Reaction of 3,6-di-tert-butyl-o-benzoquinone with dimedone. Functionalized derivatives of hinderedo-quinones and catechols

The reaction of 3,6-di-tert-butyl-o-benzoquinone with dimedone in the presence of a catalytic amount of Et₃N occurs as repeated 1,4-nucleophilic addition-oxidation and isomerization of a tricyclic quinone into quinomethane. The intermediate products were isolated and characterized. Semiquinone complexes of quinones were studied by ESR in solution. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2206–2209, December, 1997.     

A reaction of 3,6-di(tert-butyl)-4-chloro-1,2-benzoquinone with potassium ethyl xanthate. New sulfur-containing o-quinones

In a reaction of 3,6-di(tert-butyl)-4-chloro-1,2-benzoquinone with potassium ethyl xanthate, the halogen atom was replaced by the ethyl xanthate group. Acid hydrolysis of the product gave 4,7-di(tert-butyl)-5,6-dihydroxy-1,3-benzodithiol-2-one containing a pyrocatechol fragment, which was oxidized into the corresponding quinone. When irradiated in solution with visible light, this quinone quantitatively eliminated a CO molecule with contraction of the ring to a five-membered one. The quinones obtained were studied as ligands in coordination spheres of metals by ESR spectroscopy. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 683–686, April, 2006.     

Photolytic decarbonylation of o-benzoquinones

It was established that photodecarbonylation of o-benzoquinones occurs by irradiation not only by UV-light, but visible light (λ>520 nm) too. Study of the series of 4,5-di-substituted 3,6-di-tert-butyl-o-benzoquinones detected that the only product of photoreaction is the corresponding 3,4-di-substituted 2,5-di-tert-butyl-cyclopentadienone, which is formed in a yield close to quantitative. NMR monitoring of reaction of photodecarbonylation of o-benzoquinones detected that this is a two-stage process. In the first stage the photoexcited molecule of quinone rearranges into bicyclo compound (bicyclo[1.3.0]hexa-3-en-2,6-dione) containing five- and three- membered cycles, which spontaneously decomposes during the following dark stage into cyclopentadienone and a molecule of CO.     

Oxidative chemistry of the natural antioxidant hydroxytyrosol: hydrogen peroxide-dependent hydroxylation and hydroxyquinone/o-quinone coupling pathways

Oxidation of the natural antioxidant hydroxytyrosol (1) with peroxidase/H₂O₂ in phosphate buffer at pH 7.4 led to the formation of two main ethyl acetate-extractable products. These could be isolated by preparative TLC after reduction and acetylation, and were identified as the tetraacetyl derivative of 2-(2,4,5-trihydroxyphenyl)ethanol (3) and the heptaacetyl derivative of the pentahydroxybiphenyl 4 by 2D NMR and MS analysis. Similar oxidation of 4-methylcatechol gave, after the same work-up, the acetylated derivatives of 1,2,4-trihydroxy-5-methylbenzene (5) and the pentahydroxybiphenyl 6. Mechanistic experiments suggested that hydrogen peroxide affects the course of the oxidation of 1 by adding to the first formed o-quinone to give a hydroxyquinone intermediate. This could bring nucleophilic attack to the o-quinone of 1 to give the dimer 4. These results disclose novel oxidative pathways of 4-alkylcatechols and provide an improved chemical basis to enquire into the mechanism of the antioxidant action of 1.     

A new route to the synthesis of ellipticine quinone from isatin

1-(2-Oxo-2-(pyridin-4-yl)ethyl)indoline-2,3-dione can be prepared and converted by treatment with sodium hydroxide into 2-isonicotinoyl-1H-indole-3-carboxylic acid as a key intermediate which can be transformed into ellipticine quinone in a two step sequence.     

Novel synthesis of 2,2-dialkyl-3-dialkylamino-2,3-dihydro-1H-naphtho[2,1-b]pyrans

2,2-Dialkyl-3-dialkylamino-2,3-dihydro-1H-naphtho[2,1-b]pyrans were prepared from 2-naphthol, a secondary amine, and 3-hydroxy-2,2-dialkylpropanal in the presence of a catalytic amount of p-toluenesulfonic acid. This one-pot reaction involves retro-aldol disintegration of 3-hydroxy-2,2-dialkylpropanal followed by formation of a Mannich base intermediate from 2-naphthol, a secondary amine, and formaldehyde (retro-aldol product). This Mannich base then disproportionates into a quinone methide intermediate and the secondary amine is regenerated. It then forms an enamine intermediate with 2,2-dialkylacetaldehyde (another retro-aldol product). Finally, the quinone methide intermediate undergoes electrocyclic ring closure with enamines to produce the title compounds.     

A facile electrochemical method for the synthesis of phenazine derivatives via an ECECC pathway

Electrochemical oxidation of 2,3-dimethylhydroquinone 1 has been studied in the presence of o-phenylenediamines 3a-c as nucleophiles in aqueous solution, using cyclic voltammetry and controlled potential coulometry. The results indicate that the quinone 2 derived from 2,3-dimethylhydroquinone participates in Michael addition and imine condensation reactions with o-phenylenediamine via an ECECC mechanism, and is converted to the corresponding phenazine derivatives 7a-c and 7′b. The electrochemical synthesis of compounds 7a-c and 7′b has been performed successfully at a carbon rod electrode in an undivided cell with good yields and high purities.     

Lead structures for applications in photodynamic therapy. Part 2: Synthetic studies for photo-triggered release systems of bioconjugate porphyrin photosensitizers

Photodynamic therapy (PDT) selectivity and specificity can be improved by binding the photosensitizers to target receptors. One approach is to cross-link porphyrins to a biological target receptor via the photocleavable o-nitrobenzyl linker, where a controlled released of the porphyrin can be monitored upon irradiation. The synthetic pathways involved esterification of a porphyrin-carboxylic acid and a unit containing the o-nitrobenzyl alcohol moiety and the bioconjugate. Reactions of a model porphyrin and o-nitrobenzyl alcohol using the carbonyl activating carbodiimide reagent DCC gave a stable N-acyl urea porphyrin, whereas use of EDAC (1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide hydrochloride) gave the desired compounds. Further studies were carried out on the attachment of carbohydrates (i.e., potentially receptor binding ligands) through such a linker to porphyrins. Preliminary irradiation experiments of such a compound show that upon UV irradiation (350 nm) for 80 min, approximately 50% of the porphyrin was cleaved to release the carboxylic acid porphyrin photosensitizer indicating the utility of such systems as photosensitizers delivery systems.     

Differentiation between o- and p-quinol/quinone functionality with a simple test using cyclic voltammetry

Complexation with boric acid combined with cyclic voltammetry serves to prove theo-quinol/quinone functionality in emmotin H (1) and I (2). Supported by similar investigation of four reference compounds the method is suggested as a quick and reliable test ofo-quinol/quinone functionality and for differentiation fromp-quinol/quinone functionality. Voluminous groups such as 2-isopropanolyl ino-position do not interfere with the complex formation. The test requires quantities of materials in the order of 10 to 100g.     

A photolabile protection strategy for terminal alkynes

We present a strategy for photolabile protection of terminal alkynes. Several photo-caged alcohols were synthesized via mild copper(II)-catalyzed substitution between tertiary propargylic alcohols and 2-nitrobenzyl alcohol to build up robust, base stable o-nitrobenzyl (NB) photo-cleavable compounds. We compare the new photolabile protecting group with the commonly used alkyne protecting group, 2-methyl-3-butyn-2-ol and the results show that NB ethers are stable under the cleaving conditions for the cleavage of methylbutynol protected alkynes. Additionally, we present the synthesis of photo-cleavable NB derivatives containing thiol groups that can serve as agents for photoinduced surface functionalization reactions.     

Structure and Conformation of Photosynthetic Pigments and Related Compounds. 12. A Crystallographic Analysis of Porphyrin-quinones and Their Precursors

Abstract. A comprehensive crystallographic analysis of 10 porphyrin quinone precursors (dimethoxybenzene derivatives), and six porphyrin quinones has been performed. The free bases and zinc(II) complexes of the porphyrin quinones are of the 5,10,15-triaryl/alkyl-20-quinone-porphyrin type and carry various bridging and quinone units. The structural and conformational parameters were determined for all compounds; the donor-acceptor separation distances range from 6.3 to 10.9 Å. Knowledge of these data is a prerequisite for a detailed interpretation of theoretical and spectroscopic studies on such systems. Despite the obvious influence of the type and geometry of the bridging unit and quinone on the spatial arrangement of the donor and acceptor components, a large variety of different packing arrangements in the crystal were observed. These include π stacking, aggregate formation and axial ligation in the zinc(II) porphyrins. The latter often utilized the quinone (or dimethoxy) oxygen atoms for coordination to zinc(II) centers leading to porphyrin quinone dimers and even polymers.     

Hetero-Diels-Alder reaction of photochemically generated α-hydroxy-o-quinodimethanes with trifluoromethyl ketones

Hetero-Diels-Alder reaction of α-hydroxy-o-quinodimethanes photochemically generated from o-tolualdehydes with trifluoromethyl ketones gave a mixture of hemiacetals and hydroxyaldehydes in fairly good yields. Their subsequent oxidation with PCC provided 1-isochromanones as formal oxidative [4+2] cycloaddition products. In contrast, similar reaction of aromatic ketones such as o-methylbenzophenone, 1-indanone, and α-tetralone gave exclusively the corresponding ketones having (trifluoromethyl)methylol groups at the o-position.     

A new benzoquinone-bridged porphyrin

The bridged porphyrin 6 was prepared via the bis(dipyrrylmethane) compounds 3, 4 and 5, by demethylation of 6 and oxidation the benzoquinone/porphyrin system 2 was obtained. The strong fluorescence quenching of 2, as compared to 6, is explained on the basis of an electron-transfer from porphyrin to quinone in the S₁ state.     

Synthesis of meso-furyl porphyrins with N4, N3S, N2S2 and N3O porphyrin cores

A series of meso-furyl porphyrins with four different porphyrin cores (N₄, N₃S, N₂S₂ and N₃O) were synthesized and characterized. The comparison of NMR, optical and fluorescence properties of meso-furyl porphyrins with porphyrins with six-membered aryl groups indicates that electronic properties of porphyrins were changed drastically on the introduction of furyl groups at meso positions. The maximum shifts in spectral bands were observed for meso-furyl porphyrins with N₂S₂ core. On protonation, the absorption bands of meso-furyl porphyrins were further red shifted. All these changes were ascribed to the possibility of more planarity of the meso-furyl porphyrins due to the small size of the furyl groups which results in extending the π-delocalisation of the porphyrin ring in to the furyl groups.     

Synthesis, Characterization, and Conductivity Studies of Poly-o-Methoxyaniline Intercalated into V2O5 Xerogel

In this work we study the conductivity properties of poly-o-methoxyaniline/V₂O₅ intercalation compounds obtained through intercalative polymerization of o-methoxyaniline with V₂O₅·nH₂O in hydrogel form and by reacting directly with V₂O₅ film in de-hydrated form (xerogel). These new compounds were characterized using Fourier-transform infrared and ultra-violet/visible spectroscopies, electron paramagnetic resonance, elemental analysis (C, N, H), thermogravimetric analysis, scanning electron microscopy, dc-conductivity and powder X-ray diffraction. For samples formed from V₂O₅ xerogel in film form, an increase in dc-conductivity and a decrease in the thermal activation energy in comparison with pure matrix was observed. The increase in conductivity is attributed to an increase of carrier density in the vanadium oxide lattice and the contribution of polarons from the polymer. On the other hand, for the intercalation compound obtained with V₂O₅·nH₂O in hydrogel form presents a very low room temperature conductivity value. The decrease in conductivity is due to the lack of connectivity of the various parts that compose the material.     

The meso-β-linkage as structural motif in porphyrin-based donor-acceptor compounds

Synthetic strategies for using the meso-β-linkage as a structural motif in electron transfer mimics have been tested. Exploratory syntheses of directly meso-β-linked bis- and trisporphyrins and the first representative X-ray structure of a meso-β-linked bisporphyrins are reported. The structure reveals a unique form of intramolecular π-π stabilization between one porphyrin and a meso-aryl substituent in a second porphyrin unit that accounts for the stability of different atropisomers in trimers. Using β-formyl porphyrins, dipyrromethanes, and suitable quinone precursor aldehydes, mixed condensations gave convenient access to porphyrin-porphyrin-quinone (P-P-Q) donor-acceptor systems consisting of a meso-β-linked bisporphyrin, a spacer, and a quinone acceptor.     

A novel and efficient synthesis of porphine

meso-Tetra(n-hexyloxycarbonyl)porphyrin was found to be converted into porphine, the mother compound of porphyrins, in a 77% yield when heated in aqueous sulfuric acid at 180 °C over 30 min under an inert atmosphere. The observation demonstrates that the substituted porphyrin serves as a novel and useful precursor for porphine.     

Iodine and iodic acid: an efficient reagent combination for iodination of aryl hydroxy ketones

A variety of ortho-hydroxy substituted aromatic carbonyl compounds were regioselectively iodinated with iodine and iodic acid in excellent yields.     

Synthesis of photolabile o-nitroveratryloxycarbonyl (NVOC) protected peptide nucleic acid monomers

The chemical synthesis of peptide nucleic acid (PNA) monomers was accomplished using various combinations of the o-nitroveratryloxycarbonyl (NVOC) group (N-aminoethylglycine backbone) and base labile acyl-type nucleobase protecting groups (anisoyl for adenine and cytosine; isobutyryl for guanine), thus offering a photolithographic solid-phase PNA synthetic strategy compatible with photolithographic oligonucleotide synthesis conditions and allowing the in situ synthesis of PNA microarrays in an essentially neutral medium, by avoiding the use of the commonly used deprotection reagents such as trifluoroacetic acid or piperidine. Convenient methods were also explored to prepare 1-(carboxymethyl)-4-N-(4-methoxybenzoyl)cytosine and 9-(carboxymethyl)-2-N-(isobutyryl)guanine with good yields.     

A computational study of the thermal opening of benzocyclobutenes to (E)- and (Z)-xylylenes

The structures of eleven 1-substituted benzocyclobutenes and corresponding (E)-o-xylylenes and (Z)-o-xylylenes have been calculated at the Becke3LYP/6-311G(d,p) level. Some o-xylylenes are plane and even some (Z)-isomers. In three cases (substituent: methoxy, amino and formamido groups), the (Z)-isomer is more stable than the (E)-isomer. The regioselectivity of the Diels-Alder reaction between (o)-xylylenes and propene or ethylvinylether is discussed according to the frontier OM coefficients.