John N. Low

Regioselective synthesis of 4,7,8,9-tetrahydro-2H-pyrazolo[3,4-b]quinolin-5(6H)-ones. Mechanism and structural analysis

Abstract Reactions of 5-amino-3-methyl-1H-pyrazole with dimedone and aldehydes afford regioselectivelly tricyclic linear 3,7,7-trimethyl-4,7,8,9-tetrahydro-2H-pyrazolo[3,4-b]quinolin-5(6H)-ones in good yields. Several aspects on this regioselective reaction, such as the reaction mechanism and structural studies of the predominant tautomeric form, are treated.

A triclinic polymorph of benzanilide: disordered molecules form hydrogen-bonded chains

In the P-1 polymorph of benzanilide or N-phenylbenzamide, C(13)H(11)NO, the molecules are linked into simple C(4) chains by N-H...O hydrogen bonds. The molecules exhibit orientational disorder, but the donor and acceptor in a given hydrogen bond may occur, independently, in either the major or the minor orientation, such that all four possible N-H.O combinations have very similar geometries. The structure of this P-1 polymorph can be related to that of a previously reported C2/c polymorph.

Supramolecular structures in N-isonicotinoyl arylaldehydehydrazones: multiple hydrogen-bonding modes in series of geometric isomers

Sixteen N-isonicotinoyl arylaldehydehydrazones, NC(5)H(4)CONHN=CHC(6)H(4)R, have been studied and the structures of 14 of them have been determined, including the unsubstituted parent compound with R = H, and the complete sets of 2-, 3- and 4-substituted geometric isomers for R = F, Br and OMe, and two of the three isomers for R = Cl and OEt. The 2-chloro and 3-chloro derivatives are isostructural with the corresponding bromo isomers, and all compounds contain trans amide groups apart from the isostructural pair where R = 2-Cl and 2-Br, which contain cis amide groups. The structures exhibit a wide range of direction-specific intermolecular interactions, including eight types of hydrogen bonds, N-H...N, N-H...O, O-H...O, O-H...N, C-H...N, C-H...O, C-H...pi(arene) and C-H...pi(pyridyl), as well as pi...pi stacking interactions. The structures exhibit a very broad range of combinations of these interactions: the resulting hydrogen-bonded supramolecular structures range from one-dimensional when R = 2-F, 2-OMe or 2-OEt, via two-dimensional when R = 4-F, 3-Cl, 3-Br, 4-OMe or 3-OEt, to three-dimensional when R = H, 3-F, 2-Cl, 2-Br, 4-Br or 3-OMe. Minor changes in either the identity of the substituent or its location can lead to substantial changes in the pattern of supramolecular aggregation, posing significant problems of predictability. The new structures are compared with the recently published structures of the isomeric series having R = NO(2), with several monosubstituted analogues containing 2-pyridyl or 3-pyridyl units rather than 4-pyridyl, and with a number of examples having two or three substituents in the aryl ring: some 30 structures in all are discussed.

Design, synthesis and biological evaluation of (E)-2-(2-arylhydrazinyl)quinoxalines, a promising and potent new class of anticancer agents

A series of forty-seven quinoxaline derivatives, 2-(XYZC6H2CHN-NH)-quinoxalines, 1, have been synthesized and evaluated for their activity against four cancer cell lines: potent cytotoxicities were found (IC50 ranging from 0.316 to 15.749 μM). The structure-activity relationship (SAR) analysis indicated that the number, the positions and the type of substituents attached to the aromatic ring are critical for biological activity. The activities do not depend on the electronic effects of the substituents nor on the lypophilicities of the molecules. A common feature of active compounds is an ortho-hydroxy group in the phenyl ring. A potential role of these ortho-hydroxy derivatives is as N,N,O-tridentate ligands complexing with a vital metal, such as iron, and thereby preventing proliferation of cells. The most active compound was (1: X,Y=2,3-(OH)2, Z=H), which displayed a potent cytotoxicity comparable to that of the reference drug doxorubicin.

Structural and thermodynamic characterization of polyphenylbenzenes

The thermodynamic and structural study of a series of polyphenylbenzenes, from benzene, n(Ph) = 0, to hexaphenylbenzene, n(Ph) = 6, is presented. The available literature data for this group of compounds was extended by the determination of the relevant thermodynamic properties for 1,2,4-triphenylbenzene, 1,2,4,5-tetraphenylbenzene, and hexaphenylbenzene, as well as structural determination by X-ray crystallography for some of the studied compounds. Gas phase energetics in this class of compounds was analyzed from the derived standard molar enthalpies of formation in the gaseous phase. The torsional profiles relative to the phenyl-phenyl hindered rotations in some selected polyphenylbenzenes, as well as the gas phase structures and energetics, were derived from quantum chemical calculations. In the ideal gas phase, a significant enthalpic destabilization was observed in hexaphenylbenzene relative to the other polyphenylbenzenes, due to steric crowding between the six phenyl substituents. A relatively low enthalpy of sublimation was observed for hexaphenylbenzene, in agreement with the decreased surface area able to establish intermolecular interactions. The apparently anomalous low entropy of sublimation observed for hexaphenylbenzene is explained by its high molecular symmetry and the six highly hindered phenyl internal rotations. For the series of polyphenylbenzenes considered, it was shown that the differentiation in the entropy of sublimation can be chiefly ascribed to the torsional freedom of the phenyl substituents in the gas phase and the entropy terms related with molecular symmetry.

Experimental Support for the Role of Dispersion Forces in Aromatic Interactions

Herein a core scaffold of 1-phenylnaphthalenes and 1,8-diphenylnaphthalenes with different substituents on the phenyl rings was used to study substituent effects on parallel-displaced aromatic π⋅⋅⋅π interactions. The energetics of the interaction was evaluated in gas phase based on the standard molar enthalpies of formation, at T=298.15 K, for the compounds studied; these values were derived from the combination of the results obtained by combustion calorimetry and Knudsen/Quartz crystal effusion. A homodesmotic gas-phase reaction scheme was used to quantify and compare the intramolecular interaction enthalpies in various substituted 1,8-diphenylnaphthalenes. The application of this methodology allowed a direct evaluation of aromatic interactions, and showed that substituent effects on the interaction enthalpy cannot be rationalized solely on classical electrostatic grounds, because no correlation with the σ(meta) or σ(para) Hammett constants was observed. Moreover, the results obtained indicate that aromatic π⋅⋅⋅π interactions are significantly enhanced by substitution, in a way that correlates with the ability of the interacting aryl rings to establish dispersive interactions. A combined experimental and computational approach for calculation of the true aromatic π⋅⋅⋅π interaction energies in these systems, free of secondary effects, was employed, and corroborates the rationale derived from the experimental results. These findings clearly emphasize the role of dispersion and dilute the importance of electrostatic forces on this type of interactions.

Control of the reaction between 2-aminobenzothiazoles and Mannich bases. Synthesis of pyrido[2,1-b][1,3]benzothiazoles versus [1,3]benzothiazolo[2,3-b]quinazolines

Reactions between 2-aminobenzothiazoles and Mannich bases are observed to be selectively controlled by the steric hindrance in the latter. Pyrido[2,1-b][1,3]benzothiazoles 3 are produced with non-sterically hindered Mannich bases such as 3-(dimethylamino)propiophenone hydrochlorides 2, whilst [1,3]benzothiazolo[2,3-b]quinazolines are produced with bulky Mannich bases such as 2-(dimethylaminomethyl)tetralone 4. This is shown by reactions with 2-amino-5-(ethylsulfanyl)thiadiazole, which was previously reported to follow the former reaction pathway with 2, while the reaction with 4 follows the latter reaction pathway. The final structures are established by NMR and X-ray diffraction, thus confirming the cyclization processes.

N-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl) derivatives of glycine, valine, serine, threonine and methionine : interplay of molecular, molecular-electronic and supramolecular structures

In each of N-(6-amino-3, 4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)valine, C(10)H(15)N(5)O(4) (3) (orthorhombic, P2(1)2(1)2(1)), N-(6-amino-3, 4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)serine monohydrate, C(8)H(11)N(5)O(5).H(2)O (4) (orthorhombic, P2(1)2(1)2(1)), and N-(6-amino-3, 4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)threonine, C(9)H(13)N(5)O(5)(5) (monoclinic, P2(1)), the C-nitroso fragments exhibit almost equal C-N and N-O bond lengths: the C-N range is 1. 315 (3)-1.329 (3) A and the N-O range is 1.293 (3)-1.326 (3) A. In each compound there are also very short intermolecular O-H.O hydrogen bonds, in which carboxyl groups act as hydrogen-bond donors to the nitrosyl O atoms: the O.O distances range from 2.440 (2) to 2. 504 (4) A and the O-H.O angles lie between 161 and 163 degrees. An interpretation of the relationship between the unusual intramolecular bond lengths and the very short intermolecular hydrogen bonds has been developed based on database analysis and computational modelling. In each of (3)-(5) there is an extensive network of intermolecular hydrogen bonds, generating three-dimensional frameworks in (3) and (5), and two-dimensional sheets in (4).

Symmetrically 4,6-disubstituted 2-aminopyrimidines and 2-amino-5-nitrosopyrimidines: interplay of molecular, molecular-electronic and supramolecular structures.

The structures of six symmetrically 4,6-disubstituted 2-aminopyrimidines, four of them containing a 5-nitroso substituent, have been determined. The nitroso compounds, in particular, exhibit polarized molecular-electronic structures leading to extensive charge-assisted hydrogen bonding. The intermolecular interactions observed include hard hydrogen bonds of N-H...N and N-H...O types together with O-H...O and O-H...N types in 2-amino-4,6-bis(2-hydroxyethylamino)-5-nitrosopyrimidine; soft hydrogen bonds of the C-H...O type in both 2-amino-4,6-bis(morpholino)-5-nitrosopyrimidine (3) and 2-amino-4,6-bis(benzylamino)-5-nitrosopyrimidine (4), and of the C-H...pi(arene) type in both 2-amino-4,6-bis(piperidino)pyrimidine (1) and 2-amino-5-nitroso-4,6-bis(3-pyridylmethoxy)pyrimidine (5); and aromatic pi...pi stacking interactions in 2-amino-5-nitroso-4,6-bis(3-pyridylmethoxy)pyrimidine. The supramolecular structures formed by the hard hydrogen bonds are finite, zero-dimensional in (1), one-dimensional in 2-amino-4,6-bis(3-pyridylmethoxy)pyrimidine (2), two-dimensional in both (3) and (4), and three-dimensional in both (5) and 2-amino-4,6-bis(2-hydroxyethylamino)-5-nitrosopyrimidine.

Interplay of hydrogen bonds, iodo⋯nitro interactions and aromatic π⋯π stacking interactions in iodo-nitroanilines

Molecules of 2-iodo-5-nitroaniline (I) are linked by N-H...O hydrogen bonds into centrosymmetric dimers and by asymmetric three-centre iodo...nitro interactions into chains, so forming chains of fused centrosymmetric rings: these chains are linked by aromatic pi...pi stacking interactions to form a three-dimensional structure. In the isomeric 4-iodo-2-nitroaniline (II), each of the two independent molecules forms hydrogen-bonded chains that are linked by two-centre iodo...nitro interactions into sheets of two types, each containing only a single type of molecule: pi...pi stacking interactions are absent. In 2,4-diiodo-3-nitroaniline (III), where the nitro group is almost orthogonal to the aryl ring, a combination of N-H...O hydrogen bonds and two distinct two-centre iodo...nitro interactions links the molecules into a three-dimensional framework that is reinforced by aromatic pi...pi stacking interactions. Bond lengths and conformations are discussed and comparisons are drawn with related compounds.