Twist-boat Form Research Articles

Synthesis, Hirshfeld atom refinement of crystal structure, and Hirshfeld surface analysis of 5-ethoxycarbonyl-6-methyl-4-(2-(trifluoromethyl)phenyl)-3,4-dihydropyrimidin-2-(1H)-one formic acid monosolvate

We report the cyanamide-based multicomponent synthesis and crystal structure of the title compound, 5-ethoxycarbonyl-6-methyl-4-(2-(trifluoromethyl)phenyl)-3,4-dihydropyrimidin-2-(1H)-one which crystallized as a binary cocrystal with formic acid, C15H15F3N2O3·HCO2H (DHPM···FA). The molecular structure of the DHPM···FA heterosynthon was unequivocally confirmed using X-ray single crystal diffraction techniques and spectroscopic methods. X-ray diffraction analysis shows that this cocrystalline material crystallizes in the monoclinic space group P21/n, Z = 4 with the 3,4-dihydropyrimidin-2-(1H)-one ring in a twist-boat form attached to the ester and phenyl group in an s−cis/s−cis and sp−conformation, respectively. The molecular structure and crystalline packing are stabilized by intermolecular N2–H2∙∙∙O1, N1–H1∙∙∙O5, O4–H4A∙∙∙O1, C4–H4∙∙∙O5, C6–H6∙∙∙O3 and C16–H16∙∙∙O3 hydrogen bonds into a 3D supramolecular network R22(8). Furthermore, the Hirshfeld surface analysis confirms that the most important contributions for the crystal packing are the H∙∙∙O/O∙∙∙H (58.7%), C − H···C (17.8%), H∙∙∙H (30.4%) and H∙∙∙F/F∙∙∙H (18.3%) surface contacts.

Methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside] monohydrate: a solvomorphism study.

Methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside] monohydrate, C13H24O11·H2O, (I), was obtained via spontaneous transformation of methyl β-lactoside methanol solvate, (II), during air-drying. Cremer-Pople puckering parameters indicate that the β-D-Galp (β-D-galactopyranosyl) and β-D-Glcp (β-D-glucopyranosyl) rings in (I) adopt slightly distorted 4C1 chair conformations, with the former distorted towards a boat form (BC1,C4) and the latter towards a twist-boat form (O5SC2). Puckering parameters for (I) and (II) indicate that the conformation of the βGalp ring is slightly more affected than the βGlcp ring by the solvomorphism. Conformations of the terminal O-glycosidic linkages in (I) and (II) are virtually identical, whereas those of the internal O-glycosidic linkage show torsion-angle changes of 6° in both C-O bonds. The exocyclic hydroxymethyl group in the βGalp residue adopts a gt conformation (C4' anti to O6') in both (I) and (II), whereas that in the βGlcp residue adopts a gg (gauche-gauche) conformation (H5 anti to O6) in (II) and a gt (gauche-trans) conformation (C4 anti to O6) in (I). The latter conformational change is critical to the solvomorphism in that it allows water to participate in three hydrogen bonds in (I) as opposed to only two hydrogen bonds in (II), potentially producing a more energetically stable structure for (I) than for (II). Visual inspection of the crystalline lattice of (II) reveals channels in which methanol solvent resides and through which solvent might exchange during solvomorphism. These channels are less apparent in the crystalline lattice of (I).

Isopropyl 3-deoxy-α-D-ribo-hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside): evaluating trends in structural parameters.

Isopropyl 3-deoxy-α-D-ribo-hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside), C9H18O5, (I), crystallizes from a methanol-ethyl acetate solvent mixture at room temperature in a 4C1 chair conformation that is slightly distorted towards the C5SC1 twist-boat form. A comparison of the structural parameters in (I), methyl α-D-glucopyranoside, (II), α-D-glucopyranosyl-(1→4)-D-glucitol (maltitol), (III), and 3-deoxy-α-D-ribo-hexopyranose (3-deoxy-α-D-glucopyranose), (IV), shows that most endocyclic and exocyclic bond lengths, valence bond angles and torsion angles in the aldohexopyranosyl rings are more affected by anomeric configuration, aglycone structure and/or the conformation of exocyclic substituents, such as hydroxymethyl groups, than by monodeoxygenation at C3. The structural effects observed in the crystal structures of (I)-(IV) were confirmed though density functional theory (DFT) calculations in computed structures (I)c-(IV)c. Exocyclic hydroxymethyl groups adopt the gauche-gauche (gg) conformation (H5 anti to O6) in (I) and (III), and the gauche-trans (gt) conformation (C4 anti to O6) in (II) and (IV). The O-glycoside linkage conformations in (I) and (III) resemble those observed in disaccharides containing β-(1→4) linkages.

Dinuclear Copper Complexes Triply Bridged by a Tetraphosphane, rac‐Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2

Reaction of [Cu(MeCN)4]BF4 with rac‐Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2 (rac‐dpmppm) gave [Cu2(µ‐rac‐dpmppm)(MeCN)4](BF4)2 (1), and that of CuCl with rac‐dpmppm in the presence of 2,6‐xylyl isocyanide (XylNC) and NH4PF6 afforded [Cu2(µ‐rac‐dpmppm)(XylNC)4](PF6)2 (2). Complex 1 was converted by treatment with diimino ligands (N^N) to a series of CuI2 complexes, [Cu2(µ‐rac‐dpmppm)(N^N)2]2+ (N^N = phen (3), 4‐Mephen (4), 4,7‐Me2phen (5), 5,6‐Me2phen (6), 2,9‐Me2phen (7), 3,4,7,8‐Me4phen (8), 4,7‐Ph2phen (9), bpy (10), 4,4′‐Me2bpy (11), and 1,8‐naphtyridine (nap) (12)), where phen = 1,10‐phenanthroline and bpy = 2,2′‐bipyridyl. The isolated complexes were characterized by spectroscopic and X‐ray crystallographic analyses to reveal that two CuI ions are triply bridged by a rac‐dpmppm ligand in κ2,κ2 fashion resulting in robust Z‐form dicopper structures, where the Cu···Cu distances vary from 4.2 to 5.6 Å depending on the ancillary ligands through altering conformation of the fused six‐membered chelate rings. The bulky ligands of XylNC and 2,9‐Me2phen deform the conformation to boat (2) and twist boat (7) forms, leading to elongation of Cu···Cu separation (Cu···Cu = 5.3488(5) Å (2), 5.5681(6) Å (7)), and other non‐bulky ligands render them deformed chair forms with Cu···Cu = 4.2051(5) Å (8) to 4.7341(5) Å (10). The solution behaviors of 5 and 11 were examined by variable temperature 1H NMR measurements to show fluxional exchange of the two nitrogen atoms attaching to the tetrahedral Cu centers with ΔG‡ of 14 kcal/mol at room temperature. Photophysical properties of 3–12 were also investigated to show rapid fluorescence at 394–467 nm in CH2Cl2 solutions and phosphorescence at 582–608 nm in the solid states. Reaction of [Cu(MeCN)4]BF4 with rac‐dpmppm and NaBH4 in MeOH/CHCl3 afforded [Cu4(µ4‐H)(µ‐Cl)2(µ‐rac‐dpmppm)2]BF4 (13), in which two rac‐dpmppm ligands exhibited a new coordination mode of κ2,κ1,κ1 M3‐bridge to hold two {Cu2(µ‐Cl)(µ‐rac‐dpmppm)}+ units connected by a central µ4‐hydride.

A concise and efficient concurrent synthesis of 6,11-dihydrodibenzo[b,e]azepines and 5,6,11,12-tetrahydrodibenzo[b,f]azocines and their conversion to 4-oxo-8,13-dihydro-4H-benzo[5,6]azepino[3,2,1-ij]quinoline-5-carboxylates and N-acetyl-5,6,11,12-tetrahydrodibenzo[b,f]azocines: synthetic sequence, spectroscopic characterization and the structures of two products.

Reaction of 2-allyl-N-benzyl-4-fluoroaniline or 2-allyl-N-benzyl-4-chloroaniline with 98% sulfuric acid leads to the concurrent formation of halogeno-substituted 11-ethyl-6,11-dihydrodibenzo[b,e]azepines, (II), and halogeno-substituted 11-methyl-5,6,11,12-tetrahydrodibenzo[b,f]azocines, (III), in each case in (II):(III) molar ratios of ca 2:1. Further reaction of (II) leads to ethyl 13-ethyl-2-halogeno-4-oxo-8,13-dihydro-4H-benzo[5,6]azepino[3,2,1-ij]quinoline-5-carboxylate, while acetylation of (III) gives the corresponding N-acetyl derivatives. The dibenzo[b,e]azepine and dibenzo[b,f]azocine ring systems are of importance in forming the core of a variety of bioactive compounds. In ethyl 13-ethyl-2-fluoro-4-oxo-8,13-dihydro-4H-benzo[5,6]azepino[3,2,1-ij]quinoline-5-carboxylate, C22H20FNO3, (IVa), the azepine ring adopts a conformation close to the twist-boat form, and the molecules are linked into a three-dimensional framework structure by a combination of C-H...O and C-H...π(arene) hydrogen bonds. The azocine ring in 5-acetyl-2-chloro-11-methyl-5,6,11,12-tetrahydrobenzo[b,f]azocine, C18H18ClNO, (Vb), adopts the boat-boat conformation and the molecules are again linked by C-H...O and C-H...π(arene) hydrogen bonds, but this time form a sheet structure.

A Convenient Synthesis of Fused Tetrahydroazocines from Acenaphthylene-1,2-dione, Proline, and Acetylenic Esters

A synthesis of dialkyl (12E,14E)-7-oxo-10,11-dihydro-7H,9H-azocino[2,1-a]benzo[de]isoquinoline-13,14-dicarboxylates through a 1,3-dipolar cycloaddition reaction of azomethine ylides, generated in situ from proline and acenaphthylene-1,2-dione, with dialkyl acetylenedicarboxylates is described. According to the X-ray diffraction data, the tetrahydroazocine ring has a rigid twist-boat form with approximate local C2 symmetry.

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.

Diastereoisomeric forms of 11-ethyl-6,11-dihydro-5H-dibenzo[b,e]azepine-6-carboxamide: syntheses and the molecular and supramolecular structure of the minor form (6RS,11RS)-11-ethyl-6,11-dihydro-5H-dibenzo[b,e]azepine-6-carboxamide.

Compounds containing the tricyclic dibenzo[b,e]azepine system have potential activity in the treatment of a number of diseases. Continuing with our studies on the synthesis of new small and potentially bioactive molecules, a synthetic route, involving acid-catalysed intramolecular Friedel-Crafts cyclization, to the readily separable diastereoisomers of 11-ethyl-6,11-dihydro-5H-dibenzo[b,e]azepine-6-carboxamide, a potentially useful precursor in the synthesis of analogues of some anti-allergenic, antidepressant and antihistaminic drugs currently in use, has been developed starting from 2-allylphenylamine and methyl 2-bromo-2-phenylacetate and proceeding via racemic methyl 2-[(2-allylphenyl)amino]-2-phenylacetate (A) and racemic 2-[(2-allylphenyl)amino]-2-phenylacetamide (B), to give the two diastereoisomers (I) and (II), C17H18N2O. Isomers (I) and (II), and their precursors (A) and (B), have all been fully characterized spectroscopically. Structure analysis of the minor isomer (I) shows that it has the (6RS,11RS) configuration, and that the azepine ring adopts a conformation intermediate between the boat and twist-boat forms, with the carboxamide and ethyl substituents both occupying quasi-equatorial sites. The molecules of (I) are linked by a combination of N-H...O, N-H...π(arene) and C-H...π(arene) hydrogen bonds to form complex sheets. Comparisons are made with the structures of some related compounds.

Ethyl (4-benzyloxyphenyl)-6-methyl-2-sulfanylidene-1,2,3,4-tetrahydropyrimidine-5-carboxylate and a redetermination of ethyl (4RS)-4-(4-methoxyphenyl)-6-methyl-2-sulfanylidene-1,2,3,4-tetrahydropyrimidine-5-carboxylate, as its 0.105-hydrate, both at 200 K: subtly different hydrogen-bonded ribbons.

Two sulfanylidene-1,2,3,4-tetrahydropyrimidine derivatives have been synthesized using acid-catalysed cyclocondensation reactions between thiourea, ethyl 3-oxobutanoate and substituted benzaldehydes. In each of ethyl (4RS)-4-(4-benzyloxyphenyl)-6-methyl-2-sulfanylidene-1,2,3,4-tetrahydropyrimidine-5-carboxylate, C21H22N2O3S, (I), where Z' = 2, and ethyl (4RS)-4-(4-methoxyphenyl)-6-methyl-2-sulfanylidene-1,2,3,4-tetrahydropyrimidine-5-carboxylate 0.105-hydrate, C15H18N2O3S·0.105H2O, (II), the reduced pyrimidine ring adopts a conformation intermediate between the boat, screw-boat and twist-boat forms. In (I) and (II), a combination of N-H...O and N-H...S hydrogen bonds links the organic molecules into ribbons containing alternating R2(2)(8) and R4(4)(20) rings. In (I), the ribbon contains three types of ring, viz. two different R2(2)(8) rings which are both centrosymmetric and R4(4)(20) rings which are not centrosymmetric. In (II), the ribbon contains two types of ring, both of which are centrosymmetric. In compound (II), the ribbons enclose continuous channels which run along the twofold rotation axes in the space group C2/c, and the partial-occupancy water molecules lie within these channels. Structural comparisons are made with a number of related compounds.

(8RS)-4-Amino-6-(4-chlorophenyl)-8-(2,4-dichlorothiazol-5-yl)-8,9-dihydro-7H-pyrimido[4,5-b][1,4]diazepine N,N-dimethylformamide monosolvate: sheets built from N-H···N and C-H···O hydrogen bonds.

In the title compound, C16H11Cl3N6S·C3H7NO, the seven-membered ring adopts a conformation which is close to the twist-boat form. The molecular components are linked into sheets by a combination of two N-H···N hydrogen bonds and two C-H···O hydrogen bonds. Comparisons are made with other aminopyrimidine derivatives.

The molecular structure of gaseous 1,4-dioxane: An electron-diffraction reinvestigation aided by theoretical calculations

The structure of the molecule 1,4-dioxane (DIOX) has some features in common with other ring systems previously studied in this laboratory. In contrast to 1,4-cyclohexanedione, however, which consists both of a twisted boat form of D2 symmetry and a chair form of C2h symmetry, DIOX was reported, in two much earlier studies, to exist only as the chair form. The results of our work are in agreement with the earlier conclusions that gaseous DIOX exists either entirely, or essentially entirely (less than a few percent) in the chair form. Our work is much more extensive than the previous studies, and, aided by high-level theoretical molecular orbital- and normal-coordinate calculations, yielded the following bond distances (rg/Å) and bond angles (∠α/deg). 〈C–H〉=1.104 (4), C–O=1.420 (2), C–C=1.514 (4), 〈C–C–H〉=105.4 (55), H–C–H=108.0 (26), C–C–O=111.1 (3), C–O–C=110.9 (10). The “flap” angle — the angle by which the COC plane is tilted up from the plane of the four carbon atoms — is equal to 50.6 (7)°. The structure is discussed and compared with the previous work and with predictions from theory.

1,4-Cyclohexanedione. Composition, Molecular Structures, and Internal Dynamics of the Vapor: An Electron Diffraction Investigation Augmented by Molecular Orbital Calculations

Electron diffraction experiments on the vapor of 1,4-cyclohexanedione have been carried out at a nominal temperature of 435 K. The results are consistent with the presence of a mixture of a chair form of C2h symmetry and a twisted boat form of D2 symmetry. The former has the familiar dynamic properties of a semirigid molecule, but the D2 form undergoes a large-amplitude twisting motion (pseudorotation) that degrades the symmetry to C2. The analysis was designed to elucidate parameter values and internal dynamics of each conformer and the composition of the system. The large-amplitude motion of the twisted boat form was modeled by placement of 10 pseudoconformers at approximately 5° intervals along a pseudorotational coordinate that began at the D2 position and that reflected the angle between the C═O bond vectors. A Gaussian weighting of the pseudeoconformers centered on the (lowest-energy) D2 position was assumed. Differences in the interatomic distances and bond angles of these pseudoconformers were calculated via B3LYP/cc-pVTZ theory and introduced as constraints. The bond length averages over the twisted boat forms followed by values for the chair in square brackets are (rg/Å; [angle]α/deg) r(C-H) = 1.115(11) [1.124(11)], r(C═O) = 1.211(3) [1.233(6)], r(C1-C2) = 1.524(5) [1.526(5)], and r(C2-C3) = 1.533(11) [1.539(11)]. The corresponding ring angle values are [angle](C1C2C3) = 111.1(5) [111.0(4)] and [angle](C6C1C2) = 116.3(8) [115.7(8)]. In the twisted boat form, pseudorotation leads to a weighted average displacement of the angle between the C═O bond vectors, [angle]Δ(CO,CO), equal to 21.3° from the 180° value in the D2 form corresponding to an average angle between the CO bond vectors of 158.7(1)°. The amount of the chair form in the gas at 435 K is 24(10)%. The listed uncertainties are estimated at 2σ.

ChemInform Abstract: Conformational Deformation of Ring C in 14β-Estra-1,3,5(10),15- tetraen-17-ones.

Abstract High-field n.m.r. analysis of four 3-methoxy-14β-estra-1,3,5(10), 15-tetraen-17-ones provides evidence for conformational deformation of ring C to a twist-boat form in solution. These observations are supported by molecular mechanics (MM2) calculations, which predict that the ring C chair and ring C twist-boat conformers have similar steric energies, slightly favouring the latter. An X-ray crystal structure determination on 3-methoxy-14-methyl-14β-estra-1,3,5(10), 15-tetraen-17-one revealed that ring C does indeed adopt a twist-boat conformation in the solid state.

The marked difference in conformational behavior of the two diastereomers of 7-substituted-1,1-dichloro-7b-((Z)-8-chloro-6,7-dihydro-5H-benzo[7]annulen-9-yl)-1a,2,3,7b-tetrahydro-1H-cyclopropa[a]naphthalene, single crystal X-ray, 1H NMR and AM1 studies

The 7-substituted of 1 and 2 were synthesized and conformational analysis carried out. While 7-substituted of 2 show two conformers in solution, 7-substituted of 1 show only one form in solution. AM1 semi-empirical molecular orbital calculations show that the conformation of cycloheptadiene ring in 1 and 2 is a twist boat form. In this conformation, the C-7 substituents can be oriented in pseudo equatorial (exo) and pseudo axial (endo) positions. The 3 J calculation with Haasnoot equation on optimized structure of 2 reproduces the observed 3 J coupling constants in exo and endo forms. Ring inversion of cycloheptadiene moiety in substituted 2 interconvert the e′–a′ (exo–endo) positions. The 3 J calculation on optimized structure of 1 shows that 7-substitution is in pseudo equatorial (exo) direction, as found in the crystal structure of 1a by single crystal X-ray crystallography. The barrier to ring inversion in 2a is determined by dynamic 1H NMR spectroscopy to be Δ G ≠ (335K) = 68.0 ± 0.5 kJ/mol.

Crystal structure and DFT calculations of 3,4- seco-lup-20(29)-en-3-oic acid isolated from Wrightia tinctoria: Stacking of supramolecular dimers in the crystal lattice

3,4- Seco-lup-20(29)-en-3-oic acid ( 1), a rare triterpene, was isolated from Wrightia tinctoria R.Br (Family: Apocyanaceae). It appeared promising to study the structural chemistry of this compound because of its unique bio activity. X-ray diffraction analysis demonstrated that this compound consists of three six membered rings and one five membered ring. The first six membered ring (A) is in the twist boat form while the remaining two six membered rings (B, C) are in the chair form. The crystal of this compound belongs – to the monoclinic crystal system and space group P2 1. Lattice constituents are as follows: a = 13.074(6) Å, b = 11.972(5) Å, c = 17.394(7) Å; α = 90.00°, β = 98.20(13)°, γ = 90.00°; V = 2695(2) Å 3, d = 1.094 Mg/m 3, Z = 4. The optimized geometry of the reported molecule has been calculated with the DFT/B3LYP theory using 6-31G(d,p) basis set.

ChemInform Abstract: Boat Forms of Ring A in B-Norsteroids.

The X-ray crystal structures of 6β-toluene-p-sulfonyloxymethyl-5β-hydroxy-B-norandrostane-3,17-dione (7) and 6β-methoxycarbonyl-3β,5β-dihydroxy-B-norandrostan-17-one (8) reveal that ring A exists in a twisted boat form in these 5β-B-norandrostanes.

Crystal Structure of [2,13-Dibenzyl-5,16-dimethyl-2,6,13,17-tetraazatricyclo(14,4,01.18,07.12)docosane]copper(II) Diperchlorate

In the title compound, [Cu(C34H52N4)](ClO4)2, the Cu(II) ion has a square-planar coordination somewhat distorted towards tetrahedral geometry. The macrocyclic ligand adopts a less stable trans-I (RSRS) configuration. The two benzyl groups and two secondary amine H atoms are oriented towards the same side of the macrocyclic plane. The two six-membered rings are in slightly distorted chair and unsymmetrical twist-boat forms while both five-membered chelate rings are in the gauche conformation. The longer distances (2.050(4) and 2.035(4) A) of Cu–N(tertiary) compared to 2.017(4) and 1.990(4) A for Cu–N(secondary) may be due to the steric effects of the attached two benzyl groups on the tertiary N atoms. The crystal structure is stabilized by hydrogen bonds between secondary NH groups and O atoms of perchlorate counter-ions. In the title compound, [Cu(C34H52N4)](ClO4)2, the Cu(II) ion has a square-planar coordination somewhat distorted towards tetrahedral geometry. The macrocyclic ligand adopts a trans-I configuration. The benzyl groups and secondary amine H atoms are oriented towards the same side of the macrocyclic plane.

Chair−Boat Form Transformation of Piperazine-Containing Ligand toward the Preparation of Dirhenium Metallacycles

A new generation of a flexible bipyridine-based ligand, 1,4-bis(4-pyridyl)-piperazine, with two main stable states (chair and twist-boat forms) was employed in the construction of unusual dirhenium metallacycles.

Theoretical Study on the Conformational Conversion of 1,3-Dioxane Inside a Capsular Host

Abstract The mechanism for the conformational conversion of 1.3-dioxane guest encapsulated inside a capsular host was theoretically investigated using semiempirical PM3 method and DFT methods. The free-state process of the conformational conversion of 1,3-dioxane was also investigated to make a comparison between the two different states using the same theory. The influences of the inner phase of the capsule on the conformational conversion of guest molecule were discussed via analyzing the comparative results. It was found that the capsular host could accommodate 1,3-dioxane within its cavity by the weak attractive interactions between host and guest, and it responds to the conformational conversion of guest by the deformation of hydrogen-bonding seam at the middle of the capsule. When entrapped in the capsule, the guest molecule undergoes the conformational conversion from chair form to twist-boat form slower than that under the free condition. The deformation of the capsule is favorable to maximize the attractive interactions between host and guest.

Investigations into conformational transitions and solvation structure of a 7-piperidino-5,9-methanobenzo[8] annulene in water

Solvation shell structure of a 7-piperidino-5,9-methanobenzo[8] annulene (PMA) in water has been investigated in ambient conditions using both molecular dynamics (MD) and Car-Parrinello molecular dynamics (CPMD) calculations. From the MD calculations, we find that this molecule exists in three major conformational states out of which two are in twist-boat forms and one in chair form. Due to the limited time scale accessible in CPMD simulations, we have studied all the three conformational states separately using CPMD. The molecular geometry, electronic charge distribution and solvation structure for all three forms are investigated. The stability order of the chair and twist-boat conformations in water solvent has been reversed when compared to the gaseous phase results and in the case of polar aprotic solvents (J. Org. Chem., 1999, 61, 5979). From the radial distribution function, we find that the solvent density around the chair form is significantly lower, which has to be directly related to the smaller solvent accessible area for this conformation and this is in complete agreement with earlier reports. Among the findings are that the solvation shell structure around the nitrogen atom in the chair form of PMA is considerably different from the open conformational forms or the twist-boat forms. The dipole moment for the closed form is found to be significantly larger when compared to the twist-boat forms.