Xanthates rearrangement

FREUDENBERG - SCHdNBERG Xanthate Rearrangement Rearrangement of S-methyl xanthates to S-methyIdithiocarbonatas (conversion of alcohols lo thiols via xanthates, also phenols to thiophenols via thiocarbamates). 

However, as shown by Taguchi, xanthates rearrange to the corresponding dithiocarbonates when no /3-hydrogens are present (Scheme 43) [113,114]. 

For a hetero-Claisen rearrangement of allyl xanthate in the synthesis of agelasidine A see ref 369. Two sequential allyl xanthate rearrangements are described in ref 370. 

The Chugaev reaction, or thermal decomposition of the substituted esters of the xanthates, gives olefins without rearrangement (35,36). For example ... 

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

A number of catalysts of Pd(II), Pt(II), Rh(I), and Ir(I) induce rearrangements of 0-a11y1ic-.9-methy1 dithiocarbonates at 25°C (45). In a relatively low temperature procedure, olefins readily form from certain classes of xanthate esters (46) ... 

The Chugaev elimination is of synthetic value, because it proceeds without rearrangement of the carbon skeleton. Other non-thermolytic elimination procedures often lead to rearranged products, when applied to the same substrates. However applicability of the Chugaev reaction is limited if the elimination is possible in more than one direction, and if a /3-carbon has more than one hydrogen. Complex mixtures of isomeric olefins may then be obtained. For example the thermolysis of xanthate 12, derived from 3-hexanol yields 28% S-hex-3-ene 13, 13% Z-hex-3-ene 14, 29% -hex-2-ene 15 and 13% Z-hex-2-ene 16 ... 

Extension of these processes to provide enantio-enriched products was successfully applied after desymmetrization of the starting materials. An example is shown below (Reaction 76), where silane-mediated xanthate deoxygenation-rearrangement-electrophile trapping afforded the conversion of (+)-94 to (+)-95 in 56% yield. ... 

Salts of O-alkyldithiocarbonatcs ( xanthates ) are hazardous as dusts, forming explosive suspensions in air. The lower-alkyl salts are claimed to be explosive in the solid state when dry [1]. Explosions reported when drying hydrated xanthate salts are probably the consequence of release of carbon disulphide to form an inflammable atmosphere of very low autoignition temperature in the oven [2], Xanthate esters are thermally unstable by a variety of eliminations and rearrangements, all distinctly exothermic and many evolving extremely flammable gases and vapours. Free xanthic acids, which may be isolated on acidification, decompose autocatalytically and perhaps explosively [3],... 

An interesting neophyl-type radical rearrangement process has been established for the synthesis of azabicycles, which are not readily accessible by other means. Barton McCombie deoxygenation of xanthate 70 under slow addition of (TMS)3SiH and AIBN in refluxing toluene furnished the 2-azabenzonorbor-nane derivative in good yield (Reaction 7.72) [82]. 

Alcohols such as PhjCHCHjOH rearrange on treatment with acid they can be dehydrated by heating their methyl xanthates (Tschugaev reaictkMi). The pyrolysis proceeds by a cyclic transition state. Outline the steps using PhjCHCHjOH. ... 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

The thermal rearrangement of an allyl xanthate to a dithiocarbonate was used to introduce the sulfur atom on a quaternary carbon in the synthesis of agelasidine A, a natural S-containing sesquiterpene [494]. 

A Beckmann rearrangement-reduction sequence has been used to access a number of substituted H- 1-benzazepine derivatives, with the required substituted a-tetralone precursors being prepared by a xanthate-based free radical cyclization process <2006BMC6165>. 

Hu, Sun and Scott described recently an efficient synthesis of some taxadiene derivatives266. In their approach, alcohol (66) is treated with carbon disulphide followed by methyl iodide under standard conditions for the formation of xanthates, to afford the dithiocarbonate (67) in a process that presumably involved a Claisen rearrangement (Scheme 9). Interestingly, 67 undergoes smooth reaction with B SnH to provide the required products (68, 69)266. 

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