Xanthates, allylic synthesis

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 (IsQ-coordinated species [Mo(CO)3(py)3] prepared from [Mo(CO)6] and pyridine is useful for the synthesis of various jr-allyl molybdenum compounds (68JOM(13)Pl, 970M5365). The 3-allyl complexes [( 3-allyl)Mo(CO)2Br(py)2] form [( 3-allyl)Mo(CO)2(S,S)(py)] with the anionic S,S-donors, dithiocarbamates and xanthates of sodium and potassium, M (S,S) (81JOM(218)185). [( 3-C3H5)Mo(CO)2(py)2Br] (68JO M(14)375) with thallium hexafluorophosphate in acetonitrile gives... 

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. 

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. 

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. ... 

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 most important synthetic asset of the xanthate transfer methodology lies in its ability to induce carbon-carbon bond formation by intermolecular addition to unactivated olefins. Again, this is possible because the initial radical has a comparatively long lifetime in the medium. Unhindered, terminal olefins are the best substrates, but other types of olefins (especially strained or lacking allylic hydrogens) may be made to react in some cases. Three examples of additions are collected in Scheme 18. The first involves formation and capture of a trifluoroacetonyl radical, a species hitherto only studied by mass spectrometry but never employed in synthesis [34a]. This reaction represents a convenient route to various, otherwise inaccessible, trifluoromethyl ketones. In the second example a tetrazolylmethyl radical, also a previously unused intermediate, is intercepted by a latent allyl glycine [34b]. The amino acid moiety may be part of the xanthate partner as highlighted by the last example [34c]. 

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]. 

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... 

Roush et al. have reported the synthesis of a durhamydn aglycon model (Scheme 1.39) [50]. Allylation of aldehyde 223 with the aUyhc borane 224 provided 225 after the formation of a xanthate. Deoxygenation gave the requisite RCM precursor whose cycUzation proceeded reasonably well provided that Ti(Oi-Pr)4 was used as an additive. As suggested by model studies, this additive may avoid the coordination of the rathenium carbene by either the aryl ether (OBOM) or the neighboring ethers (OMe, OTCE). 

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