Dimerization naphthoquinones

Prajoubklang A et ai. Bioactive deoxypreussometins and dimeric naphthoquinones from Diospyros ehretioides fruits Deoxypteussomerins may not be plant metabolites but may be from fungal epiphytes ot endophytes, Chem Biodivers... 

A series of monomeric and dimeric naphthoquinones with potential for treatment of Leishmania infections were identified in vitro using a direct cytotoxicity assay against extracellular promastigotes of Leishmania spp. [196,197]. Lapachol was evaluated in vitro against intracellular amastigotes of Leishmania braziliensis and then tested in an animal model (hamster) to try to reproduce the leishmanicidal activity. In vitro lapachol exhibited an... 

Emadi A, Ross AE, Cowan KM, Fortenberry YM, Vuica-Ross M. A chemical genetic screen for modulators of as3nnmetrical 2,2 -dimeric naphthoquinones c5dotoxicity in yeast PLoS One. 2010 5(5) el0846. doi 10.1371/journal.pone.0010846. 

Aphids make their own pigments, called aphins, not found in any other insects. Their complex chemistry was studied by Lord Todd in the 1950s and 60s. Aphins are dimeric naphthoquinones. The two most important are protoaphin-y (isolated first from the common bean aphid Aphis... 

Side reactions that occur with intramolecular cycloaddition, such as linear oligomerization or dimerization of the nitrile oxide, are not very common when shorter chain lengths n < 1) are used due to the entropically favored intramolecular process. A rather unusual result in this regard involves the formation of a fused cyclooctane instead of the less-strained six-membered ring (also fused) in the cycloaddition of the nitrile oxide derived from p-naphthoquinone (Scheme 6.43). This result is consistent with the effect of electron-withdrawal in the enedione part, leading to increased reactivity (247), and also reflects the known sluggishness of cyclohexenes towards nitrile oxides (cf. Section 6.2.1.2). 

However, attempts by Kiefer and Carlson59 to prohibit undesired bi-molecular reactions by irradiating 2,3,3-trimethyl-l-penten-4-one adsorbed onto silica gel were unsuccessful (due probably to steric inhibition of adsorption) the product composition was the same as that previously obtained in solution. Werbin and Strom80 attempted to restrain the freedom of movement of the radicals formed from the photolysis of vitamin K3 (2-methyl-1,4-naphthoquinone) by adsorption onto silica gel, but obtained the same mixture of dimers as that obtained from the irradiation in acetone solution, viz., syn and anticyclobutanes, an oxetane dimer, and a binaphthoquinone dimer. Photolysis of the solid substrate, however, produced only the syn isomer of cyclobutane, in this case no migration of radicals is possible, hence only one product. 

It is pertinent at this point to refer briefly to the sources of quinone methides, though these have been reviewed (B-74M122400). The general approach used in chroman syntheses involves the thermal elimination of HX from an -substituted phenol. Commonly the eliminated molecules are water, methanol or dimethylamine (287 X = OH, OMe, NMe2, respectively). However, these methods are not entirely suitable because the eliminated molecules may promote side reactions. In the case of 1,2-naphthoquinone 1-methide, the thermal dissociation of the spirodimer (288) is a better source than the other methods. Its formation represents another example of dimerization by a [4+2]-cycloaddition, since it is prepared by heating l-dimethylaminomethyl-2-naphthol in dodecane or xylene with careful exclusion of moisture (73JCS(P1)120,81CJC2223). 

Oxidation. Very vigorous oxidation of the thiophene ring results in breakdown to maleic and oxalic acids and ring sulfur is oxidized to sulfuric acid. Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide 54 and 2-hydroxythiophene sulfoxide 56. These compounds can be trapped by addition to benzoquinone to give ultimately naphthoquinones 55 and 57. Dimerization via a DielsAlder reaction gives 58. 

Hence, complex (90) was obtained by reaction of [(nbd)Pd(ma)] with one equivalent of IMes and one equivalent of ma NHC-naphthoquinone and benzoquinone dimers (91) and (92) were prepared in the same manner from... 

In this chapter, we include certain natural products, such as typical anthraquinones and naphthoquinones, which are present, along with condensed dimeric structural drivatives such as dianthrones, in many medicinal plants. In addition, we deal with the much more infrequent OT-r/io-quinones. 

In 1976, using the parent quinine as a catalyst and 30% aqueous hydroperoxide as an oxidant, the asymmetric epoxidation of cyclic enones, such as naphthoquinones, was explored by Wynberg and coworkers [13]. However, the resulting enantioselec-tivity was minimal. Their further attempt under biphasic conditions with N-benzyl-quininium chloride 1 as the catalyst and t-BuOOH as the oxidant also resulted in very low enantioselectivities (up to 20% ee for the epoxidation of various cyclohexe-nones) [14]. However, the use of the dimeric form of Wynberg s catalysts 6 and 7 resulted in somewhat better (up to 63% ee with cyclohexenone) asymmetric... 

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