Bibenzyl compounds

Rycroft et al. (1999) identihed the major components of plants from six locations in western Scotland and four from the Azores using nuclear magnetic resonance (NMR) fingerprinting and GC-MS. The terpene P-phellandrene [129], which may be responsible for the aroma of material crushed in the held, was detected in all specimens. The major components, which appear in Fig. 5.6, were shown to be methyl eveminate [444], the four methyl orcellinate derivatives [445 8], the two 9,10-dihydrophenanthrene derivatives [449] and [450], the newly described phthalide killamiensolide [451], and the bibenzyl [453]. Methyl eveminate was the major compound in all 10 specimens other compounds were more varied in their occurrence. Killamiensolide was not isolated as such but was detected when extracts were acetylated yielding, among other compounds, [452]. The presence of the bibenzyl compound [453] in more than trace amounts in P. killarniensis raises the possibility that it represents contamination from P. spinulosa with which it was growing at the one site. 

The most famous of these compounds is combretastatin A-4 (CA-4,7), isolated by Pettit et al. in 1989 [30]. Pettit s research led to the isolation and structural determination of a series of phenanthrenes, dihydrophenanthrene, stilbene, and bibenzyl compounds [31]. CA-4 (7), alongside CA-1 (8), was found to be an extremely active inhibitor of tubulin polymerization [30,32]. The major problems associated with these compounds were poor bioavailability and low aqueous solubility [33,34], and hence, research in the field was turned to designing better alternatives with the hope of eradicating the negative properties of these potent compounds. 

Miyazawa, M., H. Shimamura, S.-I. Nakamura, et al. 1999. Moscathin from Dendrobium nobile, a naturally occurring bibenzyl compound with potential antimutagenic activity. /. Agric. Food Chem. 47(5) 2163-2167. 

According to the suggestion given by Gorm et al. [56], bibenzyl compounds are classified into four groups. However, a few groups of bibenzyls are new addition to... 

Our method is more efficient compared to the previous attempts to prepare functionalized bibenzyl compounds [40] in terms of mild conditions and speed of the reaction. More significantly, no transition metal catalyst (Pd or Ni) was required for completion of the reaction nor was a solvent required. 

Chromium atoms were cocondensed with benzyl sulfide at 77 K (35), the primary result being desulfurization to form bibenzyl and trans-stilbene. Coordination compounds were not characterized in this system. 

Model compound studies were also carried out in MeOH/KOH, and the results are shown in Table VI. Phenanthrene and biphenyl were quantitatively recovered unchanged by the reactions, and bibenzyl was recovered in 95% yield, with small amounts of toluene observed. Anthracene and diphenyl ether, on the other hand, were converted respectively to 9,10-dihydroanthracene and a mixture of polymethyl-phenols similar to that observed in the work with coal. The cleavage of diphenyl ether via hydrogenolysis should yield both benzene and phenol as products we saw no benzene in our study, and our... 

Model Component Studies. Model compound work with this system showed that anthracene was reduced to its 9,10-dihydro derivative (35% yield). Bibenzyl, on the other hand, was recovered unchanged, with only a trace of toluene observed. 

Note. In a recent paper, Miller and Stein have provided values for both C-C and C-0 bonds for a variety of coal model compounds, including bibenzyl and benzyl phenyl ether (11). Their rate constant for bibenzyl provides half-life values at 335°C and 400°C even larger than those discussed here, and it would seem on the basis of their data that at those low temperatures C-C scission in bibenzyl itself is too slow for thermal scission to be significant. 

DDQ was first introduced for the dehydrogenation of hydroaromatic compounds, such as tetralin and bibenzyl, which yield naphthalene and stilbene, respectively. A benzene ring or an olefinic bond provides sufficient activation, although it is sometimes difficult to force the reaction to completion. Phis high-potential quinone has since found wide a )plication, jiarticularly... 

PKSs are characterized by their ability to catalyze the formation of polyketide chains from the sequential condensation of acetate units from malonate thioesters. In plants they produce a range of natural products with varied in vivo and pharmacological properties. PKSs of particular note include acridone synthase, bibenzyl synthase, 2-pyrone synthase, and stilbene synthase (STS). STS forms resveratrol, a plant defense compound of much interest with regard to human health. STS shares high sequence identity with CHS, and is considered to have evolved from CHS more than once. ° Knowledge of the molecular structure of the CHS-like enzymes has allowed direct engineering of CHS and STS to alter their catalytic activities, including the number of condensations carried out (reviewed in Refs. 46, 51, 52). These reviews also present extensive, and superbly illustrated, discussions of CHS enzyme structure and reaction mechanism. 

It has recently been reported that radical bond breakage may be initiated by carbon black of high surface area. The conversion of a model coal compound, 4-( I-naphthyl-methyl) bibenzyl was accelerated at 375°C without hydrogen pressure in either the presence or absence of hydrogen donors (41, 42). This suggests that polarity or surface radical content of carbon black may initiate the decomposition of phenyl-methylnaphthyl linkages. 

Relative parent-peak sensitivities of the available Ci4Hi4 isomers, measured at the low-voltage conditions used in this study, are bibenzyl, 0.565, and 2-methyldiphenylmethane, 0.825. True concentrations of the C44 and presumably the C2i compounds are therefore somewhat different than estimates derived directly from isotopic distributions of the parent ions.]... 

Veith et al. [20] studied the correlation between Kow and (C is) reversed-phase HPLC retention time for a wide variety compounds, such as substituted benzenes, PAHs, and PCBs. Their calibration set consisted of benzene, bromobenzene, biphenyl, bibenzyl, DDE, and 2,2, 4,5,5 -pentachlorobiphenyl with measured log Kow values of 2.13, 2.99, 3.76, 4.81, 5.69, and 6.11, respectively. Similarly, Chin et al. 

The use of Nafion-H in de-ferf-butylation—in fact, trans-fcrf-butylation—has been extensively studied by Olah, Yamato, and co-workers. An early study established275 that ferf-butyl-substituted aromatics, when treated in the presence of a suitable acceptor compound, are easily de-ferf-butylated over Nafion-H used in catalytic amount [Eq. (5.106)]. Additional compounds including substituted biphenyls, bibenzyls, and cyclophanes, along with a range of substituted ferf-butylphenols, all gave de-ferf-butylated products in high (80-97%) yields. 

Available rate and thermochemical data allow estimates to be made for bond homolysis rate constants for virtually all covalent bonds presumed or found to be present in coal and model compound reactions. In Table III is compiled a list of coal-related homolysis rate constants, k, and bond homolysis half lives at 400°C, T1/2 (ti/2 = In 2/k is equal to the time required to break one-half of the bonds if homolysis were the only mode of reaction). Most of these values are estimated relative to measured rate constants for bibenzyl dissociation in tetralin, and relative values are expected to be only weakly dependent on solvent. It is evident from Table III, with few exceptions, that only bonds that yield two resonance stabilized radicals upon breaking are likely to undergo significant homolysis under coal liquefaction conditions. 

The compound D resulting from catalytic hydrogenation of trans-a-fluorostilbene is bibenzyl [5], and compounds E and F resulting from hydrogenation of difluoromaleic acid are fluorosuccinic acid and succinic acid, respectively [77]. Naturally similar hydrogenolyses of vinylic fluorine take place in the hydrogenation of difluorofumaric acid, fluoro-maleic and fluorofumaric acid, and many other unsaturated fluorides [77,72,73],... 

Finally, there is one aspect of the photoreduction of phenyl ketones which has been resolved only recently. It is now well established that benzyl radicals couple to form l-alkylidene-2,5-cyclohexadienes as well as bibenzyls, the ratio depending primarily on steric factors 121>. Schenck first showed that the semi-pinacol radicals involved in the photoreduction of benzophenone also form such products 122>. These compounds absorb very strongly in the near UV and often... 

Table V shows some sterically crowded salicylic acid derivatives. Compound 1 is bibenzyl and is included for comparison. The Air value indicates some degree of interaction between the aromatic rings, which can be considered as an intramolecular hydrophobic effect. Log F is known for both salicylic acid and for phenyl methyl sulfide, and these values can be added to give the expected value for Compound 2. Experimentally, log F is smaller than this, so some intramolecular interaction probably occurs.

Under the same conditions, experiments carried out with model compounds showed that benzyl sulfide underwent 100% conversion, the major product being diphenylmethane (from benzyl carbonium ion attack on benzene), whereas only 23% and 26% conversion to mixtures of unidentified compounds took place for n-heptyl sulfide and bibenzyl, respectively. 

The compounds were proposed to be dichloro C6-polychlorodibenzo-furans [46]. Similar observations from pulp mill samples were done by Kuehl et al. [47]. They suggested the structures of these compounds to be chlorinated xanthenes and xanthones. Buser et al. reported the occurrence of methyl-, polymethyl-, and alkyldibenzofurans in pulp mill sludge and sediments [48], Later, C5-PCBBs were detected as three dichloro and four trichloro congeners in pulp mill effluents, recipient sediment, and mussels incubated in recipient water [49]. Mass spectra of these congeners ruled out the structure of alkyl polychlorodibenzofurans and supported chlorinated alkyl bibenzyls instead, which was verified by model compound syntheses of the both structure types [50-52]. 

The compound (42) gives, on treatment with 3 M HCl, bibenzyl and stilbene in 79 and 7% yields, respectively, and reacts with PMes to give a 1 1 complex (43 equation 40). ... 

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