Skew-boat Conformation Research Articles

(3aS*,9bS*)-(±)-3a,9b-Dihydro-2,9b-dimethylthiazolo[5,4-c]quinolin-4(5H)-one

The stereochemistry of the title compound, C 12 H 12 N 2 OS, is established as cis. There are two very similar independent molecules in the asymmetric unit. The five-membered rings have envelope conformations and the six-membered hetero-rings are in skew-boat conformations. The molecules are linked by N-H...O hydrogen bonds with N...O distances in the range 2.996 (2)-3.114(2)A.

Synthesis and Conformational Analysis of(6R)-[6-2H1]-1,2:3,4-Di-O-isopropylidene-.alpha.-D-galactopyranose. NMR and Molecular Modeling Studies

The conformation preferences of 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose have been determined using NMR and molecular modeling (conformational searching) techniques. The pyranose ring assumes a skew-boat conformation. Coupling constants for the C 6 side chain and hydroxyl proton indicate that the predominant conformation places the oxygen anti to C 4 (gt conformation) with the hydroxyl hydrogen aligned toward the pyranose oxygen in deuteriochloroform. A similar orientation of the C 5 -C 6 bond was observed in water and toluene. In DMSO the C 6 oxygen is anti to the pyranose oxygen (tg conformation) with the hydroxyl hydrogen anti to C 5

Skew-boat conformations for two sugar-derived 5,6-dihydropyran-2-ones

Abstract The structures for two sugar-derived 5,6-dihydropyran-2-ones were determined: (5R,6S)-3.5-bisbenzoyloxy-6-methyl-5,6-dihydro-2H-pyran-2-one (1), C20H16O6, P212121, a = 27.610(9), b = 10.734 (4), c = 5.972 (2) Å, Z = 4, R = 0.033, Rw = 0.037, and (5S)-3, 5-bisbenzoyloxy-5.6-dihydro-2H-pyran-2-one (2), C19H14O6, P21, a = 15.93(1), b = 8.150(5), c = 6.238(5), β = 99.43(5)°, z = 2, R = 0.096, Rw = 0.096. They adopt skew-boat (SB) solid-state conformations of the pyranoid ring, such that C-6, and, to a lesser degree, the ring oxygen lie above a plane made up of C-(2)–C-(3)–C-(4)–C-(5).

Efficient stereochemical regulation of octahedral cobalt(III) complexes by a chiral bidentate ligand. Part 2. Remarkable effect of the chelate-ring size in the stereoselective formation of sym-cis-(ethylenediamine-N,N′-diacetato)(pentane-2,4-diamine)cobalt(III)

The chiral diamine (2R), (4R)-pentane-2,4-diamine (2R,4R-ptn) has been found efficiently to regulate the stereochemistry of the cobalt(III) complex [Co(edda)(2R,4R-ptn)]+(edda = ethylenediamine-N,N′-diacetate). The product ratio Λ-sym-cis:Δ-sym-cis is 3:97. The stereoselectivity is surprisingly high in comparison with that for the analogous complex involving (1R),(2R)-cyclohexane-1,2-diamine (R,R-chxn), Λ-sym-cis:Δ-sym-cis= 60:40. It is discussed on the basis of crystal structure determinations carried out for both diastereoisomers of sym-cis-[Co(edda)(2R,4R-ptn)]+. A notable difference was observed in the six-membered chelate ring of 2R,4R-ptn which adopts the chair conformation in Λ-sym-cis-[Co(edda)(2R,4R-ptn)]ClO4·H2O 1a but the λ skew-boat conformation in Δ-sym-cis-[Co(edda)(2R,4R-ptn)]ClO41b. The chair conformation in 1a is due to steric repulsions between the chelates, which is the important factor for the preferential formation of 1b over 1a. Such steric repulsions between the chelates are remarkably efficient for the chiral six-membered chelate compared with those for the five-membered one. N-Alkyl substitutions on the edda ligand caused preferential formation of the Λ-sym-cis form, as is the case for the [Co(R2edda)(R,R-chxn)]+ system.

Structure of dipotassium 5,6-dihydrouracil-6,6-disulfonate monohydrate

The title compound was synthesized and its crystal structure determined by single crystal X-ray diffraction techniques. It crystallizes in the monoclinic system witha=9.390(1),b=9.688(2),c=13.828(3)A,β=110.16(2)°, space groupP21/c,Z=4. The structure was solved by direct methods and refined by full-matrix least-squares calculations toR=0.032 for 1968 reflections withI>3σ(I) [MoKα radiation]. The six-membered ring of the 5,6-dihydrouracil-6,6-disulfonate dianion displays an approximate skew-boat conformation, but the ring puckering differs from that commonly observed in 5,6-dihydrouracil derivatives. The coordination to two crystallographically independent K+ ions which exhibit different coordination geometries links anions, cations and water molecules in an infinite three-dimensional network.

Solid-state conformation of 2,6- cis-and 2,6- trans-substituted dihyropyran-3-ones

2,6-Dihydropyran-3-ones carrying substituents at C-2 and C-6 in cis-arrangement invariably adopt half-chair conformations in which the ring oxygen and the carbon atom next to the carbonyl group are above and below, respectively, the plane formed by the other four carbon atoms, i.e., the 2 H o or o H 2 conformation. In the case of a trans-arrangement of 2,6-substituents, the geometry of the pyramid ring falls into the B o,6 ⇌ E o ⇌ 2 H o or inverse o,6 B ⇌ o E ⇌ o H 2 section of the conformational cycle, depending on the absolute configuration of the compound; for two of the dihydropyranones, 4B and 6, a unique skew-boat ( SB o,6) conformation, fixed between the B o,6 and E o geometries, was ascertained, which previously has only been observed for pyranoid enelactones.

Determination of the protons eliminated in the regioselective deprotonation of some optically active 3-keto steroids by a chiral lithium amide

Mono-deuterated derivatives of optically active 3-keto steroids ( 4-2α-d, 4-2β-d, 5-4α-d, 5-4β-d, 6-2α-d, and 6-2β-d) were treated with a chiral lithium amide (( S)- or ( R)- 2b) in the presence of excess trimethylsilyl chloride to give the corresponding diastereomeric Δ 2- and ΔA 3-silyl enol ethers. It is concluded that the present deprotonation reaction occurs in chair conformation in 6, while in boat (skew-boat) conformation about the A-rings in 4 and 5.

Structure and properties of iron(III) 1,3-propanediaminetetraacetate complex in aqueous solutions

The structure of iron(III) 1,3-propanediaminetetraacetate (1,3-pdta) in aqueous solutions was studied on the basis of Raman spectroscopy, nuclear magnetic relaxation, and pH titration. It has been found that, in aqueous solution, Fe(1,3-pdta) − exists in a normal sexidentate hexacoordinate geometry. The six-membered chelate ring formed by the diamine moiety of the 1,3-pdta ligand does not adopt a skew-boat conformation having C 2 symmetry (as found in sodium salt crystals) hut a twist-boat one having no C 2 symmetry (as in the lithium salt crystals). Although edta complexes of iron(III) form a dimeric complex in slightly alkaline solutions, the 1,3-pdta complex of Fe(III) does not show such a tendency. The 1,3-pdta complex is stable only in the pH region of 2–7.5; the corresponding edta complex is stable even in a strongly acidic solution.

Conformational analysis of isopropylidene-protected C-glycosyl derivatives of 3-deoxy- d- manno-2-octulosonic acid (Kdo) in the solid state and in solution

The favoured conformations of a series of C-glycosyl derivatives of 4,5:7,8-di- O-isopropylidene-3-deoxy- d- manno-2-octulosonic acid (Kdo) esters were analyzed by n.m.r. spectroscopy and in one instance by X-ray crystallography. The Kdo derivatives were found to adopt a skew-boat conformation B 3,6 + 0 S 3) in solution which corresponds well to the slid-state conformation. The skew-boat conformation of the pyranose ring appeared to be imposed solely by the 4,5-dioxolane ring. Molecular-mechanics calculations [MM2(85)] on a model compound identified two low-energy conformations of equal steric energy, namely a skew-boat conformation almost identical to the experimentally observed conformation, and a chair conformation 5 C 2).

Improved preparation of acetals of myo-inositol and its (±)-1-benzyl ether: conformational analysis of di- O-isopropylidene- myo-inositol derivatives

The acid-catalysed reactions of myo-inositol with 3–5 equiv. of 2-methoxypropene or 2,2-dimethoxypropane in methyl sulfoxide or N,N-dimethylformamide gave mixtures of the 1,2:4,5-, 1,2:5,6-, and 1,2:3,4-di- O-isopropylidene derivatives with little or none of the 1,2:3,4:5,6-triacetal 5. With ≈ 7 equiv. of 2-methoxypropene in methyl sulfoxide-hexane, 70% of 5 was obtained. Kinetic acetonation of 1- O-benzyl- myo-inositol gave mainly the 3,4:5,6-diacetal, whereas thermodynamic conditions gave mainly the 2,3:5,6-diacetal. 1- O-Benzyl-2,3:4,5-di- O-isopropylidene- myo-inositol, obtained as a minor product, exists primarily in a chair conformation in non-polar solvents and in a skew-boat conformation in polar solvents, whereas the 6-acetate adopted only a skew-boat conformation.

Synthesis and characterization of dinickel(II) and dipalladium(II) complexes of the macrocyclic binucleating ligand 3,13-dimethyl-3,13-dinitro-1,5,11,15-tetra-azacycloeicosane-8,18-dithiol (L5). Crystal structure of the complex [Ni2(L5– 2H)][NO2]2·3.5H2O

Condensation of the bis(1,5-diaminopentane-3-thiolato)dinickel(II) ion or the dipalladium(II) analogue with formaldehyde and nitroethane in basic methanol yields the dianion of the macromonocyclic ligand 3,13-dimethyl-3,13-dinitro-1,5,11,15-tetra-azacycloeicosane-8,18-dithiol (L5) as the dimetal(II) complex. Spectroscopic properties of the diamagnetic complexes are reported, together with the crystal-structure analysis of the dinickel(II) complex. The nitrite salt of the latter crystallizes in the space group Pbnm, a= 7.762(3), b= 14.921 (3), c= 25.459(3)Å, Z= 4, R= 0.057 for 1 739 ‘observed’ reflections. The complex is a syn isomer, the first time this geometry has been observed from this type of condensation reaction, with the two N2NiS2 planes folded slightly towards each other about the mirror plane through S ⋯ S in a ‘butterfly’ arrangement (interplanar angle 162.8°). Oxygen atoms of the pendant nitro groups form weak axial interactions, the six-membered rings at the ‘ends’ having skew-boat conformations.

Energy‐Minimized Structures and Calculated and Experimental Isomer distributions in the hexaamine‐cobalt(III) system [Co(L)2]3+ with the chiral facially‐coordinating triamine (l = butane‐1,2,4‐triamine)

AbstractButane‐ 1,2,4‐triamine (trab) is the smallest tridentate aliphatic unsubstituted chiral triamine. With optically pure trab, there are three, with racemic trab five isomers of [Co(trab)2]3+, One of the five isomers is centrosymmetrical, the others are chiral. For one of the isomers, there are four possible conformations (all combinations of chair and skew boat conformations for the chelate six ring of each ligand), for the others there exist only three independent conformers. All sixteen independent structures have been calculated by strain‐energy minimization. The calculated isomer distribution, based on total strain energies corrected with statistical entropy contributions (21%:16%:16%:4%:43%, and 40%:30%:30%, for racemic and optically pure trab, respectively) are in excellent agreement with the experimental data based on HPLC and 13C‐NMR analyses of equilibrium solutions of the hexaamine‐Co(III) compounds prepared by oxygenation of aqueous solutions in presence of activated charcoal. The results are also briefly discussed in relation to possible stereoselectivity upon complexation of optically pure trab and a racemic chiral ligand to a transition‐metal center.

Cyclic (ALN)nCompounds as Precursors to Aluminum Nitride: Synthesis and Structure of [(CH3)2AlNH2]3and the Planar Species [(t-C4H9)2AlNH2]3

Abstract The crystal and molecular structures of the title compounds, [(CH3)2AlNH2]3 1 and [(t-C4H9)2AlNH2]3 2, have been determined in connection with their investigation as possible precursors to aluminum nitride. Both compounds have an (AlN)3 ring-structure with distorted tetrahedral geometries for the ring Al and N atoms. The distortion from tetrahedral geometry is most pronounced for the N atoms where the endocyclic Al-N-Al bond angles average 125.3 for 1 and 134.2 for 2. The (AlN)3 ring in 1 is in a skew-boat conformation with no unusual intra- or intermolecular contacts. Compound 2 on the other hand exhibits an unprecedented planar (AlN)3 ring as required by a crystallographic three-fold symmetry axis. The effects of the Al and N substituents on the (AlN)3 ring size and conformation, as well as on the endocyclic Al-N-Al bond angles, are discussed in the context of the structural results obtained for these and other (AlN)n ring compounds.

X-Ray crystal structure of the permethylated β-cyclodextrin complex with m-iodophenol. A stabilized skew-boat pyranose conformation in the distorted macrocyclic ring

In the crystalline inclusion complex of permethylated β-cyclodextrin with m-iodophenol, the host molecule has a glucose unit with an unusual 0S2 skew-boat conformation and shows markedly distorted macrocyclic conformation.

Conformation of the piperidine ring in isomeric 1,2,5-trimethyl-4-phenyl(triorganosilyl)piperidin-4-ols from the PMR spectra

A full analysis of the PMR spectra of isomeric promedol alcohols was undertaken for the first time. It was shown that the conformational equilibrium must be taken into account only in the case of the Β isomer, where it is determined mainly by chair-chair interconversion. In the α isomers of 1,2,5-trimethyl-4-triorganosilylpiperidin-4-ols an equilibrium was found between the chair conformation and the two skew boat conformations, which are stabilized by an intramolecular hydrogen bond and by the large size of the triorganosilyl substituent.

Structure of cis-5,5-dichloro-1,3-diphenyl-1λ6,3λ6,2,4,6,5λ5-dithiatriazaphosphorine 1,3-dioxide

C12H10C12N302PS2, Mr=394.22, triclinic, P1, a = 9.8757 (4), b= 9.9319 (4), c= 17.219 (1)A, ct= 96.183 (4), fl= 96.283 (4), y= 76.525 (3) °, V= 1626.6(1)A 3, Z=4, Dx=1.610gcm -3, 2(MoKtx) = 0.71073 A, p = 7.5 cm -1, F(000) = 800, T= 295 K, R = 0.042 for 4018 observed reflections. The unit cell contains two independent molecules, both with a skew-boat conformation of the inorganic ring. Phenyl substituents are in cis positions. Mean bond lengths are N-P = 1.573 (3), N-S = 1.575 (3), S-O = 1.428 (1), S-C = 1.761 (2), P-Cl= 1.981 (2) and C-C = 1.374 (2) A. The average endocyclic angles at P and S are 118.4 (2) and 112.3 (1) ° respectively. The endocyclic angles at N range from 121.2 (2) to 122.6 (2) °.

Studies on the metal—amide bond. XVII. The crystal and molecular structure of the α-form of aqua[N,N′-bis(2′-pyridinecarboxamido)-1,3-propane] copper(II) dihydrate

α-Aqua[N,N′-bis(2′-pyridinecarboxamido)-1,3-propane]copper(II) dihydrate, C 15H 20N 4O 5Cu, is monoclinic, space group P2 1/c, with a = 11.719(2), b = 13.092(2), c = 12.663(2) Å, β = 119.56(1)°, Z = 4. The structure was refined to R = 0.026 for 2398 diffractometer data using full-matrix least-squares methods. The copper atom is five-coordinate with the N 4-tetradentate ligand encompassing the base of a distorted square-based pyramid which is appreciably distorted towards a trigonal bipyramid [average Cu-N(amide) 1.950(2), Cu-N(pyridine) 2.043(2) Å, N(amide)-Cu-N(amide) 94.5(1), N(pyridine)-Cu-N(pyridine) 100.2(1)°] and with the copper atom lying 0.27 Å above the N 4 plane towards the apical water molecule [Cu-O 2.236(2) Å]. The central six-membered chelate ring adopts a skewed boat conformation and the enforced strain in the molecule results in non-planar distortions in the pyridine rings with only small distortions in the amide groups. The molecules pack in sheets parallel to (10 1 ) and the hydrogen-bonding network involves the water molecules and the amide oxygen atoms of the ligand.

Conformational analysis of coordination compounds. XIV. Proton, carbon-13, cobalt-59 and platinum-195 n.m.r. spectra of six-membered diamine chelaterings

The conformations adopted by six-membered diamine chelate rings in complexes of the types [Co(NH3),L]3+, [Co(CN)4L]-, [Pt(bpy)L]2+, [Pt(NH3)2L]2+ and [CoL3]3+ have been studied by n.m.r. techniques. The ligands (L) used in the study were meso- and (+)-pentane-2,4-diamine, butane-1,3-diamine, propane-1,3-diamine and 2-methylpentane-2,4-diamine. It has been found that for most complexes of meso-pentane-2,4-diamineth e diequatorial chair conformation predominates, although for the bipyridineplatinum(II) complex it is unclear whether the skew-boat conformation is significantly populated or the chair structure is unusually distorted. For (+)-pentane-2,4-diamine chelates, which normally adopt the skew-boat conformation in octahedral complexes, the chair conformation with an axial and an equatorial methyl group has a mole fraction of 0.5 and 0.7 for the bipyridine- and diammine-platinum(II) complexes, respectively. In tetraammine(butane-1,3- diamine)cobalt(III) the chelate ring is in the chair conformation with the methyl equatorial.

The metabolism of gallic acid and hexahydroxydiphenic acid in plants. Part 3. Esters of (R)- and (S)-hexahydroxydiphenic acid and dehydrohexahydroxydiphenic acid with D-glucopyranose (1 C 4 and related conformations)

In a small group of plants a process of further metabolism of galloyl-D-glucose esters occurs in which oxidative coupling takes place between galloyl ester groups in the 1,6:2,4 or 3,6-position of D-glucopyranose in its least favoured 1C4(C-1) or related skew-boat conformations. This group also metabolise esters of dehydrohexahydroxydiphenic acid (1).

Conformational analysis of coordination compounds. XI. Molecular structure of tetraammine{(±)-pentane-2,4-diamine)cobalt(III) dithionate

The crystal structure of tetraammine{(�)-pentane-2,4-diamine}cobalt(III) dithionate dihydrate [Co{(�)-ptn}(NH3)4] (S2O6)1.5,2H2O has been determined by X-ray structure analysis. The crystals are triclinic with the space group Pī, with a 11.43, b 11.65, c 7.78 �, α 100.48�, β 105.95�, and γ 75.95�. The structure has been refined by least-squares methods with anisotropic temperature factors to an R value of 0.043 for 2654 reflections. The six-membered diamine chelate ring has a skew-boat conformation in which the two methyl groups are in equatorial orientations. A strain- energy-minimization calculation for the complex reveals an energy difference of 8.3 kJ mol-l between the preferred skew-boat conformation and the chair conformation. The calculated minimum-energy geometry for the skew-boat conformation is in good agreement with the observed structure for the chelate ring in the dithionate salt. The lH N.M.R. spectra of this complex and other complexes of the chiral isomer of pentane-2,4-diamine are discussed in terms of the observed structures and the conformational analysis.