Synthesis of chiral ferrocenyl aminoalcohols involving a diastereospecific oxidation: use in asymmetric catalysis

The synthesis of six new chiral ferrocenic aminoalcohols has been performed. One step of this synthesis was a diastereospecific oxidation of diastereomeric mixtures of 1-[2-(N,N-dimethylaminomethyl)ferrocenyl]alcohols by manganese dioxide: one diastereomer was oxidized, while the other stayed inert. The six new chiral compounds in association with ZnEt₂ proved to be good catalysts in the asymmetric ethylation of benzaldehyde (77 to 88% ee).     

Semicarbazones and thiosemicarbazones of the heterocyclic series

IsatinΒ-thiosemicarbazone and its alkyl derivatives containing a hydrogen atom attached to N_(2′) exist primarily in the form of the syn isomer stabilized by intramolecular hydrogen bonding in solutions and in the crystalline state. The strength of this hydrogen bond increases when both hydrogen atoms in the primary thioamide group are replaced by alkyl groups.     

Intermolecular-medium and intramolecular-weak hydrogen bonding chains in the crystals of chiral trifluoromethylated amino alcohols

A structural feature of hydrogen bonding chains found in the crystals of trifluoromethylated amino alcohols is reported. Hydrogen bondings of 3-(N,N-dialkylamino)-1,1,1-trifluoro-2-propanols construct chiral spiral hydrogen bonding chains. Lone pairs on the nitrogen atoms of the amino alcohols participate in two hydrogen bondings. Detailed structural analysis of the hydrogen bonds of the 3-(N,N-dimethylamino)-1,1,1-trifluoro-2-propanol suggested that the chain built up with alternating intermolecular-medium and intramolecular-weak hydrogen bonds. The medium intermolecular hydrogen bond, which transfers a proton from the hydroxy group to the amino nitrogen, would make a tentative zwitterionic form of the molecule. Then, electrostatic attraction between the charges in the zwitterion centers induced a weak intramolecular hydrogen bond.     

Heteronuclear NMR spectroscopic investigations of hydrogen bonding in 2‐(benzo[d]thiazole‐2′‐yl)‐N‐alkylanilines

The 2‐(benzo[d]thiazole‐2′‐yl)‐N‐alkylanilines have previously revealed the presence of a strong intramolecular hydrogen bond. This in turn gives rise to a more complicated multiplet for the protons attached to the carbon adjacent to the amino group. This intramolecular hydrogen bond was investigated by a deuterium exchange experiment using heteronuclear NMR spectroscopy (¹H, ¹³C, ¹⁵ N and ²H). We observed changes in the multiplet structure and chemical shifts providing further evidence that the deuterium replaces the hydrogen in the intramolecular hydrogen bond. A time course study of the D₂O exchange confirmed the presence of a strong hydrogen bond. The comparison of the structures obtained by X‐ray crystallography showed a very small difference in planarity between the two‐substituted and four‐substituted amino compounds. In both the cases, the phenyl ring is not absolutely coplanar with the thiazole unit. The existence of this intramolecular hydrogen bond in 2‐(benzo[d]thiazole‐2′‐yl)‐N‐alkylanilines was further confirmed by single crystal X‐ray crystallography. Copyright © 2015 John Wiley & Sons, Ltd.     

Kinetik der Bromierung von Phenolen und phenolischen Mekrkernverbindungen

The bromination of 15 dinuclear phenolic compounds (dihydroxydiphenylmethanes, methylene bisphenols) by molecular bromine in acetic acid was studied kinetically at 22°C. In all compounds the electrophilic substitution occurred inortho-position to the phenolic hydroxy group of the methyl phenol unit while the non reacting neighboring unit was differently substituted by H, CH₃,t-Bu and NO₂. A decrease in the reaction rate was observed in 2,2′-dihydroxydiphenylmethanes, where the +M-effect of the hydroxy group is diminished by an intramolecular hydrogen bond. The strength of this hydrogen bond may be influenced mainly by steric factors. Strong electron withdrawing substituents like NO₂ show a rate decreasing influence on the reactivity of the neighboring unit also in 2,4′- and 4,4′-dihydroxydiphenylmethanes.     

Amido‐3‐hydroxypyridin‐4‐ones as Iron(III) Ligands

The synthesis and physicochemical properties of a range of 2‐ and 6‐amido‐3‐hydroxypyridin‐4‐ones are described. All the amido‐substituted 3‐hydroxypyridin‐4‐ones have lower pK_a values than 1,2‐dimethyl‐3‐hydroxypyridin‐4‐one (deferiprone). This is due to the inductive effect of the amido group. Furthermore, the pK_a values of the 3‐hydroxy group in 1‐nonsubstituted pyridinones are dramatically lower than those of the corresponding 1‐alkyl analogues, indicating that a strong hydrogen bond exists between the 2‐amido function and the 3‐oxygen anion, which stabilises the anion. As a result of the decreased competition with protons, the pFe³⁺ values of this group of molecules are higher than that of deferiprone. The distribution coefficients of these molecules are also increased despite the lack of a hydrophobic 1‐alkyl substituent and this is ascribed to the intramolecular hydrogen bond. X‐ray diffraction studies confirm the existence of the intramolecular hydrogen bond.     

Deuterium isotope effects on 13C NMR chemical shifts of some α-(2-hydroxyaryl)-N-phenylnitrones (Schiff base N-oxides)

The deuterium isotope effect on the ¹³C NMR chemical shifts of some α-2-hydroxyaryl-N-phenylnitrones (Schiff base N-oxides) was studied. The existence of an intramolecular hydrogen bond with the proton localized on the phenolic oxygen atom was evidenced. Exceptionally large isotope effects ΔC-2(D) and ΔC-α(D) suggest that the substitution of the proton of the OH group by deuterium leads to a weakening of the hydrogen bond and some conformational changes in the molecule. This conclusion was drawn on the basis of a comparison of the deuterium isotope effects of Schiff base N-oxides and parent Schiff bases. Copyright © 2001 John Wiley & Sons, Ltd.     

Conformational Analysis of N-Alkyl-N-[2-(diphenylphosphoryl)ethyl]amides of Diphenylphosphorylacetic Acid: Dipole Moments,IR Spectroscopy,DFT Study

Experimental and theoretical conformational analysis of N-methyl-N-[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide, N-butyl-N-[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide, and N-octyl-N-[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide was carried out by the methods of dipole moments, IR spectroscopy, and Density Functional Theory (DFT) B3PW91/6-311++G(df,p) calculations. In solution, these N,N-dialkyl substituted bisphosphorylated acetamides exist as a conformational equilibrium of several forms divided into two groups—with Z- or E-configuration of the carbonyl group and alkyl substituent, and syn or anti arrangement of the phosphoryl-containing fragments relative to the amide plane. The substituents at the phosphorus atoms have eclipsed cis- or staggered gauche-orientation relative to the P=O groups, and cis orientation of the substituents is due to the presence of intramolecular H-contacts P=O...H−C_phenyl or p,π conjugation between the phosphoryl group and the phenyl ring. Preferred conformers of acetamides molecules are additionally stabilized by various intramolecular hydrogen contacts with the participation of oxygen atoms of the P=O or C=O groups and hydrogen atoms of the methylene and ethylene bridges, alkyl substituents, and phenyl rings. However, steric factors, such as a flat amide fragment, the bulky phenyl groups, and the configuration of alkyl bridges, make a significant contribution to the realization of preferred conformers.     

Stereochemistry of nitrogen heterocycles. 48. Synthesis,configurations, and ir spectra of 1,2-dimethyldecahydroquinoline-5-ol isomers and their acetates

Three isomers of 1,2-dimethyldecahydroquinolin-5-ol and their acetates with cis and trans fusion of the rings and different orientations of the hydroxy and acyloxy groups were obtained. The correlation of the frequencies of the C-O, O-H, and N⁺-H stretching vibrations with the configurations of these compounds was studied. The 1,2-dimethyl-cis-decahydroquinolin-5-ol isomer with a syn orientation of the amino and hydroxy groups exists in a conformation with an intramolecular hydrogen bond, the character of which is determined by the functional state of the amino group (OH...N in the base and N⁺-H...O in the hydrochloride). The hydrochloride of the acetate of this alcohol does not form an intramolecular N⁺-H...O(COCH₃) bond.See [1] for Communication 47.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 382–388, March, 1984.     

Alkyl and 2-hydroxy derivatives of imidazo]4,5-b]pyrazine and their N-oxides

N-Monoxides and N,N-dioxides of some alkyl and hydroxy derivatives of imidazo[4,5-b]pyrazine were synthesized. It was proved that the nitrogen atoms of the pyrazine ring are oxidized; alkylation of N₁ sterically hinders oxidation of N₇. The hydrolytic cleavage of imidazo[4,5-b]pyrazines and their N-oxides was studied.     

Substituent effects on regioselective intramolecular oxidation of unactivated C-H bonds: stereoselective synthesis of substituted tetrahydropyrans

Our previously reported intramolecular delta-selective C-H bond oxidation by dioxiranes, generated in situ from activated ketones, offers a novel approach to the synthesis of tetrahydropyrans. To synthesize substituted tetrahydropyrans in a stereoselective manner, we examined the effects of alkyl, nitrogen, and oxygen substituents at the alpha-, beta-, and gamma-sites of ketones on the stereoselectivities of intramolecular C-H bond oxidation reactions. Ketones 1-4 with a methyl group at the alpha-, beta-, or gamma-site showed the diastereoselectivities that agreed with the trans/cis ratio predicted by considering steric interactions in the transition states. Furthermore, ketones 5 and 6 carrying a bulky phthalimido group at the alpha- and the beta-sites, respectively, exhibited excellent stereoselectivity, each affording only one diastereomer. However, ketones 9 and 10 bearing beta-oxygen substituents gave reversed stereoselectivity as compared to those with beta-alkyl or nitrogen substituents, possibly because of the hydrogen bonding interaction in the transition state. For ketones 12 and 13, both bearing methyl and silyloxy groups, the hydrogen bonding interaction was probably more important than the steric effect on the diastereoselectivity of intramolecular oxidation of C-H bonds.     

Testing the conformational hypothesis of passive membrane permeability using synthetic cyclic peptide diastereomers

Little is known about the effect of conformation on passive membrane diffusion rates in small molecules. Evidence suggests that intramolecular hydrogen bonding may play a role by reducing the energetic cost of desolvating hydrogen bond donors, especially amide N-H groups. We set out to test this hypothesis by investigating the passive membrane diffusion characteristics of a series of cyclic peptide diastereomers based on the sequence cyclo[Leu-Leu-Leu-Leu-Pro-Tyr]. We identified two cyclic hexapeptide diastereomers based on this sequence, whose membrane diffusion rates differed by nearly two log units. Results of solution NMR studies and hydrogen/deuterium (H/D) exchange experiments showed that membrane diffusion rates correlated with the degree of intramolecular hydrogen bonding and H/D exchange rates. The most permeable diastereomer, cyclo[d-Leu-d-Leu-Leu-d-Leu-Pro-Tyr] (1), exhibited a passive membrane diffusion rate comparable to that of the orally available drug cyclosporine A.     

Raman spectroscopic study of hydrogen bonding of N-alkyl-2-pyrrolidinones in heavy water

The Raman spectra of D₂O solutions of N-methyl-2-pyrrolidinone, N-ethyl-2-pyrrolidinone, and N-n-butyl-2-pyrrolidinone under diverse conditions were measured. Using a computer fitting of the band shape of the carbonyl stretching mode at various temperatures, an enthalpy difference for the inversion motion at the nitrogen atom due to hydrogen bonding with deuterium was estimated for these compounds. The enthalpy difference of hydrogen bond formation to the nitrogen atom of N-methyl-2-pyrrolidinone at 30 wt% in D₂O (mole fraction 0.080) was greater than that of N-methyl-2-pyrrolidinone in an aqueous solution at a mole fraction of 0.406. Furthermore, the enthalpy difference of N-alkyl-2-pyrrolidinones increased with the alkyl chain length. This is interepreted as a result of the change of the hydrophobic hydration of D₂O molecules around the solute molecules.     

Intra- and intermolecular hydrogen bonding in solutions of <Emphasis Type="Italic">N</Emphasis>-(2-hydroxy-3,8-diiodocyclooctyl)trifluoroacetamide and <Emphasis Type="Italic">N</Emphasis>-(4-iodo-2,2,5,5-tetramethyltetrahydrofuran-3-yl)trifluoroacetamide

The methods of IR spectroscopy and quantum chemistry (B3LYP/DGDZVP) were applied to investigation of the types of self-associates formed in solutions of N-(2-hydroxy-3,8-diiodocyclooctyl)trifluoroacetamide and N-(4-iodo-2,2,5,5-tetramethyltetrahydrofuran-3-yl)trifluoroacetamide in CCl₄ by means of hydrogen bonding. The ОН group of N-(2-hydroxy-3,8-diiodocyclooctyl)trifluoroacetamide participates in the formation of intramolecular hydrogen bond ОH···О=С, while chain dimers are formed due to interactions between the NH and С=О groups of the neighboring molecules. Due to the formation of the intramolecular bond, the dimers of N-(2-hydroxy-3,8-diiodocyclooctyl)trifluoroacetamide are energetically less stable than the chain dimer of N-(4-iodo-2,2,5,5-tetramethyltetrahydrofuran-3-yl)trifluoroacetamide.     

Surface properties and hydrogen bonds of mono-functional polybenzoxazines with different <Emphasis Type="Italic">N</Emphasis>-substituents

A series of mono-functional benzoxazine monomers with different N-substituents were synthesized from phenol, formaldehyde, and various amines (ammonia, methylamine, n-butylamine, dodecylamine), named P-am, P-m, P-b, and P-da, respectively. The surface properties of these polybenzoxazine films were proven by contact angle measurements. The hydrogen bond network of the polybenzoxazine systems was studied using the FTIR spectra. And the results showed that the surface free energy increased with increasing the fraction of intermolecular hydrogen bonding when the N-substituent was an alkyl chain. However, the rule was not suitable when the N-substituent was H. That was because there was one more kind of intramolecular hydrogen bond in the poly(P-am). Based on these findings, we proposed that both the N-substituent alkyl group and the fraction of intermolecular hydrogen bonding had effects on the surface free energy.     

Elementary steps of ethanol oxidation on Pt in sulfuric acid as evidenced by isotope labelling

The elementary steps of the anodic oxidation of ethanol on Pt in sulfuric acid are visualized with differential electrochemical mass spectroscopy (DEMS) by means of deuterium and ¹⁸O labelling. It turns out that: (i) ethanol is oxidized directly to acetaldehyde by the cleavage of one hydrogen of the α-C-atom and the hydroxyl hydrogen; (ii) a strongly bound intermediate is formed in parallel; (iii) the intermediate is oxidized at 0.7 V to give two CO₂ molecules, one originating from the alcoholic group and still containing the alcoholic O, the other from the methyl group; (iv) CC bond splitting seems to occur during the oxidation of the adsorbate only.     

Total synthesis of nahuoic acid A via a putative biogenetic intramolecular Diels–Alder (IMDA) reaction

Inspired by the biogenetic proposal of an intramolecular Diels–Alder (IMDA) cycloaddition, the total synthesis of natural product nahuoic acid A, a cofactor-competitive inhibitor of the epigenetic enzyme lysine methyl transferase SETD8, has been carried out. A non-conjugated pentaenal precursor was synthesized with high levels of stereoselectivity at seven stereogenic centers and with the appropriate control of double bond geometries. Although the IMDA reaction of the non-conjugated pentaenal using Me₂AlCl for catalysis at −40 °C selectively afforded the trans-fused diastereomer corresponding to the Re-endo mode of cycloaddition, under thermal reaction conditions it gave rise to a mixture of diastereomers, that preferentially formed through the exo mode, including the cis-fused angularly-methylated octahydronaphthalene diastereomer precursor of nahuoic acid A. The natural product could be obtained upon oxidation and overall deprotection of the hydroxyl groups present in the Si-exo IMDA diastereomer.

The total synthesis of natural product nahuoic acid A, a cofactor-competitive inhibitor of the epigenetic enzyme lysine methyl transferase SETD8, has been carried out based on the biogenetic proposal of an intramolecular Diels–Alder (IMDA) cycloaddition.     

FT-IR and FT-Raman study of hydrogen bonding in p-alkylcalix[8]arenes

The FTIR and FT Raman spectra of p-alkylcalix[8]arenes (alkyl = tert-butyl, isononyl) were recorded. Analysis of IR spectra showed that the cyclic cooperative intramolecular hydrogen bond is realized in calix[8]arene. It was found that the strength of the cyclic cooperative intramolecular hydrogen bond in the series of alkyl derivatives of calix[8]arenes depends very little on the replacement of the p-tert-butyl groups by the more bulky isononyl group. From our data follows that the orientation of aromatic fragments in calixarene molecules depends on the type of alkyl substituent.An analysis of the changes in the IR spectra with heating and dissolution shows that the conformation of the “pleated-loop” is retained in p-tert-butylcalix[8]arene. It turned out that the intramolecular hydrogen bond is a “probe” of the conformation of calixarene molecules and IR spectroscopy is a unique method that allows one to follow the slightest nuances of changes in the H-bound system of these supermolecules.     

Solid state and solution conformation of 2-pyridinecarboxylic acid hydrazides: a new structural motif for foldamers

X-Ray crystallography and NMR show a strong preference for trans conformers of N′-phenyl or N′-(2-pyridyl) 2-pyridinecarboxylic acid hydrazides, stabilized by an NHN_pyr. intramolecular hydrogen bond both in the solid state and in solution. This allows us to extrapolate that oligomers of this unit should adopt extended linear conformations.     

Temperature-dependent polymorphism of N-(4-fluorophenyl)-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide: experimental and theoretical studies on intermolecular interactions in the crystal state

X-ray analysis of N-(4-fluorophenyl)-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide reveals the temperature-dependent polymorphism associated with the crystallographic symmetry conversion. The observed crystal structure transformation corresponds to a symmetry reduction from I4₁ /a (I) to P4₃ (II) space groups. The phase transition mainly concerns the subtle but clearly noticeable reorganization of molecules in the crystal space, with the structure of individual molecules left almost unchanged. The Hirshfeld surface analysis shows that various intermolecular contacts play an important role in the crystal packing, revealing graphically the differences in spatial arrangements of the molecules in both polymorphs. The N-oxide oxygen atom acts as a formally negatively charged hydrogen bonding acceptor in intramolecular hydrogen bond of N–H…O^? type. The combined crystallographic and theoretical DFT methods demonstrate that the observed intramolecular N-oxide N–H…O hydrogen bond should be classified as a very strong charge-assisted and closed-shell non-covalent interaction.