1,4-Quinone unit

Among the naphthalenoid and benzenoid ansamycin antibiotics, some ansamycins possess a 1,4-naphthoquinone or a 1,4-quinone unit as a chromophore, and others possess a 1,4-hydroxynaphthalene or a 1,4-hydroquinone units (or their derivatives) as a chromophore. Since these two types of chromphore are in most cases reversible, it is not appropriate to classify by the difference of the oxidation stage of the chromophore. In this review, benzenoid and naphthalenoid ansamycins are further divided according to the difference of the length of their ansa chains. Thus, naphthalenoid ansamycins are divided into 3 groups, i.e., naphthalenoid ansamycins with C17 ansa chains, naphthalenoid ansamycins with C23 ansa chains, and naphthalenoid ansamycins with C9 ansa chains. The benzenoid ansamycins are divided into 2 groups, i.e., benzenoid ansamycins with C15 ansa chains and benzenoid ansamycins with Ci7 ansa chains. The relationships between, and the antibiotics within these groups are indicated in Scheme 2. 

The best yields of 5-hydroxyindoles are obtained when equimolar amounts of the quinone and enamine are used. An excess of enamine gives rise to non-indolic products derived from reaction of two enamine units and one quinone unit or the product which results from the initial Michael addition of the enamine to the quinone. Use of excess quinone has been reported less frequently, but limited studies indicate no advantage. When 2,5-dichloro-l,4-benzoquinone (32) was treated with a 50% excess of ethyl 3-... 

Electrochemical switching of the emission of a metal complex occurs in a system in which a luminescent (tris-bipyridine)ruthenium(ll) centre is linked to a quinone unit. Interconversion of the redox couple 124-125 allows the reversible switching... 

Although calix[n]arenes with Cn symmetry are most appealing we will summarize finally some examples with inherent C2 symmetry one of which 98 was already mentioned.185b Other C2-symmetrical examples were prepared by m-sub-stitution142 or by addition reactions to calixbisquinones143 with opposite quinone units along with the isomeric Cs-symmetrical compounds. 

Benzoquinones are highly conjugated molecules and are consequently coloured. 8enzo-1,4-quinone itself is yellow and benzo-1,2-quinones are often red. Chloranil forms glistening yellow platelets on crystallization from fuming nilric acid Quinones occur naturally and some natural pigments contain a quinone unit... 

The CV of the paracyclophanes containing one quinone unit showed reversible one-electron reduction step similar to the reference compound 2,5-dimethyl-1,4-... 

Diels-Alder reactions of 1235 synthesis of 1228, 1231, 1250, 1335 Quinone oxime tautomers 251, 252 Quinones—see also Aminoquinones, Anthraquinones, Arenolquinones, Benzoquinones, CaUxquinones, Hydroquinones, Hydroxyquinones, Naphthoquinones, Perylenequinones, Scabequinone reduction of 1108 Quinone units 1396... 

Among seven tetralones known to Aristolochia so far, four tetralones, aristelegone A (547), B (548), C (549) and D (550) have been described in A. elegans collected in Taiwan [168,252], The other three, 4-hydroxy-4,7-dimethyl-l-tetralone (551), 4,7-dimethyl-6-methoxy-l-tetralone (553) and aristolindiquinone (552) were reported from A. brevipes [273], A. tagala [384], and A. indica [92,385], respectively. All the tetralones except aristolindiquinone possess a keto function at C-l. Aristolindiquinone (552) is a fully aromatic tetralone with ap-quinone unit. 

The flexibility of the quinone components in respect to the porphyrin in structure 7, however, results in folding of the quinone units toward the porphyrin ring. As a result, the spatial separation of the charge-separated products formed in the multistep ET is not sterically optimized, and the extent of stabilization of the triad ion pair, P - Q1-Q2is quite low. 

An indole quinone unit A reduced indole quinone unit... 

However, if cysteine is present, it adds to the reactive o-quinone unit of dopaquinone, forming 5-S-cysteinyldopa (isomers of this species are also formed). Cyclization and dimerization produce the yellow to violet tri-chochrome pigments (trichochrome F and C, Figure 4-30), formerly reported by Flesh and Rothman as iron-containing pigments, which they called trichosiderins [63,64]. 

In contrast to the sequential introduction of bridging straps, Weiser and Staab used a one-step synthesis to prepare a porphyrin sandwiched between two parallel quinone units. Thus, condensation of the bis-aldehyde 252 with pyrrole in refluxing propionic acid yielded a mixture of oligomers and polymers, and three doubly-strapped porphyrins. The desired cross trans-linked porphyrin, obtained in 0.1%, was easily identified by its characteristic symmetric H-NMR spectrum. Demethylation and oxidation then yielded the desired bis-quinone porphyrin 253 (Scheme 76). 

The presence of the perz-hydroxyl group can also control the regiochem-istry of the coupling reaction. That is, the chelation between one of the two carbonyl groups of the quinone unit 16 and the hydroxyl group differentiates the electronic states of the two olefinic carbons. The cycloaddition reaction then proceeds in a regioselective manner (Scheme 8). This reactivity was used in the regioselective syntheses of y-rhodomycinone [20,21] and lacton-amycin [22,23]. 

Kurreck, H. Huber, M. Model reaetions for photosynthe-sis-photoindueed eharge and energy transfer between covalently linked porphyrin and quinone units. Angew. Chem.. Int. Ed. Engl. 1995. 34, 849. 

The (CH3)4N+ 4 salt has been crystallized [9]. X-ray diffraction shows a 1-D structure of stacked semiquinone units. Each anion is flat and symmetrical, and the spacing between anions in the stack is constant (3.31 A). Each (CH3)4N+ is held between the four oxygens of two anions in separate stacks. TTiese stacks are at al ut 90 to each other. It seems likely that the conductivity is along the stacks, so that the molecular length plays a minor role. It is important, however, for conductance, that there are two quinone units in one molecule. The presence of two quinones and one electron may impart some mixed-valence character to the species in the solid. 

When the reduction process is performed in aqueous (1.5%) acetonitrile, reduction of the quinone units is followed by protonation with formation of the corresponding hydroquinone. Protonation of the reduced species amounts to its stabilization. The oxidation peak of the hydroquinone moiety is in fact at --+0.96 V under the above conditions. Therefore, electron-transfer quenching from the hydroquinone to the excited complex is endergonic (AG-0.2 V) and its rate cannot compete with the luminescence decay of the excited state (Fig. 13). 

In an attempt to construct a rigid supramolecular systems capable to exhibit the above described type of behavior, Belser, et have synthesized compound 11 which contains the classical Ru(bpy)3 moiety fused with a quinone unit. In acetonitrile or water-acetonitrile solution 11 exhibits a visible absorption spectrum practically identical to that of Ru(bpy)3 , but no luminescence. This can be accounted for by electron transfer quenching from the MLCT excited state of the Ru(bpy)3 + unit to the quinone moiety. In CH2CI2 solution, however, where electron transfer is expected to be less favorable, 11 shows the usual Ru(bpy)3 luminescence. Electrochemical, photochemical and photophysical investigations on this and related complexes are under way in our laboratories. 

Another new route for the selective synthesis of di-functionalized pillar[5]arenes is oxidation and reduction of pillar[n]arene units (Scheme 3.15). Our group and Huang s group reported that the oxidation of one pillar[5]arene unit afforded pillar[5]arenes containing one benzo-quinone unit (3.92 and 3.93). Reduction of the benzoquinone unit yielded di-hydrojylated pillar[5]arenes at the A1/A2 positions (3.84 and 3.95). This method avoids the formation of many constitutional isomers and can be used for pillar[5]arenes with one benzoquinone and two hydro)q l groups at the A1/A2 positions. A clickable di-functionalized pillar[5]arene was prepared by modification of the alkyne moieties. The A1/A2 di-all e... 

Figure 6.16 Crystal structure of a per-allqrlated pillar[6]arene containing one benzo-quinone unit (6.26).

Analysis of thermally oxidised PET by fluorescence spectroscopy has shown the presence of hy-droxylated terephthalate units [523]. These groups are formed by hydroxyl radical attack on the terephthalate units and then undergo further oxidation to produce stilbene-quinone units causing discoloration of the polymer. Fluorescence analysis has also been used to study photobleaching and photo-induced discoloration reactions in EVA formulations [22]. Fluorescence spectroscopy offers a... 

Reactions with Imines. In some cases, BSF can react with imines to provide AA -acetal derivatives. For example, treatment of the quinone methide-substituted imine 3 (R = t-Bu, X = OAc, formed via lead(IV) oxide-mediated oxidation of Schiff base 2) provides the corresponding iV-formyl-lV-imino-lV,)V-acetal 4, with concurrent rearomatization of the quinone unit (eq 13). However, when this procedure was adapted for multigram scale, the yield of desired product was very low, with 9% epimer formation. Changing of the oxidant to DDQ allowed smoother reactions... 

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