Experiments on the total synthesis of Lysolipin I. Part II. Michael addition of 1,3-cyclohexanedione to quinone acetals

Base-catalyzed reaction of 1,3-cyclohexanedione ( 3 ) with the quinone monoacetals 4 and 7 leads to the polycyclic products 5 and 8 , respectively, and in the case of 4 to variable amounts of dibenzofuranone 6 . The 2-arylcyclohexanedione 9 , on the other hand, is isolated from the reaction of 3 and bisacetal 11 catalyzed by ZnCl₂ (Scheme 2). Treatment of the adduct 8 with (CH₃O)₂SO₂/K₂CO₃ results in cleavage of teh heterocyclic ring by a retro-Michael reaction affording teh liable enone 23 which was further transformed to 24 by selective hydrogenation. The 8-acetoxydibenzofuranone 22 is obtainable from 8 by acid treatment and acetylation (Scheme 4). The reactions of the silylenol ethers 27 and 35 with quinone monoacetals were very complex (Scheme 6). The desired arylcyclohexanone derivatives 28 and 36 were formed in very low yields. Under certain conditions (elevated temperature or strong Lewis acids as catalysts), single-electron transfer or addition to the ene-acetal rather than to the enone function of the quinone monoacetals became predominant. In connection with this study, the sensitive 2-methoxy-p-benzoquinone monoacetals 15 (Scheme 3) and 29 (Scheme 6) have been prepared and characterized.     

3+2] coupling of quinone monoacetals by combined acid-hydrogen bond donor

The expeditious and efficient [3+2] coupling approach of quinone monoacetals 1 with alkene nucleophiles 2 by the action of an activated Br?nsted acid in the presence of a hydrogen bond donor perfluorinated alcohol has been achieved. With the optimized combined acid, the reaction could proceed under mild conditions by only mixing the two reactants to afford the cycloadducts 3 in a short time (within 10 min) with good to quantitative yields.     

Quinones and quinone methides—V: Further rearrangements of the dimer from 5-methoxy-2-(4-methoxyphenylmethyl)-1,4-benzoquinone

In strongly acid solutions the dimer 3 dissociates to the quinone 1 and quinone methide 2 which recombine with elimination of p-methoxybenzyl alcohol to form the xanthylium salt 9a. With zinc and acetic acid 3 yields isormeric meso- and dl-ethylenediquinol derivatives 14a. Sodium borohydride reduction of 3 yields an alcoholic quinol 5a which rapidly reoxidizes to the spiro-tetrahydrofuran derivative 24a. Possible mechanisms which may be involved in the formation of these rearranged products are discussed.     

Regioselective routes towards 14-hydroxyabietane diterpenes. A formal synthesis of immunosuppressant (−)-triptolide from (+)-abietic acid

Two procedures to introduce an oxygenated function into the C-14 of abietane diterpenes with complete regioselectivity have been developed. Utilizing these, the synthesis of the antileishmanial quinone (−)-12-deoxyroyleanone (1) and a formal synthesis of antitumour and immunosuppressant (−)-triptonide (7) and (−)-triptolide (8) from (+)-abietic acid (13) have been carried out.     

A luminescence probe for measurements of electron-exchange rates in polymer films on electrodes

The effect of metal ions on the, reduction of 1,10-phenanthroline-5,6-quinone (1), 5,8-quinolinequinone (II) and 6,7-dichloro-5, 8-quinolinequinone (III) has been investigated in 50% dimethylsulfoxide+water solvent. 1 contains the 1,10-phenanthroline structure in both its quinone and hydroquinone forms, while II and III contain the 8-hydroxyquinoline structure in the hydroquinone forms. Complexation of the hydroquinones of II and III by metal ions causes positive shifts in the quinone half-wave potentials. These shifts have been used to calculate conditional formation constants for Pb²⁺(II) and Pb²⁺ and Zn²⁺(III). The quinone form of I binds strongly to Ni²⁺, Co²⁺ and Zn²⁺ but not to Ca²⁺. Mg²⁺. Mg₂₊. Mn²⁺ and Pb²⁺. With the latter four metals, binding to the hydroquinone from of I was detected and formation constants were determined. In addition to binding both the quinone and hydroquinone forms at the nitrogen atoms, Ni₂₊ Co²⁺ and Zn²⁺ formed complexes at the 1,2-dihydroxy site of the hydroquinone of I.     

Synthesen von Dibenzo[b,e][1,4]dioxin-2,3-chinonen,einschliesslich der Ecklonochinone A und B sowie der Isoecklonochinone A und B

Syntheses of Dibenzo[b,e][1,4]dioxin-2,3-quinones Including the Ecklonoquinones A and B and the Isoecklonoquinones A and B Oxidation of monomesyloxy-substituted pyrocatechols with MnO₂ in toluene using phase-transfer conditions leads in high yield to monomesyloxy-substituted dibenzo[b,e][1,4]dioxin-2,3-quinones with loss of one mesyloxy group. In this way, ecklonoquinone A ( 2 ), ecklonoquinone B ( 3 ), isoecklonoquinone A ( 43 ), and isoecklonoquinone B ( 44 ) were prepared. Their structures are based on X-ray analyses of ecklonoquinone-A leucoacetate ( 45 ) and the mesyloxy-substituted quinone 20 . The reddish-violet dibenzodioxin-diquinone 49 was prepared from an intermediate of the iso-series. The parent compound 1 has been synthesized in yields better than 50% from pyrocatechol and methyl 2,5-dioxo-2,5-dihydrobenzoate as oxidant and 2-methoxypyridin as catalyst. To rationalize the specific effect on the dimerisation step of the mesyloxy group, the intermediacy of 1,4-quinone monoacetals is proposed. This also applies to a proposed biogenetic scheme.     

The first total synthesis of hibarimicinone,a potent v-Src tyrosine kinase inhibitor

The first total synthesis of hibarimicinone has been achieved. The polyhydroxydecalin moieties (AB and GH rings) have been synthesized from sulfonylenone 4 derived from d-arabinose. The chiral biaryl 20 was coupled with two polyhydroxydecalins 11 by Michael–Dieckmann type condensation to give the eight rings system. Aromatization and oxidation with Ag⁺ gave quinone 24, and the subsequential transannular etheration gave the hibarimicinone skeleton. Deprotection and tautomerization were performed in one pot to give hibarimicinone (1).     

Products of the addition of 1,4-naphthoquinone to an acecyclone

The major products from the exposure of tetracyclic keto-alcohol 1 to excess 1,4-naphthoquinone in the presence of acid were a hexacyclic quinone (6) and a heptacyclic dihydrobenzofuran derivative (8).     

Beobachtungen zum mechanismus der Nenitzescu-reaktion

The structure of the carbmolamine 5 was recognised as an intermediate in the Nenitzescu-reaction but until recently it had been formulated as the quinone derivate 3. The course of the Nenitzescu-reaction in acetic acid would be understood if the last step by reduction with hydroquinone of 5 to the 5-hydroxyindol derivative 6 could be clarified. The 4,4′ and 4,6′-diindole derivates 11 and 12 as well as the acetoxyhydroxy-indolederivative 10 h in the absence of hydroquinone were established.     

Efficient Coupling Reaction of Quinone Monoacetal with Phenols Leading to Phenol Biaryls

A simple and efficient synthesis of phenol biaryls by the cross‐couplings of quinone monoacetals (QMAs) and phenols is reported. The Brønsted acid catalytic system in 1,1,1,3,3,3‐hexafluoro‐2‐propanol was found to be particularly efficient for this transformation. This reaction can be extended to the synthesis of various phenol biaryls, including sterically hindered biaryls, with yields ranging from 58 to 90 % under mild reaction conditions and in a highly regiospecific manner.     

Anomalous behavior in the two-step reduction of quinones in acetonitrile

The electrochemical reduction of eight quinones, 9,10-anthraquinone (1), duroquinone (2), 2,6-di-tert-butyl-1,4-benzoquinone (3), 2,6-dimethoxy-1,4-benzoquinone (4), 9,10-phenanthrenequinone (5), tetrachloro-1,2-benzoquinone (6), tetrabromo-1,2-benzoquinone (7) and 3,5-di-tert-butyl-1,2-benzoquinone (8), have been studied in acetonitrile. In every case it was found that cyclic voltammograms differed in significant ways from those expected for simple stepwise reduction of the quinone to its radical anion and dianion. The various types of deviations for the eight quinones have been cataloged and some speculation is offered concerning their origins.     

Regioselective single and double conjugate additions to substituted cyclohexa-2,5-dienone monoacetals

[reaction: see text]. A series of quinone monoacetals bearing electron-withdrawing groups was treated with diethyl malonate and other bifunctional nucleophiles in the presence of KO-t-Bu in THF. Reactions of ethyl 3-nitropropionate or diethyl malonate resulted in single conjugate addition adducts. When ethyl acetoacetate was used as a nucleophile, bridged bicyclic products were obtained in good yields. The regiochemistry of conjugate addition was dependent on the quinone monoacetal substitution.     

Antibacterial activities of a few prepared derivatives of oleanolic acid and of other natural triterpenic compounds

Oxidizing, phosphorus, ester and amide derivatives of oleanolic acid 1 have been prepared. The antibacterial activity of compound 1, isolated from the fruit barks of Periploca laevigata (Asclepiadaceae), and its derivatives have been tested using Tween-80 as complex agent to form a water-soluble triterpenes. The same activity of maslinic acid acetate 2, β-amyrin 3, and its acetylated derivative 3a (Fig. 1), isolated from the same source as that of oleanolic acid 1, have also been investigated. To cite this article : F. Hichri et al., C.R. Chimie 6 (2003).     

Nematicidal quinone derivatives from three Rubia plants

Five new quinone derivatives, rubiasins D-F (1–3), rubialatones A and B (4 and 5), have been isolated from the roots and rhizomes of three Rubia species, Rubia alata, R. wallichiana and R. schumanniana, together with 26 known quinones (6–31). Their structures have been elucidated on the basis of NMR, MS spectra and computational methods. The compounds have been evaluated for their toxicity to the saprophytic nematode Caenorhabditis elegans and the root-knot nematode Meloidogyne incognita. Compounds 1, 6 and 7 showed toxicity to C. elegans with the LC₅₀ values at 8.50, 9.44 and 44.82?μg/mL, respectively; and 6 showed toxicity to M. incognita with the LC₅₀ value at 35.22?μg/mL. Meanwhile, compounds 1 and 6 showed inhibitory effect on egg hatch of C. elegans with the LC₅₀ values at 5.60 and 48.95?μg/mL.     

Towards lignin-protein crosslinking: amino acid adducts of a lignin model quinone methide

The polyaromatic structure of lignin has long been recognized as a key contributor to the rigidity of plant vascular tissues. Although lignin structure was once conceptualized as a highly networked, heterogeneous, high molecular weight polymer, recent studies have suggested a very different configuration may exist in planta. These findings, coupled with the increasing attention and interest in efficiently utilizing lignocellulosic materials for green materials and energy applications, have renewed interest in lignin chemistry. Here we focus on quinone methides (QMs)—key intermediates in lignin polymerization—that are quenched via reaction with cell-wall-available nucleophiles. Reactions with alcohol and uronic acid groups of hemicelluloses, for example, can lead to lignin-carbohydrate crosslinks. Our work is a first step toward exploring potential QM reactions with nucleophilic groups in cell wall proteins. We conducted a model compound study wherein the lignin model compound guaiacylglycerol-β-guaiacyl ether 1, was converted to its QM 2, then reacted with amino acids bearing nucleophilic side-groups. Yields for the QM-amino acid adducts ranged from quantitative in the case of QM-lysine 3, to zero (no reaction) in the cases of QM-threonine (Thr) 10 and QM-hydroxyproline (Hyp) 11. The structures of the QM-amino acid adducts were confirmed via 1D and 2D nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations, thereby extending the lignin NMR database to include amino acid crosslinks. Some of the QM-amino acid adducts formed both syn- and anti-isomers, whereas others favored only one isomer. Because the QM-Thr 10 and QM-Hyp 11 compounds could not be experimentally prepared under conditions described here but could potentially form in vivo, we used DFT to calculate their NMR shifts. Characterization of these model adducts extends the lignin NMR database to aid in the identification of lignin-protein linkages in more complex in vitro and in vivo systems, and may allow for the identification of such linkages in planta.     

A General Synthetic Route to Optically Active Alkylpyridines from Alkylsubstituted Succinaldehyde Monoacetals

Succinaldehyde monoacetals (I), a class of organic compounds accessible through hydroformylation of α, β-unsaturated aldehyde acetals using rhodium catalysts¹, are very useful intermediates in several organic syntheses.^1,2 On searching for more simple and efficient synthetic methods for obtaining consistent amounts of optically active mono- and dialkylpyridines (IV), we have investigated the homologation of compounds I to the corresponding glutaraldehyde monoacetals (II) and related 1,5-dioxo compounds or their precursors, which are the most usual starting products for preparing pyridine derivatives.^3,4,5     

Structure and electronic properties of quinone dimers connected with acetylene and diacetylene linkages

Quinone dimers connected with acetylene (QAQ) and diacetylene linkages (QAAQ) have been synthesized and their structure and electronic properties studied. X-ray analysis, DFT calculations, and UV-vis measurements showed that, unlike directly connected quinone dimers (QQ), they had planar and thus efficiently extended π conjugation systems. The respective reduction potentials of QAQ and QAAQ were considerably raised, and QAQ thereby behaved as a mild oxidizing agent.     

The electrochemical methoxylation of methoxybenzenes and related compounds : The mechanism of electrochemical methoxylation of 1,4-dimethoxybenzene

Anodic oxidation of benzenoid aromatic ethers in methanolic KOH soln at a platinum electrode constitutes a one step method for the synthesis of the rare class of compounds, herein designated as the quinone diketals. The mechanism of conversion of 1,4-dimethoxybenzene to the diketal of p-benzoquinone, based on the evidence accumulated, is considered to proceed via discharge of the adsorbed aromatic compound, followed by nucleophilic reaction of the solvent system with the electrogenerated cationic species. The quinone diketal products resulting from anisole, 1,4-, 1,2-, and 1,3-dimethoxybenzene, 9,10-dimethoxyanthracene and benzodioxane have been established. In contrast methyl benzoate, anisonitrite, and benzene were found to be unreactive in agreement with a direct discharge scheme which does not involve methoxy radicals.     

Reaction of diazomethane with 5-hydroxy-4-phenylabonzofurazan as a synthetic route to derivatives of a new heterocycle—pyrazolo [3,4-f]-1,2,3-benzotriazole

Diazomethane adds to 5-hydroxy-4-phenylazobenzofurazan 1 giving a compound 3 which rearranges in alkaline solution to the 8-monoxime of 2-phenyl-5(6)H-pyrazolo [3,4-f]-1,2,3-benzotriazole-4,8-dione 4, the structure of which has been demonstrated by chemical and spectroscopic methods. The probable structure of adduct 3 is discussed on the basis of the chemical reactivity of the compound. The orientation of the cycloaddition and the dipolarophilic behaviour of the 6,7 double bond of the benzofurazan are interpreted on the basis of the quinone hydrazone struture of 1.     

A selective colorimetric chemosensor for thiols based on intramolecular charge transfer mechanism

Compound 1 as an electron donor-acceptor compound with N,N-dimethylaniline and quinone units was designed for a highly selective colorimetric determination of thiol-containing amino acids and peptides, by making use of the unique reactivity of thiol towards quinone. Compound 1 shows a strong intramolecular charge transfer (ICT) band around 582 nm; but, it decreased after addition of either cysteine (Cys) or glutathione (GSH). Moreover, the ICT band intensity at 582 nm decreased linearly with the increasing concentrations of Cys or GSH. The interference from other amino acids can be neglected. Therefore, compound 1 can be employed as a selective colorimetric visual chemosensor for thiol-containing amino acids and peptides.