Zinc iodide reduction

It is not possible to use zinc for reductive debromination in the presence of (x-halo ketones and for transformations involving these intermediates, sodium iodide has been used. ° In some instances, e.g. 5,6-dihalo-3-ketones, iodide does not always give a completely halogen-free product, and zinc does not give clean debromination. The use of chromous chloride has proved advantageous in such cases and is the reagent of choice for vicinal dichlorides, which are inert to iodide ... 

In the preparation of iodides, but not bromides, PMHS may be substituted for the TMDO. Chlorides can be obtained if thionyl chloride and zinc iodide are added to suppress the formation of symmetrical ethers.314 An example of this type of reductive chlorination is shown by the TMDO-mediated conversion of p-tolualdehyde into p-methylbenzyl chloride (Eq. 201).313 To obtain chlorides from aldehydes having electron-withdrawing groups such as nitro or carbomethoxy, the initial reaction is first carried out at —70° and the mixture is then heated to reflux in order to reduce the formation of symmetrical ether by-products. Zinc chloride is substituted for zinc iodide for the synthesis of chlorides of substrates with electron-donating groups such as methoxy and hydroxy.314... 

REDUCTION WITH ZINC AND SODIUM IODIDE Reductive Cleavage of Sulfonates to Hydrocarbons [700]... 

Ketoester 208 derived from l-(2-nitrophenyl)-lH-pyrrole and ethyl oxalyl chloride can be selectively reduced at the keto group with zinc iodide and sodium cyanoborohydride. Further reduction of the nitro group and cyclization to lactam 209 has been accomplished by treatment with stannous chloride in refluxing ethanol (Scheme 43 (2003BMCL2195)). 

Reduction of the unstable alcohol 116 is achieved using sodium cyanoborohydride in the presence of zinc iodide to produce the saturated compound 117 <2001SM(119)99> (Equation 30). 

Possible chemical mechanisms of methane release have been investigated by studies of the properties of isolated coenzyme F-430 and related nickel complexes. Jaun and Pfaltz [121] demonstrated the formation of methane from the Ni(I) state of F-430 with methyl iodide or methyl sulfonium as methyl donor, though methyl CoM was not effective. With zinc as reductant the reaction became... 

Purines are some of the most ubiquitous heterocydes. The quantity of naturally occurring purines produced on the earth is enormous, as 50% of ribo- and 2 -deoxyribonucleic acid (RNA, DNA) bases are purines. Purine is a colorless, crystalline weak base which was first prepared by E. Fischer by zinc dust reduction of 2,6-diiodopurine, which had been obtained from 2,6,8-trichloropurine by reaction with hydrogen iodide and phosphonium iodide. 

Hydroxymethyl-1,4-benzodioxin (137) obtained in 80% yield by reduction of ethyl 1,4-benzo-dioxin-2-carboxylate (39) with lithium aluminum hydride in refluxing ether <91TL5525> reacted with zinc azide bis-pyridine complex under Mitsunobu conditions (triphenylphosphine, diisopropyl azodicarboxylate) to yield exclusively compound (138) in 75% yield. Otherwise, (137) was first reacted with zinc iodide under the same conditions until complete transformation of the starting material into the mixture of regioisomers (139) and (140) excess of dry piperidine was then added to the crude reaction medium to yield the alkenic analogue (141) of Piperoxan <89TL1637>. 

The keto ether (187) on treatment with diethyl carbonate in presence of sodium hydride in 1,2-dimethoxyethane afforded the keto ether (188), which was made to react with methyl-lithium in ether, to obtain the tertiary alcohol (189). This on being refluxed with methanolic hydrochloric acid yielded the phenol (190). It was methylated to yield(191) and heated with zinc, zinc iodide and acetic acid to produce pisiferol (192). Its methyl derivative (193) on oxidation with Jones reagent at room temperature, followed by esterification, furnished the keto ester (194). Reduction of (194) with metal hydride produced an alcohol whose tosyl derivative on heating with sodium iodide and zinc dust furnished the ester (195). Its identity was confirmed by comparing its spectral data and melting point with an authentic specimen [77]. The transformation of the ester (195) to pisiferic acid (196) was achieved by treatment with aluminium bromide and ethanethiol. The identity of the resulting pisiferic acid (196) was confirmed by comparison of its spectroscopic properties (IR and NMR) with an authentic specimen [77]. 

Fig. (16). The alcohol (171) was converted to the keto ether (185) applying the standard organic reactions and this on subjection to Robinson annelation. The resulting adduct on treatment with sodium methoxide in methanol afforded the tricyclic ketone (187) which is converted to another keto ether (188). It is converted to tertiary alcohol (189) by treatment with methyllithium. Acid treatment of the alcohol produced the phenol (190). Its methyl derivative (191) is converted to pisiferol (192) by treatment with zinc and zinc iodide. Its methyl derivative (193) was converted to ester (195) via oxidation, reduction, tosylation and detosylation. The reagents mentioned accomplished its conversion to pisiferic acid (196).

Tertiary alcohols are seldom amenable to direct deoxygenation. However, sodium cyanoborohydride in the presence of zinc iodide reduces tertiary alcohols to the corresponding alkanes in high yields. Primary and secondary alcohols are not reduced with this reduction reagent and thus a very desirable chemoselectivity in deoxygenation is attainable. 

It has been reported that catalytic amounts of lead (at least 1000 ppm) are necessary for successful zinc-mediated reduction of a 2-thioxo-4-thiazolidi-none to the corresponding 4-thiazolidinone (Scheme 22.2) the lead may be present in the as-supplied zinc dust, or may be added subsequently as lead dust or as lead(ii) salts (chloride, bromide, iodide, acetate), although addition of amounts greater than 2% gives reduced activity. Doping experiments established that reduction of lead(ii) to particulate lead(O) occurs in the course of the reaction, and suggests that the latter is the catalytic agent. ... 

The chiral Michael accep tor di-(—)-menthyl methylenemalonate provided an asymmetric scaffold for the diastereoselective cycloaddition with 1 or 2 in the presence of zinc iodide (eq 10). Subsequent reduction of the dicarboxylate to the diol with lithium aluminum hydride provided moderately enantioenriched cyclopropanes and cyclobutanes. 

This could indeed be achieved by samarium-II-iodide reduction, and a similar titanium-mediated zinc reduction was shown to represent a formal exchange of functional groups in aUylic alcohol 88 [31]. 

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