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Anomeric Effect (anomeric + effect)
Selected AbstractsAnomeric effect plays a major role in the conformational isomerism of fluorinated pnictogen compoundsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2008Levindo E. Martins Abstract According to our theoretical studies, the anomeric effect, an stereoelectronic interaction between lone pair and a vicinal antibonding orbital, has shown to contribute decisively for the conformational isomerism of 1-fluoro- N,N -dimethylmethanamine (1) and of its corresponding P, As and Sb analogues (2,4). CX bonds in 2,4 are larger than in the parent compound 1, thus providing a LPX/CF* interaction progressively weaker on going from 1 to 4. However, such hyperconjugation contributed by more than 1.3,kcal,mol,1 for the stabilization of anti conformer in 4 (,LPXCF,=,180°), increasing to 24.1,kcal,mol,1 in 1. An isodesmic reaction model supported these findings. Copyright © 2008 John Wiley & Sons, Ltd. [source] Anomeric effect and hydrogen-bonded supramolecular motif in 5-(3-fluoro-4-methoxyphenyl)-1-[(3-fluoro-4-methoxyphenyl)aminomethyl]-1,3,5-triazinane-2-thioneACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2009Zhenfeng Zhang In the title compound, C18H20F2N4O2S, the triazinane-2-thione ring adopts an envelope conformation, the ring substituents lie on the same side of the mean plane of the heterocyclic ring and the exo lp,N,C,Ntriaz unit (lp is a lone pair and triaz is the triazinane ring) exhibits an antiperiplanar orientation, which is shown to be governed by strong anomeric effects. Molecules are linked into a complex three-dimensional framework by a combination of two N,H...S hydrogen bonds, three C,H...F hydrogen bonds and a ,,, stacking interaction. [source] Anomeric effect for a 2,5,7-triazabicyclo[2.2.1]heptane derivativeACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2009Amir Taheri A new member of the polyazapolycyclic family of compounds, namely N3,N6,2,5,7-pentaphenyl-2,5,7-triazabicyclo[2.2.1]heptane-3,6-diamine xylene solvate, C34H31N5·C8H10, was synthesized for the first time and the crystal structure is reported. There are no hydrogen bonds joining the molecules. All four chiral C atoms have the same absolute configurations. With regard to the four N,C,N groups, anomeric effects are observed to cause a reduction of C,N bond length and N-atom pyramidality. [source] Synthesis and Comparative Glycosidase Inhibitory Properties of Reducing Castanospermine AnaloguesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2005Paula Díaz Pérez Abstract The feasibility of the intramolecular nucleophilic addition of the nitrogen atom in cyclic (thio)carbamates with a pseudo- C -nucleoside structure to the masked carbonyl group in aldose precursors in the synthesis of reducing (i.e., 5-hydroxy)6-oxaindolizidine frameworks is illustrated by the preparation of the 6- epi, 7- epi, 8- epi and 6,8a-di- epi diastereomers of the potent glycosidase inhibitor (+)-castanospermine. In all cases, the increased anomeric effect caused by the high sp2 character of the pseudoamide-type nitrogen atom resulted in the pseudoanomeric hydroxy group being anchored in an axial orientation in aqueous solution, as in the aglycons in ,-glycosides. These analogs of the natural alkaloid showed a higher selectivity in the inhibition of ,-glucosidases. Structure/glycosidase inhibitory activity studies indicated that inversion of any hydroxy group resulted in a dramatic decrease in the inhibition potency, confirming the critical importance of a correct hydroxylation profile. In the case of (+)-8- epi -6-oxacastanospermine derivatives, with a hydroxylation profile with a structural complementarity to that of D -galactose, a moderate but very selective inhibition of ,-galactosidase was observed, supporting the importance of a defined configuration at pseudoanomeric centres for anomeric specificity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Synthesis of a Fusion-Isomeric Cellobionoimidazole and Its Evaluation against the syn -Protonating Glycosidase Cel7AHELVETICA CHIMICA ACTA, Issue 12 2005Narinder Mohal The fusion-isomeric cellobinoimidazole 2, a potential inhibitor of the syn -protonating , -glycosidase Cel7A, was synthesised by Koenigs,Knorr glycosylation of the ,- D -arabinopyranoside 32, followed by selective hydrolysis. Glycosylation of 32 with acetobromoglucose 6 proceeded with poor diastereoselectivity, giving the desired 1,3-linked , - d- disaccharide 35 as minor product, besides the major 1,3-linked ,- d- disaccharide 36. Hg2+ -Promoted glycosylation of 32 led predominantly to the 1,2-ortho ester 33. Sequential removal of the silyl, acetyl, and allyl groups of 35 led to a 45,:,55 equilibrium mixture 2 and the manno -configured isomer 39. Similarly, deprotection of 36 gave a mixture of the maltonoimidazole 42 and the manno -configured isomer 43. According to a known protocol, the glycosyl acceptor 32 was synthesised in eleven steps and an overall yield of 8,13% from D -lyxose. The silylated arabinopyranosyl moiety of the ,- d- glucosides 13,19, 33, 34, and 36 adopts a 4C1 conformation, while the arabinopyranosyl moiety of the , - d- glucosides 17 and 35 exists as a 1,:,3,mixture of 4C1 and 1C4 conformers, as a result of the combined preferred axial orientation of bulky vicinal substituents and the anomeric effect. MM3* Modelling evidences a preferred 4C1 conformation of 35 and 36, and stronger steric interactions between the pyranosyl moieties of 35. The equilibrium mixture 2/39 proved a poor inhibitor of Cel7A with an IC50 value of ca. 4,mM. [source] A generalized exo -anomeric effect.ISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2000Substituent, solvent effects on the conformational equilibria of 2-(arylseleno)cyclohexanones The effects of substitution and solvent on the conformational equilibria of 2-[(4-R-substituted-phenyl)seleno]cyclohexanones are described. The conformational equilibria were determined by comparison of the linewidths of the H-2 resonances in the 1H NMR spectra of the conformationally averaged systems with those of the anancomeric (highly biased) 4-isopropyl-2-substituted cyclohexanones. The substituent (R = NMe2, OMe, Me, H, F, Cl, CF3, NO2) and solvent ((CD3)2CO, CD3CN, CD2Cl2, CDCl3) effects are discussed in terms of electrostatic effects and the possible stabilizing orbital interactions. The values of Keq (axial-equatorial) increase as the substituent becomes more electron withdrawing, in agreement with the dominance of nSe , ,*C=O or ,C-Se , ,*C=O orbital interactions in the axial conformers. The increase in the proportion of the equatorial isomers in more polar solvents for a given substituent suggests a damping of the dipolar interactions in the equatorial isomers. However, the proportion of the equatorial isomers in a given solvent increases as the substituent becomes more electron withdrawing, indicating that electrostatic interactions do not dominate in controlling the conformational equilibria. Analysis of the equilibrium data by means of a dual substituent parameter approach indicates the best correlation with ,I and ,+R substituent constants in CD2Cl2 and with ,I and ,°R substituent constants in CD3CN, with similar sensitivities to the resonance and polar effects. The correlations are interpreted in terms of accommodation of effective positive charge on the selenium atom in the axial isomers in CD2Cl2, and a lesser sensitivity to the buildup of positive charge in the more polar solvent CD3CN. Comparison of the IR ,CO -stretching frequencies for the axial and equatorial ArSe-substituted anancomeric systems (R = NO2, NMe2) indicates a higher stretching frequency for the NO2 -substituted isomers. In the case of the NMe2 -substituted compounds, ,CO appears at a higher frequency in the equatorial isomer, whereas in the case of the NO2 -substituted compounds, ,CO is less sensitive to the axial or equatorial orientation of the substituent. The results are consistent with the operation of nse , ,*c=0 or ,C-Se , ,*C=O orbital interactions in the axial isomers. The JC2-H2 values in the axially-substituted anancomeric isomers are of greater magnitude than those in the equatorially-substituted isomers, which is also consistent with the operation of the orbital interactions described above. There is, however, no marked substituent effect on the JC2,H2 values within the series of axial or equatorial isomers. We argue that this does not support the dominance of ,C-Se , ,*C=O orbital interactions. Examination of crystal structures reported in the literature for related compounds indicates a particular gauche orientation about the C2,Se bond, which lends further support to the operation of an nSe , ,*C=O orbital interaction. We suggest that the latter interaction is a manifestation of a generalized exo -anomeric effect. [source] Preparation and structure elucidation of 1,6,9,13-tetraoxadispiro(4.2.4.2)tetradecaneJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2000Bruce Gaede The tricyclic title compound, a symmetrical dispiro oxygen heterocycle, was isolated as a byproduct in the hydrogenation of furfuryl alcohol in the presence of hydrochloric acid. NMR studies and single crystal X-ray analysis have established the relative stereochemistry of the two ketal carbons. Formation of the observed trans stereoisomer under equilibrating conditions is attributed to the anomeric effect. [source] Anomeric effect plays a major role in the conformational isomerism of fluorinated pnictogen compoundsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2008Levindo E. Martins Abstract According to our theoretical studies, the anomeric effect, an stereoelectronic interaction between lone pair and a vicinal antibonding orbital, has shown to contribute decisively for the conformational isomerism of 1-fluoro- N,N -dimethylmethanamine (1) and of its corresponding P, As and Sb analogues (2,4). CX bonds in 2,4 are larger than in the parent compound 1, thus providing a LPX/CF* interaction progressively weaker on going from 1 to 4. However, such hyperconjugation contributed by more than 1.3,kcal,mol,1 for the stabilization of anti conformer in 4 (,LPXCF,=,180°), increasing to 24.1,kcal,mol,1 in 1. An isodesmic reaction model supported these findings. Copyright © 2008 John Wiley & Sons, Ltd. [source] Probing the influence of stereoelectronic effects on lithium affinity in 1,3- and 1,4-dioxa systemsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2001Bishwajit Ganguly Abstract A combined computational and structural study of the lithium affinity (LA) of O,C,O systems exhibiting the anomeric effect and of O, C,C,O systems exhibiting the gauche effect is presented. QM ab initio calculations using the MP2/6,31G* basis set were carried out on the gas-phase lithium affinities of dimethoxymethane (DMOM), dimethoxyethane (DMOE), 1,3-dioxane (DOX) and cis - and trans -tetraoxadecalin (TOD), along with that of dimethyl ether and of its dimer as reference compounds. Structural parameters were retrieved from the Cambridge Structural Database (CSD) for diethyl ether dimer and O,C,C,O lithium complexes and these agreed well with the calculated data. The computed lithium affinities of dimethoxymethane and dimethoxyethane were found to be conformationally dependent. The LAs are conformationally dependent (wherever applicable) and decrease in the order: (Me2O)2 >,DMOE >,DMOM >,DOD >,DOX >,trans -TOD, but cis -TOD restores the high LA (better than DMOE) by virtue of multiple coordination. Copyright © 2001 John Wiley & Sons, Ltd. [source] Peracetylated ,- d -glucopyranosyl fluoride and peracetylated ,-maltosyl fluorideACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2010Simone Dedola The X-ray analyses of 2,3,4,6-tetra- O -acetyl-,- d -glucopyranosyl fluoride, C14H19FO9, (I), and the corresponding maltose derivative 2,3,4,6-tetra- O -acetyl-,- d -glucopyranosyl-(1,4)-2,3,6-tri- O -acetyl-,- d -glucopyranosyl fluoride, C26H35FO17, (II), are reported. These add to the series of published ,-glycosyl halide structures; those of the peracetylated ,-glucosyl chloride [James & Hall (1969). Acta Cryst. A25, S196] and bromide [Takai, Watanabe, Hayashi & Watanabe (1976). Bull. Fac. Eng. Hokkaido Univ.79, 101,109] have been reported already. In our structures, which have been determined at 140,K, the glycopyranosyl ring appears in a regular 4C1 chair conformation with all the substituents, except for the anomeric fluoride (which adopts an axial orientation), in equatorial positions. The observed bond lengths are consistent with a strong anomeric effect, viz. the C1,O5 (carbohydrate numbering) bond lengths are 1.381,(2) and 1.381,(3),Å in (I) and (II), respectively, both significantly shorter than the C5,O5 bond lengths, viz. 1.448,(2),Å in (I) and 1.444,(3),Å in (II). [source] 2,3,4,6-Tetra- O -acetyl-,- d -glucopyranosyl azideACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2008Simone Dedola The CuI -catalysed 1,3-dipolar cycloaddition of an azide and a terminal alkyne is becoming an increasingly popular tool for synthetic chemists. This is the most representative of the so-called `click reactions' and it is used to generate 1,4-disubstituted triazoles in high yield. During studies on such cycloaddition reactions, a reduced reactivity of an ,-glucosyl azide with respect to the corresponding ,-anomer was observed. With the aim of understanding this phenomenon, the structure of the title compound, C14H19N3O9, has been determined at 140,K. The glucopyranosyl ring appears in a regular 4C1 chair conformation with all the substituents in equatorial positions, except for the anomeric azide group, which adopts an axial orientation. The observed bond lengths are consistent with a strong anomeric effect, which is reflected in a change in dipolar character and hence reduced reactivity of the ,-glucosyl azide. [source] DFT Studies of the Molecular Structures and Conformational Processes of 1,2-, 1,3- and 1,4-DithiepaneCHINESE JOURNAL OF CHEMISTRY, Issue 3 2008Mina HAGHDADI Abstract In this study density functional theory (DFT) calculations at B3LYP/6-31G(d), B3LYP/6-31+G(d) and B3LYP/6-311+G(2df,2p) levels for geometry optimization and total energy calculation were applied for investigation of the important energy-minimum conformations and transition-state of 1,2-, 1,3-, and 1,4-dithiepanes. Moreover, ab initio calculations at HF/6-31G(d) level of theory for geometry optimization and MP2/6-311G(d)//HF/ 6-31G(d) level for a single-point total energy calculation were reported for different conformers. The obtained results reveal that, the twist-chair conformer is a global minimum for all of these compounds. Also, two local minimum were found in each case, which are twisted-chair and twisted-boat conformers. The boat and chair geometries are transition states. The minimum energy conformation of 1,2-dithiepane is more stable than the lowest energy forms of 1,3-dithiepane and 1,4-dithiepane. Furthermore, the anomeric effect was investigated for 1,3-dithiepane by the natural bond orbital method. The computational results of this study shows that all conformers of 1,3-dithiepane have a hypercojugation system. Finally, the 13C NMR chemical shifts for the conformers of 1,4-dithiepane were calculated, which have good correlation with their experimental values. [source] 1H, 13C, and 15N NMR stereochemical study of cis -fused 7a(8a)-methyl and 6-phenyl octa(hexa)hydrocyclopenta[d][1,3]oxazines and [3,1]benzoxazinesCHIRALITY, Issue 2-3 2002Petri Tähtinen Abstract Four 7a-methyl octa(or hexa)hydrocyclopenta[d][1,3]oxazines, five 8a-methyl octa(or hexa)hydro[3,1]benzoxazines, two 6-phenyl hexahydro[3,1]benzoxazinones, and 8a-methyl hexahydro[1,3]benzoxazinone, all cis -fused, were prepared and their stereostructures studied by various one- and two-dimensional 1H, 13C, and 15N NMR spectroscopic methods. In solution, the cyclopentane-fused 2-oxo derivatives and the 1,3-benzoxazinone were found to attain exclusively the N-in/O-in conformation, whereas the 6-phenyl 2-oxo/thioxo derivatives were found to be present predominantly in the N-out conformation. The C-2 unsubstituted and the 2-oxo/thioxo 7a/8a-methyl derivatives were all present in solution as a rapidly interconverting equilibrium of the N-in and N-out conformations. The C-2 methyl derivatives were each found to be interconvertable mixtures of epimers (at C-2) with the N-in conformer predominating for one epimer and the N-out conformer predominating for the other, with both predominating conformers having the C-2 methyl group equatorially orientated. The substituent on the nitrogen (H or Me) was found to be always predominantly equatorial with respect to the heteroring, except for the epimeric 2-methyl derivatives with N-out conformations where steric constraints and the generalized anomeric effect resulted in the axial orientation of the C-2 methyl being favored. Chirality 14:187,198, 2002. © 2002 Wiley-Liss, Inc. [source] | ||||||||||||||||||||||