Stereodynamics and edge-to-face CH-pi aromatic interactions in o-phenethyl-substituted biaryls

As part of a search for systems that exhibit intramolecular aromatic edge-to-face interactions, a series of four biaryl compounds containing a phenethyl side chain were prepared. These compounds exist as a slowly interconverting mixture of two atropisomers due to steric hindrance to rotation about the biaryl bond. The more thermodynamically stable isomer exhibits an abnormal shielding of an ortho-proton in solution indicative of an edge-to-face CH-pi interaction with the terminal phenyl ring on the side chain. This tilted-T type of geometry was observed in the X-ray crystal structure of one of the compounds. The edge-to-face conformation in solution is estimated by variable temperature NMR studies to be energetically favored by ca. 1.6 kcal mol(-1) but entropically disfavored by ca. 5.0 cal K(-1) mol(-1).     

Separation characteristics of phenyl-containing stationary phases for gas chromatography based on silarylene-siloxane copolymer chemistries

The solvation parameter model is used to characterize the retention properties of five open-tubular column stationary phases (ZB-5 ms, DB-5 ms, DB-XLB, DB-17 ms, and DB-35 ms) based on silarylene-siloxane copolymer chemistries at five equally spaced temperatures over the range 60-140 degrees C. System constant differences and regression models for varied compounds are used to establish the selectivity equivalence of the silarylene-siloxane copolymer stationary phases and to compare their separation characteristics with poly(dimethyldiphenylsiloxane) stationary phases containing a nominally similar concentration of phenyl groups. These studies demonstrate that ZB-5 ms and DB-5 ms are selectivity equivalent. DB-XLB is significantly more dipolar and polarizable than DB-5 ms. In general terms, the silarylenesiloxane copolymer stationary phases are slightly less cohesive and more dipolar and polarizable with similar hydrogen-bond basicity to the poly(dimethyldiphenylsiloxane) stationary phases they were designed to replace. None of the silarylenesiloxane copolymer or poly(dimethyldiphenylsiloxane) stationary phases are hydrogen-bond acidic. Selectivity differences between the two types of stationary phase are temperature dependent and tend to be smaller at higher temperatures within the temperature range studied. Consequently, selectivity differences cannot be globalized without reference to the temperature for the comparison.     

The synthesis and single-crystal X-ray structures of palladium(II) and platinum(II) complexes of the difluorovinyl and 1-chloro-2-fluorovinyl-substituted phosphines, PPh2(Z-CFCFH) and PPh2(E-CClCFH)

The coordination chemistry of the fluorovinyl substituted phosphines PPh₂(Z-CFCFH) and PPh₂(E-CClCFH) with K₂MX₄ (M = Pd, Pt; X = Cl, Br, and I) salts has been investigated resulting in the first reported palladium(II) and platinum(II) complexes of phosphines containing partially fluorinated vinyl groups. The complexes have been characterised by a combination of multinuclear [¹H, ¹³C{¹H}, ¹⁹F, ³¹P{¹H}] NMR spectroscopy, and IR/Raman spectroscopy. The single-crystal X-ray structures of trans-[PdX₂{PPh₂(CFCFH)}₂], X = Cl (1), Br (2), I (3), trans-[PdCl₂{PPh₂(CClCFH)}₂] (4), cis-[PtX₂{PPh₂(CFCFH)}₂], X = Cl (5), Br (6), trans-[PtI₂{PPh₂(CFCFH)}₂] (7), and both cis- and trans-[PtCl₂{PPh₂(CClCFH)}₂] (8), have been determined. Results obtained from spectroscopic and crystallographic data suggest that replacement of a β-fluorine by hydrogen, whilst reducing the steric demand of the ligand, has little effect on the electronic character of the ligand. The presence of a proton in the vinyl group results in short proton-halide secondary interactions in the solid state (d(H?X) = 2.72(3) for 1, and 2.92(5) Å for 2) forming an infinite chain ribbon motif.     

On partitioning the orbitals of a transitive permutation group

Let be a permutation group on a set with a transitive normal subgroup . Then acts on the set of nontrivial -orbitals in the natural way, and here we are interested in the case where has a partition such that acts transitively on . The problem of characterising such tuples , called TODs, arises naturally in permutation group theory, and also occurs in number theory and combinatorics. The case where is a prime-power is important in algebraic number theory in the study of arithmetically exceptional rational polynomials. The case where exactly corresponds to self-complementary vertex-transitive graphs, while the general case corresponds to a type of isomorphic factorisation of complete graphs, called a homogeneous factorisation. Characterising homogeneous factorisations is an important problem in graph theory with applications to Ramsey theory. This paper develops a framework for the study of TODs, establishes some numerical relations between the parameters involved in TODs, gives some reduction results with respect to the -actions on and on , and gives some construction methods for TODs.

     

Isolation and characterisation of transition and main group metal complexes supported by hydrogen-bonded zwitterionic polyphenolic ligands

Reaction of Zr(O(i)Pr)4 or Sn[N(SiMe3)2]2 with the tris-phenol amine ligand H3L(Me/Me) results in the formation of zirconium or tin complexes containing the new C3-symmetric zwitterionic ammonium-trisphenolate ligand HL2-, while increasing the steric bulk of the ligand results in the isolation of a zirconium complex containing the known trianionic ligand L3-.     

On Locally Projective Graphs of Girth 5

Let be a graph and G be a 2-arc transitive automorphism group of . For a vertex x let G(x)^(x) denote the permutation group induced by the stabilizer G(x) of x in G on the set (x) of vertices adjacent to x in . Then is said to be a locally projective graph of type (n,q) if G(x)^(x) contains PSL_n(q) as a normal subgroup in its natural doubly transitive action. Suppose that is a locally projective graph of type (n,q), for some n 3, whose girth (that is, the length of a shortest cycle) is 5 and suppose that G(x) acts faithfully on (x). (The case of unfaithful action was completely settled earlier.) We show that under these conditions either n=4, q=2, has 506 vertices and , and contains the Wells graph on 32 vertices as a subgraph. In the latter case if, for a given n, at least one graph satisfying the conditions exists then there is a universal graph W(n) of which all other graphs for this n are quotients. The graph W(3) satisfies the conditions and has 2²⁰ vertices.     

Molecular beacon probes of photodamage in thymine and uracil oligonucleotides

Molecular beacons (MB) are becoming more common as sequence-selective detectors of nucleic acids. Although they can easily detect single-base mismatches, they have never been used to directly detect DNA or RNA damage. To measure the degree of ultraviolet (UV) light damage in oligonucleotides, we report a novel MB approach for general detection of photoproducts in UV-irradiated rU17 and dT17 oligonucleotides. With monochromatic UV light irradiation at ca 280 nm under anoxic conditions, the oligonucleotide absorption decays with a single-exponential time constant of 123+/-1 min for rU17 and with double-exponential time constants of 78+/-0.5 min (99%) and 180+/-5 min (0.05%) for dT17 oligonucleotides. Under the same conditions, the MB fluorescence decays more quickly, with single-exponential time constants of 19+/-2 and 26+/-3 min for rU17 and dT17, respectively. Similar kinetics were observed with broadband UV light irradiation of oligonucleotides. The differences in the UV damage kinetics of dT17 and rU17 and their detection by absorption and fluorescence techniques will be discussed in the context of differential instabilities introduced in the nucleic acid-MB duplex by the different photoproducts formed.     

Synthesis of aromatic bisabolene natural products via palladium-catalyzed cross-couplings of organozinc reagents

Aromatic bisabolene derivatives were prepared by two methods involving cross-coupling of organozinc reagents. The first synthesis of (+/-)-glandulone A (10), as well as syntheses of (+/-)-curcuhydroquinone (8) and (+/-)-curcuquinone (9), were accomplished via coupling of a secondary alkyl zinc reagent (1,5-dimethyl-4-hexenylzinc halide, 18) to protected bromohydroquinones using Pd(dppf)Cl(2) as catalyst. Coupling of arylzinc halides with alkenyl triflate 16 using Pd(PPh(3))(4) catalyst provided a number of bisabolene derivatives and led to syntheses of dehydro-alpha-curcumene (2), (+/-)-curcuphenol (3), and (+/-)-elvirol (13). A high-yield synthesis of the (+/-)-heliannuol D precursor 29 is also reported using this method.     

Crystal and molecular structures of benzophenone phenylhydrazone derivatives with anticancer activity

The X-ray crystal structures of 4,4-dihydroxybenzophenone-2,4-dinitrophenylhydrazone (I) and 2,2-dihydroxybenzophenone-2,4-dinitrophenylhydrazone (II) have been determined in order to study the structural characteristics of these molecules that may contribute to their antiestrogenic and cytotoxic properties. These structures have been compared to other hydrazone derivatives as well as tamoxifen, an antiestrogen drug presently used clinically for the treatment of breast cancer.Crystal data: (I) C₁₉H₁₄N₄O₆ · C₄H₁₀O; MW=468.0; monoclinic,P2₁;a=8.601(2), b=15.502(8), c=16.851(4) Å,=98.58(2)°;Z=8; finalR=0.036 for 1904 observed reflections. (II C₁₉H₁₄N₄O₆ · H₂O; MW=410.0; monoclinic,P2₁/c;a=7.603(2),b=19.552(4),c=12.493(3) Å,=92.11(1)°;Z=4; finalR=0.045 for 1171 observed reflections.     

A standard protocol for single nucleotide primer extension in the human genome using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Analysis of single nucleotide polymorphisms (SNPs) has become an increasingly important area of research, with numerous applications in medical genetics, population genetics, forensic science, and agricultural biotechnology. Large-scale SNP analyses require the development of methodologies that are economical, flexible, accurate and capable of automation. Primer extension in conjunction with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is currently emerging as a potential method for high-throughput SNP genotyping. We have evaluated a number of published primer extension methods and refined a simple and robust protocol to analyze human autosomal disease-causing mutations and population genetic markers on the Y-chromosome. Twelve different variant sites were examined, and homozygotes, heterozygotes and hemizygotes were accurately typed. A 100% concordance was observed between SNP genotypes obtained using the MALDI-TOFMS technique and alternative genotyping methods, such as restriction fragment length polymorphism (RFLP) assays and denaturing high-performance liquid chromatography (DHPLC). Since multiple polymorphisms can be detected in single reactions, the method provides a cost-effective approach for SNP analysis. The protocol is also extremely flexible (able to accommodate new markers) and can be adapted to a number of platforms without the use of commercial kits.