Exocyclic Oxygen Research Articles

Photoelectron spectral study of the thermolysis of benzofuroxan

Photoelectron spectroscopy data have shown that thermolysis of benzofuroxan vaporin vacuo results in thermal abstraction of the exocyclic oxygen to afford a nonvolatile product, which decomposes to give nitrogen monoxide, carbon monoxide, and carbon dioxide.

Very strong hydrogen bonds in hydrated 1:1 adducts of an olefinic double betaine with perchloric and nitric acids: L·HClO4·H2O and L·HNO3·H2O [L=cis-(p-Me2N+C5H4N)2C2(COO−)2

The hydrated 1:1 adducts of an olefinic double betaine with perchloric and nitric acids, L·HClO4·H2O I and L·HNO3·N2O2 [L=cis-(p-Me2N+C5H4N)2C2(COO−)2], have been shown to be nearly isostructural by X-ray crystallography and infrared spectroscopy. The common cationic unit HL+ of both compounds exhibits significant semi-quinonoid character and possesses symmetrym, ist two coplanar carboxylate groups being linked by a very strong, nearly linear intramolecular O−H−O hydrogen bond [O...O 2.397(9)A and O−H−O 167.1° for1; 2.388(7)A and 167.4° for2]. The HL+ units are further connected by intermolecular O−H...O hydrogen bonds between the exocyclic carboxyl oxygen atoms and the water molecules to generate polymeric zigzag chains running parallel to thec axis, and the disordered ClO4 or NO3− anions occupy channels running parallel to thea axis in the resulting host lattice.

Dipole moments and electron distribution of furoxans and furazans

Dipole moments of a set of substituted 1,2,5-oxadiazoles (furazans) and 1,2,5-oxadiazole-2-oxides (furoxans) were measured in benzene solution. The dipole moment in furazan is oriented with the negative end towards oxygen in agreement with common opinion but contrary to the literature. Comparison of the two classes of compounds revealed a strong electron shift in furoxans, from the exocyclic oxygen back into the heterocyclic system, corresponding to a mesomeric moment of approximately 3D. However, the electron redistribution cannot be described in terms of simple mesomeric structures. The molecule of furoxan is well characterized as electron-overcrowded, particularly in the vicinity of the nitrogen atom N(2).

14N and 15N NMR study of 2,1,3-thiadiazolium-5-olate and related compounds

Abstract The 1 H, 13 C, 14 N and 15 N NMR data of 2,1,3-thiadiazolium-5-olate and some related structures have been collected. The nitrogen NMR data suggest that the compound exists in a cyclic B type mesoionic form. Protonation of 2,1,3-thiadiazolium-5-olate occurs on the exocyclic oxygen. Contrary to type B mesoionic diazoles and tetrazoles, the 14 N NMR data suggest a relatively high density of positive charge on the N3 atom.

Effect of Sulfur Substitution on the Floral Odor of Tetrahydropyranyl and Tetrahydrofuranyl Ethers

Sulfur-containing analogues of tetrahydropyranyl and tetrahydrofuranyl derivatives of selected alcohols and phenols have been prepared and their odor properties evaluated. The compounds include derivatives in which either one or both of the oxygen atoms had been replaced with sulfur. Derivatives with two sulfur atoms do not exhibit pleasant odors; those with an endocyclic oxygen and an exocyclic sulfur generally exhibit green notes. The floral character is found only in a few derivatives with an endocyclic sulfur and an exocyclic oxygen

A convenient strategy for replacement of the anomeric hydroxyl group by difluoromethyl functionality in carbohydrate derivatives

A series of carbohydrate gem-difluoroenol ethers are readily prepared by the reaction of the corresponding lactones with dibromodifluoromethane, tris(dimethylamino)phosphine and zinc. Subsequent catalytic hydrogenation provides difluoromethyl C-glycoside analogues in which the exocyclic oxygen atom has been replaced by a difluoromethylene unit.

Cell-permeable non-hydrolyzable cAMP derivatives as tools for analysis of signaling pathways controlling gene regulation in Dictyostelium.

A novel class of cAMP derivatives were tested for binding to surface cAMP receptors (CAR), protein kinase A (PKA), and cAMP-phosphodiesterase (PDE) and for induction of three classes of cAMP regulated genes in Dictyostelium discoideum. These derivatives carry sulfur substitutions for either the axial (Sp) or equatorial (Rp) exocyclic oxygen atoms, while further modifications were introduced to provide specificity for binding to either CAR or PKA, and/or to increase lipophilicity and render the derivatives membrane-permeable. All derivatives bind weakly to PDE and are almost not degraded during incubation with Dictyostelium cells. One cAMP derivative, 6-thioethyl-purineriboside 3',5'-monophosphorothioate, Sp-isomer (Sp-6SEtcPuMPS), fulfills the criteria for selective activation of PKA in vivo. The compound enters Dictyostelium cells and reaches an intracellular concentration of 1 microM, sufficient to activate PKA, at an extracellular concentration of 30 microM, which is insufficient to activate CAR. Expression of cAMP-regulated prespore and prestalk genes and the aggregative PDE gene are effectively induced by CAR agonists and very poorly by PKA agonists. Even Sp-6SEtcPuMPS is ineffective to induce gene expression. These data not only indicate that surface cAMP receptors are the first targets for cAMP-induced gene expression, but argue against direct induction of expression of these genes by cAMP-induced PKA activation.

Structural features of substituted purine derivatives compatible with depletion of human O6-alkylguanine-DNA alkyltransferase

A series of O6- and S6-substituted purine derivatives were tested for their ability to deplete the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) in cell-free extracts from HT29 colon tumor cells and intact HT29 cells. The order of potency was O6-(p-Y-benzyl)-guanine (Y = H, F, Cl, and CH3) > O6-benzyl-2'-deoxyguanosine > O6-(p-Y-benzyl)guanosine (Y = H, Cl, and CH3) > or = a series of 9-substituted O6-benzylguanine derivatives > or = O6-allylguanine > O6-benzylhypoxanthine > O6-methylguanine. A series of 7-substituted O6-benzylguanine derivatives, 2-amino-6-(p-Y-benzylthio)purine (Y = H, CH3), 2-amino-6-[(p-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine, and 7-benzylguanine were inactive. It is concluded that for efficient AGT depletion, an allyl or benzyl group attached through exocyclic oxygen at position 6 of a 2-aminopurine derivative is required. Activity is preserved with a variety of substituent groups attached to position 9 while substitution at position 7 leads to a complete loss of activity.

Mapping of the epitope/paratope interactions of a monoclonal antibody directed against adenosine 3',5'-monophosphate.

A series of systematically modified cyclic AMP (cAMP) analogues, including newly synthesized benzimidazole ribofuranosyl 3',5'-monophosphates was used to map the essential molecular interactions between cAMP and the monoclonal antibody 4/2C2 (mab 4/2C2) directed against 2'-O-succinoyl cAMP [Colling, Gilles, Nass, Moka & Jaenicke (1988) Second Messengers Phosphoproteins 12, 123-133]. Its paratope binds the purine base in syn conformation by dipole-dipole interactions and hydrophobic forces and/or stacking interactions. The ribose phosphate moiety is recognized by a combination of charge interactions and H-bonds to the exocyclic and the 5'-oxygen atoms and a hydrophobic interaction at the 2'-position. There is no regioselectivity for the exocyclic oxygen atoms. Compared with the known types of binding, mab 4/2C2 thus shows a new combination of molecular interactions which may be the basis of its strikingly specific recognition and binding of the cyclic adenylates. On this account mab 4/2C2 may become an important tool in studies on cAMP metabolism.

The cyclic nucleotide specificity of three cAMP receptors in Dictyostelium.

cAMP receptors mediate signal transduction pathways during development in Dictyostelium. A cAMP receptor (cAR1) has been cloned and sequenced (Klein, P., Sun, T. J., Saxe, C. L., Kimmel, A. R., Johnson, R. L., and Devreotes, P. N. (1988) Science 241, 1467-1472) and recently several other cAR genes have been identified (Saxe, C. L., Johnson, R., Devreotes, P. N., and Kimmel, A. R. (1991a) Dev. Genet. 12, 6-13; Saxe, C. L., Johnson, R. L., Devreotes, P. N., and Kimmel, A. R. (1991b) Genes Dev. 5, 1-8). We have expressed three receptor subtypes, cAR1, cAR2, and cAR3, in growing cells and have investigated their affinity and pharmacological specificity in a series of [3H]cAMP binding studies. In phosphate buffer, there were two affinity states of about 30 and 300 nM for cAR1 and 20 and 500 nM for cAR3 but no detectable affinity for cAR2. In the presence of 3 M ammonium sulfate, there was one affinity state of 4 nM for cAR1 and 11 nM for cAR2 and two affinity states of approximately 4 and 200 nM for cAR3. The relative affinities of 14 cyclic nucleotide derivatives were tested for each cAR in ammonium sulfate. These studies suggest a model (Van Haastert, P. J. M., and Kien, E. (1983) J. Biol. Chem. 258, 9636-9642) in which cAMP binds to all three receptor subtypes by maintaining hydrogen bond interactions at the N6 and O3' positions. Interactions at the exocyclic oxygens of cAMP varied between the receptors; cAR2 and cAR3 lacked a stereoselective interaction at the axial oxygen which was present in cAR1. The cleft, which binds the adenine ring of cAMP, was hydrophobic in cAR1 and cAR3 but relatively polar in cAR2. The analog specificity of cAR1 and cAR3 in phosphate buffer was similar to that measured in ammonium sulfate though the derivatives' relative affinity to cAMP was reduced. We conclude that these cAMP receptor subtypes can be distinguished by distinct pharmacological properties which will allow selective activation of each cAR during development.

Crystal structures of [AgHgL2(NO3)]n, where LH = 2-oxazolidone and β-propiolactam: Formation of helical chains and new 12-membered Ag/Hg metallamacrocyclic rings in the propiolactam complex

Reaction of 2-oxazolidone (C3H5NO2) or β-propiolactam (C3H5NO) with mercury(II) acetate and silver(I) nitrate in methanol leads to the formation of the polymeric heterometallic complexes [AgHg(C3H4NO2)2(NO3)]n (1) and [AgHg(C3H4NO)2(NO3)]n (2), respectively; the structures of which have been determined by X-ray diffraction methods. In each case, the deprotonated organic ligands bind to the mercury via nitrogen with nearly linear NHgN bonding and to the silver atom via oxygen. Despite the similar nature of the ligands, totally different three-dimensional assemblies are produced. Complex 1 is dimeric with a folded Ag2O2 ring in which the bridging oxygen atoms are provided by the exocyclic oxygen atoms of the 2-oxazolidone anions. These dimers are further linked in pairs to produce a “stepped” Ag4O4 species. Each silver atom is six-coordinate. Weak electrostatic, anion-mediated interact ions bridge these tetrameric units so as to produce loosely linked polymeric sheets. In compound 2 the silver atoms are linked by twisted Hg(β-propiolactam)2 bridges to form a helical chain. Each turn of the helix is bridged by a nitrate anion linking between adjacent silver atoms forming a 12-membered metallamacrocycle. Here the silver atoms are tetrahedrally coordinated.

Identifying the molecular switches that determine whether (Rp)-cAMPS functions as an antagonist or an agonist in the activation of cAMP-dependent protein kinase I.

Previous investigations revealed that under physiological conditions in the presence of MgATP the phosphorothioate analogue of cAMP, (Rp)-cAMPS, is a competitive inhibitor and antagonist for cAMP for cAMP-dependent protein kinases I and II [DeWit et al., (1984) Eur. J. Biochem. 142, 255-260]. For the type I holoenzyme, the antagonist properties of (Rp)-cAMPS are shown here to be absolutely dependent on MgATP. In the absence of MgATP, (Rp)-cAMPS serves as a weak agonist with a Ka of 7.9 microM. The high-affinity binding of MgATP imposes a barrier on cAMP-induced activation of the homoenzyme--a barrier that both cAMP and (Sp)-cAMPS, but not (Rp)-cAMPS, can overcome. In the absence of MgATP, this barrier no longer exists, and (Rp)-cAMPS functions as an agonist. The holoenzyme also was formed with mutant regulatory subunits. Replacing the essential arginine, predicted to bind the exocyclic oxygens of cAMP, in site A with lysine abolishes high-affinity binding of cAMP to site A. The holoenzyme formed with this mutant R-subunit is activated by (Rp)-cAMPS in both the presence and absence of MgATP. These results suggest that the stereospecific requirements for holoenzyme activation involve this guanidinium side chain. Mutations that eliminate the high-affinity binding of MgATP, such as the introduction of an autophosphorylation site in the autoinhibitory domain, also generate a holoenzyme that can be activated by (Rp)-cAMPS. In the case of the type II holoenzyme, (Rp)-cAMPS is an antagonist in both the presence and absence of MgATP, emphasizing distinct roles for MgATP in these two forms of cAMP-dependent protein kinase.

Synthesis and properties of quasi‐rigid polyesters containing 2,5‐disubstituted tetrahydropyran rings in their main chains

AbstractA polyester composed of trans‐2,5‐linked tetrahydropyran rings (4t) was synthesized by direct polycondensation of trans‐5‐hydroxytetrahydropyran‐2‐carboxylic acid (5t) using N‐methyl‐2‐chloropyridinium iodide or triphenylphosphine and hexachloroethane as condensing agents in pyridine at 25 or 50°C. The hydroxycarboxylic acid 5t was polymerized also with dibutyltin oxide, although longer reaction times were required. The polyester 4t is soluble in m‐cresol and halogenated protic solvents such as 2,2,2‐trifluoroethanol, trifluoroacetic acids and dichloroacetic acid, but insoluble in chloroform and dichloromethane in which the corresponding cis‐isomer (4c) composed of cis‐2,5‐linked tetrahydropyran rings is soluble. According to conformational energy calculations, the repeating unit exists predominantly as a conformer in which both the exocyclic oxygen and the carbonyl carbon in the backbone of the chain are located in the equatorial positions of the tetrahydropyran ring; in other words, the chain of 4t takes a linear conformation. On the other hand, the exocyclic oxygen in the repeating unit of 4c occupies an axial position of the tetrahydropyran ring and the chain is winding. Conformational energies for dimer models for 4c and 4t were also estimated by means of force field calculations. These polyesters are partially crystalline according to their X‐ray diffraction patterns. Both polyesters show birefringence at or above 200°C. The polyesters 4c and 4t decompose at 300°C in nitrogen atmosphere, approximately 150°C higher than the isomeric polyester 2.

A 13C and 15N NMR study of some mesoionic 4-hydroxy pyrazole derivatives in various solvents

13C and 15N NMR data are given for some mesoionic 4-hydroxy pyrazole derivatives. The data support the given cyclic structures for these compounds with a delocalised positive charge in the five-membered ring. Evidence is presented for the site of preferential protonation being at the exocyclic oxygen.

Role of magnesium-ATP in the activation and reassociation of cAMP-dependent protein kinase I: consequences of replacing the essential arginine in cAMP-binding site A

The type I form of cAMP-dependent protein kinase binds MgATP with a high affinity, and binding of MgATP decreases the affinity of the holoenzyme for cAMP [Hofmann et al. (1975) J. Biol. Chem. 250, 7795]. Holoenzyme was formed here with a mutant form of the bovine recombinant type I regulatory subunit where the essential arginine in site A, Arg-209, was replaced with Lys. Although this mutation does not significantly change the high-affinity binding of MgATP to the holoenzyme, it does abolish high-affinity binding of cAMP to site A. In the absence of MgATP, binding of cAMP to site B is sufficient to promote dissociation of the holoenzyme complex and activation of the catalytic subunit [Bubis et al. (1988) J. Biol. Chem. 263, 9668]. In the presence of MgATP however, holoenzyme formed with this mutant regulatory subunit is very resistant to cAMP. The Kd(cAMP) was greater than 1 microM, and the Ka(cAMP) increased 60-fold from 130 nM to 6.5 microM in the presence of MgATP. Thus, MgATP serves as a lock that selectively stabilizes the holoenzyme and inhibits activation. Both site A and site B are shielded from cAMP in the presence of MgATP. These results suggest that Arg-209 may play a role in stabilizing the MgATP.holoenzyme complex in addition to its role in binding the exocyclic oxygens of cAMP when cAMP is bound to the regulatory subunit. The catalytic subunit also reassociates rapidly with this mutant regulatory subunit, and in contrast to the wild-type regulatory subunit, holoenzyme formation does not require MgATP.

The cyclic diguanylic acid regulatory system of cellulose synthesis in Acetobacter xylinum. Chemical synthesis and biological activity of cyclic nucleotide dimer, trimer, and phosphothioate derivatives.

An unusual compound, cyclic-bis(3'----5') diguanylic acid (c-di-GMP or cGpGp), is involved in the regulation of cellulose synthesis in the bacterium Acetobacter xylinum. This cyclic dinucleotide acts as an allosteric, positive effector of cellulose synthase activity in vitro (Ka = 0.31 microM) and is inactivated via degradation by a Ca2(+)-sensitive phosphodiesterase, PDE-A (Km = 0.25 microM). A series of 13 analogs cyclic dimer and trimer nucleotides were synthesized, employing a phosphotriester approach, and tested for the ability to mimick c-di-GMP as activators of cellulose synthase and as substrates for PDE-A. Seven of the synthetic compounds stimulate cellulose synthase activity and all of these activators undergo the Ca2(+)-inhibited degradation reaction. The order of affinities for synthase activators is cGpGp approximately cdGpGp approximately cGp(S)Gp (S-diastereomer) greater than cIpGp greater than cdGpdGp greater than cXpGp greater than cIpIp greater than cGp(S)Gp (R-diastereomer). Three cyclic dinucleotides of negligible affinity for either enzyme are cApAp, cUpUp, and cCpCp. This same order of affinities essentially pertains to the analogs as inhibitors of PDE-A activity, but at least one cyclic dinucleotide, cXpXp, which does not bind to cellulose synthase, is also a substrate for the degradation reaction, demonstrating that although the two enzymes share a similar, high degree of specificity for c-di-GMP, their cyclic dinucleotide binding sites are not identical. Phosphodiester bonds of activators in which an exocyclic oxygen is replaced with an atom of sulfur (cGp(S)Gp isomers) resist the action of PDE-A, and such derivatives may be prototypes for synthetic non-hydrolyzable c-di-GMP analogs.

Lactam and acid amide acetals 57. Synthesis, protonation, and acid-base properties of condensed derivatives of 4-oxo-1,4-dihydro-1,8-naphthyridine

A number of 4-oxo-1,4-dihydro-1,8-naphthyridine derivatives that differ with respect to the sizes of the azaand carbocycles were synthesized by the reaction of 3-amino-4-ethoxycarbonyl-5,6-dihydro-7H-pyrindine and 3-amino-4-ethoxycarbonyl-5,6,7,8-tetrahydroisoquinoline with N,N-dimethylacetamide diethylacetal and N-methylbutyrolactam, N-methylvalerolactam, and N-methylcaprolactam diethylacetals and subsequent cyclization of the intermediate amidines. It was established by UV and 1H and 13C NMR spectroscopy that the protonation of these compounds takes place at the exocyclic oxygen atom. The dependence of the ionization constants of the compounds obtained in 70% DMFA on the size of the saturated cyclic fragments of the molecules was established.

Alkali-nucleobase interactions: Involvement of exocyclic oxygens of 1-methyluracil and 1-methylthymine in Na + binding

Co-crystallization of Na[AuCl 4] with 1-methyluracil, C 5H 6N 2O 2 (1-MeUH), and 1-methylthymine, C 6H 8N 2O 2 (1-MeTH), at pH 0.8 yields predominently salts of composition [Na(1-MeUH) 4][AuCl 4] ( 1) and [Na(1-MeTH)(H 2O) 4][AUCl 4]·1-MeTH·2H 2O ( 2). Both 1 and 2 crystallize in space group P 1 with cell dimensions a = 10.531(3), 1, 8.436(5), 2; b = 9.194(3), 1, 10.521(7), 2; c = 7.924(2), 1, 15.917(11) Å, 2; α = 96.68(3), 1, 102.61(5), 2; β = 85.09(3), 1, 90.45(5), 2; γ = 103.88(3), 1, 108.10(5)°, 2; U = 738.4, 1, 1306(2) Å 3, 2; Z = 1, 1, 2, 2. Na + binding to 1-MeUH is through O(4) in 1 and O(2) in 2. The packing of cations and anions in 1 is special in that infinite stacks of alternating [Na(1-MeUH) 4] + and [AuCl 4] − are formed with the metals 3.962(1) Å apart.

Platinum coordination chemistry with nucleobases

Abstract

Anomeric effect and infra-red υ CD stretching frequency in tetrahydropyran compounds

The stretching frequencies of equatorial and axial fused tetrahydropen C 2-D bonds appear around 2190 and 2100 cm −1, with a wide multiplicity. These frequencies are lowered by 20 to 70 cm −1 when an OR group is added on C2. An sp 2 hybridization of the exocyclic oxygen may explain this variable effect.