Oligonucleotide Analogues with Integrated Bases and Backbone. Part 16

The self‐complementary, ethylene‐linked U*[c_a]A⁽*⁾ dinucleotide analogues 8, 10, 12, 14, 16 , and 18 , and the sequence‐isomeric A*[c_a]U⁽*⁾ analogues 20, 22, 24, 26, 28 , and 30 were obtained by Pd/C‐catalyzed hydrogenation of the corresponding, known ethynylene‐linked dimers. The association of the ethylene‐linked dimers was investigated by NMR and CD spectroscopy. The U*[c_a]A⁽*⁾ dimers form linear duplexes and higher associates (K between 29 and 114M ^?1). The A*[c_a]U⁽*⁾ dimers, while associating more strongly (K between 88 and 345M ^?1), lead mostly to linear duplexes and higher associates; they form only minor amounts of cyclic duplexes. The enthalpy–entropy compensation characterizing the association of the U*[c_x]A⁽*⁾ and A*[c_x]U⁽*⁾ dimers (x=y, e, and a) is discussed.     

Oligonucleotide Analogues with Integrated Bases and Backbones. Part 24

Inspection of Maruzen models and force‐field calculations suggest that oligonucleotide analogues integrating backbone and bases (ONIBs) with an aminomethylene linker form similar cyclic duplexes as the analogous oxymethylene linked dinucleosides. The self‐complementary adenosine‐ and uridine‐derived aminomethylene‐linked A*[n ]U dinucleosides 15 – 17 were prepared by an aza‐Wittig reaction of the aldehyde 10 with an iminophosphorane derived from azide 6 . The sequence‐isomeric U*[n ]A dinucleosides 18 – 20 were similarly prepared from aldehyde 3 and azide 12 . The N‐ethylamine 5 , the acetamides 7 and 14 , and the amine 13 were prepared as references for the conformational analysis of the dinucleosides. In contradistinction to the results of calculations, the N‐ethylamine 5 exists as intramolecularly H‐bonded hydroxyimino tautomer. The association in CDCl₃ of these dinucleosides was studied by ¹H‐NMR and CD spectroscopy. The A*[n ]U dinucleosides 16 and 17 associate more strongly than the sequence isomers 19 and 20 ; the cyclic duplexes of 16 form preferentially Watson–Crick‐type base pairs, while 17, 19 , and 20 show both Watson–Crick‐ and Hoogsteen‐type base pairing. The cyclic duplexes of the aminomethylene‐linked dinucleosides prefer a gg‐orientation of the linker. No evidence was found for an intramolecular H‐bond of the aminomethylene group. The CD spectra of 16 and 17 show a strong, those of 19 a weak, and those of 20 almost no temperature dependence.     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 15

The self‐complementary (Z)‐configured U*[c_e]A⁽*⁾ dinucleotide analogues 6, 8, 10, 12, 14 , and 16 , and the A*[c_e]U⁽*⁾ dimers 19, 21, 23, 25, 27 , and 29 were prepared by partial hydrogenation of the corresponding ethynylene linked dimers. Photolysis of 14 led to the (E)‐alkene 17 . These dinucleotide analogues associate in CDCl₃ solution, as evidenced by NMR and CD spectroscopy. The thermodynamic parameters of the duplexation were determined by van't Hoff analysis. The (Z)‐configured U*[c_e]A⁽*⁾ dimers 14 and 16 form cyclic duplexes connected by Watson–Crick H‐bonds, the (E)‐configured U*[c_e]A dimer 17 forms linear duplexes, and the U*[c_e]A⁽*⁾ allyl alcohols 6, 8, 10 , and 12 form mixtures of linear and cyclic duplexes. The C(6/I)‐unsubstituted A*[c_e]U allyl alcohols 19 and 23 form linear duplexes, whereas the C(6/I)‐substituted A*[c_e]U* allyl alcohols 21 and 25 , and the C(5′/I)‐deoxy A*[c_e]U⁽*⁾ dimers 27 and 29 also form minor amounts of cyclic duplexes. The influence of intra‐ and intermolecular H‐bonding of the allyl alcohols and the influence of the base sequence upon the formation of cyclic duplexes are discussed.     

Oligonucleotide Analogues with Integrated Bases and Backbone

The thiomethylene‐linked U*[s]U⁽*⁾ dimers 9 – 14 were synthesized by substitution of the 6‐[(mesyloxy)methyl]uridine 6 by the thiolate derived from the uridine‐5′‐thioacetates 7 and 8 followed by O‐deprotection. Similarly, the thiomethylene‐linked A*[s]A⁽*⁾ dimers 9 – 14 were obtained from the 8‐(bromomethyl)adenosine 15 and the adenosine‐5′‐thioacetates 16 and 17 . The concentration dependence of both H? N(3) of the U*[s]U⁽*⁾ dimers 9 – 14 evidences the formation of linear and cyclic duplexes, and of linear higher associates, C(8 or 6)CH₂OH and/or C(5′/II)OH groups favouring the formation of cyclic duplexes. The concentration dependence of the chemical shift for both H₂N? C(6) of the A*[s]A⁽*⁾ dimers 18 – 23 evidences the formation of mainly linear associates. The heteroassociation of U*[s]U⁽*⁾ to A*[s]A⁽*⁾ dimers is stronger than the homoassociation of U*[s]U⁽*⁾ dimers, as evidenced by diluting equimolar mixtures of 11 / 20 and 13 / 22 . A 1 : 1 stoichiometry of the heteroassociation is evidenced by a Job's plot for 11 / 20 , and by mole ratio plots for 9 / 18, 10 / 19, 12 / 21, 13 / 22 , and 14 / 23 .     

Oligonucleotide Analogues with Integrated Bases and Backbones. Part 26

The self‐complementary aminomethylene‐linked A*[n] U* dinucleosides 23 – 26 were prepared by reductive coupling of aldehyde 10 and azide 8 . The U*[n] A* sequence isomers 19 – 21 were similarly prepared from aldehyde 14 and azide 3 . The substituents at C(6/I) of 23 – 26 and at C(8/I) of 19 – 21 strongly favour the syn‐conformation. The A*[n] U* dinucleoside 23 associates more strongly than the sequence‐isomeric U*[n] A* dinucleoside 19 . The A*[n] U* dinucleosides 23 and 24 associate more strongly than the analogues devoid of the substituent at C(6/I), while the U*[n] A* dinucleoside 19 associates less strongly than the analogue devoid of the substituent at C(8/I). While 23 and 24 form cyclic duplexes mostly by Watson–Crick‐type base pairing, 25 only forms linear associates. The U*[n] A* dinucleoside 19 forms mostly linear duplexes and higher associates, and 21 forms cyclic duplexes showing both Watson–Crick‐ and Hoogsteen‐type base pairing. The cyclic duplexes of the aminomethylene‐linked dinucleosides show both the gg‐ and gt‐orientation of the linker, with the gg‐orientation being preferred.     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 22

The tritylated and silylated self‐complementary A*[s]U*[s]A*[s]U* and U*[s]A*[s]U*[s]A* tetramers 18 and 24 , linked by thiomethylene groups (abbreviated as [s]) between a nucleobase and C(5′) of the neighbouring nucleoside unit were prepared by a linear synthesis based on S‐alkylation of 5′‐thionucleosides by 6‐(chloromethyl)uridines, 7 or 10 , or 8‐(chloromethyl)adenosines, 12 or 15 . The tetramers 18 and 24 were detritylated to the monoalcohols 19 and 25 , and these were desilylated to the diols 20 and 26 , respectively. The association of the tetramers 18 – 21 and 24 – 26 in CDCl₃ or in CDCl₃/(D₆)DMSO 95 : 5 was investigated by the concentration dependence of the chemical shifts for H? N(3) or H₂N? C(6). The formation of cyclic duplexes connected by four base pairs is favoured by the presence of one and especially of two OH groups. The diol 20 with the AUAU sequence prefers reverse‐Hoogsteen, and diol 26 with the UAUA sequence Watson–Crick base pairing. The structure of the cyclic duplex of 26 in CDCl₃ at 2° was derived by a combination of AMBER* modeling and simulated annealing with NMR‐derived distance and torsion‐angle restraints resulting in a Watson–Crick base‐paired right‐handed antiparallel helix showing large roll angles, especially between the centre base pairs, leading to a bent helix axis.     

Oligonucleotide Analogues with Integrated Bases and Backbone (ONIB). Part 31

The self‐complementary guanosine‐ and cytidine‐derived aminomethylene‐linked C*[n ]G dinucleoside 9 was synthesized by reductive amination of aldehyde 3 with an iminophosphorane derived from azide 7 . Deacylation of 9 gave the isopropylidene‐protected dinucleoside 10 . The sequence‐isomeric G*[n ]C dinucleoside 11 was similarly prepared from aldehyde 8 and azide 5 , and deacylated to 12 . The association of 10 and 12 in CHCl₃ or in CHCl₃/DMSO mixtures, and the structure of the associates were studied by ¹H‐NMR, ESI‐MS, CD, and vapor pressure osmometry (VPO). Broad ¹H‐NMR signals of dinucleosides 10 and 12 evidence an equilibrium between duplexes and quadruplexes (Hoogsteen base pairing between the Watson? Crick base‐paired duplexes). The quadruplex dominates for the G*[n ]C dinucleoside 12 between ?50° and room temperature. The sequence‐isomeric C*[n ]G 10 forms mostly only a cyclic duplex in CDCl₃ and in CDCl₃/(D₆)DMSO 9 : 1.     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 14

The self‐complementary tetrameric propargyl triols 8, 14, 18 , and 21 were synthesized to investigate the duplex formation of self‐complementary, ethynylene‐linked UUAA, AAUU, UAUA, and AUAU analogues with integrated bases and backbone (ONIBs). The linear synthesis is based on repetitive Sonogashira couplings and C‐desilylations (34–72% yield), starting from the monomeric propargyl alcohols 9 and 15 and the iodinated nucleosides 3, 7, 11 , and 13 . Strongly persistent intramolecular H‐bonds from the propargylic OH groups to N(3) of the adenosine units prevent the gg‐type orientation of the ethynyl groups at C(5′). As such, an orientation is required for the formation of cyclic duplexes, this H‐bond prevents the formation of duplexes connected by all four base pairs. However, the central units of the UAUA and AAUU analogues 18 and 14 associate in CDCl₃/(D₆)DMSO 10 : 1 to form a cyclic duplex characterized by reverse Hoogsteen base pairing. The UUAA tetramer 8 forms a cyclic UU homoduplex, while the AUAU tetramer 21 forms only linear associates. Duplex formation of the O‐silylated UUAA and AAUU tetramers is no longer prevented. The self‐complementary UUAA tetramer 22 forms Watson–Crick‐ and Hoogsteen‐type base‐paired cyclic duplexes more readily than the sequence‐isomeric AAUU tetramer 23 , further illustrating the sequence selectivity of duplex formation.     

Oligonucleotide Analogues with Integrated Bases and Backbones. Part 23

The sulfone 2 , and the sulfoxides (S)‐ 3 and (R)‐ 3 were obtained by oxidation of the thiomethylene‐linked A*[s ]U dinucleoside 1 . Conformational analysis and association studies of 2 , (S)‐ 3 , and (R)‐ 3 reveal a strong influence of the configuration on the conformation of the linking unit and on the self‐association of the dinucleosides.     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 30

The protected G*[s ]C*[s ]U*[s ]A*[s ]U*[s ]A*[s ]G*[s ]C* octanucleoside 24 was prepared by S‐alkylation of the thiolate derived from tetranucleoside 23 with the methanesulfonate 22 , and transformed to the silylated and isopropylidenated 25 , and further into the fully deprotected octanucleoside 26 . Compound 22 was derived from the methoxytrityl‐protected tetranucleoside 21 , and 21 was obtained by S‐alkylation of the thiolate derived from the dinucleoside 19 with methanesulfonate 17 derived from 16 by detritylation and mesylation. Similarly, tetranucleoside 23 resulted from S‐alkylation of the thiolate derived from 18 with the methanesulfonate 20 derived from 19 . Dinucleosides 16 and 18 resulted from S‐alkylation of the thiolate derived from the known cytidine‐derived thioacetate 15 with the C(8)‐substituted guanosine‐derived methanesulfonates 12 and 14 , respectively, that were synthesized from the protected precursors 4 and 7 by formylation, reduction, protection, and mesylation. The structures of the duplexes of 25 and 26 were calculated using AMBER* modelling and based on the known structure of the core tetranucleoside U*[s ]A*[s ]U*[s ]A*. The former shows a helix with a bent helix axis and strong buckle and propeller twists, whereas the latter is a regular, right‐handed, and apparently strain‐free helix. In agreement with modelling, the silylated and isopropylidenated octanucleoside 25 in (CDCl₂)₂ solution led to a mixture of associated species possessing at most four Watson? Crick base pairs, while the fully deprotected octanucleoside 26 in aqueous medium forms a duplex, as evidenced by a decreasing CD absorption upon increasing the temperature and by a UV‐melting curve with a melting temperature of ca. 10° below the one of the corresponding RNA octamer, indicating cooperativity between base pairing and base‐pair stacking.     

Oligonucleotide Analogues with Integrated Bases and Backbones. Part 19.

The A*[s]U⁽*⁾ dinucleosides 1 and 2 form thermoreversible gels in organic solvents. The basis of the gelation is the formation of linear aggregates by base pairing following desolvation of the nucleobases. This is evidenced by the absence of gel formation by the C(6)‐deaminated analogue 3 of 1 , the correlation of gelation with the anti‐conformation, as preferred for 1 , and the temperature‐, concentration‐, and time‐dependent CD spectra. The gels were also characterized by the minimum gelation concentration, the gel–sol transition (melting) temperature, and rheological properties.     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 13

The self‐complementary UA and AU dinucleotide analogues 41 – 45, 47, 48 , and 51 – 60 were prepared by Sonogashira coupling of 6‐iodouridines with C(5′)‐ethynylated adenosines and of 8‐iodoadenosines with C(5′)‐ethynylated uridines. The dinucleotide analogues associate in CDCl₃ solution. The C(6/I)‐unsubstituted AU dimers 51 and 54 prefer an anti‐oriented uracilyl group and form stretched linear duplexes. The UA propargyl alcohols 41 and 43 – 45 possess a persistent intramolecular O(5′/I)? H???N(3/I) H‐bond and, thus, a syn‐oriented adeninyl and a gt‐ or tg‐oriented ethynyl moiety; they form corrugated linear duplexes. All other dimers form cyclic duplexes characterized by syn‐oriented nucleobases. The preferred orientation of the ethynyl moiety (the C(4′),C(5′) torsion angle) defines a conformation between gg and one where the ethynyl group eclipses O(4′/I). The UA dimers 42, 47 , and 48 form Watson–Crick H‐bonds, the AU dimers 56 and 58 – 60 H‐bonds of the Watson–Crick‐type, the AU dimers 53 and 55 reverse‐Hoogsteen, and 57 Hoogsteen H‐bonds. The pairing mode depends on the substituent of C(5′/I) (H, OSiⁱPr₃; OH) and on the H‐bonds of HO? C(5′/I) in the AU dimers. Association constants were derived from the concentration‐dependent chemical shift for HN(3) of the uracilyl moiety; they vary from 45–104 M ^?1 for linear duplexes to 197–2307 M ^?1 for cyclic duplexes. The thermodynamic parameters were determined by van't Hoff analysis of the temperature‐dependence of the (concentration‐dependent) chemical shift for HN(3) of the uracilyl moiety. Neglecting stacking energies, one finds an average energy of 3.5–4.0 kcal/mol per intermolecular H‐bond. Base stacking is evidenced by the temperature‐dependent CD spectra. The crystal structure of 54 shows two antiparallel chains of dimers connected by Watson‐Crick H‐bonds. The chains are bridged by a strong H‐bond between the propargylic OH and O?C(4) and by weak reverse A ? A Hoogsteen H‐bonds.     

Stability of Hoogsteen‐Type Triplexes – Electrostatic Attraction between Duplex Backbone and Triplex‐Forming Oligonucleotide (TFO) Using an Intercalating Conjugate

Syntheses are described for two novel twisted intercalating nucleic acid (TINA) monomers where the intercalator comprises a benzene ring linked to a naphthalimide moiety via an ethynediyl bridge. The intercalators Y and Z have a 2‐(dimethylamino)ethyl and a methyl residue on the naphthalimide moiety, respectively. When used as triplex‐forming oligonucleotides (TFOs), the novel naphthalimide TINAs show extraordinary high thermal stability in Hoogsteen‐type triplexes and duplexes with high discrimination of mismatch strands. DNA Strands containing the intercalator Y show higher thermal triplex stability than DNA strands containing the intercalator Z . This observation can be explained by the ionic interaction of the protonated dimethylamino group under physiological conditions, targeting the negatively charged phosphate backbone of the duplex. This interaction leads to an extra binding mode between the TFO and the duplex, in agreement with molecular‐modeling studies. We believe that this is the first example of an intercalator linking the TFO to the phosphate backbone of the duplex by an ionic interaction, which is a promising tool to achieve a higher triplex stability.     

Hoogsteen-Duplex DNA: Synthesis and base pairing of oligonucleotides containing 1-deaza-2′-deoxyadenosine

Oligodeoxyribonucleotides containing 1-deaza-2′-deoxyadenosine ( = 7-amino-3-(2-deoxy-β-D -erythro-pentofuranosyl)-3H-imidazo[4, 5-b]pyridine; 1b ) form Hoogsteen duplexes. Watson-Crick base pairs cannot be built up due to the absence of N(1). For these studies, oligonucleotide building blocks – the phosphonate 3a and the phosphoramidite 3b – were prepared from 1b via 4a and 5 , as well as the Fractosil-linked 6b , and used in solid-phase synthesis. The applicability of various N-protecting groups (see 4a – c ) was also studied. The Hoogsteen duplex d[(c¹A)₂₀] · d(T₂₀) ( 11 · 13 ; T_m 15°) is less stable than d(A₂₀) · d(T₂₀) ( 12 · 13 ; T_m 60°). The block oligomers d([c¹A)₁₀–;T₁₀] ( 14 ) and d[T₁₀–(c¹A)₁₀] ( 15 ) containing purine and pyrimidine bases in the same strand are also able to form duplexes with each other. The chain polarity was found to be parallel.     

Novel copper(II) heterochelate: synthesis,structural features and fluorescence studies

Fluorescence properties of four based derivatives [Aⁿ] (where n = 1–4) and their Cu(II) heterochelates of the type [Cu(Aⁿ)(CQ)(OH)]^?xH₂O {where A¹ = 3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one, A² = 8‐methyl‐3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one, A³ = 6‐methyl‐3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one, A⁴ = 8‐chloro‐3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one and x = 3, 2, 4, 1} were studied at room temperature. The fluorescence spectra of heterochelates show red shift, which may be due to the chelation by the ligands to the metal ion. It enhances ligand ability to accept electrons and decreases the electron transition energy. The kinetic parameters such as order of reaction (n), energy of activation (E_a), entropy (ΔS^#), pre‐exponential factor (A), enthalpy (ΔH^#) and Gibbs free energy (ΔG^#) have been reported. The antimicrobial activity of Clioquinol and Cu(II) heterochelates have been determined and described. All the heterochelates showed a more effective antimicrobial activity than the free ligand. Copyright © 2010 John Wiley & Sons, Ltd.     

A new one‐dimensional ZnII coordination polymer based on 2‐[(1H‐imidazol‐1‐yl)methyl]‐1H‐benzimidazole and benzene‐1,2‐dicarboxylate

Multidentate N‐heterocyclic compounds form a variety of metal complexes with many intriguing structures and interesting properties. The title coordination polymer, catena‐poly[zinc(II)‐bis{μ‐2‐[(1H‐imidazol‐1‐yl)methyl]‐1H‐benzimidazole}‐κ²N³:N^3′;N^3′:N³‐zinc(II)‐bis(μ‐benzene‐1,2‐dicarboxylato)‐κ²O¹:O²;κ³O¹,O^1′:O²], [Zn₂(C₈H₄O₄)₂(C₁₁H₁₀N₄)₂]_n, has been synthesized by the reaction of Zn(NO₃)₂ with 2‐[(1H‐imidazol‐1‐yl)methyl]‐1H‐benzimidazole (imb) and benzene‐1,2‐dicarboxylic acid (H₂bdic) under hydrothermal conditions. There are two crystallographically distinct imb ligands [imb(A) and imb(B)] in the structure which adopt very similar coordination geometries. The imb(A) ligand bridges two symmetry‐related Zn1 ions, yielding a binuclear [(Zn1)₂{imb(A)}₂] unit, and the imb(B) ligand bridges two symmetry‐related Zn2 ions resulting in a binuclear [(Zn2)₂{imb(B)}₂] unit. The above‐mentioned binuclear units are further connected alternately by pairs of bridging bdic²⁻ ligands, forming an infinite one‐dimensional chain. These one‐dimensional chains are further connected through N—H...O hydrogen bonds, leading to a two‐dimensional layered structure. In addition, the title polymer exhibits good fluorescence properties in the solid state at room temperature.     

Magnetic Exchange Interactions in Dinuclear Copper(II) and Nickel(II) Complexes with μ‐Oxalato Bridges. The Structure of μ‐Oxalato(O,O′, O″, O‴)‐bis{[1, 8‐di(2‐pyridyl)‐3, 6‐dithiaoctane‐N,N′, S,S′]nickel(II)} Dinitrate Dihydrate

A copper(II) and two nickel(II) dinuclear oxalato‐bridged compounds of formulae [{Cu(bpdto)}₂(μ‐ox)](ClO₄)₂ ( 1 ), [{Ni(bpdto)]₂(μ‐ox)](ClO₄)₂( 2 ), and [{Ni(bpdto)}₂(μ‐ox)](NO₃)₂·2H₂O ( 3 ), where bpdto = 1, 8‐bis(2‐pyridyl)‐3, 6‐dithiaoctane and ox = oxalate = C₂O₄^2— anion, have been synthesized and characterized. The crystal structure of 3 was determined by single‐crystal X‐ray analysis. It is a dinuclear complex with i symmetry in which the oxalate ligand is coordinated in bis(didentate) fashion to the inversion centre‐related nickel atoms. The distorted octahedral environment of each nickel atom is completed by two sulphur atoms in the equatorial plane and by two pyridyl nitrogen atoms in axial positions. Magnetic susceptibility measurements over the range 5 — 299K, show antiferromagnetic interactions that are weak in 1 (J = —12.8 cm^—1) and strong in 2 and 3 (J = —37.8 and —40.9 cm^—1, respectively), which in the case of 3 is in keeping with the observed structural parameters.     

Orientational Preference of Long,Multicenter Bonds in Radical Anion Dimers: A Case Study of π‐[TCNB]22− and π‐[TCNP]22−

The similar shape and electronic structure of the radical anions of 1,2,4,5‐tetracyanopyrazine (TCNP) and 1,2,4,5‐tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon?carbon bond in π‐[TCNP]₂^2? and in π‐[TCNB]₂^2?, in good accord with the Maximin Principle predictions. Instead, the two known structures of π‐[TCNP]₂^2? have a D_2h(θ=0°) and a C₂(θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six‐membered rings). The only known π‐[TCNB]₂^2? structure has a C₂(θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (E_int) and the variation of the E_int components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the E_int(θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of E_int(θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the E_int(θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)^.? ??? cation⁺ interactions are also included in the model aggregate are the experimental data reproduced computationally.     

Synthesis of Heteroleptic Cerium(IV) Complexes Using a Heptadentate (N4O3) Tripodale Schiff‐base Ligand

The Cerium(IV) complexes [{N[CH₂CH₂N=CH(2‐O‐3,5‐^tBu₂C₆H₂)]₃}CeCl] ( 1 ) and [{N[CH₂CH₂N=CH(2‐O‐3,5‐^tBu₂C₆H₂)]₃}Ce(NO₃)] ( 2 ) were derived from the condensation of tris(2‐aminoethyl)amine and 3,5‐di‐tert‐butylsalicylaldehyde and the appropriate Ce starting material CeCl₃(H₂O)₆ and (NH₄)₂[Ce(NO₃)₆], respectively. Single crystal X‐ray diffraction studies reveal monomeric complexes.     

Oligodeoxyribonucleotides Containing 4-Aminobenzimidazole in Place of Adenine: Solid-phase synthesis and base pairing

Oligonucleotides containing 4-aminobenzimidazole 2′-deoxyribofuranoside (1,3-dideaza-2′-deoxyadenosine; c¹c³A_d, 1 ) were synthesized. For this purpose, various NH₂-protecting groups were investigated, and the [(9H-fluoren-9-yl)methoxy]carbonyl group was selected for phosphoramidite protection (→ 4c ). Apart from the phosphoramidite 3 , the phosphonate 2 was prepared. Compound 1 was incorporated in a homooligonuclectide as well as in oligomers containing naturally occurring nucleosides. The T_m values and the thermodynamic data of various duplexes ( 11 · 10 , 17 · 10 , 18 · 10 ) containing 4-aminobenzimidazole were determined. Although d[(c¹c³A)₂₀] ( 11 ) does not form a Hoogsteen duplex with d(T₂₀) ( 10 ) as observed with d[(c¹A)₂₀], it destabilizes the Watson-Crick duplexes to a much smaller extent than it was expected from a bulged loop structure. Apparently, 4-aminobenz-imidazole interacts with regular nucleoside residues within a Watson-Crick duplex structure, most likely by vertical stacking. According to the low basicity of the amino group, only weak H-bonding is expected.