Xanthates group transfer

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. 

Scheme 14. Degenerative radical chain transfer of a xanthate group...

Alkoxycarbonyl radical cyclization leads to direct formation of lactones. For example, the lactone 154 was obtained from the selenocarbonate 153 in Corey s synthesis of atractyligenin (155) [103] (Scheme 53). 5-Alkoxycarbonyl xanthates are also viable precursors for alkoxycarbonyl radicals the lactone 157 was prepared from the xanthate 156 via group transfer radical cyclization en route to methyl-enolactocin (149) [104] (Scheme 54). 

Homolytic additions. Xanthates in which the 5-bearing carbon site can support a free radical readily undergo homolysis by heating with t-Bu202. The two radical fragments can be taken up by an unsaturated compound. Group transfer to a remote double bond concurrent with cyclization of certain unsaturated xanthates is an intramolecular version. An oxocane precursor of lauthisan can be acquired (35%) in this manner. ... 

Thermal )6-elimination reactions of acetates, benzoates, xanthates, sulfoxides, selenoxides, and N-oxides are also group transfer reactions. All these elimination reactions are yn-stereospecific and proceed through a cyclic six membered—or five membered—ring transition state of 6e process by intramolecular transfer of hydrogen atom, where all the participating orbitals have suprafacial interactions. These reactions are fundamentally retro-group transfer reactions. 

The transfer of a trifluoromethyl group of xanthate R—O—(C=S)—S—CF3 is a recent and powerful method for the trifluoromethylation of olefin.An important and interesting point of this method is that the reaction product is a xanthate itself, able to undergo further radical reactions and permitting cascade reactions, as exemplified in Figure 2.40. The starting xanthates R—O—(C=S)—S—CF3 are easily prepared from trifluoroacetic anhydride and sodium xanthate. 

The radical deoxygenation of secondary alcohols through tri-n-butyltin hydride reduction of their xanthate derivatives was introduced in 1975 [227]. The reaction proceeds by thiophilic radical addition to the thiocarbonyl group of a tin radical followed by collapse of the carbon intermediate, as shown in the accompanying scheme. Hydrogen transfer completes the process. 

Lactones may in principle be obtained if the olefinic trap is placed on the oxygen side of the xanthate. The more complex sequence in Scheme 11, discovered seren-dipitously, illustrates the construction of such a lactone starting with an aryl thio-carbonate [20]. It also involves the transfer of the aryl group in the penultimate step. 

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