Acyl xanthates

This problem is not so severe when acyl xanthates are used as precursors because these substrates absorb in the visible region, while the products do not (however, the products might still be recycled to the radical pool by radical addition-elimination). Visible light photolysis of benzoyl xanthane (42) and allyl acetate provides (43) in 60% yield. Standard (ionic) 3-elimination of the xanthane is a facile reaction that gives (44). When the tertiary acyl xanthane (45) is irradiated in the presence of W-benzylmaleimide... 

Briggs et al. proposed a new strategy for the synthesis of tricyclic structures using acyl xanthates as precursor for acyl radicals [121]. Irradiation with visible light of a solution of acyl xanthate in presence of 1,6-diene 126 afforded czs-fused bicyclic compound 127 in a good yield (Scheme 38). Radical reduction of xanthate and subsequent aldol condensation leads to the formation of [5.5.5]-fused ring systems similar to those of the triquinane terpene family. 

The second set of examples pictured in Scheme 4 illustrates a route to a S-tri-fluoromethyl xanthate 9, which cannot be made by a simple displacement reaction [10]. In this case the corresponding S-acyl xanthate 8 is first prepared... 

Delduc et al. have reported that the photolysis of S-alkyl and -acyl xanthates provides a useful source of free alkyl and acyl radicals. Photolysis of the imide (30) gives tetramethyl-cyclobutadiene (Kashima et al.). 

Acyl radical sources, other than aldehydes, are also available. The group transfer addition of an acyl radical has been reported by Zard and co-workers, where S-acyl xanthates serve as acyl radical sources [44]. Crich and co-workers reported that an acyl radical, generated from an aromatic acyl telluride by photolysis, adds to an allylic sulfide which contains an ethoxycarbonyl group to form the corre-.sponding y-unsaturated ketones [45]. The addition pathway involves Sh2 type reaction with extrusion of a /ert-butylthiyl radical. 

Sodium 0-ethyl xanthate in acetone added with stirring at —35° during 30 min. to phenacetyl chloride in the same solvent, after 1 hr. allowed to warm to room temp. 0-ethyl S-phenacetyl xanthate (Y 97%) in refluxing dioxane irradiated 3 hrs. with a Hg-arc lamp, and the resulting S-benzyl 0-ethyl xanthate refluxed 1 hr. in 1 9 aq. ale. 1 N NaOH a-toluenethiol (Y 96%). The acyl xanthates are smoothly converted into acyl and xanthate radicals. The former are de-carbonylated and recombine with the xanthate radicals. F. e. and limitations s. D. H. R. Barton, M. V. George, and M. Tomoeda, Soc. 1962, 1967. 

Mixed aiihydiosulfides can be leaddy obtained by adding an acetone solution of the xanthate to an acetone solution of the acyl halide at —35 C (53) ... 

Tris[(2-perfluorohexyl)ethyl]tin hydride has three perfluorinated segments with ethylene spacers and it partitions primarily (> 98%) into the fluorous phase in a liquid-liquid extraction. This feature not only facilitates the purification of the product from the tin residue but also recovers toxic tin residue for further reuse. Stoichiometric reductive radical reactions with the fluorous tin hydride 3 have been previously reported and a catalytic procedure is also well established. The reduction of adamantyl bromide in BTF (benzotrifluoride) " using 1.2 equiv of the fluorous tin hydride and a catalytic amount of azobisisobutyronitrile (AIBN) was complete in 3 hr (Scheme 1). After the simple liquid-liquid extraction, adamantane was obtained in 90% yield in the organic layer and the fluorous tin bromide was separated from the fluorous phase. The recovered fluorous tin bromide was reduced and reused to give the same results. Phenylselenides, tertiary nitro compounds, and xanthates were also successfully reduced by the fluorous fin hydride. Standard radical additions and cyclizations can also be conducted as shown by the examples in Scheme 1. Hydrostannation reactions are also possible, and these are useful in the techniques of fluorous phase switching. Carbonylations are also possible. Rate constants for the reaction of the fluorous tin hydride with primary radicals and acyl radicals have been measured it is marginally more reactive than tributlytin hydrides. ... 

The relative rates of acylation and of deoxygenation have been determined with these various reagents [44]. As expected, the pentafluoro reagent reacts the fastest with an alcohol under standard conditions, followed by the 4-fluoro reagent, and the phenyl derivative is the slowest. However, for the deoxygenation reaction the fastest group is the methyl xanthate. The slowest is the pentafluorophenyl derivative. This is not important because all of the thiocarbonyl derivatives mentioned give very fast radical reactions 144],... 

This chapter shows how radical chemistry based on thiocarbonyl derivatives of secondary alcohols can be useful in the manipulation of natural products and especially in the deoxygenation of carbohydrates. From the original conception in 1975, the variety of thiocarbonyl derivatives used has increased, but the methyl xanthate function still remains the simplest and cheapest, when other functionality in the molecule does not interfere. Otherwise, selective acylation with aryloxythiocarbonyl reagents is important. Many of the functional groups present in carbohydrates and other natural products do not interfere with radical reactions. 

A variety of different sources of radicals have been used in several heteroaromatic substitution reactions [2] these include acyl peroxides, oxaziridines, thiohydroxa-mic Barton esters, the Gif reaction, alkyl xanthates, and ketones/H202 (Scheme 8). 

Zard and colleagues have developed an interesting radical chain reaction based on the thionocarbonyl derivatives, where S-alkyl, S-acyl and S-alkoxyacyl xanthates are employed as new and very useful sources of alkyl and acyl radicals, as demonstrated in Scheme 53.168 The alternate undesirable pathways available to an alkyl or alkoxyacyl radical are not important in this reaction due to the reversible and degenerate (path A) nature of the adduct radicals.168 This reaction does not therefore complete with the expulsion of carbon dioxide (path B), in sharp contrast to previous processes based on stannane... 

Not only acyl, but also alkoxycarbonyl radicals can be generated from the corresponding xanthates, and their capture by addition to olefins produces esters or lactones, depending on whether the addition reaction is inter-or intra-molecular [53]. Grainger and Innocenti found that xanthates derived from carbamoyl chlorides were difficult to make and handle but, by replacing the xanthate salt by a dithiocarbamate, better precursors for the desired aminocarbonyl radicals were obtained [54]. Irradiation with a tungsten lamp proved more efficient than chemical initiation with lauroyl peroxide and lactams of various sizes could be readily obtained, as illustrated by the... 

A Mo—O Insertion of S into Mo-C acyl bond to form metal-bound xanthate 20000M261... 

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