Zinc reduction

This cleavage reaction is more often seen in structural analysis than in synthesis The substitution pattern around a dou ble bond is revealed by identifying the carbonyl containing compounds that make up the product Hydrolysis of the ozonide intermediate in the presence of zinc (reductive workup) permits aide hyde products to be isolated without further oxidation... 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

Bartonand Wolft achieved a similar transformations after introduction of a 5,6-double bond by zinc reduction of a 5,6-halohydrin. 

Although 3y5-hydroxy-A -l9-aldehydes can be obtained from the corresponding oxime by acid treatment, the most efficient method for the production of A -19-aldehydes is nitrosation of the oximes followed by zinc reduction of the 5a-halo hemiacetal. ... 

Unsaturated 19-norsteroids are also obtained by thermal decarboxylation of A -19-acids (obtained by zinc reduction of the 5a-halo-6/3,19-lactones). 

Carbonates, like esters, can be cleaved by basic hydrolysis, but generally are much less susceptible to hydrolysis because of the resonance effect of the second oxygen. In general, carbonates are cleaved by taking advantage of the properties of the second alkyl substituent (e.g., zinc reduction of the 2,2,2-trichloroethyl carbonate). The reagents used to introduce the carbonate onto alcohols react readily with amines as well. As expected, basic hydrolysis of the resulting carbamate is considerably more difficult than basic hydrolysis of a carbonate. 

Other zinc reductions have been used extensively. Zinc dust in aqueous ammonium chloride is a standard reagent for the reductive cyclization of nitro esters to hydroxamic acids. These reactions are usually carried out at low temperatures (0°-10°) to avoid further reduction. Despite the fact that good yields can often be obtained, these reductions are highly capricious, depending on the quality of the zinc (impurities seem to improve the reaction) and other unknown factors. 

Knighton, J. B. Steunenberg, R. K., "Preparation of Metals By Magnesium Zinc Reduction, Part II, Reduction of Plutonium Dioxide," ANL-7059, Argonne National Laboratory, Argonne Illinois, June 1965. 

It is through selecting a proper level of doping zinc oxide wildi elementary zinc that one can completely counterbalance the processes of zinc reduction and fixation on the adsorbent s surface to passivate thus the ZnO film as to the interaction with Hg. The film can be passivated... 

Reduction of the ketone (147), followed by elimination gave the olefin (148). The olefin (148) was subjected again to a second annelation, and as expected, dichloroketene addition, ring expansion and zinc reduction gave the tricyclic compound (149). Compound (149) could be converted to ( )-hirsutic acid C (150) 52K... 

A related Heck reaction of substituted o-bromoacetanilides with styrenes followed by selenium-induced cyclization of the resulting o-styiylacetanilides gives 2-arylindoles [378], Substituted o-bromonitrobenzenes react with ethyl vinyl ether under the influence of Pd(OAc)2 to give the corresponding o-ethoxyethenylnitrobenzenes. Zinc reduction then yields indoles [379]. The one-step Pd-catalyzed conversion of o-bromoanilines to indoles 302 with enamines (or with A/-vinyl-2-pyrrolidone) has been reported [380]. 

Cyclopentenones. Some time ago, Martin et al.1 reported that a,(3-unsaturated acid chlorides react with acetylene in the presence of 1 equiv. of A1C1, to give 5-chlorocyclopentenones. More recent research2 shows that this reaction is a convenient route to 4- and 5-substituted cyclopentenones after zinc reduction (equation... 

Lie and coworkers31 reported the synthesis and NMR properties of all geometrical isomers of conjugated linoleic acids. Pure geometric isomers of conjugated linoleic acid (CLA) were prepared from castor oil as the primary starting material. Methyl octadeca-9Z, 11 /i-dienoate (36) and methyl octadeca-9Z,llZ-dienoate (38) were obtained by zinc reduction of methyl santalbate (35, methyl octadec-11 -en-9-ynoatc) and methyl... 

Zinc Reduction. (This is a mild procedure used with great success on highly ring substituted derivatives.) CPB, 16(2), 217. 

Another Zinc Reduction. Prepare or activate the zinc as follows 400 g of mossy zinc is treated with 800 ml of 5% aqueous solution of mercuric chloride for 1 hour. Decant the solution off and use the zinc right away. Add. 834 mole of compound to be reduced to the zinc amalgam, followed by as much HCl acid (.834 mole) diluted in as much water as is required to cover all the zinc. Reflux for 6 hours while adding small portions of dilute HCL acid. Cool, separate the upper, wash free of acid (a few portions of dilute sodium hydroxide), dry and distill to get about a 79% yield of product. 

To reduce to the active formula, see the zinc reduction as given in the reduction section, CPB, 16, 217 (1968). This reduction is specifically matched to 3-bromo-4,5-methylenedioxy-B-nitrostyrene and other highly substituted ring type styrenes and propenes. Zinc reductions carried out properly are very gentle and do not destroy delicate ring substituents, while some reductions do. Zinc reductions can reduce any nitrostyrene or propenes, but some of these compounds must use the zinc reduction. Which compounds Compounds with lots of ring substituents, like 2,5-dimethoxymethylenedioxy, 3-methoxy-4-0-carbethoxy, etc. This is not to say that some of the other reductions are not capable of gentle reductions. 

Zinc in the presence of ammonium chloride reduces primary, secondary and tertiary aliphatic nitro compounds but yields of hydroxylamines are moderate and formation of coupling products is common. Zinc with or without ammonium chloride reduces aromatic nitro compounds (e.g. 75, equation 49) into hydroxylamines in moderate to good yield. However, it has been mentioned that the reaction is sensitive to the grade and quality of zinc dust (equation 50) and aromatic amines have been obtained as major products in zinc reduction reactions. ... 

The cyclodimerization of 1,3-butadiene and isoprene by zinc reduction of an iron nitrosyl complex dispersed in [BMIM]BF4 (and alternatively [BMIMJPFg) showed... 

Aristolactam (13) (in some papers, aristololactam) was first prepared by catalytic hydrogenation or zinc reduction in acetic acid from aristolochic acid I (25). It has been isolated from seveizl Aristolochic plants, including A. debilis and A. fangchi (24). Kupchan and Merianos isolated the first aristolactam iV-glucoside (19) from A. indica (37). 

Ethynylcarbazole was apparently formed by sodamide treatment of 9-dichlorovinylcarbazole (97) and zinc reduction of 97 gave 98. Where no reaction occurred with the Z isomer, 99 gave the dimer 100 with boron trifluoride the process presumably involves Lewis acid-catalyzed alkylation of one double bond by another double bond, complexed to boron trifluoride, followed by intramolecular Friedel-Crafts type alkylation at the carbazole 1-position. ... 

Another limitation is seen when extra strain is included in the compound to be reduced. Dehalogenation of 3,3-dichlorobicyclo[2.2.0]hexan-2-one with zinc/ammonium chloride in methanol gave, at best, a 25% yield of 3-chlorobicyclo[2.2.0]hexan-2-one (14) together with cyclohexenone and 6-chlorohex-5-enoic acid.128 The best results were achieved with the zinc/ acetic acid system, while addition of water, silver-promoted zinc reduction in methanol, tri-butyltin hydride reduction or hydrogenolysis with palladium in methanol did not result in formation of 14, but various other ring-opened products. 

In fact, the sensitive disubstituted alkene of 14 turned out to not be stable to the subsequent A1C1, coupling conditions, so the alkene and the secondary alcohol were protected together as the bromoether 15. Condensation of the derived enol ether 16 with the sulfone 2 in the presence of DTBMP (2,6-di-/-butyl-4-methylpyridine) then gave 17. Yamaguchi lactonization followed by regeneration of the alkene by zinc reduction completed the synthesis of 1. 

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