Exocyclic Nitrogen Atom Research Articles

Effect of vinylene and 1,4-phenylene spacers on efficiency of the ground-state intramolecular charge-transfer in enlarged 4-dimethylamino-1-methylpyridinium cations

15N NMR chemical shifts of the exo- and endocyclic nitrogen atoms show how efficient is the ground-state intramolecular charge transfer between these sites in 4-dimethylamino-1-methylpyridinium cation (increased contribution of the quinoid resonance form results in a shielding and deshielding effect of their NMR signals, respectively). As it was anticipated, insertion of vinylene and/or 1,4-phenylene spacers to the cation considerably hinders the ground-state charge transfer. This hypothesis is further supported by an analysis of the C–NMe2 bond lengths (X-ray data show that spacers elongate this bond). The selected valence angles in the compounds studied are also linearly dependent on δ(15Nendo) and δ(15Nexo) values. Although the correlation coefficient for the δ(15Nendo) versus δ(15Nexo) dependence is equal to 0.983, decrease of the net charge on one nitrogen atom is not compensated entirely by its increase on another nitrogen atom. This shows that exocyclic nitrogen atom is not the only acceptor of the positive charge in the molecule. The natural population analysis shows that the positive charge is transferred not only to the exocyclic N but also to, e.g., 1- and N-methyl C as well as to C3 and C5 atoms in pyridine ring. Ground-state charge transfer through the p-phenylene moiety was found to be less effective than through the trans-vinylene bridge.

Fragmentation of Isomeric Intrastrand Crosslink Lesions of DNA in an Ion-Trap Mass Spectrometer

The collision-induced dissociation pathways of isomeric cytosine-guanine and cytosine-adenine intrastrand crosslink-containing dinucleoside monophosphates were investigated with the stable isotope-labeled compounds to gain insights into the effects of chemical structure on the fragmentation pathways of these DNA modifications. A Dimroth-like rearrangement, which was reported for protonated 2'-deoxycytidine and involved the switching of the exocyclic N4 with the ring N3 nitrogen atom, was also observed for the cytosine component in the protonated ions of C[5-8]G, C[5-2]A, and C[5-8]A, but not C[5-N(2)]G or C[5-N(6)]A. In these two sets of crosslinks, the C5 of cytosine is covalently bonded with its neighboring purine base via a carbon atom on the aromatic ring and an exocyclic nitrogen atom, respectively. On the contrary, the rearrangement could occur for the deprotonated ions of C[5-N(2)]G, C[5-N(6)]A, and unmodified cytosine, but not C[5-8]G, C[5-2]A, or C[5-8]A. In addition, ammonia could be lost more readily from C[5-N(2)]G and C[5-N(6)]A than from C[5-8]G, C[5-2]A, and C[5-8]A. The results from the present study afforded important guidance for the application of mass spectrometry for the structure elucidation of other intrastrand/interstrand crosslink lesions.

Insertion of Isocyanides, Isothiocyanates, and Carbon Monoxide into the Pd−C Bond of Cyclopalladated Complexes Containing Primary Arylalkylamines of Biological and Pharmaceutical Significance. Synthesis of Lactams and Cyclic Amidinium Salts Related to the Isoquinoline, Benzo[g]isoquinoline, and β-Carboline Nuclei

Cyclometalation of 3-(2-naphthyl)-d-alanine methyl ester is achieved by reacting the corresponding hydrochloride salt and Pd(OAc)2 in a 1:1 molar ratio (acetonitrile, room temperature, 6 days). Although the chloro-bridged dimer (A) cannot be isolated in a pure form, addition of RNC to the reaction mixture affords the mononuclear derivatives (R)-[Pd{κ2(C,N)-C10H6CH2CH(CO2Me)NH2-2}Cl(CNR)] (R = tBu (1a-tBu), Xy (C6H3Me2-2,6) (1a-Xy)). Similar complexes (S)-[Pd{κ2(C,N)-C8H5N(CH2CH(CO2Me)NH2)-2}Cl(CNR)] (R = tBu (1b-tBu), Xy (1b-Xy)) are prepared from RNC and a previously reported cyclometalated derivative of l-tryptophan methyl ester, (S,S)-[Pd2{κ2(C,N)-C8H5N(CH2CH(CO2Me)NH2)-2}2(μ-Cl)2] (B). Compound 1b-Xy reacts with XyNC to give the iminoacyl complex (S)-[Pd{κ2(C,N)-C(═NXy)C8H5N(CH2CH(CO2Me)NH2)-2}Cl(CNXy)] (2b), whose crystal structure has been determined by X-ray diffraction studies. The cyclopalladated dimer B or those containing phenethylamine (C) or phentermine (D; see Chart ) reacts with RNC (R = tBu, Xy) in refluxing chloroform or toluene to render, depending on the reaction conditions, the cyclic amidines (3c-tBu, 3d-Xy) or the amidinium salts (4b-tBu, 4c-tBu, 4c-Xy, 4d-tBu, 4d-Xy). The amidines 3a-tBu and 3a-Xy and the amidinium salts 4a-tBu and 4a-Xy are synthesized from complexes 1a-tBu and 1a-Xy. When D reacts with isothiocyanates RNCS (R = Me, To (C6H4Me-4)), the cyclic amidinium salts 4d-Me and 4d-To are isolated. Amidinium triflates derived from phentermine with an aryl substituent at the exocyclic nitrogen atom (4d-Xy, 4d-To) present E/Z isomerism in CHCl3 or CH2Cl2 solution. CO reacts with A−D to give, after depalladation, the corresponding lactams: (R)-1-oxo-3-(methoxycarbonyl)-1,2,3,4-tetrahydrobenzo[g]isoquinoline (5a), (S)-1-oxo-3-(methoxycarbonyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole (5b), 1-oxo-1,2,3,4-tetrahydroisoquinoline (5c), and 1-oxo-3,3-dimethyl-1,2,3,4-tetrahydroisoquinoline (5d). The crystal structures of compounds 4b-tBu, 4c-tBu, 4d-tBu, 4d-Xy, 4d-Me, 4d-To, and 5a−d have been determined by X-ray diffraction studies.

Vanadium(V) Compounds with the Bis-(hydroxylamino)-1,3,5-triazine Ligand, H2bihyat: Synthetic, Structural, and Physical Studies of [V2VO3(bihyat)2] and of the Enhanced Hydrolytic Stability Speciescis-[VVO2(bihyat)]−

Reaction of the ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H(2)bihyat) with NaV(V)O(3) in aqueous solution followed by addition of either Ph(4)PCl or C(NH(2))(3)Cl, respectively, gave the mononuclear vanadium(V) compounds Ph(4)P[V(V)O(2)(bihyat)].1.5H(2)O (1) and C(NH(2))(3)[V(V)O(2)(bihyat)] (2). Treatment of V(IV)OSO(4).5H(2)O with the ligand H(2)bihyat in methyl alcohol under specific conditions gave the oxo-bridged dimer [V(V)(2)O(2)(mu(2)-O)(bihyat)(2)] (3). The structures for 1 and 3 were determined by X-ray crystallography and indicate that these compounds have distorted square-pyramidal arrangement around vanadium. The ligand bihyat(2-) is bonded to vanadium atom in a tridentate fashion at the pyridine-like nitrogen atom and the two deprotonated hydroxylamino oxygen atoms. The high electron density of the triazine ring nitrogen atoms, which results from the resonative contribution of electrons of exocyclic nitrogen atoms (Scheme 4 ), leads to very strong V-N bonds. The cis-[V(V)O(2)(bihyat)](-) species exhibits high hydrolytic stability in aqueous solution over a wide pH range, 3.3-11.0, as it was evidenced by (1)H and (51)V NMR spectroscopy and potentiometry. The high affinity of the H(2)bihyat ligand for the V(V)O(2)(+) unit, its tridentate character, as well as its small size, paves the way for potential applications in medicine, analysis, and catalysis for the C(NH(2))(3)[V(V)O(2)(bihyat)] compound. The molecular structures, vibrational and electronic spectra, and the energetics of the metal-ligand interaction for compounds 1 and 3 have been studied by means of density functional calculations.

Phosphino‐Aminothiazoline Platinum(II) and Platinum(II)/Gold(I) Complexes: Structural, Chemical and Vapoluminescent Properties

Phosphino-amino-thiazolines and -thiazoles can exist in solution in two tautomeric forms, in which the N-H proton involves the endo-cyclic or exo-cyclic nitrogen atom. The two tautomers show different reactivities toward alcoholysis; the imino form degrades more rapidly. Their bischelated platinum complexes were studied in the solid state by single crystal X-ray diffraction. Thus, the unique stereoelectronic features of the [Pt(PN(th))] (PN(th)=diphenylposphino-aminothiazoline) moiety were revealed. The complex cis-[Pt(PN(th))(2)] reacts with gold(I) salts to yield dimetallic compounds, the molecular structures of which have been determined by X-ray diffraction. Solid cis-[Pt(PN(th))(2)] shows vapoluminescent properties if exposed to alcohol vapors. A combined photophysical and crystallographic investigation has been carried out to clarify the unprecedented rigidochromic role of the alcohol in this phenomenon.

Identification of Adducts Formed in the Reactions of 2′‐Deoxyguanosine and Calf Thymus DNA with Glutaraldehyde

AbstractGlutaraldehyde (1,5‐pentanedial) is a widely used industrial chemical that has been found to be mutagenic in bacteria and mammalian cells. It has been claimed that glutaraldehyde forms adducts with 2′‐deoxyguanosine, 2′‐deoxyadenosine and 2′‐deoxycytidine, but until recently none of the adducts had been identified. In a very recent contribution from our laboratories, the structures were determined of three glutaraldehyde adducts formed in the reaction with 2′‐deoxyadenosine, and it was shown that one of the adducts is formed in calf thymus DNA. In the current work, we have studied the reaction of glutaraldehyde with 2′‐deoxyguanosine and calf thymus DNA in aqueous buffered solutions. Among the numerous adducts formed in the reaction with 2′‐deoxyguanosine, six compounds were found to be stable and could be isolated by semi‐preparative liquid chromatography and identified by UV absorbance and 1H and 13C NMR spectroscopic and mass spectrometric studies. Adduct dG350b, comprising two diastereomers, was found to be derived from one unit of glutaraldehyde which has been transformed to a tetrahydropyran ring so that C‐2 and C‐6 of the ring are attached to N‐1 and N2 of the purine unit resulting in a bridged structure. The adducts dG349a and b are analogous to dG350b, but the oxygen atom in the bridgehead position has been replaced by a nitrogen atom resulting in a piperidine ring. Adducts dG349a and b bear a diastereomeric relationship to one another and were not only separable, but were configurationally assigned. Adduct dG599, also comprising two diastereomers, is similar to dG349a and b, but the nitrogen atom in the piperidine ring in this case is the exocyclic amino nitrogen atom of another 2′‐deoxyguanosine unit. It was found that adducts dG350b and dG349a and b were formed in calf thymus DNA. Plausible mechanisms for the formation of these adducts are presented.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

N- VersusP-Coordination in Bis(amino)cyclodiphosph(III)azane Complexes of Aluminum

The reaction of the bis(amino)cyclodiphosph(III)azane, cis-{(tBuNH)(2)(PNtBu)(2)}, with AlMe(3), AlClMe(2), AlCl(2)Me, and AlCl(3) is reported. The less Lewis acidic compound AlMe(3) forms the adduct cis-[(tBuNH)(2)(PNtBu){P.(AlMe(3))NtBu}] (1), in which the aluminum atom is exclusively coordinated to one phosphorus atom. At elevated temperatures AlMe(3) undergoes migratory exchange between the two phosphorus atoms, but no methane elimination is observed. By using the more Lewis acidic compound AlClMe(2) the P-coordinated compound cis-[(tBuNH)(2)(PNtBu){P(AlClMe(2))NtBu}] (2) can be obtained at low temperatures. Compound 2 rearranges irreversibly to a product in which the AlClMe(2) group is coordinated by one exo-cyclic nitrogen atom. A concomitant 1,2-H shift from this nitrogen atom onto the phosphorus atom is observed. The N-coordinated rearrangement product slowly decomposes via a P-N bond cleavage in solution. Reaction of the even more Lewis acidic compounds AlCl(2)Me and AlCl(3) finally led to stable adducts, cis-[(tBuNH)(PNtBu)(tBuNAlCl(2)Me){P(H)NtBu}] (3), and cis-[(tBuNH)(PNtBu)(tBuNAlCl(3)){P(H)NtBu}] (4), in which the aluminum atoms are N-coordinated by a tBuN=PH unit.

Synthesis, spectroscopic and thermal studies on platinum(II) complexes of 5-nitrosouracil derivatives

Three complexes were obtained during reactions of 6-amino-1-methyl-5-nitrosouracil, 6-methylamino-1-methyl-5-nitrosouracil and 6-methylamino-1-benzyl-5-nitrosouracil with K2PtCl4. The complexes were isolated in good yields as powdery precipitates and characterized through elemental analysis, infrared and 1H NMR spectroscopies, and thermal analysis. The pyrimidine bases easily substitute chloro ligands as a neutral monodentate ligand form. The exocyclic oxygen atoms are the probable binding sites rather than ring or exocyclic nitrogen atoms. trans Square planar structures were proposed in all cases.

The guanine cation radical: investigation of deprotonation states by ESR and DFT.

This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G*+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2'-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation radical, G*+ (pH 3-5), singly deprotonated species, G(-H)* (pH 7-9), and doubly deprotonated species, G(-2H)*- (pH > 11), are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N-substituted derivatives at N1, N2, and N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G*+, G(-H)*, and G(-2H)*-. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)*. Using the B3LYP/6-31G(d) method, the geometries and energies of G*+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)* and G(N2-H)*, were investigated. In a nonhydrated state, G(N2-H)* is found to be more stable than G(N1-H)*, but on hydration with seven water molecules G(N1-H)* is found to be more stable than G(N2-H)*. The theoretically calculated hyperfine coupling constants (HFCCs) of G*+, G(N1-H)*, and G(-2H)*- match the experimentally observed HFCCs best on hydration with seven or more waters. For G(-2H)*-, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until nine or 10 waters of hydration are included.

Synthesis of novel N2-adamantyl derivatives of 2-amino-6-methyl-4(3H)-pyrimidinone as potential activators of tumor necrosis factor (TNF) release

A method has been developed for the synthesis of N2-adamantyl-2-amino-6-methyl-4(3H)-pyrimidinones based on the reaction of (adamant-1-yl)alkylamine with 2-(ethylsulfanyl)-6-methyl-4(3H)-pyrimidinone which can lead to the corresponding derivatives in which the exocyclic nitrogen atom and the adamantyl radical are separated by a hydrocarbon fragment.

2-Amino-5,5-bis(hydroxymethyl)-1,3-thiazol-4(5H)-one and its spirodioxane compounds in the Mannich reaction

It was found that the aminomethylation of 2-amino-5,5-bis(hydroxymethyl)-1,3-thiazol-4(5H)-one and its spiro analogs using primary amines is accompanied by cyclization at the amidine fragment of the molecule with annelation of a tetrahydrotriazine ring. When using secondary amines the aminomethylation occurs at the exocyclic nitrogen atom.

Tautomerism of acridin-9-amines substituted at the exocyclic nitrogen atom: Spectroscopic investigations and theoretical studies

In the ground electronic state, the first two ( 1 and 2) of the compounds investigated – 9-(methoxyamino)acridine ( 1), 9-hydrazinoacridine ( 2), N-(2-chloroethyl)acridin-9-amine ( 3) and N-(5-methylpyridin-2-yl)acridin-9-amine ( 4) – exist principally in the imino tautomeric form, while the other two ( 3 and 4) can coexist as amino and imino tautomers in solvents of various polarities and abilities to enter into specific interactions. These features of the molecules are reflected in the experimental absorption spectra and the predicted thermodynamic and spectral data. The predicted thermodynamic characteristics suggest that in the S 1 state, the imino tautomers of 1, 2 and 4 and the amino tautomer of 3 are more stable than their tautomeric counterparts. However, the predicted rates of intersystem crossing suggest that the imino tautomers of 1– 3 and the amino tautomer of 4 lose excitation energy very rapidly, so that only their counterpart tautomers in fact emit radiation. This explains why 1 and 2 do not fluoresce and why the amino form of 3 and the imino form of 4 are the emitters. 3 and 4 thus represent acridin-9-amines whose imino forms are preferred in the ground state, but whose respective amino and imino forms are preferred in the excited state. Because 3 and 4 are capable of tautomeric transformations in the ground and excited states, and also enter into specific interactions with solvents, these compounds could be potent spectral indicators or probes of environmental properties.

Structural and physicochemical features, as well as abilityto tautomerisation of acridin-9-amines substituted at the exocyclic nitrogen atom

The results of MP2 geometry optimisation for 9-(methoxyamino)acridine ( 1), 9-hydrazinoacridine ( 2), N-(2-chloroethyl)acridin-9-amine ( 3) and N-(5-methylpyridin-2-yl)acridin-9-amine ( 4) were compared with X-ray structure investigations done on 1. The structural properties of the amino tautomer of 3 and the imino tautomer of 1 extracted from X-ray data were well predicted by the computational method applied. Dipole moments, electrostatic features of the substituents at the exocyclic N atom (mean Mulliken charges) and equilibrium constants for the amino→imino tautomerisation were also predicted reasonably well by this method. This information was used to analyse the relations between the ability to tautomerise and the properties of substituents or features of the medium. It was confirmed that electron-attracting and weakly electron-donating substituents at the exocyclic N atom cause imino tautomers to be more stable, while the presence of electron-donating ones means that amino forms prevail. Equilibrium constants account well for the tautomeric equilibria in which 4 participates, and are reflected in the NMR investigations that we carried out earlier. Each tautomeric form of a given acridin-9-amine exhibits a unique distribution of electrostatic potential around its molecules, which is an important feature worth taking into account in investigations of interactions of these compounds with other molecules and their recognition by other molecules.

Isomerization/Recyclization of Some 5‐Ethoxycarbonyl‐Pyrimidines.

AbstractFor Abstract see ChemInform Abstract in Full Text.

NMR and DFT investigations of the substituent and solvent effect on amino—imino tautomerism in acridin‐9‐amines substituted at the exocyclic nitrogen atom

AbstractThe 1H and 13C NMR spectra of 9‐(methoxyamino)acridine (1) and 9‐hydrazinoacridine (2) show that these compounds exist principally in the imino tautomeric form in CDCl3, acetone‐d6, CD3CN, DMSO‐d6 and Py‐d5, all solvents with different polarities and abilities to participate in specific interactions. The spectra of the other two compounds investigated—N‐(2‐chloroethyl)acridin‐9‐amine (3) and N‐(5‐methylpyridin‐2‐yl)acridin‐9‐amine (4)—indicate that they coexist in the amino and imino forms. The amino tautomer of compound 3 predominates in CDCl3, CD3CN and Py‐d5 and that of compound 4 in CDCl3 and Py‐d5. On the other hand, the amino and imino forms of compound 3 coexist in acetone‐d6 and probably DMSO‐d6, whereas those of compound 4 coexist in acetone‐d6 and DMSO‐d6. The positions of the signals in the NMR spectra compare qualitatively with those predicted computationally at the GIAO/DFT level of theory. The equilibrium constants predicted by the DFT(PCM) method are in agreement with the results of NMR spectral analysis. In general, both the data predicted at the DFT level of theory and x‐ray structural data show that the imino tautomers display a ‘butterfly’‐type geometry, whereas the amino forms are characterized by an almost flat acridine moiety. Electron‐attracting substituents at the exocyclic N atom improve the stability of the imino form, and electron‐withdrawing substituents do likewise for the amino form. The importance of tautomeric phenomena in the context of the ability of acridin‐9‐amines to participate in specific interactions is outlined in brief, as are the possible applications of these compounds as probes of environmental properties. Copyright © 2005 John Wiley & Sons, Ltd.

Molecular and crystal structure of N-[2H-1,4-benzoxazin-3(4H)-ylidene]-cyclohexane aminium chloride

X-ray diffraction investigation showed that protonation of 4H-benzo-[1,4]-oxazin-3-ylidene-cyclohexylamine occurs at the exocyclic nitrogen atom. In crystal, the cation has a structure with a distinct exocyclic C=N bond.

Nucleophilic Catalysis by 4‐(Dialkylamino)pyridines Revisited — The Search for Optimal Reactivity and Selectivity

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nucleophilic catalysis by 4-(dialkylamino)pyridines revisited--the search for optimal reactivity and selectivity.

4-(Dimethylamino)pyridine (4-DMAP, 1) is well known as a catalyst for the esterification of alcohols by acid anhydrides and for various other synthetically useful transformations involving acyl transfer. 2] Its catalytic potential was first discovered by the groups of Litvinenko and Steglich in the late 1960s and its synthetic utility and that of its congeners, including polymeric variants, have been reviewed. Recently, attention has been focused on the development of enantiomerically pure chiral 4-(dialkylamino)pyridines for the kinetic resolution of alcohols and related enantioselective transformations. As a result of this interest, the detailed mechanism of catalysis by 4-(dialkylamino)pyridines and the factors that influence their reactivity have come under renewed scrutiny. In particular, Steglich and co-workers reported pyridonaphthyridine 3 as being the most catalytically active 4-DMAP analogue yet prepared for the acetylation of tertiary alcohols, and work by Kattnig and Albert has illustrated the key role of the anion and general base catalysis in regulating the rate and regioselectivity of polyol acetylation by 1 (Scheme 1). Herein, the detailed mechanism of catalysis of esterification by 4-(dialkylamino)pyridines is reexamined in light of these findings, and the complexity associated with this apparently straightforward process is highlighted. That pyridine and 4-substituted derivatives act primarily as nucleophilic rather than general base catalysts for alcohol esterification follows from the dramatic loss of activity that accompanies 2-alkyl substitution despite the relatively marginal effect that this substitution has on the pKa value of these derivatives. Such steric inhibition of catalysis was first shown to be characteristic of nucleophilic catalysis in work by Gold and Jefferson in the early 1950s on the hydrolysis of Ac2O with a series of methyl-substituted pyridines. The effect was quantified by Litvinenko and co-workers in 1981 for the catalysis of benzoylation of benzyl alcohol with BzCl. In addition to confirming the nucleophilic nature of the catalysis, this work also highlighted the particularly high catalytic activity of 1, which exhibits a rate of 3.4 ; 10 relative to the uncatalyzed reaction (Scheme 2). The high catalytic reactivity of 1 had previously been noted by Litvinenko and co-workers in the benzoylation of 3chloroaniline and subsequently, but independently, 1 was shown by Steglich and co-workers to enable esterification of even hindered tertiary alcohols with Ac2O. [4] Esterification reactions of tertiary alcohols are relatively slow and particularly susceptible to steric factors and therefore proved to be useful for exploring structure–activity relations for catalysis by 4-DMAP analogues. Accordingly, Hassner et al. found that 4pyrrolidinopyridine (4-PPY, 2) was the most efficient of a series of 4-aminopyridine derivatives, including 1, for the acetylation of 1-methylcyclohexanol with Ac2O (Scheme 3). [17] Hassner et al. noted the lack of correlation between the pKa value and the catalytic activity; they suggested that the relative efficiencies of the various catalysts reflected the stabilities of the respective derived acyl pyridinium intermediates in a mechanistic scenario involving equilibrium formation of these salts followed by rate-determining reaction with the alcohol (Scheme 4). Additionally, Hassner et al. noted that the order of catalytic activity of 4aminopyridine derivatives [4-pyrrolidino (2)> 4-dimethylamino (1)> 4-piperidino (5)> 4-morpholino (7)] mirrored the order of reactivity of cyclohexanone-derived enamines towards electrophiles. This order has been rationalized as a balance of stereoelectronic (nN!p*C=C) and steric effects (A strain) which dictates the efficiency with which the lone pair of electrons on the enamine nitrogen atom interacts with the C C double bond. By analogy with the enamine series, Hassner et al. noted that there was a qualitative correlation between the degree of shielding of the pyridyl b-hydrogen atoms in the H NMR spectra of the catalytically active 4-aminopyridine derivatives and their catalytic efficiency (see Scheme 3). They inferred that the extent of electronic communication between the lone pair of electrons of the exocyclic nitrogen atom and the carbonyl function through the pyridyl ring was a key factor in stabilizing the acyl pyridinium intermediate. The design of pyridonaphthyridine 3 (Scheme 1), recently disclosed by Steg[*] Dr. A. C. Spivey, S. Arseniyadis Department of Chemistry South Kensington Campus Imperial College London SW7 2AZ (UK) Fax: (+44)20-7594-5841 E-mail: a.c.spivey@imperial.ac.uk Highlights

Spectroscopic studies on Co(II) and Cu(II) complexes of 6-amino-1-methyl-5-nitrosouracil and its 6-methylamine derivative

Four complexes are obtained during the reactions of 6-amino-1-methyl-5-nitrosouracil and its 6-methylamine derivative with Co(II) and Cu(II) ions. Theses complexes were characterized through their elemental, thermal analysis, infrared and 1H NMR spectroscopes. The obtained results indicate that, the exocyclic oxygen and nitrogen atoms are the most probable binding sites rather than ring nitrogen atoms. For cobalt complexes, the two pyrimidine bases act as bidentate ligands in the anionic form with the dissociation of iminic or N(3) proton depending upon the nature of substituents on the pyrimidine ring. For copper complexes, the pyrimidine bases interact in the neutral form as monodentate ligands. Octahedral geometries are proposed for all of these complexes.

Recyclization of 5‐Ethoxycarbonylpyrimidines Occurring with Substitution of a Carbon Atom in the Heterocycle by an Exocyclic Carbon Atom.

AbstractFor Abstract see ChemInform Abstract in Full Text.