Extraction of cesium-137 from nitric acid medium in the presence of macrocyclic polyethers

Extraction behavior of¹³⁷Cs was studied from nitric acid medium using dibenzo 18 crown 6 (DB18C6), 4,4'(5')di-acetylbenzo 18 crown 6 (DAB18C6), 4, 4'(5')di-hexanoylbenzo 18 crown 6 (DHB18C6), 4,4'(5')di-nonanoylbenzo 18 crown 6 (DNB18C6) and 4,4'(5')di-t-butylbenzo 18 crown 6 (DTBB18C6) in nitrobenzene medium. The stoichiometry of the species extracted with dibenzo 18 crown 6 (L) conformed to ML⁺. NO₃ ⁻ TheD _Cs values were found not to be affected by the presence of aluminium nitrate in the aqueous phase. The separation behavior of fission products obtained from an irradiated natural uranium target was also studied. Presence of 0.004M phosphotungstic acid found to enhance theD _Cs values at lower acidities.     

Dynamic stereochemistry of erigeroside by measurement of 1H-1H and 13C-1H coupling constants

Erigeroside was extracted from Satureja khuzistanica Jamzad (Marzeh Khuzistani in Persian, family of lamiaceae), and (1)H, (13)C, (13)C{(1)H}, (1)H-(1)H COSY, HMQC and J-HMBC were obtained to identify this compound and determine a complete set of J-coupling constants ((1)J(C-H), (2)J(C-H), (3)J(C-H) and (3)J(H-H)) values within the exocyclic hydroxymethyl group (CH(2)OH) and anomeric center. In parallel, density functional theory (DFT) using B3LYP functional and split-valance 6-311++G** basis set has been used to optimized the structures and conformers of erigeroside. In all calculations solvent effects were considered using a polarized continuum (overlapping spheres) model (PCM). The dependencies of (1)J, (2)J and (3)J involving (1)H and (13)C on the C(5')-C(6') (omega), C(6')-O(6') (theta) and C(1')-O(1') (phi) torsion angles in erigeroside were computed using DFT method. Complete hyper surfaces for (1)J(C1',H1'), (2)J(C5',H6'R), (2)J(C5',H6'S), (2)J(C6',H5'), (3)J(C4',H6'R), (3)J(C4',H6'S) and (2)J(H6'R-H5'S) as well as (3)J(H5',H6'R) were obtained and used to derive Karplus equations to correlate these couplings to omega, theta and phi. These calculated J-couplings are in agreement with experimental values. These results confirm the reliability of DFT calculated coupling constants in aqueous solution.     

Synthesis and deprotonation of 2-(halophenyl)pyridines and 1-(halophenyl)isoquinolines

The ability of the 2-pyridyl and 1-isoquinolyl groups to direct metalation was studied in the benzene series. For this purpose, 2-(halophenyl)pyridines and 1-(halophenyl)isoquinolines were prepared. Interestingly, nucleophilic addition reactions on the azine ring were not observed under kinetic control using butyllithium, and the substrates were cleanly deprotonated on the benzene ring: the azine ring acidifies the adjacent hydrogen H2' (N-H2' interaction through space and/or inductive electron-withdrawing effect) and probably favors the approach of butyllithium (chelation). Under thermodynamic conditions using lithium dialkylamides, the presence of the azine group makes the lithio derivative at C2' more stable (chelation and/or inductive electron-withdrawing effect). This was evidenced in two ways: (1) syntheses of 2-halophenyllithiums (F, Cl, Br) substituted at C6 by a 2-pyridyl or 1-isoquinolyl group without elimination of lithium halide and (2) iodine migration from C2' to C4' when treating 2-(3-halo-2-iodophenyl)pyridines or 1-(3-fluoro-2-iodophenyl)isoquinoline with LTMP. Comparisons between the 2-pyridyl and fluoro units showed the latter was the stronger directing group for deprotonation.     

Synthesis and deprotonation of 2-(pyridyl)phenols and 2-(pyridyl)anilines

2-(2- and 3-Pyridyl)anilines (1, 2), 2,2-dimethyl-N-[2-(2- and 3-pyridyl)phenyl]propanamides (3, 4), and 2-, 3- and 4-(2-methoxyphenyl)pyridines (7-9) are readily synthesized using cross-coupling reactions. Whereas the amines 1, 2 undergo side reactions, the corresponding amides 3, 4 are deprotonated with lithium 2,2,6,6-tetramethylpiperidide (LTMP): the compound 3 at C6' under in situ quenching, and the compound 4 at C4'. When the ether 7 is subjected to the same reagent, lithiation occurs at C6'.     

Stereoselective synthesis of the eastern quinolizidine portion of himeradine A

The synthesis of the C(15)-C(17)/N(1')-C(11') quinolizidine portion of himeradine A is disclosed. An intramolecular, heteroatom Michael addition was employed to establish the C(6') stereogenic center with high diastereoselectivity. The quinolizidine ring was constructed using microwave-induced cyclization at the N(1')-C(2') position. The C(17) stereogenic center was introduced through a diastereoselective Overman rearrangement.     

Spin transition at the mesophase transition temperature in a cobalt(II) compound with branched alkyl chains

A cobalt(II) compound, [Co(C5C12C10-terpy)2](BF4)2 [C5C12C10-terpy = 4',5' '-decyl-1' '-(heptadecyloxy)-2,2':6',2' '-terpyridine] with branched alkyl chains, based on a terpyridine frame, was synthesized. The cobalt(II) compound exhibits a spin transition between low-spin and high-spin with a thermal hysteresis loop (T(1/2) upward arrow = 288 K and T(1/2) downward arrow = 284 K) at the liquid-crystal transition temperature. It is the first example in the cobalt(II) compounds in which the spin transition occurs at the crystal-liquid crystal transition temperature.     

Practical synthesis of an open geodesic polyarene with a fullerene-type 6:6-double bond at the center: diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene

Diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene (1), the smallest possible alkene-centered C60 substructure with a curved pi-system, is obtained in 25-35% yield by flash vacuum pyrolysis of the twisted 1,1'-dibromobifluorenylidene (2) on a 100 mg scale at 1050 degrees C. At 1200 degrees C, the bowl-shaped hydrocarbon 1 rearranges to the planar isomer diindeno[5,6,7,1-defg;5',6',7',1'-lmnop]chrysene (14) by a double 5/6 ring-expansion/ring-contraction. X-ray crystallography establishes that the central carbon atoms of 1 are nearly 80% as pyramidalized as the carbon atoms of C60 (POAV angles = 9.0 degrees and 11.6 degrees for 1 and C60, respectively). A four-step synthesis has been developed to prepare the pyrolysis precursor (2) as a mixture of (E)- and (Z)-isomers in 39% overall yield from commercially available 9-fluorenone-1-carboxylic acid (10).     

Synthesis of polyphenylene dendrimers related to "cubic graphite"

Four large, 6-fold symmetric, polyphenylene hydrocarbons have been prepared by short syntheses that chiefly employed alkyne trimerization, palladium-catalyzed coupling, and Diels-Alder reactions. The two largest of these molecules, hexakis[4'-(pentaphenylphenyl)biphenyl-4-yl]benzene (4, C(294)H(198)) and hexakis[4'-(2,3,4,5-tetraphenylphenyl)biphenyl-4-yl]benzene (5, C(258)H(174)) are substructures of "phenylogous cubic graphite", and the other two, hexakis(2',3',4',5',6'-pentaphenylbiphenyl-4-yl)benzene (26, C(258)H(174)) and hexakis(2',3',4',5'-tetraphenylbiphenyl-4-yl)benzene (25, C(222)H(150)) are strongly pitched, six-bladed molecular propellers. The X-ray crystal structure of compound 26 has also been determined; dendrimer 26 is at present the largest crystallographically characterized hydrocarbon.     

The mechanism of the reaction catalyzed by ADP-beta-L-glycero-D-manno-heptose 6-epimerase

ADP-l-glycero-d-manno-heptose 6-epimerase (AGME, RfaD) is a bacterial enzyme that is involved in lipopolysaccharide biosynthesis and interconverts ADP-beta-l-glycero-d-manno-heptose (ADP-l,d-Hep) with ADP-beta-d-glycero-d-manno-heptose (ADP-d,d-Hep). AGME is known to require a tightly bound NADP+ cofactor for activity and presumably employs a mechanism involving transient oxidation of the substrate. Four mechanistic possibilities are considered that involve transient oxidation at either C-7' ', C-6' ', or C-4' ' of the heptose nucleotide. In this contribution, the use of solvent isotope incorporation studies and alternate substrates provides strong evidence for a mechanism involving nonstereospecific oxidation/reduction directly at C-6' '. It was found that the epimerization proceeds without any detectable incorporation of solvent-derived deuterium or 18O-isotope into the product. This argues against mechanisms involving either proton transfers at carbon or dehydration/rehydration events. In addition, the deoxygenated analogues, 7' '-deoxy-ADP-l,d-Hep and 4' '-deoxy-ADP-l,d-Hep, were both found to serve as substrates for the enzyme, indicating that oxidation at either C-7' ' or C-4' ' is not required for catalysis.     

Reversible carboxamide-mediated internal activation at C(6) of 2-chloro-4-anilino-1H-pyrrolo[2,3-d]pyrimidines

A synthetic route to bisanilino-1H-pyrrolo[2,3-d]pyrimidines has been discovered, wherein the C(6)-chloride reactivity is necessarily enhanced via reversible acid-catalyzed internal activation of the pyrimidine ring by a C(1')-carboxamide moiety. Subsequent selective nucleophilic displacements at C(6) and C(1') constitute a one-pot tandem protocol for the rapid assembly of bisanilino-1H-pyrrolo[2,3-d]pyrimidines.     

Reversible molecular switching of ruthenium bis(bipyridyl) groups bonded to oligothiophenes: effect on electrochemical and spectroscopic properties

We report the preparation of complexes in which ruthenium(II) bis(bipyridyl) groups are coordinated to oligothiophenes via a diphenylphosphine linker and a thienyl sulfur (P,S bonding) to give [Ru(bpy)(2)PT(3)-P,S](PF(6))(2) (bpy = 2,2'-bipyridyl, PT(3) = 3'-(diphenylphosphino)-2,2':5',2' '-terthiophene), [Ru(bpy)(2)PMeT(3)-P,S](PF(6))(2) (PMeT(3) = 3'-(diphenylphosphino)-5-methyl-2,2':5',2' '-terthiophene), [Ru(bpy)(2)PMe(2)T(3)-P,S](PF(6))(2) (PMe(2)T(3) = 5,5' '-dimethyl-3'-(diphenylphosphino)-2,2':5',2' '-terthiophene), and [Ru(bpy)(2)PDo(2)T(5)-P,S](PF(6))(2) (PDo(2)T(5) = 3,3' ' '-didodecyl-3' '-diphenylphosphino-2,2':5',2' ':5' ',2' ':5' ',2' ' '-pentathiophene). These complexes react with base, resulting in the complexes [Ru(bpy)(2)PT(3)-P,C]PF(6), [Ru(bpy)(2)PMeT(3)-P,C]PF(6), [Ru(bpy)(2)PMe(2)T(3)-P,C]PF(6), and [Ru(bpy)(2)PDo(2)T(5)-P,C]PF(6), where the thienyl carbon is bonded to ruthenium (P,C bonding). The P,C complexes revert back to the P,S bonding mode by reaction with acid; therefore, metal-thienyl bonding is reversibly switchable. The effect of interaction of the metal groups in the different bonding modes with the thienyl backbone is reflected by changes in alignment of the thienyl rings in the solid-state structures of the complexes, the redox potentials, and the pi --> pi transitions in solution. Methyl substituents attached to the terthiophene groups allow observation of the effect of these substituents on the conformational and electronic properties and aid in assignments of the electrochemical data. The PT(n)() ligands bound in P,S and P,C bonding modes also alter the electrochemical and spectroscopic properties of the ruthenium bis(bipyridyl) group. Both bonding modes result in quenching of the oligothiophene luminescence. Weak, short-lived Ru --> bipyridyl MLCT-based luminescence is observed for [Ru(bpy)(2)PDo(2)T(5)-P,S](PF(6))(2), [Ru(bpy)(2)PT(3)-P,C]PF(6), [Ru(bpy)(2)PMeT(3)-P,C]PF(6), and [Ru(bpy)(2)PMe(2)T(3)-P,C]PF(6), and no emission is observed for the alternate bonding mode of each complex.     

Total synthesis of 2',3',4',5',5' '-(2)h(5)-ribonucleosides: the key building blocks for NMR structure elucidation of large RNA

The diastereospecific chemical syntheses of uridine-2',3',4',5',5' '-(2)H(5) (21a), adenosine-2',3',4',5',5' '-(2)H(5) (21b), cytidine-2',3',4',5',5' '-(2)H(5)(2)H(5) (21c), and guanosine-2',3',4',5',5' '-(2)H(5) (21d) (>97 atom % (2)H at C2', C3', C4', and C5'/C5' ') have been achieved for their use in the solution NMR structure determination of oligo-RNA by the Uppsala "NMR-window" concept (refs 4a-c, 5a, 6), in which a small (1)H segment is NMR-visible, while the rest is made NMR-invisible by incorporation of the deuterated blocks 21a-d. The deuterated ribonucleosides 21a-d have been prepared by the condensation of appropriately protected aglycone with 1-O-acetyl-2,3,5-tri-O-(4-toluoyl)-alpha/beta-D-ribofuranose-2,3,4,5,5'-(2)H(5) (19), which has been obtained via diastereospecific deuterium incorporation at the C2 center of appropriate D-ribose-(2)H(4) derivatives either through an oxidation-reduction-inversion sequence or a one-step deuterium-proton exchange in high overall yield (44% and 24%, respectively).     

Overcrowding motifs in large PAHs. An ab initio study.

Planar and overcrowded LPAHs C(34)H(18) anthra[9,1,2-cde]benzo[rst]penaphene (1), benzo[rst]phenanthro[10,1,2-cde]pentaphene (2), tetrabenzo[a,cd,j,lm]perylene (3), tetrabenzo[a,cd,lm,o]perylene (4), and LPAHs C(38)H(18) anthra[2,1,9,8-klmno]naphtho[3,2,1,8,7-vwxyz]hexaphene (5), dianthra[2,1,9,8-stuva;2',1',9',8'-hijkl]pentacene (6), dibenzo[jk,uv]dinaphtho[2,1,8,7-defg;2',1',8',7'-opqr]perylene (7), diphenanthro[5,4,3-abcd;3',4',5'-lmno]perylene (8), potential products of peri-peri reductive couplings of benzanthrone and of naphthanthrone, respectively, were subjected to an ab initio study with emphasis on overcrowding motifs. The HF and DFT B3LYP methods were employed to calculate energies and geometries of the minima conformations of these LPAHs. The most stable LPAHs in these series were found to be planar C(2)(v)()-1 and C(2)(v)()-5, respectively. Among overcrowded LPAHs, twisted-folded C(2)-3 and C(2)-7 with two cove regions were found to be more stable than their respective isomers twisted-folded C(2)-4 and C(2)-8 with one fjord region each, in contrast to the semiempirical predictions. The energy differences between the most stable planar isomer and the overcrowded isomers were significantly smaller in the C(38)H(18) series, than in the C(34)H(18) series. Overcrowded twisted-folded C(2)-7 with two coves was found to be more stable than planar C(2)(h)()-6 by 2.0 kJ/mol (at B3LYP/6-311G), indicating enhanced role of aromatic stabilization and decreased destabilization due to overcrowding, with increasing the number of aromatic rings. Heats of formation of LPAHs 1-8 were derived from the ab initio total energies (at B3LYP/6-31G). A search of the conformational spaces of 3 and 4 revealed an anti-folded local minimum C(i)()-3 and a syn-folded transition state C(s)()-4, 23.7 and 120.3 kJ/mol higher in energy than the twisted-folded C(2)-3 and C(2)-4, respectively (at B3LYP/6-31G). The cove and fjord torsion angles in the C(38)H(18) series were found to be smaller than in the C(34)H(18) series. The nonbonding distances between carbon atoms at cove and fjord regions of the overcrowded LPAHs were found to be smaller than the sum of the van der Waals radii of two carbon atoms     

End-group-confined chain walking within a group 4 living polyolefin and well-defined cationic zirconium alkyl complexes for modeling this behavior

Living polymers derived from the polymerization of 1-butene using the cationic zirconium initiator, {Cp*ZrMe[N(Et)C(Me)-N(tBu)]}[B(C6F5)4] (Cp* = eta5-C5Me5) (1), have been shown to undergo end-group-confined chain walking that is competitive with direct beta-hydride elimination and chain release at -10 degrees C. The well-defined complexes, {Cp*Zr(iBu)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2) and {Cp*Zr(2-ethylbutyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (3), were prepared, and each was found to possess a strong beta-hydrogen agostic interaction that is absent in the living polymer. The isotopically single- and double-labeled derivatives, {Cp*Zr(2-d-2-methylpropyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2') and {Cp*Zr(1-13C-2-d-2-methylpropyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2' '), were also prepared and found to undergo isotopic label scrambling at 0 degrees C. For 2' ', the observation that after scrambling each deuterium label is located on a 13C-labeled carbon atom is consistent with the Busico mechanism for chain-end epimerization rather than the Resconi mechanism. Decomposition of 3 yielded olefinic products also consistent with chain walking prior to beta-hydride elimination and chain release. Finally, the unexpected decrease in stability of the living polymer relative to that of the model complexes reveals the importance of subtle differences in steric and electronic factors in controlling beta-hydride elimination and chain release.     

DFT analysis of NMR scalar interactions across the glycosidic bond in DNA

The relationship between the glycosidic torsion angle chi, the three-bond couplings (3)J(C2/4-H1') and (3)J(C6/8-H1'), and the one-bond coupling (1)J(C1'-H1') in deoxyribonucleosides and a number of uracil cyclo-nucleosides has been analyzed using density functional theory. The influence of the sugar pucker and the hydroxymethyl conformation has also been considered. The parameters of the Karplus relationships between the three-bond couplings and chi depend strongly on the aromatic base. (3)J(C2/4-H1') reveals different behavior for deoxyadenosine, deoxyguanosine, and deoxycytidine as compared to deoxythymidine and deoxyuridine. In the case of (3)J(C6/8-H1'), an opposite trans to cis ratio of couplings is obtained for pyrimidine nucleosides in contrast to purine nucleosides. The extremes of the Karplus curves are shifted by ca. 10 degrees with respect to syn and anti-periplanar orientations of the coupled nuclei. The change in the sugar pucker from S to N decreases (3)J(C2/4-H1') and (3)J(C6/8-H1'), while increasing (1)J(C1'-H1') for the syn rotamers, whereas all of the trends are reversed for the anti rotamers. The influence of the sugar pucker on (1)J(C1'-H1') is interpreted in terms of interactions between the n(O4'), sigma*(C1'-H1') orbitals. The (1)J(C1'-H1') are related to chi through a generalized Karplus relationship, which combines cos(chi) and cos(2)(chi) functions with mutually different phase shifts that implicitly accounts for a significant portion of the related sugar pucker effects. Most of theoretical (3)J(C2/4-H1') and (3)J(C6/8-H1') for uracil cyclo-nucleosides compare well with available experimental data. (3)J(C6/8-H1') couplings for all C2-bridged nucleosides are up to 3 Hz smaller than in the genuine nucleosides with the corresponding chi, revealing a nonlocal aspect of the spin-spin interactions across the glycosidic bond. Theoretical (1)J(C1'-H1') are underestimated with respect to the experiment by ca. 10% but reproduce the trends in (1)J(C1'-H1') vs chi.     

Determination of DNA structure in solution: enzymatic deuteration of the ribose 2' carbon

An enzymatic solution to the problem of obtaining 13C/15N-labeled nucleotides that are deuterated uniquely at the H2' ' position within the ribose ring is presented. Selective deuteration occurs with an overall yield of >80%. The deuteron at the H2' ' position allows measurement of the scalar and residual dipolar couplings for the bond vectors attached to the C2' carbon of each ribose sugar. These data allow the accurate determination of sugar conformation. Interesting DNA double helices of 2-3 turns are now within the reach of solution NMR spectroscopy. As an example, these labeled nucleotides are incorporated uniquely at positions 6-14 in a 20-bp DNA sequence containing the adenovirus major late promoter.     

Studies on the conformational flexibility of α-L-rhamnose-containing oligosaccharides using 13C-site-specific labeling, NMR spectroscopy and molecular simulations: implications for the three-dimensional structure of bacterial rhamnan polysaccharides

Bacterial polysaccharides are comprised of a variety of monosaccharides, L-rhamnose (6-deoxy-L-mannose) being one of them. This sugar is often part of α-(1 → 2)- and/or α-(1 → 3)-linkages and we have therefore studied the disaccharide α-L-Rhap-(1 → 2)-α-L-Rhap-OMe to obtain information on conformational preferences at this glycosidic linkage. The target disaccharide was synthesized with (13)C site-specific labeling at C1' and at C2', i.e., in the terminal group. 2D (1)H,(13)C-HSQC-HECADE and (1)H,(13)C-J-HMBC NMR experiments, 1D (13)C and (1)H NMR spectra together with total line-shape analysis were used to extract conformationally dependent hetero- and homonuclear spin-spin coupling constants. This resulted in the determination of (2)J(C2',H1'), (3)J(C1',C1), (3)J(C1',C3), (3)J(C2',C2), (2)J(C1',C2), (1)J(C1',C2'), and (1)J(C1',H1'). These data together with previously determined J(CH) and (1)H,(1)H NOEs result in fourteen conformationally dependent NMR parameters that are available for analysis of glycosidic linkage flexibility and conformational preferences. A 100 ns molecular dynamics (MD) simulation of the disaccharide with explicit water molecules as solvent showed a major conformational state at φ(H)≈ 40° and ψ(H)≈-35°, consistent with experimental NMR data. In addition, MD simulations were carried out also for α-L-Rhap-(1 → 3)-α-L-Rhap-OMe and a rhamnan hexasaccharide. The gathered information on the oligosaccharides was used to address conformational preferences for a larger structure, a 2- and 3-linked nonasaccharide, with implications for the 3D structure of rhamnan polysaccharides, which should be regarded as flexible polymers.     

N(1)-C(5')-linked dimer hydrates of 5-substituted uracils produced by anodic oxidation in aqueous solution

Electrochemical dimerization reactivity has been studied for 5-substituted uracils (5XU) including thymine (1a: X = Me) and 5-halouracil derivatives (1b: X = F; 1c: X = Cl; 1d: X = Br; 1e: X = I). Upon galvanostatic electrolysis of Ar-saturated aqueous solution 1a underwent anodic oxidation to produce N(1)-C(5')- and N(1)-C(6')-linked dimer hydrates, 1-(6'-hydroxy-5',6'-dihydrothymin-5'-yl)thymine (5a) and 1-(5'-hydroxy-5',6'-dihydrothymin-6'-yl)thymine (6a), as the major products. These N-C-linked dimerizations were accompanied by the formation of novel stereoisomeric C(5)-C(5')-linked dimers (meso isomer: 13a[meso]; racemic isomer: 13a[rac]) with a condensed tetrahydrofuran ring skeleton. Similar electrolyses of 5-fluorouracil (1b) and 5-chlorouracil (1c) also afforded the corresponding N(1)-C(5')-linked dimer hydrates, 1-(5'-fluoro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-fluorouracil (5b) and 1-(5'-chloro-6'-hydroxy-5',6'-dihydrouracil-5'-yl)-5-chlorouracil (5c), respectively, while resulting in neither N(1)-C(6')-linked dimer analogues nor C(5)-C(5')-linked dimers, unlike the reactivity of 1a. In contrast to 1a-c, no dimeric products were obtained from 5-bromouracil (1d) and 5-iodouracil (1e). The present electrochemical method was applicable to the cross-dimerization into N(1)-C(5')-linked heterodimer hydrates composed of binary 5-substituted uracils that occurred in competition with the formation of homodimer hydrates. A mechanism of the N(1)-C(5')-linked dimerization of 1a-c has been proposed, by which allyl-type radical intermediates with limiting mesomeric forms of N(1)-centered and C(5)-centered pyrimidine radicals (2a-c [N(1)]/2a-c [C(5)]) are generated via anodic one-electron oxidation and subsequent deprotonation at N(1) and undergo a head-to-tail coupling.     

Synthesis of the TT pyrimidine (6-4) pyrimidone photoproduct-thio analogue phosphoramidite building block

The phosphoramidite building block synthesis of the thio analogue at the 5,6-dihydropyrimidine C5 position of the thymidylyl(3'-5')thymidine (6-4) photoproduct 1 is presented. This compound was readily obtained from the appropriately protected dinucleotide P-methyl-5'-O-dimethoxytritylthymidilyl(3' --> 5')-4-thiothymidine 2 after irradiation at 366 nm, then S-sulfenylmethylation of the thiol function of the resulting (6-4) adduct, and phosphitylation of the 3'-hydroxyl group.     

Synthesis of soluble bis-terpyridine ligands bearing ethynylene-phenylene spacers

Soluble and rigid terpyridine-based ditopic ligands bearing one to five phenylene/ethynylene modules have been synthesized by way of a stepwise procedure. Each module is attached to the terpyridine unit via an ethynylene fragment and functionalized at the 4-position with an additional ethynylene connector and in the 2,5-positions with two flexible dodecyloxy chains. The synthetic protocol is based on sequential Pd(0)-catalyzed cross-coupling reactions between a terpyridine subunit grafted with the necessary diethynyl/phenyl or ethynylphenyl/bromide appendage. For ditopic ligands displaying an even number of phenyl/ethynylene modules, the final step involves a single cross-coupling reaction between 4'-ethynylene-2,2':6',6' '-terpyridine and the appropriate bromo derivative. In the case of the ligands having an odd number of phenylene/ethynylene fragments, a double cross-coupling reaction between an extended dibromopolyphenylene intermediate and 4'-ethynylene-2,2':6',6' '-terpyridine or 1-(4'-ethynylene-2,2':6',2' '-terpyridine)-4-ethynylene-2,5-didodecyloxy-benzene is required. For ligands I-V, optimal preparative conditions were found with [Pd(0)(PPh(3))(4)] (6 mol %) in n-propylamine at 70 degrees C. Oxidative dimerization of the 1-(4'-ethynylene-2,2':6',2' '-terpyridine)-4-ethynylene-2,5-didodecyloxybenzene derivative in the presence of cupric salts and oxygen gives the corresponding homoditopic ligand II(2)() bearing a central diphenyldiacetylene spacer. Spectroscopic data for the new oligomers are discussed in terms of the extent of pi-electron conjugation. Upon increasing the number of phenylene/ethynylene modules, there is a progressive lowering in energy of absorption and fluorescence transitions.