Imino Hydrogen Research Articles

Organotin mefenamic complexes—preparations, spectroscopic studies and crystal structure of a triphenyltin ester of mefenamic acid: novel anti‐tuberculosis agents

AbstractThe triphenyltin adduct of mefenamic acid, [SnPh3L] (1), the monophenyltin complex [PhSnOL] n (2), and the dibutyltin complex [SnBu2L2] (3), where HL is 2‐[bis(2,3‐dimethylphenyl)amino]benzoic acid (mefenamic acid), have been prepared and structurally characterized by means of vibrational, 1H and 13C NMR spectroscopies. The crystal structure of 1 has been determined by X‐ray crystallography. X‐ray analysis revealed a pseudo‐pentacoordinated structure containing Ph3Sn coordinated to the carboxylato group. The structural distortion is a displacement from the tetrahedron toward the trigonal bipyramid. Significant CH–π interactions and intramolecular hydrogen bonds stabilize the structure 1. The polar imino hydrogen atom participates in intramolecular hydrogen bonds. Complex 1 is self‐assembled via CH–π and stacking interactions. Vibrational and NMR data are discussed in terms of the crystal structure and the proposed structures for 1–3. Compounds 1 and 3 were tested for antimycobacterial activity against Mycobacterium tuberculosis H37Rv. Copyright © 2002 John Wiley & Sons, Ltd.

Copper(II) acetate complexes, [CuLm(OAc)2]n (L = HNPPh3), stable in the solid state either as a dimer (m = 1, n = 2) or a monomer (m = 2, n = 1).

The synthesis and X-ray structures of two novel copper(II) phosphinimine acetate complexes, monomeric Cu(HNPPh3)2(OAc)2 and dimeric [Cu(HNPPh3)(OAc)2]2, are described. Intramolecular hydrogen bonding between acetate ligands and the imino hydrogen stabilizes Cu(HNPPh3)2(OAc)2, whereas the copper(II) tetraacetato core of [Cu(HNPPh3)(OAc)2]2 possesses neither intra- nor intermolecular hydrogen bonding.

Synthesis, crystal structures and spectroscopic studies of diorganotin derivatives with mefenamic acid. Crystal and molecular structures of 1,2:3,4-di-μ 2-2-[(2,3-dimethylphenyl)amino]-benzoato- O, O-1,3-bis-2-[(-[(2,3-dimethylphenyl)amino]benzoato- O-1,2,4:2,3,4-di-μ 3-oxo-tetrakis[di-methyltin(IV)] and 1,2:3,4-di-μ 2-2-[(-[(-[(2,3-dimethylphenyl)amino]-benzoato- O, O-1,3-bis-2-[(-[(-[(2,3-dimethylphenyl)amino]benzoato- O-1,2,4:2,3,4-di-μ 3-oxo-tetrakis[di- n-butyltin(IV)

The complexes [Me 2LSnOSnLMe 2] 2 ( 1) and [Bu 2LSnOSnLBu 2] 2 ( 2) where HL is 2-[(2,3-dimethylphenyl)amino]benzoic acid (mefenamic acid), have been prepared and structurally characterized by means of 119Sn Mössbauer, vibrational and NMR ( 1H and 13C) spectroscopies. The crystal structures of complexes 1 and 2 have been determined by X-ray crystallography. Each structure is centro-symmetric and features a central rhombus Sn 2O 2 unit with two additional tin atoms linked at the O atoms. Pairs of tin atoms are bridged by bidentate carboxylate ligands and the ‘external’ tin atoms have their coordination geometry completed by a bridging bidentate carboxylate ligand for 1 and by a monodentate carboxylate ligand for 2. Five rings, each containing two tin atoms, are present in the dimeric tetraorganodistannoxane 1 and the geometry around the four tin centers is distorted octahedral for Sn(2) and Sn(2a) and trigonal bipyramidal for Sn(1) and Sn(1a). Three such rings are present in 2 and the geometry around the four five-coordinated tin centers, Sn(1), Sn(1a), Sn(2) and Sn(2a), is distorted square-bipyramidal. Significant π→π, CH→π stacking interactions and intramolecular hydrogen bonds stabilize structures 1 and 2. The polar imino hydrogen atom participates in intramolecular hydrogen bonds. Complexes 1 and 2 are self-assembled via π→π, CH→π and stacking interactions.

Synthesis and spectroscopic studies of diorganotin derivatives with tolfenamic acid. Crystal and molecular structure of the first complex of tolfenamic acid, 1,2:3,4-di-μ 2-2-[(3-chloro-2-methylphenyl)amino]-benzoato- O, O-1,3-bis-2-[(3-chloro-2-methylphenyl)amino]benzoato- O-1,2,4:2,3,4-di-μ 3-oxo-tetrakis[di- n-butyltin(IV)

The complexes [Me 2LSnOSnLMe 2] 2 ( 1) [Bu 2LSnOSnLBu 2] 2 ( 2) and Bu 2SnL 2.H 2O ( 3), where HL is 2-[bis(3-chloro-2-methylphenyl)amino]benzoic acid (tolfenamic acid), have been prepared and characterized structurally by means of 19Sn Mössbauer, vibrational and 1H- and 13C-NMR spectroscopies. The crystal structure of complex 2 has been determined by X-ray crystallography. Three distannoxane rings are present to the dimeric tetraorganodistannoxanes of planar ladder arrangement with distorted trigonal-bipyramidal geometry about the five-coordinated tin centers. The structure, which has twofold symmetry, features a central Sn 2O 2 unit with two additional tin atoms linked at O. Pairs of tin atoms are bridged by bidentate carboxylate ligands and the external tin atoms have their coordination geometry completed by a monodentate carboxylate ligand. The tin atom geometries are similar and are based on a trigonal bipyramidal arrangement. Significant π→π stacking, C–H–π interactions and intrarmolecular hydrogen bonds stabilize this structure. The polar imino hydrogen atom on N(1) and N(2) participate in a bifurcate intramolecular hydrogen bond. In this case complex 2 is self-assembled via C–H–π and π→π stacking interactions. Tin-119 Mössbauer, vibrational and NMR data are discussed in terms of the crystal structure and the proposed structures for 1 and 3. From the variable-temperature Mössbauer effect, the Debye temperatures for 1– 3 were determined.

Solution structure of Cobalt(III)Hexammine complexed to the GAAA tetraloop, and metal-ion binding to G·A mismatches

The solution structure of a 22 nt RNA hairpin and its complex with Co(NH 3) 6 3+ bound to the GAAA tetraloop has been determined by NMR spectroscopy. Co(NH 3) 6 3+ has a similar geometry to Mg(H 2O) 6 2+ and can be used as a probe for binding sites of completely solvated magnesium ions. The hairpin contains tandem G·A mismatches, similar to the P5abc region of a group I intron, and is closed by a GAAA tetraloop. The tandem G·A mismatches are imino hydrogen bonded in contrast with the sheared G·A mismatches found in a different context in the crystal structure of the P4-P6 domain. Chemical shift changes of the imino protons upon titration of the RNA hairpin with Mg 2+ and with Co(NH 3) 6 3+ were used to identify ion-binding sites. Paramagnetic resonance broadening upon titration with Mn 2+ was also used. The titration curves gave dissociation binding constants for the magnesium ions in the millimolar range, similar to the binding in the major groove of RNA at tandem G·U base-pairs. Although the largest chemical shift change occurred at an imino proton of one of the G·A base-pairs, no nuclear Overhauser enhancement cross-peaks between the cobalt ligand and neighboring RNA protons were seen, presumably due to the high mobility of the Co(NH 3) 6 3+ at this site. Nuclear Overhauser enhancement cross-peaks between Co(NH 3) 6 3+ and the GAAA tetraloop were observed, which allowed the determination of the structure of the tetraloop binding site. The Co(NH 3) 6 3+ is bound in the major groove of the GAAA tetraloop with hydrogen bonds to guanine base N7 and to phosphate oxygen atoms of the tetraloop.

PROTONATION CONSTANTS AND SOLVENT DEPENDENT SPIN-CROSSOVER BEHAVIOUR OF IRON(II)-2-(2′-PYRIDYL)BENZIMIDAZOLE COMPLEXES

2-(2′-Pyridyl)benzimidazole (pybzim = LH) coordinates to iron(II) as a bidentate and forms the tris-ligated complex, [Fe(pybzim)3]2+ as isolated in the solid. Titration of [Fe(pybzim)3]2+ with base demonstrates the successive deprotonation of the imino hydrogens of the coordinated ligands. Protonation constants for the free ligand, pybzim (Iog10 K H = 11.33) and the complex, [Fe(pybzim)3]2+ (log10 K H 1 = 9.58, log10 K H 2 = 8.13 and log10 K H 3 = 6.97) were measured in 30% (v/v) H2O/EtOH. Results show that coordination to iron(II) increases the acidity of the imino hydrogen of the ligand. Spin-crossover behaviour of the complex were studied in different solvents ME, AC, AN, NM, NB, DMF, DMSO and ANL. The complex shows strong spin-crossover behaviour which is solvent dependent. Values of the spin-equilibrium constant (K sc) and the associated thermodynamic parameters (ΔH sc = 18.1–21.3 kJ mol−1 and δS sc = 69.6–84.4JK−1 mol−1) were calculated. An increase of the enthalpy is observed with increasing donor number (DN) of the solvent.

Microwave Spectrum and Molecular Conformation of δ-Valerolactam

Microwave rotational spectra for normal and N-deuterated species of δ-valerolactam (2-piperidinone) were observed in the frequency range from 8 to 40 GHz. The spectra of the ground vibrational states for two isotopic species and the excited vibrational states for normal species were assigned, and the rotational constants (MHz) in ground vibrational state were determined using the Watson A-reduced Hamiltonian: A = 4590.96(11), B = 2495.03(2), C = 1731.06(2) for normal species and A = 4436.79(13), B = 2484.49(1), C = 1703.83(1) for deuterated species, respectively. The comparison of the rotational constants and rs coordinates of the imino hydrogen atom with ones from the abinitio MO calculation at the MP2/6-31G(d, p) level of theory led to the conclusion that the spectra assigned were due to a half-chair conformer. One vibrationally excited state was observed and its vibrational frequency was 95(64) cm−1.

Direct identification of NH...N hydrogen bonds in non-canonical base pairs of RNA by NMR spectroscopy.

It is shown that the recently developed quantitative J(NN)HNN-COSY experiment can be used for the direct identification of hydrogen bonds in non-canonical base pairs in RNA. Scalar(2h)J(NN)couplings across NH.N hydrogen bonds are observed in imino hydrogen bonded GA base pairs of the hpGA RNA molecule, which contains a tandem GA mismatch, and in the reverse Hoogsteen AU base pairs of the E-loop of Escherichia coli 5S rRNA. These scalar couplings correlate the imino donor(15)N nucleus of guanine or uridine with the acceptor N1 or N7 nucleus of adenine. The values of the corresponding(2h)J(NN)coupling constants are similar in size to those observed in Watson-Crick base pairs. The reverse Hoogsteen base pairs could be directly detected for the E-loop of E.coli 5S rRNA both in the free form and in a complex with the ribosomal protein L25. This supports the notion that the E-loop is a pre-folded RNA recognition site that is not subject to significant induced conformational changes. Since Watson-Crick GC and AU base pairs are also readily detected the HNN-COSY experiment provides a useful and sensitive tool for the rapid identification of RNA secondary structure elements.

Density functional theory studies of hetero-Diels–Alder reactions

Transition structures for hetero-Diels–Alder reactions involving the heteroatoms O, S and N in dienes as well as in dienophiles have been determined at the MP2 (MP2/6-31G*//MP2/6-31G*) and hybrid DFT (B3LYP/6-31G*//B3LYP/6-31G*) levels of theory. The transition structures are predicted to be relatively early, concerted and asynchronous in all cases, with the DFT transition structures being more asynchronous than the MP2 ones. The reactions of butadiene with formaldehyde, thioformaldehyde and formaldimine proceed by diene HOMO–dienophile LUMO interactions whereas those of ethylene with acrolein, 1-thiabutadiene and 1-azabutadiene proceed by reverse electron demand interactions. In all the hetero-Diels–Alder reactions, the C–C bond is more fully formed than the heteroatom–carbon bond in the transition structure with the exception of the reaction between formaldimine and butadiene for which C–N bond formation is ahead of C–C bond making. All the reactions are highly exothermic. The reactions are facilitated by heteroatoms in both the diene and dienophile, and bond formation between two heteroatoms is disfavored. The reaction of formaldimine shows an endo preference whereas that of 1-azabutadiene leads to an exo preference of the imino hydrogen; these preferences of ca. 4 kcal mol-1 are caused mostly by inter-hydrogen steric effects in the transition structures. We conclude that the DFT calculations provide an economical way of accounting for electron correlation effects at nearly the same level as the MP2 ones in the investigations of hetero-Diels–Alder reactions.

A streptavidin mutant with altered ligand-binding specificity.

The biotin-binding site of streptavidin was modified to alter its ligand-binding specificity. In natural streptavidin, the side chains of N23 and S27 make two of the three hydrogen bonds with the ureido oxygen of biotin. These two residues were mutated to severely weaken biotin binding while attempting to maintain the affinity for two biotin analogs, 2-iminobiotin and diaminobiotin. Redesigning of the biotin-binding site used the difference in local electrostatic charge distribution between biotin and these biotin analogs. Free energy calculations predicted that the introduction of a negative charge at the position of S27 plus the mutation N23A should disrupt two of the three hydrogen bonds between natural streptavidin and the ureido oxygen of biotin. In contrast, the imino hydrogen of 2-iminobiotin should form a hydrogen bond with the side chain of an acidic amino acid at position 27. This should reduce the biotin-binding affinity by approximately eight orders of magnitude, while leaving the affinities for these biotin analogs virtually unaffected. In good agreement with these predictions, a streptavidin mutant with the N23A and S27D substitutions binds 2-iminobiotin with an affinity (Ka) of 1 x 10(6) M-1, two orders of magnitude higher than that for biotin (1 x 10(4) M-1). In contrast, the binding affinity of this streptavidin mutant for diaminobiotin (2.7 x 10(4) M-1) was lower than predicted (2.9 x 10(5) M-1), suggesting the position of the diaminobiotin in the biotin-binding site was not accurately determined by modeling.

A hydrogen-bonded model for the syn epoxidation of cyclic allylic ethers under Payne conditions

The syn selectivity observed for the Payne ( PhCN H 2O 2 ) epoxidation of cyclohexenyl ethers is consistent with an hydrogen-bonded model implying the allylic ether oxygen and the imino hydrogen of the in situ generated perbenzimidic acid. The geometry of the two most stable conformers, deduced from AM1 calculations, allows both hydrogen bonding and oxygen transfer.

The Secondary Structure of a Fourteen-Nucleotide Fragment of the Hairpin Ribozyme

A fourteen nucleotide RNA has been synthesized and its secondary structure investigated by non-denaturing polyacrylamide gel electrophoresis and1H nuclear magnetic resonance spectroscopy. This fourteen nucleotide RNA corresponds to the hairpin loop end of the proposed secondary structure of the (−)sTRSV hairpin ribozyme. Non-denaturing polyacrylamide gel electrophoresis indicates that the fourteen nucleotide RNA exists predominantly as a monomer at a 1 mM strand concentration. Four peaks are found in the imino hydrogen region of the1H NMR spectrum of the fourteen nucleotide RNA at this concentration. One-dimensional nuclear Overhauser effect spectroscopy of the imino hydrogen region of the1H NMR spectrum gives results consistent with a model of the secondary structure of the fourteen nucleotide RNA having a three nucleotide hairpin loop and two double-stranded stems separated by a single bulged adenosine.

Use of shaped pulses for semi‐selective excitation of imino 1H resonances in duplex DNA and RNA

AbstractThe application of shaped pulses, e.g. TOPHAT and hyperbolic secant, for the semi‐selective excitation of the imino 1H resonances of duplex DNA and RNA in aqueous solution is explored. The excitation profiles for the shaped pulses are calibrated on a sample containing a mixture of A:U base pairs in deuteriochloroform solution. The data obtained by performing a semi‐selective inversion‐recovery pulse sequence using a TOPHAT waveform for the 90° pulse and a hyperbolic secant waveform for the 180° pulse are fitted well by a single exponential function, are independent of transmitter offset over a 1 kHz bandwidth and are in good agreement with the value obtained with hard rectangular pulses. This approach for measuring T1 values was applied to a 2 mM sample of a self‐complementary oligoribonucleotide containing the nucleoside analogue 5‐fluorouridine in 90% H2O. The T1, values were calculated as a function of Tris concentration, a catalyst added to promote imino hydrogen exchange. The T1, values were obtained by good fits of single exponential functions to the inversion‐recovery data. The T1, values depend on the concentration of added Tris. Shaped pulses provide a convenient alternative to long rectangular pulses and hard pulse trains for the semi‐selective excitation of imino 1H resonances.

Preparation, crystal structure and single-crystal polarized spectra of bis(1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinido)zinc(II)–tetrahydrofuran (1/4)

The complex bis(1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinido)zinc(II), as its tetrahydrofuran (thf) adduct, Zn(C11N7H2)2·4thf, has been synthesized and its crystal structure determined. The zinc atom, which lies on a crystallographic symmetry centre, co-ordinates four imino groups in a square-planar arrangement [Zn–Nimino 2.070(8)A] and two thf units in the apical positions [Zn–Othf 2.249(7)A]; the other two thf molecules are equatorially linked to the imino hydrogens. The electronic absorption spectra have been derived through a Kramers–Kronig transformation of the single crystal polarized specular reflectance spectra and then resolved via Gaussian deconvolution. They show an extensive mixed polarization, since their prominent components are distributed along different molecular directions. The low-energy band (Q) is polarized mainly along the in-plane molecular short axis (y), the next band (B) along the long axis (x) and the third band (N) along z. The interpretation is based on the only available theoretical model, that is INDO/S molecular orbital calculations, carried out on the analogous NiII complex (D2h). The agreement between the observed and predicted transitions is qualitatively quite good, in the sense that several spectroscopic states have been calculated within the energy range investigated, of symmetry mainly B2u(y) at low energy, B3u(x) at intermediate energy and B1u(z) at high energy. While crystal optics may contribute slightly to polarization mixing, there is evidence for vibronic coupling between states of different symmetry; the proposed assignment (D2h) is thus based on the extrapolation of the above calculations by using the vibronic coupling, with the frequent occurrence of the type B2u× b1g× B3u.

Synthesis of 5'-O-(4,4'-Dimethoxytrityl)-2'-O-(tert-butyldimethylsilyl)-5-fluorouridine 3'-(Cyanoethyl N,N-diisopropylphosphoramidite) and Its Use in the Synthesis of RNA

5-Fluorouridine (FUrd) has been successfully converted to 5'-O-(4,4'-dimethoxytrityl)-2'-O-(tertbutyldimethylsilyl)-5-fluorouridine 3'-(cyanoethyl N,N-diisopropyl(phosphoramidite) by methods similar to those employed for the preparation of other ribonucleoside phosphoramidites. The desired product, as well as its precursors and the incorrect regioisomers, are prepared and fully characterized by 1 H NMR, 19 F NMR, 31 P NMR, FAB-MS, UV absorbance, elemental analysis, and melting points. The purified material is used for the production of RNA by solid-phase methods. Coupling yields of 94% are obtained with this material. The resulting RNA is readily purified and used for biophysical studies. The self-complemetary RNA decamer 5'-rGCGAAU(FU)CGC is prepared and purified. The material adopts an A-form duplex in solution at neutral pH as characterized by CD spectroscopy. The thermal stability of the duplex is similar to the parent duplex oligoribonucleotide prepared with uridine instead of 5-fluorouridine. The control duplex has a T m of 53.8 and the FUrd substituted duplex has a T m of 56.6 ×b0° C as determined by UV hyperchromicity at 260nm. The CD spectra and UV hyperchromicity data for the duplex oligoribonucleotide containing FUrd have a slight pH dependence indicating that ionization of FUrd has a slight but measurable impact on RNA duplex stability. 1D NMR spectroscopy of the imino hydrogens for this duplex in H 2 O solution confirms the formation of Watson-Crick base pairs. The imino hydrogen resonance for the FUrd-A base pair is moved downfield and broadened compared with a control duplex oligoribonucleotide as a consequence of the electron-withdrawing inductive effect of fluorine

Fine structure of 5,10,15,20-tetrakis(m-hydroxyphenyl)chlorin (m-THPC): a 1H, 13C and 15N NMR study

The 1H, 13C and natural abundance 15N NMR spectra of tetrakis(m-hydroxyphenyl)chlorin (m-THPC) are assigned through the complementary analysis of the data from one-dimensional, and both homo- and hetero-nuclear two-dimensional NMR experiments. The analysis of the 1H and 13C spectra showed that the symmetry of the structure includes an effective mirror plane, σv, and the 1H–15N coupling constant of 99 Hz (measured from the ‘inverse-mode’1H–15N 2D experiment) showed that the central imino protons undergo slow intramolecular tautomeric exchange. The single 15N chemical shift (δ 111.8 relative to 15NH4+) of the proton-bearing nitrogens is characteristic of a ‘pyrrole’ type, and the long range coupling 1H chemical shift correlation (2D) spectrum showed that, on the NMR time scale, the imino hydrogens are localised on five-membered rings that are not in the σv plane. Taken together these data prove that the fine structure of m-THPC is correctly represented with the imino hydrogens placed on opposite rings which are not reduced, as shown in structure 4.

Extraction Equilibria of Nickel(II) with Several Heterocyclic Substituted-Hydrazones

The extraction of nickel(II) with seven N, N, N-tridentate heterocyclic hydrazones, a neutral form of which is abbreviated as HL, using chloroform or benzene as an extraction solvent has been studied, the extraction equilibria being analyzed on the basis of the data for the proton dissociation of H3L2+, the formation of Ni(HL)2+ and Ni(HL)22+ complexes and the proton dissociation of Ni(HL)22+ complexes in aqueous solution. Nickel(II) is extracted as NiL2, although it forms such complexing ions as Ni(HL)2+ or Ni(HL)22+ in the aqueous phase. The distribution constants of HL and NiL2 (the abbreviations of which are Kd and Kdm, respectively) have been determined under the same condition as in aqueous solution. The logarithmic plots of Kdm vs. Kd displayed two straight lines with slopes of 1.9; one is for hydrazones bearing one pyridyl or one quinolyl ring in the aldehyde moiety and the other for hydrazones bearing another pyridyl ring introduced to the aldehyde moiety. This is attributed to an intramolecular hydrogen bond between the imino hydrogen and the nitrogen of pyridine of the latter hydrazones.

Ab initio stability study of the geometric isomerism of acyclic imines and O-protonated carbonyl compounds: Part II. Model compounds for additive schemes of anti-syn conversion energies: imino- and O-protonated derivatives of propynal, butynone and butynonal

The problem of decomposing the conversion energies of the geometrical isomers of acyclic isoelectronic imines and O-protonated carbonyl compounds into intuitive contributions associated with the repulsive intramolecular interactions of imino hydrogen atoms (hydroxyl hydrogen atoms) with hydrogen atoms bound to neighbouring carbon atoms is considered. The investigation was based on quantum chemical computations at the Hartree-Fock level with the 6-31G ∗ and 6-31G ∗∗ basis sets for 14 acyclic imines, 14 O-protonated carbonyl species, 10 model imines and 10 model O-protonated systems. Definitions of specific (1,4 and 1,5) hydrogen contributions ( E 14 and E 15) and rules for decomposing the conversion energies into additive contributions are proposed. Application of the procedure to the geometrical isomers of model compounds (imines HCXXXC·CH:NH, HCXXXC·C(:NH)·CH:CH 2, HCXXXC·C(:NH)CHO, HCXXXC·C(:NH)CH:NH, HCXXXC·C(:NH)CH 3, and O-protonated species HCXXXC·CH:Op, HCXXXC·C(:Op)CH:CH 2, HCXXXC·C(:Op)CH 3 HCXXXC·C(:Op)CH:O and HCXXXC·C(:Op)CH:NH) (p denotes a proton) gives zeroth-order estimates of particular contributions. These estimates were used to determine a set of specific contributions, which in turn permit one to reconstruct the conversion energies of the geometrical isomers of acyclic isoelectronic imines and O-protonated carbonyl compounds by using a simple additive scheme. The conversion energies were reproduced to within 0.2–0.3 kcal mol −1. The contribution terms are expected to be fairly generally applicable.

Two acetylamino derivatives of 2,4-bis(trichloromethyl)-1,3-benzdioxin showing different types of hydrogen bonding: (a) (C13C)CH?O=C and (b) N-H?O=C

8-Acetylamino-6-methyl-2,4-bis(trichloromethyl)-1,3-benzdioxin, (I), is monoclinic,P21/c,a=15.174(4),b=11.977(7),c=9.911(3)A,β=99.72(2)°. 6-Acetylamino-2,4-bis(trichloromethyl)-1,3-benzdioxin, (II), is monoclinic,P21/n,a=5.927(4),b=40.623(1),c =7.120(3)A,β=91.39(4)°. In compound (I) the imino hydrogen atom is locked in an intramolecular hydrogen bond to the proximate oxygen atom of the heterocyclic ring and is not available for intermolecular hydrogen bonding. Instead the weakly acidic hydrogen atom [Cl3C-C(2)]H takes part in a hydrogen bond to the carbonyl oxygen atom in another molecule. In compound (II) a “normal” intermolecular hydrogen bond of the type N-H⋯O=C occurs. The 6-acetylam-ino group is twisted about the (CAr-N) bond such that the angles N′H′⋯O=C, CArHAr⋯O=C, N′H′⋯O⋯HArCAr, at the carbonyl oxygen group total 360° (where N′H′ is in the related molecule). The packing in both compounds takes the form of infinite chains and in compound (II) partial overlap of the aromatic ring and the acetylamino group, with very little offset, also occurs.

Selective long‐range 1H13C polarization transfer in nucleosides

AbstractSelective INEPT (INAPT) experiments were used to correlate the nitrogen‐ and carbon‐bound hydrogens of nucleosides with 13C resonances via 2J(C, H) and 3J(C, H). Such experiments are useful as part of a strategy to distinguish the imino hydrogens of uridine and thymidine, which cannot be accomplished on the basis of chemical shift. The strategy involves correlation of the imino hydrogen with C‐5 using INAPT and testing whether or not the C‐5 resonance is quaternary. Polarization transfer is ineffecient for most positions in nucleosides owing to the small long‐range couplings, except the H‐6C‐5 transfer in adenosine, which results in enhancement of the 13C resonance over its equilibrium value. The efficiency of polarization transfer from H‐3 to C‐5 of thymidine depends on the base‐paired status of the nucleoside indicating a change in the 3J(C, H) on base‐pair formation. This sensitivity to electronic reorganization on base‐pair formation makes the INAPT experiment a useful probe for the base‐paired status of nucleosides and of the electronic properties of uridine and thymidine analogues.