Xanthates cleavage

Several, oxidatively coupled xanthates (64-66), compounds (also called xanthides) containing the photochemically reactive, sulfur-sulfur bond, have been studied.130 Homolytic cleavage of this reactive bond is the primary reaction for these compounds, although this process is normally masked by recombination of the radicals produced. This primary, light-initiated process becomes apparent when a mixture of the xanthide 64 and ethyl xanthide (67) is irradiated in cyclohexane, because an equilibrium between 64, 67, and the mixed xanthide 68 is rapidly established. 

Scheme 12.—Proposed Mechanism for the Photochemical Cleavage of Dimethylthiocar-bamates (R = NMej, Routes a and b) and xanthates (R = SMe, Route a).

The keto group of the trisubstituted dioxanone 133 generated by ozonolysis was removed by radical deoxygenation according to the Barton-McCombie protocol [80] via the alcohols 134 and the corresponding xanthate, leading after deben-zylation to the dioxane 135 in excellent yield. After conversion to the tosylate, cleavage of the acetonide and protection of the secondary alcohol function as a TBS ether provided access to oxirane 128 by cyclization with NaH in 99% yield and in virtually diastereo- and enantiomerically pure form (de, ee > 96%). 

Much more critical, however, is the introduction of the 2 -deoxy function. This can be achieved after selective 3 -0-benzoylation to 170, xanthation at the 2 position to 171 and reductive cleavage (Bu3SnH) to 172 following Barton s method [91]. Further radical ring opening by the Hullar-Hanessian procedure and final reduction leads to the D-C disaccharide 173 [92],... 

For rayon production, a controlled cleavage of the cellulose molecule is required to reduce the viscosity of the cellulose xanthate so that it can be forced through fine spinnerets. A similar reduction of viscosity is required for cellulose which will be sprayed as a lacquer. This cleavage is brought about by the air oxidation of alkali cellulose, and the process is speeded by the addition of a few parts per million of a transition metal compound which functions as an oxidation catalyst. 

Compounds 55 are subjected to cleavage of the xanthate part by H2NCH2CH2NH2 and acid-catalyzed cyclization, which provide dihydrothiophenes 56 (Scheme 13) <2005TL8053>. 

A further example of the utility of this approach involved the polymerization and manipulation of cholanic-acid-derived xanthate 59 (Scheme 14) [10]. Treatment of xanthate 59 with 15 equivalents of styrene provided polymer-bound steroid 60 in 85% yield. In this case a spacer unit was incorporated between the polymer and the substrate. The substrate was then modified at both the xanthate and carboxylic acid termini to provide N-cyclopropylamide 61. Cleavage from the polymer support was achieved using KOH in methanol and THF, and provided amide 62 in 62% yield. 

Lusinchi [11] illustrated the utility of the soluble polymer by examining both the reaction of immobilized xanthates with acceptors in solution and the reaction of immobilized alkenes with xanthates in solution. Firstly, immobilized xanthate 69 was prepared by chloroacetylation of 68, followed by displacement of chloride with 0-ethyl xanthate (Scheme 16). Treatment of 69 with excess protected allylamine 70 in the presence of 40 mol % lauroyl peroxide gave, after acid cleavage and coliunn chromatography, adduct 71 in good overall yield. Although H NMR indicated that the immobiUzed xanthate 69 was entirely consumed in the reaction, by-products were obtained due to the product xanthate formed on radical addition reacting further. 

Cleavage of bromophenyldiazirine 35 with a xanthate nucleophile has allowed the synthesis of a novel heterocycle (Scheme 9) 19994 L/29. Exposure of 35 to potassium ethyl xanthate 39 produced the adduct 40 in lotv yield. 1 he... 

The Chugaev reaction of S methyl xanthates is a synthetically useful p5u olysiss process. The corresponding electron impact induced elimination is a prominent feature of xanthate mass spectra, (45). > The electron impact induced cleavage is at least partially cfs-stereospecific as the thermal reaction is... 

The use of a low-molecular-weight sulphur-containing photoinitiator, such as S-benzoyl O-ethyl xanthate (BEX) is described for the polymerization of MMA [133], Laser flash photolysis of BEX reveals [134] that the primary photoprocess involves a C(=0)-S cleavage to generate both the benzoyl and the ethoxythio-caibonylthiyl radicals which initiate the polymerization (Scheme 34). 

The extent of El character in the transition state increases in the following order of ester types acetate < phenylacetate < benzoate < carbamate < carbonate.Cleavage of xanthates (17-5), cleavage of sulfoxides (17-12), the Cope reaction (17-9), and reaction 17-8 are probably very close to straight E mechanisms. 

Conversion of the precursor 22 by initial cleavage of the xanthate unit, and subsequent acid-induced cyclization provided a route to the dihydrothiophene 23. Similar cyclization of substrates where the pivaloyl group was replaced with TBDPS or acetyl groups gave considerably lower yields of the corresponding dihydrothiophenes <05TL8053>. 

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