Conformational behavior of nonfused biheterocycles: Part XVI. Isomeric phenyl-tetrazines and bitetrazines

We present here an ab-initio STO-3G and advanced semiempirical AM1 calculation for the structure and the conformational behavior of phenyltetrazine and bitetrazine molecules. Results for phenyltetrazine isomers show that in the absence of H-H interactions, the preferred conformation is the planar one. Instead, in the case of bitetrazines, the systems dominated by the lone pair-lone pair interactions are not planar. In the presence of direct H-H interaction as for 5-5-1,2,3,4 bitetrazine, the antiplanar conformation is the preferred one. Comparison with other systems (including one or more nitrogen lone pairs) shows that the compounds with the same ortho substituents have very similar conformational behavior irrespective of their composition. The AM1 results indicate that this method is able to give reliable conformational information for compounds in which H-H interactions are involved. A Fourier expansion of the torsional potential suggests that the first two terms fit the calculated torsional potentials well except in the case of two H-H interactions in the same molecule.     

Head-to-head polymers—II. Dilute solution properties of head-to-head polypropylene

The behaviour in dilute solution of head-to-head (H-H) polypropylenes, covering the range $\text{M}{}_{\mathrm{n}}\text{= 0.22}$ to 4.19 × 10⁴, has been studied in cyclohexane as solvent at 303 K. The data could be represented by two distinct Mark-Houwink equations describing the low and the high molecular weight regions. The unperturbed dimensions of (H-H) polypropylenes were found to be less than those for conventional atactic polypropylene in agreement with theoretical predictions. The theta temperature for (H-H) polypropylene in isoamylacetate was found to be 316 K. At this temperature the characteristic ratio was estimated to be 4.5 compared with 6.4 for atactic and syndiotactic polypropylene. The conclusion is that (H-H) placements in polypropylene increase the flexibility of the chain.     

On the ground state and first excited state geometries of syn-sesquinorbornene

Extende Hückel calculations on the title compound ($\text{1}$) predict hinge-like bending of the double bond corresponding to interplanar angles of 167° for the ground state and 210° for the first excited state. The predictions are discussed in terms of hyperconjugative interactions.     

Latrunculins: NMR study,two new toxins and a synthetic approach

A complete ¹H and ¹³C NMR assignment of one of the latrunculins (B) was accomplished with the aid of 2D NMR COSY and CH shift-correlation experiments. The various H-H Coupling constants have been datermined and a conformation of the macrolide and the tetrahydropyran (THP) ring suggested on basis of the J-values and measured NOE's. The absolute configuration of latrunculin-A($\text{1}$) was determined on the grounds of its earlier X-Ray analysis, and a chemical degradation to a known compound. Two novel latrunculins, -C($\text{3}$) and -D($\text{4}$), were isolated from the Red Sea sponge $\text{L. magnifica}$ and their structures elucidated. Starting with L-cysteine a synthon for the latrunculins has been synthesized.     

Magnetic properties and structures of equiatomic rare earth-platinum compounds RPt (R = Gd,Tb, Dy,Ho, Er,Tm)

The magnetic properties and structures of RPt compounds (R = Gd, Tb, Dy, Ho, Er, and Tm) are presented. Below their Curie temperature the compounds exhibit ferromagnetic behavior. In GdPt, the spontaneous magnetization at 4.2°K ($\text{6.7 μ}{}_{\mathrm{<mtext>B</mtext>}}\text{Gd}$) and the small superimposed susceptibility suggest that the gadolinium moments are parallel and the exchange interactions are positive. In the three types of noncollinear magnetic structures observed in the other compounds the rare earth atoms are divided into two sublattices with different magnetization directions. They give rise to a ferromagnetic component associated with an antiferromagnetic component. These structures, which are analyzed in terms of crystal field effects, result from a competition between a magnetocrystalline anisotropy and positive exchange interactions of Heisenberg type.     

Synthesis and conformational analysis of 68,11-dihalo[5]metacyclophanes

The synthesis of the 8,ll-dihalo[5]metacyclophanes $\text{1b-1d}$ (halogen = chlorine or bromine) from 1,2-dimethylenecycloheptane ($\text{2}$) is described. Spin saturation transfer and line shape analysis of the temperature dependent ¹H NMR spectra showed $\text{1b-1d}$ to occur in two conformations A (85-89%) and B (15-11%, respectively). The conformation of the pentamethylene bridge was fully analyzed from vicinal proton coupling constants which obey the Karplus relation inspite of severe distortions of the carbon valence angles. The thermodynamic parameters of the ground and transition state were determined for the equilibrium between the conformers A and B and are discussed in terms of steric interactions in these highly strained molecules.     

Crystal and molecular structure of a novel hexa-host inclusion compound with squalene as guest

An X-ray study of the adduct formed by hexakis ($\text{p}$-$\text{t}$-butylphenylthiomethyl)benzene (I) with squalene (II) establishes the first example of channel-type inclusion behaviour for a hexahost molecule, the guest triterpene (II) being observed in a novel conformation controlled by host-guest interactions.     

Interactions between frenkel excitons. I. Formalism and preliminary results

For the tightest-bound Frenkel excitons a many-particle hamiltonian is adopted which includes terms representing interactions between the excitons. The parameters of the model are the excitation energy of an isolated localized excitation, the excitation-transfer matrix elements, and the interaction energies. Several special cases, involving particular relations between the latter two sets of parameters, are treated qualitatively. Equations are translated into the language of spin-$\text{1}\text{2}$ lattices, so that use may be made of the results of that theory. Favorable conditions for observation of polyexcitons and of phase transition to a liquid of excitons are discussed. A standard formalism for the determination of absorption and emission spectra and of their moments is adapted to the present problem. Possible generalizations of the model are briefly discussed.     

Cross-conjugated biradicals: Kinetics and kinetic isotope effects in the thermal isomerizations of cis- and trans-2,3-dimethylemethylnecyclopropane,cis- and trans-3,4-dimethyl-1,2-dimethylenecyclobutane,and of 1,2,8,9-decatetraene

cis- and trans - 2,3 - Dimethylenemethylenecyclopropane ($\text{C}$ and $\text{T}$) interconvert at 160.0° with a small normal kinetic isotope effect (KIE) when the exo-methylene is deuterated, but the 1,3-shift products, 2-methylethylidenecyclopropane, show a large normal KIE, 1.35 and 1.31, when formed from $\text{C}$ and $\text{T}$, respectively. This data can be interpreted in terms of either parallel reactions or a common trimethylenemethane diradical intermediate formed with a normal KIE of 1.11 and closing to 1,3-shift product with a normal KIE of 1.29 due to the effect of deuterium in the required 90° rotation of the exo-methylene carbon.The kinetics of the thermal 1,3- and 3,3-shifts of cis- and rans-3,4-dimethyl-1,2-dimethylenecyclobutane ($\text{CB}$ and $\text{TB}$) were determined in a flow reactor. The first order rate constants are log k_CB (sec^?1) = 13.7 ? 42,200/2.3 RT and log k_TB (sec^?1) = 13.6 ? 41,900/2.3 RT (E_a in kcal/m) which compare favorably to that from the parent hydrocarbon. 1,2-dimethylenecyclobutane, after reasonable correction for dimethyl substitution.Rearrangement of $\text{TB}$ and its bis(dideuteriomethylene) derivative at 230.0° revealed a normal KIE of 1.08. This KIE could be interpreted in terms of either a methylene rotational isotope effect in a concerted reaction or formation of a bisallyl diradical with the expected normal rotational IE on closure to the 1,3-shift product of 1.12 with no IE in the ring opening when the result is corrected for return of the biradical to starting material.The kinetics of intramolecular 2 + 2 cycloaddition of 1,2,8,9-decatetraene were determined in a flow reactor. The first order rate constant is log k(sec^?1) = 9.4 ? 30,800/2.3 RT (E_a in kcal/m). These energetics are compared with those of other 2 + 2 cycloadditions. The major product is 3,4-dimethylenecyclooctene ($\text{DC}$) which is also found from the minor product, cis-7,8-dimethylenebicvyclo[4.2.0]octane ($\text{CO}$), at higher temperatures. The trans isomer, $\text{TO}$, also gives $\text{DC}$ at about the same rate as $\text{CO}$.     

Stereospecific total synthesis of 12-(r)- and 12- (s)-forms of 6-trans leukotriene B

Non enzymatic hydration of leukotriene A ($\text{1}$), the biogenetic precursor of leukotriene B ($\text{2}$) affords among other products two diastereomeric 6-$\text{trans}$-isomers of 2, the first stereospecific and efficient syntheses of which are recorded herein.     

Copper(II) in organic synthesis. IV Reaction of the copper(II) acetate complex of isatin-3-arylhydrazones with dimethyl acety-lenedicarboxylate.

The copper(II) acetate complex of isatin-3-phenylhy-drazone (2a) reacted with dimethyl acetylenedicarboxylate (DMAD) by two competitive pathways: a Cu⁺⁺ oxidation of the ligand and a [2+2] cycloaddition. The former reaction gave 3-car- benindolin-2-one (13) which reacted with DMAD in a l,3-dipolar cycloaddition, to give 3a. This was synthesized by an independent route. The phenyl radical, generated in the same process, was trapped by three DMAD and gave $\text{4a}$. The [2+2] cycloaddition gave a spiro adduct 10 which, by electrocyclic ring opening and intramolecular cyclization, allowed isolation of a pyridazino [3,4-b] indole (|5a). The reaction was performed on $\text{p}$-chlorophe-nylhydrazone 1b and the structure of 5b was demonstrated by X-ray analysis. A rationalization of the reactivity was attempted in terms of MO interactions of the reactants.     

Identification of Cu(II)/Cu(III) couples in binuclear copper(II) complexes

A new series of binuclear copper(II) complexes were synthesised and studied by magnetic, spectral, ESR and cyclic voltammetry methods. The μ_eff values per copper atom correspond to the values observed for mononuclear copper(II) complexes. ESR spectral data in solution indicate weak interactions resulting from the electron delocalisation through the ligand system. Two nearly reversible red-ox couples are identified at $\text{+}\text{?}$0.50 V and $\text{+}\text{?}$0.75 V vs SCE. They correspond to Cu(II)αCu(III) red-ox processes, successively occurring at the two copper sites in the binuclear complexes.     

Stereochemistry of organic sulphur compounds : Part. 17 conformational analysis of some erythro- and threo-thio-derivatives of β-fluorinated and β-oxygenated ethanes

The synthesis and conformational analysis of 2-fluoro, 2-hydroxy and 2-methoxy thioderivatives (thioethers, sulphoxides, sulphones and sulphonium salts) of 1,2-dimethyl and 1,2-diphenylethanes ($\text{erythro}$ and $\text{threo}$) are reported. Steric effects in thioethers and electrostatic interactions in sulphonium salts are the main factors determining the stability rotamers. A balance of these factors controls the conformational equilibria in sulphoxides and sulphones. Electronic interactions between the $\text{π}$-aromatic electrons or the unshared β-heteroatomic electrons and the sulphur atom are suggested to explain large differences in rotamer populations induced by changes in the carbon sketelon or in the relative configuration of the sulphinylic sulphur.     

EPR Spectrum of Kr2F

An intense EPR spectrum observed in UV-irradiated solid Kr/CF₃OF solutions at 10 K shows anisotropic hyperfine interactions of a single unpaired electron with one nuclear spin of $\text{1}\text{2}$ and (using isotopically enriched ⁸³Kr) two equivalent spins of $\text{9}\text{2}$. The spectrum is attributed to the linear radical KrFKr having a ²Σ_u ground state.     

Substituent effects on carbohydrate coupling reactions promoted by insoluble silver salts

Coupling reactions were performed in the α-D-glucopyranosyl bromide series (i.e. compounds $\text{1}$ a–c, $\text{2}$ a, b, $\text{3}$ and $\text{4}$) and the α-D-mannopyranosyl bromide series (i.e. compounds $\text{7}$ a–c, $\text{8}$ a, b, $\text{9}$) with aglycons $\text{5}$ or $\text{10}$ in the presence of insoluble silver salt promotors. (i.e. silver silicate and silver zeolite). The insoluble silver salt promotes the formation of the β-glycosidic linkage, while a non-participating group is present at C-2 of the glycon. It was found that in both series 4-O-acyl functions increase the β/α ratio of the glycosidic bond formation relative to 4-O-alkyl functions, whereas 3-O and 6-O-acyl functions decrease this ratio. We assume that inductive effects are responsible for the influence exerted by 3-O-substituents, but that through-bond interactions are essential for the effects exerted by 4-O and 6-O substituents. Another unexpected finding was that coupling of α-D-mannopyranosyl bromide derivatives gave much higher β/α ratios than the corresponding reactions of α -D-glucopyranosyl bromides.     

On the interchain interaction in the (SN)x polymer

The interchain interactions in the (SN)_x polymer are discussed on the basis of one-dimensional tight-binding SCF MO calculations of two chains of (SN)_x. We conclude that the interchain interaction in the (100) crystallographic plane is more significant than that in the ($\text{1}$02) plane in accordance with the previous result by Messmer and Salahub and that, from more quantitative analysis, these interactions are mainly caused by the shortest interchain SS coupling in both planes.     

Synthesis and 13C nmr characterization of some π-excessive heteropolyaromatic compounds

Several π-excessive heteropolyaromatic compounds, which contain furan and thiophen ring and are possible antifungal agents, have been synthesized in good yields according to two general methods. The first method has been used to prepare compounds possessing thiophens linked by their 2- and 5-positions, such as the ter-aryls $\text{2b}$, $\text{2d}$ and $\text{2a}$. Two precursors of these compounds have been obtained either by the Glaser reaction, or using a novel Pd-mediated reaction. The second method,which consists of the Ni-or Pd-catalyzed heteroarylation of heteroarene halides via cross-coupling with heteroaryl Grignard reagents or zinc halides,has been used to prepare the bi-aryls $\text{1a-e}$, which contain two heteroaromatic units, and the ter-aryl $\text{2c}$. Compound $\text{1e}$ has been also prepared starting from 2-(2-thienyl) furan ($\text{1c}$) by selective lithiation, followed by bromination.The ¹³C NMR signals of $\text{1a-e}$ and $\text{2a-d}$ have been assigned on the basis of the literature data and by relaxation measurements. Relaxation data have been also used to obtain qualitative informations on the conformational equilibria of the bi-aryls $\text{1a}$, $\text{1c}$ and the ter-aryls $\text{2a-d}$.     

β- and α-lithiaton of methyl β-methoxyacrylate: efficient synthesis of α,γ-substituted methyl tetronates - structure of aspertetronins and gregatins

Methyl β-methoxyacrylate ($\text{1}$) can be lithiated successively in β- and α-positions. Reaction with two electrophiles (at first a carbonyl compound) leads to α, γ-substituted methyl tetronates in a two step synthesis. Application of this method to the synthesis of methyl tetronate whose structure was assigned as that of gregatin B indicates that the gregatins and aspertetronins have the isomeric structures $\text{17}$.     

The cybrodins: new seco-illudalanes from the bird's nest fungus cyathus bulleri brodie.

The structures of the $\text{seco}$-illudalane sesquiterpenoids cybrodol $\text{2}$), isocybrodol ($\text{3}$), cybrodic acid ($\text{6}$), cybrodal ($\text{4}$), and trisnorcybrodolide ($\text{5}$), are presented. These are the first $\text{seco}$-illudalanes to be reported.