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Biomedical subjects

A L Rheingold

Publications and source records attributed to A L Rheingold.

At least 55 records · Page 3Linked to original sources
PubMed2000

Sterically encumbered systems for two low-coordinate phosphorus centers.

Tetraarylphenyls of the form 2,3,5,6-Ar4C6 (Ar = p-tert-butylphenyl) are investigated as sterically demanding ligands for the syntheses of compounds having two p-phenylene-bridged phosphorus centers. The precursor to such materials, 1,4-diiodo-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (1), is readily obtained via a one-pot procedure in 68% yield. Compound 1 is then used to provide the bis(dichlorophosphine) 1,4-bis(dichlorophosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (2) and the derived bis(phosphine) 1,4-bis(phosphino)-2,3,5,6-tetrakis(p-tert-butylphenyl)benzene (3) in yields of 56 and 94% respectively. These materials provide access to novel materials containing two low-coordinate phosphorus centers bridged by a sterically encumbered phenylene unit. Compound 2 reacts with benzaldehyde and 2,6-dichlorobenzaldehyde in the presence of excess trimethylphosphine and zinc to produce the new pale yellow crystalline bis(phosphaalkenes) (E,E)-PhC(H)=PAr4C6P=C(H)Ph (4a; 42%) and (E,E)-Ar'C(H)=PAr4C6P=C(H)Ar' (4b; 46%; Ar' = 2,6-dichlorophenyl). The crystal structure of 4a shows a P=C bond length of 1.676(5) A. Compound 2 is also used to provide the unusual red-orange bis(diphosphene) DmpP=PAr4C6P=PDmp (5; 55%; Dmp = 2,6-Mes2C6H3). Compound 5 is structurally characterized, and a P=P bond length of 2.008(2) A is ascertained.

Journal Article↗
PubMed2000

Formation of a tetranuclear copper (II) cluster assembled by metal-assisted hydrolysis and desulfurization of bis(3,5-dimethylpyrazolyl)methylphosphine sulfide, MeP(S)(3,5-Me2Pz)2.

The reaction of methylphosphine sulfide, MeP(S)(3,5-Me2Pz)2, 1, with anhydrous CuCl2 affords a tetranuclear copper cluster[Cu2Cl2(3,5-Me2Pz)3(MePO3)]2, 2, and a dinuclear compound Cu2Cl4(3,5-Me2Pz)4, 3. This reaction involves a metal-assisted desulfurization along with concomitant hydrolysis of P-N bonds. The X-ray structures of 1-3 have been determined, and the crystal parameters for these are the following. 1: space group = C2/c, a = 15.2552(1) A, b = 8.7364(2) A, c = 21.4490(3) A, beta = 93.349(1) degrees, V = 2853.74(8) A3, and Z = 8. 2: space group = P2(1)/n, a = 12.5964(4) A, b = 15.7773(4) A, c = 13.9781(4) A, beta = 116.6280(10) degrees, V = 2483.32(12) A3, and Z = 2. 3: space group = P2(1)/c, a = 8.7137(8) A, b = 13.5493(14) A, c = 11.8847(12) A, beta = 106.179(2) degrees, V = 1347.6(2) A3, and Z = 2. The structure of 2 shows that it comprises two dinuclear copper cores bridged together by two tripodal methylphosphinate, MePO3, ligands. 2 is weakly antiferromagnetically coupled, as revealed by variable temperature magnetic susceptibility measurements.

Journal Article↗
PubMed2000

Synthesis and characterization of Rhodium(III) dichloro complexes with unsymmetrically bound salen-type ligands.

We have synthesized a series of novel octahedral Rh(III) salen-type complexes where the salen ligand is unsymmetrically bound to the Rh(III) dichloride center. This mode of bonding left one intact phenol group coordinating to the rhodium center and has never before been observed in salen-metal chemistry. These remarkably stable complexes possess unique coordination geometry and represent the first time that Rh(III) salen complexes have been successfully isolated from the direct combination of RhCl(3).3H2O and the salen ligand in the absence of a nucleophilic base. The (salen)Rh(III) dichloride complex can be converted to the analogous monochloride complex by reaction with metal carbonate salts.

Journal Article↗
PubMed2000

Synthesis, structure, and magnetic properties of Co2([N(CN)2]4bpym).H2O and M([N(CN)2]2bpym).H2O (M = Mn, Fe, Co; bpym = 2,2'-bipyrimidine).

Co2([N(CN)2]4bpym).H2O (1) and M([N(CN)2]2bpym).H2O [M = Mn (2a), Fe (2b), Co (2c); bpym = 2,2'-bipyrimidine] have been synthesized and characterized structurally and magnetically. All of the compounds crystallize in the orthorhombic space group Pnma. The unit cell parameters for 1 are a = 16.1684(5) A, b = 12.9860(3) A, c = 10.4207(3) A, and Z = 4. Compound 1 is a 2-D layered structure with water intercalated between sheets. The sheets are composed of ...M-[NCNCN]2-M-bpym-M-[NCNCN]2-M... chains, which are linked together by dicyanamides. 2a-c are isomorphic with the unit cell parameters a = 17.5112(4) A, b = 11.9955(4) A, c = 7.4684(2) A for 2a, a = 17.5814(7) A, b = 11.9453(5) A, c = 7.3292(3) A for 2b, a = 17.8642(2) A, b = 11.9216(2) A, c = 7.2860(2) A for 2c, and Z = 4 for all. They crystallize as chains containing metal centers coordinated to two bridging dicyanamides, one terminal dicyanamide, one terminal chelating bpym, and one water molecule. 2a-c are the first examples of compounds containing terminal and mu-bound dicyanamides in the same structure. The broad maximum in the magnetic susceptibility of 1 could not be fit to any known dimer models. However, the high-temperature data were fit to the Curie-Weiss expression with g = 2.86 and theta = -42 K. 2a-c could best be modeled as uniform 1-D chains with g = 2.04, theta = -0.76, and J/kB = -0.15 K for 2a, g = 2.34, theta = -7.6, and J/kB = -0.42 K for 2b, and g = 2.58, theta = -5.4, and J/kB = -1.42 K for 2c. Because of small exchange coupling throughout the extended networks, no long-range magnetic ordering was observed.

Journal Article↗
PubMed2000

Syntheses and structures of methyltris(pyrazolyl)silane complexes of the group 6 metals.

The methyltris(3,5-dimethylpyrazolyl)silane ligand, TpsMe2, was readily prepared by the metathesis reaction of methyltrichlorosilane with 3 equiv of lithium 3,5-dimethylpyrazolate. The octahedral tricarbonyl complexes (TpsMe2)M(CO)3 were synthesized either by ligand exchange with the labile nitrile adducts M(CO)3(NCR)3 (M = Cr, Mo, R = Me; M = W, R = Et) or thermally by direct substitution on the hexacarbonyls M(CO)6 (M = Cr, Mo). The three new complexes were characterized by a combination of analytical and spectroscopic techniques, including electrospray ionization mass spectrometry and single-crystal X-ray diffraction. They are all isostructural and display in the solid state the expected distorted octahedral geometries with facially coordinated tris(pyrazolyl)silane ligands. Crystallographic data were used to calculate the ligand cone angles (251-264 degrees) in (TpsMe2)M(CO)3 and also to estimate a value of 1.59 A for the covalent radius of octahedral W(0).

Journal Article↗
PubMed1999

A new cryptand: synthesis and complexation with paraquat.

[formula: see text] Inspired by folded, nonpseudorotaxane complexes of bis(m-phenylene)-32-crown-10 systems, we synthesized a new bicyclic crown ether containing two 1,3,5-phenylene units linked by three tetra(ethyleneoxy) units. The new cryptand forms a "pseudorotaxane-like" inclusion complex with N,N'-dimethyl-4,4'-bipyridinium bis(hexafluorophosphate) with association constant Ka = 6.1 x 10(4) M-1, 100-fold greater than that of an analogous simple crown ether.

Ethers, Cyclic↗
PubMed1999

Unique "cradled barbell" complex between a secondary diammonium ion and bis(m-phenylene)-32-crown-10.

[formula: see text] The complexation between N,N'-dibenzyl(m-xylylene)diammonium bis(hexafluorophosphate) (2) and bis(m-phenylene)-32-crown-10 (5) was shown to occur in solution by nuclear magnetic resonance with 1:1 stoichiometry and a Ka value of 189 +/- 19 M-1. A crystal structure of 2:5 revealed a unique 1:1 "exo" or "cradled barbell" complex, instead of the expected pseudorotaxane. This unexpected result illustrates that caution be used in interpreting the results from these types of complexes in the solution and "gas" phases on the basis of crystal structures.

Crown Ethers↗
PubMed1999

A functional model for O-O bond formation by the O2-evolving complex in photosystem II.

The formation of molecular oxygen from water in photosynthesis is catalyzed by photosystem II at an active site containing four manganese ions that are arranged in di-mu-oxo dimanganese units (where mu is a bridging mode). The complex [H2O(terpy)Mn(O)2Mn(terpy)OH2](NO3)3 (terpy is 2,2':6', 2"-terpyridine), which was synthesized and structurally characterized, contains a di-mu-oxo manganese dimer and catalyzes the conversion of sodium hypochlorite to molecular oxygen. Oxygen-18 isotope labeling showed that water is the source of the oxygen atoms in the molecular oxygen evolved, and so this system is a functional model for photosynthetic water oxidation.

Catalysis↗
PubMed1998

Bis(acetylacetonato-O,O')(eta 4-trans-2,4-hexadiene)ruthenium(II) 1/6-ethanol solvate.

The title compound, [Ru(C5H7O2)2(C6H10)].1/6C2H6O, has a pseudo-octahedrally coordinated RuII center, where two coordination sites are occupied by a diene ligand. The diene ligand exhibits a eta 4-trans-diene coordination mode. The compound crystallizes in space group R3, incorporating disordered ethanol in cylindrical voids.

Crystallography, X-Ray↗
PubMed1998

Specificity of diastereomers of [99mTc]TRODAT-1 as dopamine transporter imaging agents.

Recently, we reported the first human study of [99mTc]TRODAT-1, technetium, 2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3.2.1]oct-2- yl]methyl](2-mercaptoethyl)amino]ethyl]amino]-ethanethiolato(3-)-o xo- [1R-(exo-exo)]-, as an imaging agent of central nervous system (CNS) dopamine transporters. Due to the existence of several chiral centers on this molecule, upon the formation of [99mTc]TRODAT-1 complex (2) several diastereomers could be created. Two major diastereomers of [99mTc]TRODAT-1 (2), designated as peak A (2A) and peak B (2B), were separated by HPLC. Biodistribution of the purified diastereomers 2A,B was evaluated in rats. It appears that 2A displayed a higher lipophilicity than 2B (PC = 305 and 229, respectively), and a similar trend was observed for the initial brain uptake at 2 min postinjection (0.50% and 0.28% dose/organ for 2A,B, respectively). At 60 min post-iv-injection, the specific uptakes, as measured by [striatum - cerebellum]/cerebellum ([ST-CB]/CB) ratio, were 1.72 and 2.79 for 2A,B, respectively. The higher [ST-CB]/CB ratio observed for 2B was corroborated by the results of an in vitro binding assay. Higher binding affinity for dopamine transporters was observed for 3B (Ki = 13.87 and 8.42 nM for the analogous rhenium complexes 3A,B, respectively). The structure of the [99mTc]TRODAT-1 complexes was deduced using nonradioactive rhenium as a surrogate for radioactive technetium complex. Reacting free TRODAT-1 ligand with [Bu4N][ReOCl4] yielded two major complexes: Re-TRODAT-1A (3A) and Re-TRODAT-1B (3B) (corresponding with peaks A and B of [99mTc]TRODAT-1, respectively), whose structures were determined by X-ray analysis. The X-ray structures show that both complexes have a pseudo-square-pyramidal structure of [RevO]3+N2S2 core with oxygen occupying the apical position and the N-alkyl substitution in syn-configuration to the oxo-rhenium bond. In conclusion, TRODAT-1 formed at least two diastereomers after complexing with a metal(V)-oxo (M = 99mTc, Re) center core. The two isomers display different binding affinities toward dopamine transporters and distinct properties of localization in the striatum area of the brain where the transporters are located.

Animals↗
PubMed1995

Specific competitive inhibitor of secreted phospholipase A2 from berries of Schinus terebinthifolius.

Two structurally related triterpenoids 1 and 2 from pink peppercorn (berries of Schinus terebinthifolius) are identified and characterized as active site-directed specific competitive inhibitors of the three classes of secreted 14 kDa phospholipase A2. The inhibitors not only protect the active site histidine from alkylation but also inhibit the action of secreted phospholipase A2 from pig pancreas, human synovial fluid, and bee venom. Detailed X-ray crystallographic results on the structures of the inhibitors are provided. By physical methods and X-ray crystallography the triterpenoids were identified as masticadienoic acid and masticadienolic acid (schinol). Several other triterpenoids were ineffective as inhibitors of phospholipase A2; however certain ganoderic acid derivatives showed noticeable inhibition. Results show that the side chain of these acidic tetracyclic terpenoids can access the catalytic-site region of phospholipase A2, whereas the acyclic nucleus is at the interfacial recognition region. The selectivity of the assay protocol used here is demonstrated by the fact that the original screen of ethyl acetate extracts of 60 commercially available spices and herbs was carried out with phospholipase A2 from pig pancreas, and only one extract showed inhibitory action on the hydrolytic activity in the scooting mode. Under such assay conditions the enzyme remains tightly bound to the surface of the substrate vesicles. In this way, nonspecific effects of additives that promote desorption of the enzyme from the substrate vesicle surface, under conditions in which the binding of the enzyme to the vesicle is weak, are avoided. The assay protocol is useful for the kinetic characterization of the inhibitors of phospholipase A2, and it does not give false positive results with amphiphilic and hydrophobic compounds, as is the case with virtually all assay systems in use.

Animals↗
PubMed1993

Dicarboxylate diamide dimercaptide (N2S2) technetium-99m complexes: synthesis and biological evaluation as potential renal radiopharmaceuticals.

Novel diamide dimercaptide (N2S2) ligands 4, 5, and 8 have been synthesized and evaluated as potential renal radiopharmaceuticals. The target compounds were prepared in modest overall yields of 22%, 19%, and 20%, respectively, using readily available starting materials. Following in situ deprotection, 99mTc complexes of high radiochemical purity were obtained in excellent yield and were found to be stable for up to 6 h. The 99Tc complex of ligand 8 was isolated as the AsPh4 salt. The X-ray crystallographic data for [99TcO(8)]AsPh4 (space group P2(1)/n: Z = 4, a = 9.342(3) A; b = 18.594(5) A; c = 18.417(7) A; beta, deg = 90.61(3); V, A3 = 3199.1(20)) show that the Tc is bound to both thiolate sulfur atoms and to two deprotonated amide nitrogen atoms. The coordination geometry about the Tc is square-pyramidal with an -yl oxygen atom in the apical position. The Tc-N bond distances (2.002(12) and 1.984(12) A), the Tc-S bond distances (2.300(5) and 2.286(5) A), and the Tc-O bond distance (1.667(11) A) are in good agreement with bond lengths reported for similar complexes. The carboxylate groups are not bonded to the Tc atom in the solid state, nor in CDCl3 solution, as evidenced by X-ray crystal data and solution NMR data, respectively. In the solid state, [99TcO(8)]AsPh4 is monoanionic, therefore, at physiological pH, [99mTcO(8)] is presumably trianionic. Biodistribution studies performed in rats with the 99mTc complexes revealed slow blood clearance and high muscle uptake for these agents. Modest hepatobiliary excretion was observed, and low quantities of the complexes were found in the heart, lungs, and spleen after 1 h. The urinary excretion of the 99mTc complexes of ligands 4, 5, and 8 was found to be slow when compared to the excretion of [131I]OIH in rats (22%, 22%, and 32% vs 85-86%, respectively). Protein binding of 99mTc complexes of ligands 4, 5, and 8 in both rat and monkey plasma was found to be similar to MAG3. While the synthetic schemes reported here supply facile routes to novel N2S2 ligands, biodistribution studies of the 99mTc complexes performed on rats revealed slow renal excretion rates, accompanied by slow blood clearance and high uptake in muscle tissue. Preliminary planar imaging studies in monkeys also revealed slow renal excretion for these agents. The 99mTc complexes evaluated here are poor candidates as renal radiopharmaceuticals.

Animals↗
PubMed1989

X-ray structural and n.m.r.-spectral studies of methyl alpha-L-evalopyranoside: reassignment of anomeric configuration for the methanolysis product of methyl 6-deoxy-3-C-methyl-alpha-L-mannofuranoside.

The methanolysis product of methyl 6-deoxy-3-C-methyl-alpha-L-mannofuranoside has been reassigned as methyl 6-deoxy-3-C-methyl-alpha-L-mannopyranoside by X-ray crystallographic and n.m.r.-spectral analyses. The crystals of methyl alpha-L-evalopyranoside are monoclinic, space group C2, with cell dimensions: a = 12.913(2), b = 8.052(1), c = 9.766(2) A, B = 105.13(2) degrees. The pyranoside ring exists in the 1C4 conformation, with the methoxyl and 3-C-methyl groups axial. Nuclear Overhauser effects were measured for selected proton resonances in the 1H-n.m.r. spectrum. Irradiation of the 3-C-methyl and 5-C-methyl group proton signals resulted in enhancements for H-2, H-4, H-5, and the methoxyl group hydrogen atoms, but not for H-1.

Carbohydrate Conformation↗