Synthesis of Novel Nucleic Acid Mimics via the Stereoselective Intermolecular Radical Coupling of 3'-Iodo Nucleosides and Formaldoximes(1)

A highly convergent free radical coupling of alkyl iodides and oximes, mediated by bis(trimethylstannyl) benzopinacolate (8), has been utilized to prepare a series of dimeric nucleosides as mimics of natural nucleic acids. The systematic optimization of the reaction conditions allowed for the single-step conversion of the appropriate iodides and oximes into the 2'-deoxy dimers 9 in moderate to excellent yields. For example, the reaction of 3'-deoxy-3'-iodo-5'-(triphenylmethyl)thymidine (6a) with 3'-O-(tert-butyldiphenylsilyl)-5'-O-(methyleneimino)thymidine (7a) in the presence of 8 in degassed benzene gave an 81% yield of 3'-de(oxyphosphinico)-3'-(methyleneimino)-5'-O-(triphenylmethyl)thymidylyl-(3'-->5')-3'-O-(tert-butyldiphenylsilyl)thymidine (9a). Similarly prepared were dimers containing both pyrimidine (thymine, 5-methylcytosine) and purine (adenine, guanine) bases. The reaction was highly stereoselective, giving only a single dimeric species having the ribo-configuration of the newly introduced C-3'-branched methylene moiety. Also prepared were dimers 16, incorporating 2'-O-methyl ribonucleosides in both halves of the dimer. This required the synthesis of 3'-deoxy-3'-iodo-2'-O-methyl nucleosides 12 as well as 2'-O-methyl-5'-O-methyleneimino nucleosides 15. For example, 5'-O-(tert-butyldiphenylsilyl)-3'-deoxy-3'-iodo-2'-O-methyl-5-methyluridine (12e) was prepared in 80% yield by displacement of the corresponding triflate with Bu(4)NI. Also prepared were the suitably protected 3'-deoxy-3'-iodo adenosine and guanosine derivatives. Compounds 15 were prepared in high yield by a regioselective Mitsunobu reaction to give the corresponding 5'-O-phthalimido nucleosides 13, which were subsequently converted to the requisite oximes 15. In the 2'-O-methyl series, the pinacolate coupling reaction proceeded with efficiency equal to that observed for the 2'-deoxy series 9, but with slightly less stereoselectivity, giving predominantly the C-3'ribo products 16, contaminated with 5-25% of the epimeric material. Mixed base dimers containing both pyrimidine and purine bases at all possible positions, including purine-purine dimers were prepared. The hydroxylamine or methyleneimino (MI) backbone of several representative dimers so prepared was converted via methylation to give the corresponding methylenemethylimino (MMI)-linked compounds, which are novel phosphate surrogates for use in antisense oligonucleotides.     

Chemical modification of fluorinated self-assembled monolayer surfaces by low energy reactive ion bombardment

Reactive collisions of low energy (<100-eV) mass-selected ions are used to chemically modify fluorinated self-assembled monolayer surfaces comprised of alkanethiolate chains CF₃(CF₂)₁₁(CH₂)₂S— bound to Au. Typical experiments were done by using 1-nA/cm² beams and submonolayer doses of reactant ions. Characterization of the modified surface was achieved by in situ chemical sputtering (60-eV Xe^+·) and by independent high mass resolution time-of-flight-secondary ionization mass spectrometry (TOF-SIMS) (15–25-keV, Ga⁺) experiments. Treatment with Si³⁵C1 ₄ ^+· produced a surface from which Xe⁺ sputtering liberated CF₂ ³⁵C1⁺ ions, which suggested Cl-for-F halogen exchange at the surface. Isotopic labeling studies that used Si³⁵Cl₂ ³⁷Cl ₂ ^+· ; and experiments with bromine-containing and iodine-containing projectiles, confirmed this reaction. High mass resolution TOF-SIMS spectra, as well as high spatial resolution images, provided further evidence as to the existence of halogen-exchanged species at the bombarded surface. Analogous Cl-for-F halogen substitution was observed in a model gas-phase reaction. The ion-surface reaction is suggested to proceed through an intermediate fluoronium ion in which the projectile is bonded to the target molecule. The most significant conclusion of the study is that selective chemical modification of monolayer surfaces can be achieved by using reactive ion beams, which lead to new covalent bonds at the surface and in the scattered ions.     

The beta-fluorine effect. Electronic versus steric effects in radical deoxygenations of fluorine-containing pentofuranose nucleosides

Stereoselective pyramidalization of free radicals by a vicinal fluorine substituent, the beta-fluorine effect, was invoked to rationalize a 77:23 anti/syn ratio of 2-deuterio-1-fluorocyclopentanes obtained by radical reduction of trans-2-fluoro-1-bromocyclopentane with tributyltin deuteride (Dolbier, W. R., Jr.; Bartberger, M. D. J. Org. Chem. 1995, 60, 4984-4985). We have evaluated analogous reductions of the four possible stereoisomers of some adenine 2'(3')-fluoro-3'(2')-O-phenoxythiocarbonyl nucleoside derivatives. In all cases, the steric effect of adenine on the beta face directs deuterium transfer from the stannane to C2'(C3') on the alpha face of the furanose ring. However, the beta-fluorine effect enhances ratios of deuterium transfer anti to the vicinal fluorine substituent.     

Tetronic acids and derivatives. Part XI . Structure of coupling products of tetronic acids with benzenediazonium sulfate

¹³C Nmr, ¹H nmr and ir parameters of coupling products of tetronic acids (4-hydroxy-5H-furan-2-ones) with benzene diazonium sulfate are only consistent with a phenylhydrazone formulation with an almost equal ratio of the syn and anti configurations. These results contrast with those of coupling products of acyclic 3-ketoesters, existing mainly as anti tautomers, and more sharply with the 3-parabromophenylhydrazone of L-dehydro ascorbic acid, which is exclusively syn.     

Kinetics evidence for a complex between peroxynitrous acid and titanium(IV)

The reaction between TiO(2+) and ONOOH in 0.9 M H(2)SO(4) provides evidence for direct formation, previously unobserved, of a HOONO-metal complex. The reaction proceeds via formation of an end-on complex (k = 3.0 x 10(2) M(-1) s(-1)) that rearranges to form a side-on complex (k approximately equal to 20 s(-1)). With ONOOH in excess, this rearrangement proceeds more slowly (k approximately equal to 0.1 s(-1)), probably because multiple hydrogen oxoperoxonitrate molecules form end-on complexes with oxotitanium(IV) and hinder rearrangement to the side-on complex. The absorption spectrum of the final product is that of TiO(2)(2+). Presumably, during the rearrangement or later, NO+ is lost.     

Selective alterations of the antibody response to HIV-1

HIV infection leads to progressive alterations of humoral immune functions, including B-cell hyperplasia, hypergammaglobulinemia, elevated autoantibody titers, a poor response to neoantigens and mitogens, polyclonal B-cell activation, monoclonal gammopathies, and a significant deterioration of the antigen-specific humoral response. There is also an important isotypic imbalance of the antibody (Ab) response in the systemic compartment and a profound modification of mucosal immune functions. These abnormalities may contribute to disease progression and development of opportunistic infections, despite the presence of serum-neutralizing anti-HIV Abs. Equally important are the abnormal selection mechanisms of the Ab repertoire that seem to be responsible for B-cell clonal deletions. The V_H3 gene family, which encodes for approx 50% of immunoglobulins expressed by peripheral B-cells from normal adults, is underrepresented in human monoclonal antibodies to HIV-1 and in the peripheral B-cells of AIDS patients. These abnormalities, together with features of germinal center alteration, could be responsible for the clonal elimination of a subset of B-cells, and could contribute to HIV pathogenesis.     

Structure, bonding, and reactivity of Ti and Zr amidate complexes: DFT and X-ray crystallographic studies

Easily prepared and highly modular organic amide proligands have been used to synthesize a series of new bis(amidate)-bis(amido) Ti and Zr complexes via protonolysis. These complexes have been structurally characterized by NMR spectroscopy and X-ray crystallography. The solid-state molecular structures of these complexes indicate that the amidate ligands bind to the metal centers in an exclusively bidentate fashion, resulting in discrete monomeric species. Geometric isomerism in these species is highly dependent upon the steric characteristics of the proligands utilized in the synthesis. In solution, these complexes are observed to isomerize on the NMR time scale, with one isomer being predominant. Bonding in the bis(amidate)-bis(amido) complexes was investigated by DFT calculations. The geometric isomers predicted by theory matched the experimentally observed results, within experimental error. The orbitals associated with amidate-metal bonding are energetically well below the frontier orbitals. The HOMO in these complexes is a pi orbital associated with amido ligand-to-metal bonding character, while the LUMO in all cases is a vacant d orbital on the metal center.     

Detection of metal ions using ion-channel sensor based on self-assembled monolayer of thioctic acid

Gold electrodes were chemically modified with thioctic acid monolayer designed to mimic biological ion-channel membranes. The technique was then used in the determination of alkali, alkaline earth, thallium(I), and lanthanum metal cations as analytes. Cyclic voltammograms (CV) of [Fe(CN)6]³⁻ an electroactive marker, were measured in the presence of the various types of analyte cations. In the absence of the analyte cation, electrostatic repulsion between the marker anions and the carboxylate groups of the receptor monolayer hindered the approach of the marker anion to the electrode surface and hence hindered its reduction. The modified electrodes responded well to the metal cations except the alkali metal cations. The sensors could detect the trivalent cation La³⁺ at concentrations as low as 10⁻⁸ M. The response of the sensor to the metal cations increase in the order alkali metal3+ can be discriminated in the ratio 1:100. This makes it possible to determine the trivalent ion in a sample matrix containing monovalent and divalent cations. Thallium(I) ion showed marked deviation in its response as compared to monovalent ions of the alkali metals. The ion-channel sensor based on self-assembled monolayer of thioctic acid therefore offers a potential alternative technique for the selective determination of metal ions.     

Extraction of proteins from plant tissues for two-dimensional electrophoresis analysis

To increase the number of proteins detectable by two-dimensional electrophoresis (2-DE) in plants, we present a new procedure for extracting total proteins from plant tissue. This method avoids any loss of proteins in the course of sample preparation and results in two different fractions, one comprising mainly the cytoplasmatic proteins, the other one containing predominantly structure bond proteins. 2-DE patterns obtained from these two fractions show that the total number of different protein spots detected exceeds the degree of resolution commonly reported for plant proteins threefold.     

Nucleic acid related compounds. 31. Smooth and efficient palladium-copper catalyzed coupling of terminal alkynes with 5-iodouracil nucleosides

Coupling of terminal alkynes with protected 5-iodouracil nucleosides in the presence of dichlorobis(triphenylphosphine)palladium and copper(I) iodide in triethylamine gives the corresponding 5-(alkyn-1-yl)uracil nucleosides in 72–92% yields.