Zinc compounds accessibility

Organomercury and -zinc compounds, R2Hg, R2Zn, are no longer of interest for the syntheses of RBXj or RjBX . In some cases, however, there are organomercurials accessible from which organoboron chlorides can be prepared as shown in the synthesis of 9-borafluorenes ... 

A variety of l,2,3,4-tetrahydro-j8-carbolines have been prepared from 3-piperidone phenylhydrazone derivatives. Used initially to obtain pentacyclic derivatives (35) related to the yohimbe alkaloids, this route was later extended to the synthesis of tetracyclic compounds (36). l-Methyl-5,6,7,8-tetrahydro-j8-carboline (37) was prepared in low yield by heating cyclohexanone 2-methyl-3-pyridylhydrazone with zinc chloride, a synthesis probably based on the similar preparation of the tetracyclic compound 38 starting from the corresponding quinolylhydrazine. Abramovitch and Adams extended this approach to the cyclization of cyclohexanone 3-pyri-dylhydrazone (39) itself. The main product was 6,7,8,9-tetrahydro-8-carboline (40), a smaller amount of the j8-isomer (41) also being obtained. This provides a convenient and readily reproducible route to the otherwise difficultly accessible 8-carboline ring system. The favored attack at carbon-2 over carbon-4 of the pyridine nucleus... 

Whereas the utility of these methods has been amply documented, they are limited in the structures they can provide because of their dependence on the diazoacetate functionality and its unique chemical properties. Transfer of a simple, unsubstituted methylene would allow access to a more general subset of chiral cyclopropanes. However, attempts to utilize simple diazo compounds, such as diazomethane, have never approached the high selectivities observed with the related diazoacetates (Scheme 3.2) [4]. Traditional strategies involving rhodium [3a,c], copper [ 3b, 5] and palladium have yet to provide a solution to this synthetic problem. The most promising results to date involve the use of zinc carbenoids albeit with selectivities less than those obtained using the diazoacetates. 

In 1991, Li and Chan reported the use of indium to mediate Barbier-Grignard-type reactions in water (Eq. 8.49).108 When the allylation was mediated by indium in water, the reaction went smoothly at room temperature without any promoter, whereas the use of zinc and tin usually requires acid catalysis, heat, or sonication. The mildness of the reaction conditions makes it possible to use the indium method to allylate a methyl ketone in the presence of an acid-sensitive acetal functional group (Eq. 8.50). Furthermore, the coupling of ethyl 2-(bromomethyl)acrylate with carbonyl compounds proceeds equally well under the same reaction conditions, giving ready access to various hydroxyl acids including, for example, sialic acids. 

The authors describe experiments on six NR compounds commonly used for latex gloves to determine the potential hazards of skin contact with reference to curing compounds and accelerators, including mercaptobenzthiazole and zinc dialkyldithiocarbamates. Simulation methods and the influence of a common hand disinfectant were investigated. In the case of tetramethylthiuram disulphide, it was discovered that decomposition products could be formed on the skin which are not present in the original rubber. 6 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. Accession no.849747... 

The addition of carbonyl compounds towards lithiated 1-siloxy-substituted allenes does not proceed in the manner described above for alkoxyallenes. Tius and co-work-ers found that treatment of 1-siloxy-substituted allene 67 with tert-butyllithium and subsequent addition of aldehydes or ketones led to the formation of ,/i-unsaturated acyl silanes 70 (Scheme 8.19) [66]. This simple and convenient method starts with the usual lithiation of allene 67 at C-l but is followed by a migration of the silyl group from oxygen to C-l, thus forming the lithium enolate 69, which finally adds to the carbonyl species. Transmetalation of the lithiated intermediate 69 to the corresponding zinc enolate provided better access to acylsilanes derived from enolizable aldehydes. For reactions of 69 with ketones, transmetalation to a magnesium species seems to afford optimal results. 

The zinc promoted cycloaddition of a,a -dibromoketones to 1,3-dienes is enhanced by ultrasound to such an extent that compounds such as the highly hindered bicy-clo[3.2.1]oct-6-en-3-ones (13) become easily accessible (Eq. 3.12) [96]. Reactions of this type, in the absence of ultrasound, give only low to moderate yields and require much longer reaction times of the order of 24 h. 

The requisite hydroxylamine function for such cyclizations can also be generated from a precursor having a nitro group. This novel route has provided access to hitherto unknown l-hydroxy-6-allyl-, and -6,6-bisallyl-piperazine-2,5-diones (91UP1). The starting material is an W-nitroacetyl amino acid ester that can be either mono-or bis-allylated at the methylene adjacent to the nitro group. Reduction of the N02 to NHOH using zinc/ ammonium chloride, followed by cyclization, leads to the desired products (Scheme 76). Compound (215) is unique in that it possesses a chiral center at C-3 and a quaternary carbon at C-6 on a l-hydroxypiperazine-2,5-dione system. 

A large variety of reducing agents have been proposed for this reduction. However, zinc and sodium hydroxide offer the most common system, and lithium aluminum hydride merits consideration. The reduction of azoxy compounds with lithium aluminum hydride has value mainly in structural determinations. Its importance as a preparative procedure is limited normally such a reaction sequence would be a matter of putting the cart before the horse. The reduction of azines has potential value because of the accessibility of azines unfortunately, only under specialized circumstances has it been possible simply to add the required gram-molecule of hydrogen to the structure. Usua-ally, chlorine is added to an azine structure to produce dichloro azo compounds. An extension of the reaction permits the preparation of a,a -diacyl-oxyazoalkanes from azines. 

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

Because 6-deoxyhex-5-enopyranose derivatives can be readily converted into 2-deoxyinososes,91 5-bromides (for example, 164) give useful access to such carbocyclic compounds.92 In this way, octa-0-acetyl-/ -maltose was converted by treatment with zinc-acetic acid into the 5-ene by way of the corresponding 5-bromide 30 and thence into the pseudo-disaccharide compound (169) which is related to components of members of the aminoglycoside antibiotic series.36... 

Compound 32 may be removed, after the Ugi reaction, under particularly mild conditions, thanks to sulfur activation by soft electrophiles, such as mercury salts. The yields obtained in zinc-mediated Ugi reactions are excellent and the diastereo-meric ratios are in line with those obtained with 27. Cleavage of the chiral auxiliary can be performed, after methylamine-promoted deacylation of the sugar hydroxy groups, by a diluted solution of CF3CO2H in the presence of Hg(OAc)2. Under these conditions the acyl group on nitrogen is retained. However, the enantiomer of 32 is not easily accessible. 

Of the several types of enals derivable from aldoses (see 80 and 81), the 5,6-dideoxy-hex-5-enoses are of particular significance because of the efficient cyclization to give cyclopentane derivatives that they undergo on treatment with A-alkylhydroxylamines. The reactions are spontaneous and involve intramolecular 1,3-dipolar cycloadditions undergone by intermediate nitrones. For example, compound 272, made by treatment of 6-bromo compound 271 with zinc in moist alcohol, on reaction with A-methylhydroxylamine gives the bicyclic product 273 in 80% yield (Scheme 28).256 This process gives simple access to many functionalized cyclopentanes. 

Triaryl telluronium compounds can be prepared by the quick addition of a solution of tellurium tetrachloride (1 mol) in diethyl ether to an excess of the Grignard reagents (5 mol) (Vol. IX, p. 1080). Trialkyl telluronium compounds do not seem to be accessible in this manner6. In contract, tellurium tetrachloride and diethyl zinc produced triethyl telluronium chloride (Vol. IX, p. 1080). 

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