Allylic xanthates, rearrangement

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]. 

The thermal rearrangement of allylic xanthates to allylic dithiocarbonates is a well-known reaction which results in the creation of a new stereogenic carbon center when the starting xanthate contains an unsymmetrically substituted C-C double bond. Moreover, in the case of chiral xanthates this rearrangement is accompanied by the transfer of chirality from the stereogenic carbon atom in the substrate to the newly created stcrcogcnic center in the product. 

Fully stereoselective allylic xanthate to allylic dithiocarbonate rearrangement was first observed with diastereomeric 2,3-unsaturated sugar derivatives 1 and 3 which on heating (140 UC for 1 to 2 h) gave the diastereomers of 3,4-unsaturated sugar dithiocarbonates 2 and 4, respectively43. 

A crucial step in the synthesis of thiotetronic acids involves a fully stereoselective allylic xanthate to dithiocarbonate rearrangement45. Rearrangement of ethyl ( )-(4S)-2-methyl-4-[methylthio-(thiocarbonyl)thio]-2-pentenoate (7) gives ethyl (/f)-(2.S)-2-methyl-2-[methylthio(curbonyl)thio]-3-pentenoate (8) as a single diastereomer with an -configurated double bond and at least 98% ee. This value was determined by Mosher derivatization of the alcohol prepared from the dithiocarbonate in three steps. The remainder of the (5S)-thiotetronic acid synthesis is shown in the scheme. 

The utility of the [3,3]-sigmatropic rearrangement of eight-membered thionocarbon-ates for the highly stereo-controlled synthesis of Z- or -double bonds in 10-membered thiolcarbonates has been reviewed.5,5-Disubstituted thiotetronic acids (139) have been synthesized using an allylic xanthate (137) to dithiocarbonate (138) rearrangement which has permitted the introduction of a sulfur at a tertiary centre with concomitant deconjugation of the double bond (see Scheme 39). 

There is one other synthetically interesting O S transformation based on a hetero-Cope reaction, which is worth mentioning here. This rearrangement (equation 18) is so easy that on attempting to prepare allylic xanthates (20) one isolates instead the rearranged ( )-thiocarbonates (21), which can be used in synthesis, e.g. as protected thiol derivatives or as precursors for allylic radicals. ... 

Deoxygenation of ailylic alcohols.3 A method for conversion of allylic alcohols to 1-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an O-allyl 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 route to terminal alkcnes.4... 

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 synthesis of (5 S)-thiolactomycin (792), an enantiomer of an antibacterial agent, makes use of a Wittig olefination early in the sequence as a way of preparing a,jS-unsaturated ester 786 (Scheme 106). The key step in the synthesis is an allyl xanthate-dithiocarbonate rearrangement of 788 to 789. This process occurs upon distillation of 788 at 145 °C (0.4 mm Hg) and gives the desired product 789 in nearly quantitative yield. Chirality transfer is equally efficient, with an enantiomeric excess of at least 98% [222]. 

Allyl thioncarbamates and allyl xanthates also rearrange easily in the same manner. 

From the first-order rate constants obtained in different solvents (in sealed ampoules), it is apparent that this isomerization is not very sensitive to the polarity of the medium, in accordance with an isopolar, six-membered activated complex [156]. A similar small solvent effect has been observed for the [3,3]sigmatropic rearrangement of allyl S-methyl xanthate to allyl methyl dithiol carbonate [559]. 

A free-radical approach has also been successfully applied to the synthesis of primary allylic tributylstannanes (Eq. 8) [10]. The sequence involves a thermal [3,3] rearrangement of an allylic methyl xanthate then addition of a BusSn radical to the double bond of the derived dithiocarbonate intermediate and subsequent loss of COS in a chain-propagating step. 

Sugar allyltin derivatives are very useful synthetic intermediates. The most convenient and reliable method for their preparation is a so-called xanthate procedure. The first compound of this class was prepared in 1988 by Mortlock and Thomas [74]. 1,2-0-isopropylidene-D-glycer-aldehyde (80) was converted into allylic alcohol and further transformed into the corresponding xanthate 81. This compound underwent thermal [3,3] rearrangement into the dithiocarbon-ate 81a, which upon reaction with tri-n-butyltin hydride provided the final sugar allyltin 82 as a mixture of isomers with the E-one strongly predominating (O Scheme 35). 

Asbase. KF on alumina effects the formation of trichloroacetimino ethers, and those derived from allylic alcohols readily undergo rearrangement to afford N-allyl trichloroacetamides. Xanthates are similarly prepared." ... 

A number of reports have focused interest on the synthesis of various allylic sulphur compounds. A one-pot procedure for the preparation of allylic sulphides from the corresponding alcohols involves initial rearrangement of the xanthate followed by... 

Xanthates, allyl, rearrangement 702 diaryl, rearrangement 700 intermediates in formation of thiols 194-211... 

Harano and Taguchi have studied the thermal rearrangement of 2-alkenyl S-alkyl xanthates to the corresponding dithiolocarbonates/ The rearrangement was shown to be strictly intramolecular and, in the general case, to take place accompanied by an allylic shift (Scheme 16). For sterically more... 

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