structure and in solution are discussed. A 1T2-like conformation was found to be the preferred form for these lactones in both the crystal lattice and in solution.

Synthesis and conformational studies on methyl 4-O-acetyl-3-azido-2,3,6-trideoxy-hex-5-enopyranosides of the l series

Methyl 3-azido-2,3-dideoxy-alpha-D-xylo-, -alpha-D-lyxo-, and -beta-D-xylo-hexopyranosides were converted into 4-O-acetyl-3-azido-6-iodo-2,3,6-trideoxy analogues via 6-O-p-tolylsulfonyl compounds. The elimination of hydrogen iodide from 6-iodo glycosides yielded methyl 4-O-acetyl-3-azido-2,3,6-trideoxy-beta-L-erythro-, -alpha-L-threo-, and -beta-L-threo-hex-5-enopyranosides. The configuration and conformation of all products are evaluated in depth on the basis of (1)H and (13)C NMR data. Factors determining conformational energy in 4-O-protected-3-azido-2,3,6,-trideoxy-hex-5-enopyranosides are discussed.

N-Alkyl derivatives of 2-amino-2-deoxy-d-glucose

Mono- and di-N-alkylated derivatives of 1,3,4,6-tetra-O-acetyl-2-amino-2-deoxy-beta-D-glucose (alkyl=methyl, ethyl, propyl, butyl, pentyl, hexyl, benzyl) were synthesised by the reductive alkylation of per-O-acetyl-d-glucosamine. (N-ethyl, N-propyl, N-butyl, N-pentyl and N-hexyl)-1,3,4,6-tetra-O-acetyl-2-amino-2-deoxy-beta-D-glucoses were deacetylated in order to attempt an enzymatic phosphorylation. All products were characterised by means of IR, NMR and MS spectra. N-Ethyl- and N-pentyl-d-glucosamines were found to exhibit weak antifungal activity.

Synthesis, crystal structure, and high-resolution NMR spectroscopy of methyl 3-azido-2,3-dideoxy-4,6-di-O-p-tolylsulfonyl-α-d-xylo-hexopyranoside

Synthesis of methyl 3-azido-2,3-dideoxy-4,6-di-O-p-tolylsulfonyl- and -6-O-p-tolylsulfonyl-alpha-D-xylo-hexopyranosides is presented. High-resolution 1H and 13C NMR spectral data for both compounds and their precursors, and the single-crystal X-ray diffraction analysis for methyl 3-azido-2,3-dideoxy-4,6-di-O-p-tolylsulfonyl-alpha-D-xylo-hexopyranoside are reported. The influence of the O-protective group on the chemical shift of adjacent atoms in the 1H and 13C NMR spectra is discussed.

X-ray diffraction and high-resolution NMR spectroscopy of methyl 3-azido-2,3-dideoxy-alpha-D-lyxo-hexopyranoside

The single-crystal X-ray diffraction and high-resolution 1H and 13C NMR spectral data for the title compound are reported. The influence of the ring oxygen atom on the J(1,2e) and J(4,5) coupling constants for 2-deoxy-D-lyxo- and -D-xylo-hexopyranosides is discussed.

The use of tri-O-acetyl-D-glucal and -D-galactal in the synthesis of 3-acetamido-2,3-dideoxyhexopyranoses and -hexopyranosides

Addition of hydrazoic acid to alpha,beta-unsaturated aldehydes derived from tri-O-acetyl-D-glucal and -D-galactal gave 3-azido-2,3-dideoxyhexopyranoses. These were converted into 1,4,6-tri-O-acetyl-3-azido-2,3-dideoxyhexopyranoses as well as methyl and ethyl glycosides. Hydrogenation of the proamine group in 3-azido-2,3-dideoxy derivatives provided different 3-amino and 3-acetamido sugars. The configuration and conformation of all products were established on the basis of the 1H and 13 C NMR, IR and polarimetric data.

New procedure of the Mitsunobu reaction as the key step in peptide nucleic acid (PNA) monomers synthesis

PNAs are relatively novel DNA analogues, intensively studied due to their potential as gene-targeted drugs with antigene and antisense properties. In 1996 we elaborated a new method of synthesis of PNA monomer backbones based on the Mitsunobu reaction with N-tosyl-protected (Tos) amino acid esters as acidic components of the reaction. Since the method used for the Tos group removal requires conditions incompatible with various functional groups, here we modified the procedure by replacing the tosyl group with o-nitrobenzenesulfonyl (o-NBS) group. Using the new procedure we obtained protected PNA monomer backbones with various amino acid side chains. The pseudodipeptide secondary amine groups were then deprotected by thiolysis, and after standard work-up acylated with thymin-1-ylacetic acid, to give the protected monomers. Since the deprotection of the secondary amine group occurs under mild conditions, the procedure is of general applicability and allows various modifications of PNA structure by using diverse beta-amino alcohols and alpha-amino acid esters.

The synthesis and structure of the derivatives of 2-deoxy-2-hydroxyimino-D-lyxo-hexopyranosyl-L-cysteine and -thiophenol

3,4,6-Tri-O-acetyl-2-deoxy-2-hydroxyimino-beta and -alpha-D-lyxo-hexopyranosides of thiophenol (3, 4) and the methyl ester of N-benzoyl-L-cysteine have been synthesised by condensation of 3,4,6-tri-O-acetyl-2-deoxy-2-nitroso-alpha-D-galactopyranosyl chloride with thiophenol and the L-cysteine derivative, respectively. The conformation of the sugar residue and configuration of the anomeric centre as well as of the hydroxyimino group were established on the basis of the 1H NMR (DQF-COSY, ROESY, TOCSY) spectrometric techniques and polarimetric data. Additionally, the structure of S-[3,4,6-tri-O-acetyl-2-deoxy-2-(Z)-hydroxyimino-beta-D-lyxo -hexopyranosyl]-thiophenol (3) was supported by X-ray diffraction data.

Conformation of cyclo-(D-phenylalanyl-trans-4-fluoro-D-prolyl)

cyclo(D-Phenylalanyl-trans-4-fluoro-D-prolyl), c(D-Phe-D-FPro), was synthesized and its conformation determined both in solution and in the solid state by 1H NMR and X-ray analysis, respectively. In the crystals the 2.5-diketopiperazine (DKP) ring assumes the uncommon conformation, for cyclodipeptides containing Pro residue, of a flattened chair, which seemingly results from a compromise between, on the one hand, the DKP-aromatic intramolecular ring-ring attraction (folding), requiring the C alpha--C beta bond of the Phe to be axial, and, on the other hand, the intrinsic tendency of the Pro residue to have its C alpha--C beta bond equatorial. Unlike the solid state, the 1H NMR data in CDCl3 and C6D6 demonstrate that in both solutions the DKP ring assumes a boat-like shape, typical for the Pro-containing cyclodipeptides, with the equatorial C alpha--C beta bonds in both amino acid residues, which preclude ring-ring folding. A similar conformation was encountered in the closest analog of c(D-Phe-D-FPro), viz, in c(Phe-Pro), both in solution (21, 22, 26) and in the solid state (12). A subtle interplay of intramolecular interactions introduced into a cyclodipeptide by a Pro-type and a Phe-type residue is emphasized.