Lanthanide(III) complexes of a pyridine N-oxide analogue of DOTA: exclusive M isomer formation induced by a six-membered chelate ring

The presence of a six-membered chelate ring involving a pyridine N-oxide moiety induces exclusive M isomer formation throughout the whole lanthanide series endowed with a fast water exchange in the case of the Gd(III) complex.     

The application of a new aromaticity index to six-membered ring heterocycles

A previously described index of aromatic character has now been applied to six-membered ring heterocycles. Of particular interest is its ability to indicate the extent of aromatic character retained by the amide form of potential hydroxyheterocycles.     

Five- and six-membered ring opening of pyroglutamic diketopiperazine

A variety of ring-opening reactions of pyroglutamic diketopiperazine at both the five-membered and six-membered rings is described. Mild, basic conditions facilitate nucleophilic attack by amines at the diketopiperazine carbonyls giving pyroglutamides in excellent yield. Reaction with nucleophiles under acidic conditions give bis-glutamate derivatives of 2,5-diketopiperazine (DKP). These reactions provide simple, two-step sequences to pyroglutamides and symmetrical diketopiperazines from commercial pyroglutamic acid with control of product dictated by reaction conditions, catalyst, and nucleophile.     

The effect of replacing a benzene ring with a saturated six-membered ring on the mesomorphic properties of 4,4'-disubstituted diphenyldiacetylenes

A limited selection of ring modified diphenyldiacetylenes of the type where A=, Y=C_nH_2n+1, CF₃, F, COMe, NH₂, and NMe₂, and A=, trans and cis with Y=F and trans with Y=C₃H₇, were synthesized. Mesomorphic properties were determined by hot stage polarizing microscopy and DSC. These properties were generally poorer than those found in the parent benzene compounds. This was also true of some pyrimidine analogues reported earlier. Birefringence values also decreased as expected.     

Mechanism for hydrolysis of double six-membered ring tetraborate anion

[B₄O₅(OH)₄²⁻] is a representative borate anion with a double six-membered ring structure, but there is limited knowledge about the hydrolysis mechanisms of [B₄O₅(OH)₄²⁻]. Density functional theory-based calculations show that the tetraborate ion undergoes three-step hydrolysis to form [B(OH)₄⁻] and an ring intermediate, [B₃O₂(OH)₆⁻]. Other new structures, such as linear trimer, branched tetraborate, analogous linear tetraborate, are observed, but they are not stable in neutral systems and change to ring structures. [B₃O₂(OH)₆⁻] hydrolyzes to [B(OH)₄⁻] and [B(OH)₃] in the last two steps. The structure of borate anion and the coordination environment of the bridge oxygen atom control the hydrolysis process. [B₄O₅(OH)₄²⁻] always participates in the hydrolysis reaction, even with a decrease in concentration. [B₃O₃(OH)₄⁻], [B(OH)₄⁻], and [B(OH)₃] have different roles in “water-poor” and “water-rich” zones. Concentration and pH of solution are the key factors that affect the distribution of borate ions.     

Five- and six-membered ring formation from the intramolecular reaction of a protonated oxirane and alkene

Computational studies of competing five- and six-membered cyclisation of alkenyloxiranes 1a-d show that intramolecular reaction of a protonated oxirane and alkene is a concerted, single-step, exothermic process. The reactions proceed via reactant-like transition states, but where the oxirane C-O bond is considerably stretched. Two factors are seen to affect the regiochemistry: (1) stabilisation of the transitory positive charge in the transition state favours cyclisation to the more highly substituted oxirane carbon; and (2) there is an inherent stereoelectronic preference for six-membered cyclisation over five-membered cyclisation. The inherent preference for six-membered cyclisation has a parallel in Baldwin's rules for six-membered ring closure of a carbocation with an alkene, rather than Baldwin's rule for intramolecular nucleophilic reaction of three-membered rings, suggesting that the protonated oxirane mimics a carbocation. The electronic and stereoelectronic effects for cyclisation are modified by steric interactions of axial methyl groups. These systems provide a model for the A-ring cyclisation of oxidosqualene.     

Solution- and soluble-polymer supported asymmetric syntheses of six-membered ring prostanoids

An asymmetric synthesis of prostanoids containing a six-membered ring core structure (11a-homoprostaglandins), both in solution and using non-cross-linked polystyrene (NCPS) as a soluble support, was developed. Target molecule 1 was generated in a convergent fashion using a three-component coupling strategy, wherein chiral enone (R)-2 was the precursor of the central ring and the cuprate 3 and triflate 4 were used to introduce the side chains. The chiral center of (R)-2 directed the facial selectivity of the conjugate addition reaction which then dictated the stereochemical outcome of the subsequent alpha alkylation. Attachment of a six-membered ring scaffold to NCPS facilitated purification without compromising synthetic yields, still allowed 1H-NMR analysis of the intermediates in the synthesis, and provided an avenue for the construction of six-membered ring prostanoid libraries.     

Induction of an aromatic six-membered nitrogen ring via cation-pi interaction

Nitrogen clusters have been intensively studied for their potential application as high-energy density materials, but a six-membered nitrogen ring (N6) was not found to be stable and aromatic. To explore the possibility of inducing an aromatic N6 ring via cation-pi interaction, quantum chemistry calculations were performed on the systems of Ca2N6, CaN6, CaN6(2-), N6, and N6(4-) at the B3LYP/6-311+G level. The optimized geometries reveal that the planar structure of the N6 ring is stable only in the Ca2N6 complex. The computed NBO and CHelpG charges demonstrate that the planar N6 moiety in the Ca2N6 complex is almost a 10pi-electron system. The predicted nucleus-independent chemical shift (NICS) values demonstrate that the N6 moiety is aromatic in comparison with the NICS values of benzene. The estimated enthalpy of formation for the Ca2N6 complex is 100.4 kcal/mol for the reaction of 2Ca and 3N2. The binding energy between the Ca2+ cation and the N6(4-) moiety is -1928.8 kcal/mol, with electrostatic interaction serving as the predominant component. When all the calculated results are taken into account, including the planar structure, 10pi-electron system, identical bond length, and negative NICS value of the N6(4-) moiety in the Ca2N6 complex, it is deduced that the alkaline earth metal Ca is capable of inducing an aromatic N6 ring through the cation-pi interaction formed by electron transfer from the Ca atom to the N6 ring.     

Hydrogen exchange in a six-membered ring transition state. Evidence for a stepwise McLafferty rearrangement

Mass spectra of 2-methyl-5-chloro-3-N-ethyl-benzothiazolium iodide and labeled counter-parts are discussed with particular emphasis on the loss of ethylene from the moiety produced by thermal elimination of HI from this salt. This process demonstrates the stepwise nature of a McLafferty rearrangement involving a hydrogen transfer to an olefinic carbon.     

A luminescent Pt(II) complex with a terpyridine-like ligand involving a six-membered chelate ring

The ligand 2-(8'-quinolinyl)-1,10-phenanthroline (1) was prepared in 79% yield by the Friedlander condensation of 8-amino-7-quinolinecarbaldehyde and 8-acetylquinoline. The complex [Pt(1)Cl]+ was prepared and compared with the isomeric 2-(2'-quinolinyl)-1,10-phenanthroline (2) complex. An X-ray analysis indicated that the six-membered chelate ring in the tridentate complex resulted in a relief of angle strain as well as some non-planarity in the bound ligand 1. The control system for photophysical studies is [Pt3Cl]+ where denotes 2-(2'-pyridyl)-1,10-phenanthroline. Relative to the complex of 3, in dichloromethane solution [Pt(1)Cl]+ exhibits noticeably higher energy charge-transfer absorption but slightly lower energy emission. The gap between the onset of absorption and emission is larger because the emission from [Pt(1)Cl]+ originates from a triplet excited state with substantial intra-ligand character. At room temperature in deoxygenated dichloromethane, [Pt(1)Cl]+ has an excited-state lifetime of 310 ns vs. 230 ns for [Pt(1)Cl]+. Within the series, [Pt(1)Cl]+ also exhibits the largest activation barrier for thermally induced quenching at 2730 cm(-1) in fluid dichloromethane solution. However, the barrier is only about 50% larger than that found for [Pt(1)Cl]+. There is reduced ring strain in [Pt(1)Cl]+, but inter-ligand steric interactions weaken the ligand field.     

5,10,15-Triaryl-21,23-dioxacorrole and its isomer with a protruding furan ring

An oxa-analogue of 5,10,15-triarylcorrole, i.e., 5,10,15-triaryl-21,23-dioxacorrole (21,23-O(2)Cor)H, where two pyrrole rings are replaced by furan moieties, has been produced by condensation of 2,5-bis(arylhydroxymethyl)furan, 2-phenylhydroxymethylfuran, and pyrrole. 2-Phenylhydroxymethylfuran serves as the suitable synthone to introduce a furan ring with the ability create a direct pyrrole-furan alpha-alpha bond. The replacement of 2-phenylhydroxymethylfuran by 3-phenylhydroxymethylfuran led to a nonaromatic isomer of (21,23-O(2)Cor)H, i.e., (iso-21,23-O(2)Cor)H, which accommodates two furan rings. The protruding furan is built into the macrocycle via beta and beta' carbon atoms with the oxygen atom pointing outward. Crystal structures of the [(21,23-O(2)Cor)H(2)](2)[ZnCl(4)] and [(iso-21,23-O(2)Cor)H(2)]Cl have been studied by X-ray crystallography. The complex ZnCl(4)(2-) anion is located in a clam-shell-like cavity formed by two 21,23-dioxacorrole cations of the [(21,23-O(2)Cor)H(2)](2)[ZnCl(4)] unit. The 21,23-dioxacorrole cation is only slightly distorted from planarity. In [(iso-21,23-O(2)Cor)H(2)]Cl, the macrocycle is strongly puckered as the internal ring is contracted by two carbon atoms when compared to regular porphyrin. The chloride anion is located over the center of the macrocycle and is involved in two intra (N)H...Cl and two intermolecular (C)H...Cl interactions to be classified as a tetrafurcate system. The (21,23-O(2)Cor)H molecule preserves aromaticity of the parental corrole with characteristic downfield positions of furan and pyrrole resonances in (1)H NMR accompanied the NH resonance at the upfield position (-2.53 ppm). The temperature-dependent features detected in (1)H NMR spectra of (21,23-O(2)Cor)H are consistent with the existence of a tautomeric equilibrium which involves two tautomers alternatively protonated on N(22) or N(24) nitrogen atoms. The density functional theory (DFT) has been applied to model the molecular and electronic structure of two tautomers of 21,23-dioxacorrole [22-N, 24-NH], [22-NH, 24-N]. The total energies calculated using the B3LYP/6-31G**//B3LYP/6-31G* approach demonstrate a very small energy difference (1.4 kcal/mol) between tautomers suggesting their simultaneous presence in equilibrium. Insertion of nickel(II) into (21,23-O(2)Cor)H yields five-coordinate (21,23-O(2)Cor)Ni(II)Cl--the first high-spin nickel(II) in a corrole-like macrocyclic environment.     

Valence isomer of a beta-diketiminate-supported phosphinidene: a case of C-H activation and ring contraction

A valence isomer of a beta-diketiminate-supported phosphinidene has been prepared by potassium metal reduction of the corresponding chlorophosphenium cation.     

Catalytic asymmetric cyclocarbonylation of o-isopropenylphenols: enantioselective synthesis of six-membered ring lactones

Cyclocarbonylation of o-isopropenylphenols with CO (500 psi) and H(2) (100 psi), using Pd(OAc)(2) and (+)-DIOP as the chiral catalyst, in CH(2)Cl(2) affords 3,4-dihydro-4-methylcoumarins in 60-85% yield and in up to 90% enantiomeric excess. The stereoselectivity is influenced by the structure of the chiral phosphine ligands and substrates, as well as by the reaction conditions.     

Conformational mobility of the six-membered ring in 5-oxo-1,3-cyclohexadiene and its derivatives

Molecular mechanics and MNDO calculations showed that the six-membered ring in the molecule of 5-oxo-1,3-cyclohexadiene possesses high conformational mobility. The transition from a planar equilibrium conformation to a distorted sofa conformation in which the C(sp²)-C(=O)-C(sp³)-C(sp²) torsion angle is equal to ±30° increases the energy of the molecule by less than 1 kcal mol^–1. The influence of steric (R = Me, Et, Prⁱ, Bu^t) and electronic (R = NH₂, NO₂) effects of substituents R on the equilibrium conformation and mobility of the carbocycle has been analyzed. Both types of substituents at unsaturated C atoms do not change the equlibrium conformation or flexibility of the six-membered ring. Substituents at saturated C atoms cause the transition of the carbocycle to the distorted sofa conformation and significantly restrict its mobility. The electronic structures of 5-oxo-1,3-cyclohexadiene and its amino and nitro derivatives have been analyzed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 849–854, May, 1995.     

Zinc halide-promoted cyclization of propargyl amide enynes: novel six-membered ring formation

Highly unusual zinc halide-promoted cyclization of propargyl amide enynes in the presence of an alcohol or a Brønsted acid afforded novel six-membered rings.     

Conformational analysis of six-membered ring dioxaphosphinanes. Part 1: Anancomeric thiophosphates

A study of the conformation of a series of anancomeric axial and equatorial 2-aryloxy-2-thio-1,3,2λ⁵-dioxaphosphinanes 2-12 in solution and solid state is reported. In accord to the stereoelectronic theory, aryl thiophosphates substituted with electron-withdrawing (EW) groups will tend to occupy axial positions in chair ring conformations due to the stabilizing endo-anomeric (n_πO-σ_P-X^*) hyperconjugative interaction. The antiperiplanar orientation of the orbitals involved in the stereoelectronic interaction is a requirement that is fulfiled in the axial series of compounds when the ring adopts a chair conformation. Therefore, in the equatorial series of thiophosphates, the axial seeking characteristics of aryloxy-EW groups might render the molecule with distortion of the chair conformation. An opposite trend is anticipated for the less axial seeking aryl thiophosphates substituted with electron releasing (ER) groups. A detailed analysis of the ³J_HH, ³J_PH and ³J_CP coupling constants allowed us to conclude that there is no contribution of high energy twist-boat conformations in the equatorial thiophosphates substituted with aryl-EW groups in solution. In consequence, single chair conformations were found in solid state for aryl thiophosphates in both configurations. X-ray geometrical analysis of bond distances and bond angles supports clearly the participation of hyperconjugative endo-anomeric (n_πO-σ_P-OAr^*) effect in the stabilization of axial series of compounds and the participation of endo-anomeric (n_πO-σ_PS^*) effect in the stabilization of the equatorial thiophosphates in chair conformations.