Twist-boat Conformation Research Articles

Ethyl 2-[9-(5-bromo-2-hydroxyphenyl)-3,3,6,6-tetramethyl-1,8-dioxo-1,2,3,4,5,6,7,8,9,10-decahydroacridin-10-yl]acetate

In the title compound, C27H32BrNO5, the central 1,4-dihydropyridine ring adopts a shallow sofa conformation (with the C atom bearing the bromophenol ring as the flap), whereas the pendant cyclohexene rings both have twisted-boat conformations. The molecule features an intramolecular O—H...O hydrogen bond, which closes anS(8) ring. In the crystal, molecules are linked by C—H...O interactions, formingC(12) chains along thec-axis direction. The ethyl acetate grouping is disordered over two sets of sites in a 0.719 (11):0.281 (11) ratio.

Briarenols C–E, New Polyoxygenated Briaranes from the Octocoral Briareum excavatum

Three new polyoxygenated briarane diterpenoids, briarenols C–E (1–3), were isolated from the octocoral Briareum excavatum. The structures of briaranes 1–3 were elucidated by interpretation of spectroscopic data, and the methylenecyclohexane ring in 1 was found to exist in a twisted boat conformation. Briarenol D (2) displayed an inhibitory effect on the release of elastase by human neutrophils with an IC50 value of 4.65 μM. Briarenol E (3) was found to inhibit the protein expression of pro-inflammatory inducible nitric oxide synthase (iNOS) in a murine macrophage-like cell line, RAW 264.7, stimulated with lipopolysaccharides (LPS).

Crystal structure of 4,10-dimeth-oxy-13-methyl-6H,12H-6,12-epimino-dibenzo[b,f][1,5]dioxocine.

The title compound, C17H17NO4, lacks crystallographic symmetry with one mol-ecule per asymmetric unit. The mol-ecule exists in a folded butterfly-like conformation; the benzene rings form a dihedral angle of 84.72 (7)°. The central eight-membered imino-bridged dioxocin ring adopts a twisted-boat conformation. In the crystal, inversion-related mol-ecules are linked by pairs of weak C-H⋯O hydrogen bonds, forming double-stranded chains parallel to the a axis.

1-Allyl-3′-phenyl-6′H-spiro[indoline-3,4′-isoxazolo[4′,5′:5,6]pyrido[2,3-d]pyrimidine]-2,5′,7′(8′H,9′H)-trione dimethyl sulfoxide monosolvate

In the title solvated compound, C24H17N5O4·C2H6OS, the solvent molecule, dimethyl sulfoxide, is linked to the title molecule by an N—H...O hydrogen bond. The pyridine ring adopts a twist-boat conformation. The isoxazole ring is inclined to the indoline ring system, the pyrimidine ring, and the phenyl ring by 82.31 (7), 10.41 (8) and 53.77 (10)°, respectively. There is an intramolecular C—H...π interaction present involving the phenyl ring and the indoline ring system. In the crystal, molecules are connected by two pairs of N—H...O hydrogen bonds, forming chains along the b-axis direction, and enclosing R 2 2(8) and R 2 2(14) ring motifs. The chains are linked by C—H...O and C—H...N hydrogen bonds and offset π–π interactions, between the pyrimidine and isoxazole rings of inversion-related molecules [centroid–centroid distance = 3.7140 (9) Å], forming a three-dimensional structure.

Perhydrohelicenes and other diamond-lattice based hydrocarbons: the choreography of inversion.

Overall inversion in fused cyclohexane oligomers 2, 3, and 4 (all based on cis-decalin 1) occurs by a rolling process involving no more than two adjacent rings in twist-boat conformations at any time. These inverting rings move along the oligomer in processes that are precisely choreographed by the adjacent chairs. Actual inversion mechanisms can be stepwise [CC → TC → TT → C'T → C'C'], as for cis-decalin, but it is shown that a concerted alternative [CC → TC → C'T → C'C'] is enforced in 2. The all-cis,anti,cis-isomers of perhydrohelicenes 4 are based on the diamond lattice and have remarkably low strain energies. Helix inversion in 4 is compared with that in helicenes 5. For both, the intermediates and transition states have shapes broadly like kinked old-style telephone cables. In both cases barriers increase with the length of the system to eventually reach a plateau value of ca. 120 kJ mol-1 for 4, much lower than that for 5 (320-350 kJ mol-1). While rolling inversion only requires two adjacent rings in twist-boat conformations at any instant, inversion in propellane 6 requires all three rings be converted to twist-boats, and the S4 symmetric hydrocarbon 7 requires all four rings to be converted to twist-boats. As a consequence, 7 probably has the highest barrier of any non-oligomeric cis-decalin derived structure (87.3 kJ mol-1 at B3LYP/6-31G*).

Three tetracyclic dibenzoazepine derivatives exhibiting different molecular conformations, different patterns of intermolecular hydrogen bonding and different modes of supramolecular aggregation.

The biological potential of compounds of the tricyclic dibenzo[b,e]azepine system has resulted in considerable synthetic efforts to develop efficient methods for the synthesis of new derivatives of this kind. (9RS,15RS)-9-Ethyl-11-methyl-9,13b-dihydrodibenzo[c,f]thiazolo[3,2-a]azepin-3(2H)-one, C19H19NOS, (I), crystallizes as a kryptoracemate with Z' = 2 in the space group P21, with one molecule each of the (9R,15R) and (9S,15S) configurations in the asymmetric unit, while (9RS,15RS)-9-ethyl-7,12-dimethyl-9,13b-dihydrodibenzo[c,f]thiazolo[3,2-a]azepin-3(2H)-one, C20H21NOS, (II), crystallizes with Z' = 1 in the space group C2/c. Ethyl (13RS)-2-chloro-13-ethyl-4-oxo-8,13-dihydro-4H-benzo[5,6]azepino[3,2,1-ij]quinoline-5-carboxylate, C22H20ClNO3, (III), exhibits enantiomeric disorder in the space group P-1 such that the reference site is occupied by the 13R and 13S enantiomers, with occupancies of 0.900 (6) and 0.100 (6). In each of the two independent molecules in (I), the five-membered ring adopts an envelope conformation, but the corresponding ring in (II) adopts a half-chair conformation, while the six-membered ring in the major form of (III) adopts a twist-boat conformation. The conformation of the seven-membered ring in each of (I), (II) and the major form of (III) approximates to the twist-boat form. The molecules of compound (I) are linked by two C-H...O hydrogen bonds to form two independent antiparallel C(5) chains, with each type containing only one enantiomer. These chains are linked into sheets by two C-H...π(arene) hydrogen bonds, in which the two donors are both provided by the (9R,15R) enantiomer and the two acceptor arene rings form part of a molecule of (9S,15S) configuration, precluding any additional crystallographic symmetry. The molecules of compound (II) are linked by inversion-related C-H...π(arene) hydrogen bonds to form isolated cyclic centrosymmetric dimers. The molecules of compound (III) are linked into cyclic centrosymmetric dimers by C-H...O hydrogen bonds and these dimers are linked into chains by a π-π stacking interaction. Comparisons are made with some related structures.

3-Hydroxy-4-phenyl-2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one:cisisomer

In the title compound, C15H14N2O2, the seven-membered benzodiazepine ring adopts a twist-boat conformation and the two aromatic rings are inclined to one another by 81.06 (15)°. In the crystal, molecules are linked by N—H...O hydrogen bonds, forming chains propagating along the [10-1] direction. The chains are linked by C—H...O hydrogen bonds, forming sheets parallel to theacplane. Within the sheets, there are N—H...π interactions present, and C—H...π interactions link the sheets to form a three-dimensional structure.

(4Z)-1-Dodecyl-4-(2-oxopropylidene)-2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one

In the title compound, C24H36N2O2, the orientation of the 2-oxopropylidene substituent is determined by the formation of an intramolecular N—H...O hydrogen bond. The benzodiazepine seven-membered ring adopts a slightly twisted boat conformation. The molecules pack in a bilayer fashion with the dodecyl chains intercalated to form the inner portion, and the benzodiazepine moieties on the outer surfaces.

N-Benzyl-2,7-diphenyl-1,4-diazepan-5-one analogues: Synthesis, spectral characterization, stereochemistry, crystal structure and molecular docking studies

Three novel N-benzyl-2,7-diphenyl-1,4-diazepan-5-ones 10–12 have been synthesized via two routes starting from 2,7-diphenyl-1,4-diazepan-5-ones 4–6 and N-benzyl-2,6-diphenylpiperidin-4-ones 7–9. The structural characterization and conformational analysis of these synthesized compounds have been carried out using IR, mass and 1H, 13C, DEPT-135 and 2D (COSY and HSQC) NMR spectral techniques. The N-benzyldiazepan-5-one 10 is found to prefer chair conformation with equatorial orientation of alkyl and phenyl groups while N-benzyldiazepan-5-ones 11 &12 prefer to adopt twist-boat conformation with phenyl rings at C-2 & C-7 occupying equatorial and pseudo-axial orientations, respectively. The single crystal X-ray structure of compound 12 has been determined which also supports the twist-boat conformation. In silico molecular docking study has also been performed and the results show that the compounds 10–12 might exhibit inhibitory activity against HIV-1 protease. All the compounds are screened for their antibacterial activity against three bacterial strains (Staphylococcus aureus, Escherichia coli and Bacillus cereus) and only compound 11 shows moderate activity.

Computations Reveal That Electron-Withdrawing Leaving Groups Facilitate Intramolecular Conjugate Displacement Reactions by Negative Hyperconjugation.

Intramolecular conjugate displacement (ICD) reactions, developed by the Clive group, form carbocycles and polycyclic amines by intramolecular nucleophilic attack on a Michael acceptor with an allylic leaving group. Quantum mechanical investigations with density functional theory show that ICDs involve a stepwise addition, forming an intermediate stabilized carbanion, followed by elimination. The electron-withdrawing nature of the allylic leaving group facilitates the addition by negative hyperconjugation; the twist-boat conformation of the addition and intermediate is stabilized by this interaction. In the absence of an activating electron-withdrawing group as part of the Michael acceptor, a high energy concerted SN2' reaction occurs. The reactions of carbon nucleophiles have lower activation energies than those of amines.

3-tert-Butyl-4-(4-chlorophenyl)-1-phenyl-1H-pyrazolo[3,4-e][1,4]thiazepin-7(4H,6H,8H)-one

In the title molecule, C22H22ClN3OS, the fused 1,4-thiazepinone ring adopts a near twist-boat conformation and the chlorobenzene ring is inclined to the phenyl ring by 88.38 (12)°. In the crystal, molecules are linked by pairs of N—H...O hydrogen bonds, forming inversion dimers with anR22(8) ring motif. The dimers are linkedviaC—H...O hydrogen bonds forming ribbons, enclosingR42(20) loops, propagating along [001].

Molecular Motion and Conformational Interconversion of Ir(I)·COD Included in Rebek's Self-Folding Octaamide Cavitand.

We report experimental and theoretical evidence of restrained axial rotation for heteroleptic L2·Ir(I)·1,5-cyclooctadiene (COD) complexes included in the aromatic cavity of Rebek's self-folding octaamide cavitand. At 298 K, the axial spinning motion of the included organometallic guests was slow on the (1)H NMR time scale and produced a proton spectrum for the bound host indicative of C2 symmetry. Signals corresponding to aromatic protons of the bound host coalesced at 323 K, indicating that the spinning process of the included guest became fast on the (1)H NMR time scale and that the complex approached C4 symmetry. Surprisingly, lowering the temperature of the solution to 193 K induced an additional splitting of the proton signals observed at room temperature for both the bound host and the included guest. We propose the emergence of a new element of chirality in the complexes, which was associated with a slow interconversion, on the (1)H NMR time scale between the two chiral twisted-boat conformers of the chelated COD included in the already chiral cavity of the container. This leads to the inclusion complexes existing in solution as pairs of two racemic diastereomers. We estimated that the racemization barrier for the two cyclochiral conformers of the Ir(I) chelated COD was 5 kcal mol(-1) higher as an included organometallic complex than as free in solution. Furthermore, we performed a van't Hoff plot and determined that the inclusion of the organometallic complex in the cavitand was endothermic and exclusively driven by entropy (ΔH = 5.9 kcal mol(-1) and ΔS = 33.9 cal mol(-1) K(-1)).

Limonin isolated from the seeds ofCitrus limetta(Indian sweet lemon)

The title molecule, C26H30O8[systematic name: (4aS,14bR)-12-(furan-3-yl)-6,6,8a,12a-tetramethyldecahydro-1H,3H-oxireno[2,3-d]pyrano[4′,3′:3,3a]isobenzofuro[5,4-f]isochromene-3,8,10(6H,9aH)-trione], commonly known as limonin, is an antiproliferative polyoxgenated bioactive molecule. It comprises a fused cyclic system containing a cyclohexane (D), a cyclohexanone (C), a furan (B), an epoxide and two lactone rings (AandE). In addition to these rings, a second furan ring (F) is present as a substituent. Only the cyclohexane (D) ring has a chair conformation; the other six-membered rings (A,CandE) have twist-boat conformations due to steric hinderance or bonding strain. In the crystal, molecules are linked by C—H...O hydrogen bonds, forming a three-dimensional structure.

Absolute configuration of stegane lignans by vibrational circular dichroism

Novel steganes (−)-(P,8R,8′R)-5′-desmethoxystegane 4, (−)-(P,7R,8R,8′R)-5′-desmethoxysteganol 5, (−)-(P,7R,8R,8′R)-5′-desmethoxysteganacin 6, and (−)-(P,8R,8′R)-5′-desmethoxysteganone 7 were synthesized from (+)-(M,8R,8′R)-5′-desmethoxyisostegane 2, which in turn was obtained from natural (−)-(8R,8′R)-5′-desmethoxyyatein 1 together with the novel (+)-(M,8R,8′R)-3,4-dihydroxy-3′,4′-dimethoxyisosteganolide 3. Their stereostructures were determined by 1D and 2D NMR spectroscopy, while their absolute configuration was assigned by vibrational circular dichroism (VCD) measurements in combination with density functional theory calculations. Thus, the VCD spectrum of 2 was contrasted with that of its atropisomeric derivative 4 obtained by thermal inversion of the chiral axis. These compounds exhibited numerous opposite VCD transitions due to the predominance of the biaryl core, thus allowing the identification of bands arising from local (8R,8′R) and non-local (M/P) chirality. The conformational analysis of these compounds revealed that the eight-membered ring of 2 adopts a single twist-boat–chair conformation, while in 4 it exists in an equilibrium between two different twist-boat conformations. The introduction of an acetyl group at C-7 in 4 to give 6 shifted the conformational equilibrium toward a single twist-boat conformation which was verified by VCD spectroscopy.

The first crystal structures of six- and seven-membered tellurium- and nitrogen-containing (Te—N) heterocycles: 2H-1,4-benzotellurazin-3(4H)-one and 2,3-dihydro-1,5-benzotellurazepin-4(5H)-one

There is a paucity of data concerning the structures of six- and seven-membered tellurium- and nitrogen-containing (Te-N) heterocycles. The title compounds, C8H7NOTe, (I), and C9H9NOTe, (II), represent the first structurally characterized members of their respective classes. Both crystallize with two independent molecules in the asymmetric unit. When compared to their sulfur analogs, they exhibit slightly greater deviations from planarity to accommodate the larger chalcogenide atom, with (II) adopting a pronounced twist-boat conformation. The C-Te-C angles of 85.49 (15) and 85.89 (15)° for the two independent molecules of (I) were found to be somewhat smaller than those of 97.4 (2) and 97.77 (19)° for the two independent molecules of (II). The C-Te bond lengths [2.109 (4)-2.158 (5) Å] are in good agreement with those predicted by the covalent radii. Intermolecular N-H...O hydrogen bonding in (I) forms centrosymmetric R2(2)(8) dimers, while that in (II) forms chains. In addition, intermolecular Te...O contacts [3.159 (3)-3.200 (3) Å] exist in (I).

Chemometric and conformational approach to the analysis of the aggregation capabilities in a set of bile salts of the allo and normal series

Bile salts are steroid biosurfactants that have a significant role in fat digestion, cholesterol micellar solubilization, and regulation of metabolism. They are important in pharmaceutical studies as modulators of the transport-permeability of drugs or as ligands for certain receptors. For the rational application of bile salts in medicine, it is necessary to have detailed knowledge of their aggregation capabilities (which determine their membranotoxicity and solubilization capacity). From the examination of bile salt derivatives, the in plane of lnk (RPHPLC) and micelle aggregation number n, as well as the anion of 7-oxodeoxycholic acid (7-OxD) and anion of cholic acid (C), are considered to be outliers, related to linear hydrophobic congeneric groups, which means that their micelles, in addition to being determined by hydrophobic interactions, are determined by hydrogen bonds, i.e., they form micelles with higher aggregation numbers than would be expected from the hydrophobicity of their steroid skeleton. For bile salts of the normal series in the formation of hydrogen bonds in secondary micelles, the crucial structural elements of the steroid skeleton are: α-equatorial-C3OH group and α-axial-C12OH group. Bile salts of the allo series, including allocholic (aC), allodeoxycholic (aDC) and allochenodeoxycholic acid (aCD), belong to the linear hydrophobic congeneric group. Their micelles are determined by hydrophobic interactions. It is assumed that for the analyzed allo derivatives, the A ring of the steroid skeleton is in the twisted boat conformation, which explains the spatial sheltering of their C3OH group in micelles.

Synthesis and structural investigation of new isothiochromen-4-one 2,2-dioxide derivatives

This article presents a combined experimental and computational study of novel isothiochromene 2,2-dioxide derivatives, which were synthesized by the electrophilic substitution reactions. The compounds have been studied by NMR, IR spectroscopy and single-crystal X-ray analysis. According to X-ray diffraction data, the six-membered ring of 6 adopts a distorted half-chair conformation, but in case of compounds 7 and 8, we have a twist-boat conformation of the six-membered ring with deviation of the S1 and C8 atoms from the mean plane of the remaining atoms of the ring, respectively. Results of quantum-chemical calculations of possible isomers and tautomers of the isolated oxime 8 and azo compounds 10 at the B3LYP/aug-cc-pVDZ level indicate that the oxime-oxo tautomer (8a) is more stable than the nitroso-hydroxy tautomer (8b) in agreement with the crystal data and that the keto tautomer (II) is more stable than the hydroxyl tautomer (I) and zwitterion tautomer (III), because of a strong intramolecular hydrogen bond N–H···O.

Crystal structure of 4-benzyl-2H-benzo[b][1,4]thia-zin-3(4H)-one.

In the title compound, C15H13NOS, the thia­zine ring adopts a twisted boat conformation and the dihedral angle between the aromatic rings is 86.54 (4)°. In the crystal, mol­ecules are linked by weak C—H⋯O inter­actions, resulting in chains along [010].

Crystal structure of ethyl 2-[9-(5-bromo-2-hy-droxy-phen-yl)-1,8-dioxo-1,2,3,4,5,6,7,8,9,10-deca-hydro-acridin-10-yl]acetate.

In the title compound, C23H24BrNO5, the central 1,4-di­hydro­pyridine ring of the 1,2,3,4,5,6,7,8,9,10-deca­hydro­acridine ring system adopts a half-chair conformation. The two cyclo­hexene rings fused to the central ring both have a twisted-boat conformation. The mean planes of the bromo­hydroxy­phenyl ring and the major and minor components of the disordered ethyl amino­acetate moiety make dihedral angles of 78.99 (12), 85.9 (2) and 88.3 (9)°, respectively, with the 1,4-di­hydro­pyridine ring. The terminal ethyl group of the ethyl amino­acetate moiety is disordered over two sets of sites with refined occupancies of 0.768 (17) and 0.232 (17). The mol­ecular conformation is stabilized by an intra­molecular O—H⋯O hydrogen bond, forming an S(8) ring motif. In the crystal, C—H⋯O hydrogen bonds connect the mol­ecules into layers parallel to (001), enclosing R 1 2(7) ring motifs.

Crystal structure of 1,2,3,4-di-O-methyl-ene-α-d-galacto-pyran-ose.

The title compound, C8H12O6, was synthesized by de­acetyl­ation of 6-acetyl-1,2,3,4-di-O-methyl­ene-α-d-galactose with sodium methoxide. The central part of the mol­ecule consists of a six-membered C5O pyran­ose ring with a twist-boat conformation. Both fused dioxolane rings adopt an envelope conformation with C and O atoms as the flap. In the crystal, O—H⋯O and C—H⋯O hydrogen bonds are present between adjacent mol­ecules, generating a three-dimensional network.