Mesoionic Derivatives Research Articles

Preparation of fused meso-ionic compounds from 1-amino-4,6-diphenyl-2-pyridone

A number of meso-ionic compounds derivatives of the bicyclic system 1,3,4-oxadiazolo[3,2-a]pyridine have been prepared from 1-amino-4,6-diphenyl-2-pyridone, either by thermal way or via iminophosphorane ( 4 )

Nucleophilic addition at the photoexcited flavin cation: synthesis and properties of 6- and 9-hydroxy-flavocoenzyme chromophores.

1 The method of “photohydroxylation” of azaheteroaromatics, known in the phenazine and alloxazine series, has been applied to flavin (= isoalloxazine) derivatives FloxH. The products habe been identified as 6- and 9-hydroxyflavins (Flox-6/9-OH). On this basis it was possible to assign a 6-hydroxyflavin structure to the new flavocoenzyme chromophores described by Mayhew et al. in the accompanying paper. 2 In agreement with the data obtained by Dekker et al. in a recent laser study on alloxazine, we find that the reaction proceeds via addition of water, alcohols or carboxylic acids by the lowest excited singlet state of flavin with synchronous intramolecular hydrogen shift from the site of RO-addition [= C(6) or C(9)] towards N(5) and subsequent oxidation of the 6/9-hydroxy-1,5-dihydroflavin formed. 3 The photoaddition reaction occurs in competition with photodehydrogenation of available CH-bonds by the lowest excited flavin triplet, in which case the CH-substrate may be provided by the solvent ROH or, when R = H, by methyl groups of a second flavin molecule (= flavin autophotolysis). This photodehydrogenation may be suppressed by O2-dependent triplet quenching and/or by applying a positive charge to the flavin (RFIOxH+, R = H or alkyl) in order to inhibit the reaction with itself. 4 Differentiation of 6- and 9-hydroxyflavin isomers was possible by double resonance evaluation of the proton coupling between the residual Ar-H and the adjacent methyl group, by evaluation of the electron paramagnetic resonance hyperfine coupling for the residual Ar-H in the radical cations 1,5-RHFl-6/9-OH, and by formation of cupric chelates from 6-hydroxy-flavin as compared to the lack of metal affinity in the 9-isomer. 1H-Nuclear magnetic resonance, electron paramagnetic resonance and visible spectra and their dependence on the state of ionisation of hydroxyflavins are discussed. Hydroxyflavin anions as well as their N(1)-alkylated meso-ionic derivatives are shown to exhibit characteristic long-wavelength absorption bands ranging from 50 to 800 nm with maxima around 600 nm and low molar absorption coefficients between 500 and 3000 M−1cm−1 (as compared to ∼ 104 for the first π, π* transition at ∼ 400nm), which might be readily mistaken for “flavin-substrate charge transfer” bands. 5, 9-Methoxy-flavins could be spectroscopically related to 9:10-bridged cyclic hemiacetals obtained upon ring closure of 9-hydroxy groups with 10β-carbonyl functions. Such cyclic hemiacetals can be trapped in the acid photolysis of 10-(hydroxyalkyl)-flavins, e.g. riboflavin or flavocoenzymes, in a sequence of 10-side-chain dehydrogenation and N(1)-cycloaminal cation formation of the 10β-carbonyl formed, which results in a quenching of the further photolysis in favor of 9-photoaddition.

Photochemistry of quaternary salts of aromatic heterocycles and some of their mesoionic derivatives (Review)

The photochemical methods far the synthesis of quaternary salts of aromatic heterocycles, their photochemical reduction reactions, photoreactions with water, some photochemical transformations that proceed without a change in the charged heterocycle, and photoreactions involving valence isomerization and fragmentation of the mesoionic derivatives are examined.

Unstable mesoionic oxazolium-5-oxides

Deprotonation of seven 2,3-disubstituted 5-oxo-Δ2-oxazolinium perchlorates yielded solutions of the corresponding mesoionic oxazolones. These compounds are too unstable to be isolated; they were trapped as stable 4-acyl derivatives by treatment with aroyl chlorides or trifluoroacetic anhydride and as pyrroles by cycloaddition to dimethyl acetylenedicarboxylate. It is shown by i.r. spectroscopy that the oxazolium-oxides decompose by a process of self-acylation to form dimeric mesoionic 4-acylaminoacyl derivatives, but in many cases the isolated products were symmetrical 1,3-di(acylamino)propan-2-ones. The relative rates of dimerisation of eight oxazolium-oxides indicate that the mesoionic system is stabilised by aryl substitution at C-2, the presence of a methyl group on the nitrogen atom, and fusion to a pyridine ring.

The Preparation and Properties of Mesoionic 5-Iminothiazole Derivatives

Abstract Mesoionic 5-iminothiazoles were prepared by the cyclization of N-thiobenzoyl-methylaminoacetonitriles with hydrogen chloride or acyl chlorides. The reaction between a 5-aminothiazolium chloride and acyl chlorides, isocyanate, sulfonyl chloride, and nitrous acid gave the corresponding N-substituted mesoionic 5-iminothiazole derivatives. The spectral data of these compounds as well as the ready bromination of the ring support the view that this ring system is aromatic.

Chemical and X-ray crystallographic study of meso-ionic derivatives of s-triazolo[4,3-a]pyridine

The rearrangement product of the toluene-p-sulphonylester of N-hydroxy-N′N″-diphenylguanidine reacts with pyridine or γ-picoline to give anhydro-1-phenyl-3-phenylimino-s-triazolo[4,3-a]pyridinium hydroxide or its 7-methyl derivative; the structure of the 7-methyl derivative has been unambiguously assigned by X-ray analysis and chemical considerations.

Mesoionic derivatives of the 1,3-dithiole system

Anhydro-5-hydroxy-2-phenyl-1,3-dithiolium hydroxide has been synthesized and the structure of the product previously described as this compound has been elucidated.

Hydrazones—III

Abstract Reaction of o-nitro-ω-halobenzaldehyde arylhydrazones with bases gives 3-arylazoanthranil 1-oxides; reduction of these oxides involves rearrangement and leads to mesoionic indazole derivatives. These structures should replace the triazirinoindazole formulations (R.I. 1223, R.R.I. 2097) hitherto used to describe both classes of compounds. A simple route to two known 1,3,4-oxadiazoles from semicarbazones is also described.