Substituent and solvent effects on the tautomer ratios between the hydrazone imine and diazenyl enamine forms in p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines

The p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-i and 2a-d exhibited tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The substituent and solvent effects on the tautomer ratios of A to B in a series of mixed media were studied for compounds 1a-i and 2a-d by the nmr spectroscopy. The linear correlation of the Hammett σp and σm values with the tautomeric equilibrium constants K_T ([A]/[B]) was found in the dimethyl sulfoxide media of compounds 1a-i and 2b-d . On the other hand, the linear correlation of the Hammett σp and σm values with the log C'(A:B = 1:1) was also observed in a series of mixed media of compounds 1a-h and 2a-c , wherein C'(A:B = 1:1) indicated the concentrations of trifluoroacetic acid (mol/l) giving 1:1 tautomer ratios in a series of mixed media. The increase in the Hammett σp or σm values decreased the K_T values in dimethyl sulfoxide media and augmented the C'(A:B = 1:1) values in a series of mixed media. The Hammett σp or σm values controlled the electron density of the side chain nitrogen atom, which influenced the C'(A:B = 1:1) values. In the K_T value temperature dependence, the higher temperature provided the larger K_T values in dimethyl sulfoxide media regardless of the Hammett σp or σm values.     

Progress in the chemistry of quinoxaline N-oxides and N,N′-dioxides

This review describes the 1,3-dipolar cycloaddition reaction, deoxygenation, deoxygenative transformation, ring transformation and photochemical reaction of quinoxaline N-oxides and N,N′-dioxides.     

A convenient synthesis and antifungal activity of 1-aryl-1H- and 1-aryl-3-heteroaryl-1H-pyrazolo[3,4-b]quinoxalinesA Convenient Synthesis and Antifungal Activity

Novel 1-aryl-1H- and 1-aryl-3-heteroaryl-1H-pyrazolo[3,4-b]quinoxalines (flavazoles) 9a-c, 12, 13 were synthesized from 3-methyl-2-oxo-1,2-dihydroquinoxaline 5 and the 3-triazolylmethylene-2-oxo-1,2,3,4-tetrahydroquinoxaline 6, respectively, via a facile hydrazone synthesis using aryl diazonium salts. Some of the above flavazoles and their related compounds exhibited the antifungal activity in some extent. The above results are described.     

An influence of a basic side chain moiety on the tautomer ratio between the enamine and methylene imine forms in azole condensed quinoxalines

The reaction of 7-chloro-4-(2-cyano-2-hydroxyvinyl)tetrazolo[1,5-α]quinoxaline 2a with 4-aminopyridine, p-toluidine or p-aminophenol gave 7-chloro-4-(4-pyridylcarbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]-quinoxaline 7a , 7-chloro-4-(p-tolylcarbamoylmethylene)4, 5-dihydrotetrazolo[1,5-α]quinoxaline 8a or 7-chloro-4-(p-hydroxyphenylcarbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]quinoxaline 9a , respectively. The reaction of 7-chloro-4-(2-cyano-2-hydroxyvinyl)-1,2,4-triazolo[4,3-α]quinoxaline 2b with 4-aminopyridine, p-toluidine or p-aminophenol afforded 7-chloro4-(4-pyridylcarbamoylmethylene)-4,5-dihydro-1,2,4-triazolo-[4,3-α]quinoxaline 7b , 7-chloro-4-(p-tolylcarbamoylmethylene)-4,5-dihydro-1,2,4-triazolo[4,3-α]quinoxaline 8b or 7-chloro-4-(p-hydroxyphenylcarbamoylmethylene)-4,5-dihydro-1,2,4-triazolo[4,3-α]quinoxaline 9b , respectively. The reaction of compound 2a with 2-aminopyridine or 3-aminopyridine provided 7-chloro-4-(2-pyridyl-carbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]quinoxaline 10 or 7-chloro-4-(3-pyridyl-carbamoylmethylene)-4,5-dihydrotetrazolo[1,5-α]quinoxaline 11 , respectively. Compounds 7a,b (4-pyridylcarbamoyl) predominated as the enamine tautomer A in a trifluoroacetic acid solution, while compounds 8a,b (p-tolylcarbamoyl) and compounds 9a,b (p-hydroxyphenylcarbamoyl) coexisted as the enamine A and methylene imine B tautomers in a trifluoroacetic acid solution. Moreover, the ratio of the enamine tautomer A elevated in an order of compound 11 (3-pyridylcarbamoyl), compound 10 (2-pyridylcarbamoyl) and compound 7a (4-pyridylcarbamoyl), reflecting an order of the increase in the pKa values of the aminopyridine side chain moieties. In general, the ratio of the enamine tautomer A was higher in the basic carbamoyl derivatives 7–11 than in the neutral ester derivatives 3a,b . From these results, the basic side chain moiety of the tetrazolo[1,5-α]quinoxalines 7a-11 or 1,2,4-triazolo[4,3-α]quinoxalines 7b-9b was found to increase the ratio of the enamine tautomer A in trifluoroacetic acid media.     

1H-NMR study on the tautomer ratios between the hydrazone imine and diazenylenamine forms in 3-(arylhydrazono)-methyl-2-oxo-1,2-dihydroquinoxalines

The o-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-c and 2a , c were synthesized to investigate the tautomeric behavior between the hydrazone imine A and diazenylenamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The chemical shifts of the hydrazone NH, N₄-H, hydrazone CH, and diazenyl CH protons for o-, m-, and p-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1 and 2 synthesized so far are summarized in Tables 3 and 4, respectively, which are found to be useful for the specification of the proton signals due to the hydrazone imine form A (hydrazone NH, hydrazone CH) and diazenylenamine form B (N₄-H, diazenyl CH).     

Reaction of 6-chloro-2-[1-methyl-2-(N-methylthiocarbamoyl)]-hydrazinoquinoxaline 4-oxide with dimethyl acetylenedicarboxylate

The reaction of 6-chloro-2-(1-methylhydrazino)quinoxaline 4-oxide 4a with methyl or phenyl isothiocyanate gave 6-chloro-2-[1-methyl-2-(N-methylthiocarbamoyl)hydrazino]quinoxaline 4-oxide 7a or 6-chloro-2-[1-methyl-2-(N-phenylthiocarbamoyl)hydrazino]quinoxaline 4-oxide 7b , respectively, whose reaction with dimethyl acetylenedicarboxylate afforded 6-chloro-2-[N-methyl-N-(5-methoxycarbonylmethylene-3-methyl-4-oxo-2-thioxoimidazolidin-1-yl)]aminoquinoxaline 4-oxide 8a or 6-chloro-2-[N-methyl-N-(5-methoxycarbonylmethylene-4-oxo-3-phenyl-2-thioxoimidazolidin-1-yl)]aminoquinoxaline 4-oxide 8b , respectively.     

Pressure control of enantiodifferentiating photoisomerization of cyclooctenes sensitized by chiral benzenepolycarboxylates. The origin of discontinuous pressure dependence of the optical yield

Pressure effects on enantiodifferentiating geometrical photoisomerizations of (Z)-cyclooctene and (Z,Z)-cycloocta-1,5-diene sensitized by chiral benzene-1,2,4,5-tetracarboxylate were investigated over a pressure range of 0.1-750 MPa. Enantiomeric excesses (ee's) of the (E)- and (E.Z)-isomers obtained displayed discontinuous pressure dependencies, affording distinctly different differential activation volumes (delta delta V++) for each range, indicating alteration of the enantiodifferentiation mechanism. The switching of delta delta V++ occurred at essentially the same pressures of 200 and 400 MPa, which are shared by all the chiral sensitizers, irrespective of the chiral auxiliary employed. Circular dichroism spectral examinations at pressures of up to 400 MPa also revealed that the chiral sensitizers undergo discontinuous conformational changes at 200 MPa, which most likely lead to switching of the enantiodifferentiating sensitization mechanism in the exciplex intermediate.     

Nonracemic dopant-mediated hierarchical amplification of macromolecular helicity in a charged polyacetylene leading to a cholesteric liquid crystal in water

Here we show the first example of a helical polyacetylene that forms a lyotropic liquid crystal (LC) through a hierarchical amplification of a macromolecular helicity process in water. The macromolecular helicity with an excess of one helical sense was first induced in the positively charged polyacetylene upon complexation with an extremely small oppositely charged nonracemic dopant through electrostatic interaction in water. Subsequently, the helicity was significantly amplified in the polymer backbone as an almost perfect single-handed helix through self-assembly into supramolecular helical arrays in a lyotropic cholesteric state. The present results will allow the detection of a tiny imbalance in chiral molecules and also provide new approaches for the design of novel water-soluble helical architectures and the construction of new chiral materials in areas such as biotechnology and materials science.     

13C NMR Spectroscopy on the Complexation of α-Cyclodextrin with 1-Alkanols and 1-Alkanoate Ions

Binding constants for -cyclodextrin ( -CD) complexes with 1- alkanols and 1- alkanoate ions were determined by analyzing changes ( ) in chemical shifts of guest ¹³C NMR signals with -CD concentration. The guests gave well-separated ¹³C NMR signals. The curve-fitting analyses of for individual carbons upon assumption of a simple 1:1 complexation gave virtually the same binding constants (K₁) in the cases of guests with relatively short alkyl chains, such as 1-propanol, 1-butanol, 1- pentanol , 1- butanoate , 1- pentanoate , and 1-hexanoate. However, thus obtained individual K₁ values were significantly different from one another, when the guests had relatively long alkyl chains such as 1- hexanol , 1- heptanol , and 1-octanoate. In these guests, satisfactorily consistent K₁ values were obtained by the curve-fitting analyses of for individual carbons upon an assumption that not only 1:1 but also 2:1 (host:guest) complexation occurs.     

Pressure-driven,solvation-directed planar chirality switching of cyclophano-pillar[5]arenes (molecular universal joints)

Planar chiral cyclophanopillar[5]arenes with a fused oligo(oxyethylene) or polymethylene subring (MUJs), existing as an equilibrium mixture of subring-included (in) and -excluded (out) conformers, respond to hydrostatic pressure to exhibit dynamic chiroptical property changes, leading to an unprecedented pressure-driven chirality inversion and the largest ever-reported leap of anisotropy (g) factor for the MUJ with a dodecamethylene subring. The pressure susceptivity of MUJs, assessed by the change in g per unit pressure, is a critical function of the size and nature of the subring incorporated and the solvent employed. Mechanistic elucidations reveal that the in–out equilibrium, as the origin of the MUJ''s chiroptical property changes, is on a delicate balance of the competitive inclusion of subrings versus solvent molecules as well as the solvation of the excluded subring. The present results further encourage our use of pressure as a unique tool for dynamically manipulating various supramolecular devices/machines.

Pressure switches the in/out conformation of cyclophano-pillararenes with accompanying inversion of the chiroptical properties.