Desulfurative -allylic rearrangement

Diallylsulfonium salts undergo intramolecular allylic rearrangement with strong bases to yield 1,5-dienes after reductive desulfurization. The straight-chain 1,5-dienes may be obtained by double sulfur extrusion with concomitant allylic rearrangements from diallyl disulfides. The first step is achieved with phosphines or phosphites, the second with benzyne. This procedure is especially suitable for the synthesis of acid sensitive olefins and has been used in oligoisoprene synthesis (G.M. Blackburn, 1969). 

Desulfurization. Evans et al.1 noted that dialkenyl disulfides, for example (I), are converted by reaction with triphenylphosphine into dialkenyl sulfides (II) with an accompanying allylic rearrangement. Ollis2 used this reaction in one step... 

The concentration of zinc accelerator-thiolate complexes in the rubber is not the only factor determining the balance of the two reactions in NR. Both the rate of desulfuration of polysulfide crosslinks and the rate of their thermal decomposition depend upon the positions of attachment of the sulfur chains to the backbone rubber chains and the detailed structure of the hydrocarbon at the ends of the crosslinks. In the course of normal accelerated vulcanization there are three different positions of attack on the polyisoprene backbone two of these are methylene groups in the main chain (labelled d and a in 3), and the third is the side chain methyl group (labelled b in 3). Direct analysis of the distribution of the sites of attack cannot yet be made on actual rubber vulcanizates, and information has had to be obtained solely by sulfuration of the model alkene 2-methyl-2-pentene and, more recently, 2,6-dimethyl-2,6-octadiene. The former (4) models the a-methylic site but only one of the two a-methylenic sites of polyisoprene the latter (5) models all three sites, but at the present time these are not all supported by the synthesis of relevant sulfides. Because allylic rearrangements are common in subsequent reactions of the sulfurated rubber, sulfur substituents appear not only on allylic carbon atoms but on isoallylic carbon atoms. Thus, from 2-methyl-2-pentene, the groups shown in Scheme 2 are formed. 

Owing to the reversible nature of the allylic sulfenate/allylic sulfoxide interconversion, the stereochemical outcome of both processes is treated below in an integrated manner. However, before beginning the discussion of this subject it is important to point out that although the allylic sulfoxide-sulfenate rearrangement is reversible, and although the sulfenate ester is usually in low equilibrium concentration with the isomeric sulfoxide, desulfurization of the sulfenate by thiophilic interception using various nucleophiles, such as thiophenoxide or secondary amines, removes it from equilibrium, and provides a useful route to allylic alcohols (equation 11). 

Synthesis of P-Keto Sulfoxides. Optically active p-keto sulfoxides are very useful building blocks (eq 4) because they can be stereoselectively reduced to afford either diastereomer of the corresponding p-hydroxy sulfoxide under appropriate conditions (Diisobutylaluminum Hydride or Zinc ChloridefDlBALf and thus give access to a wide variety of compounds chiral carbinols by desulfurization with Raney Nickel or LithiumJethyhmme ini the case of allylic alcohols epoxides via cyclization of the derived sulfonium salt butenolides by alkylation of the hydroxy sulfoxide 1,2-diols via a Pummerer rearrangement followed by reduction of the intermediate. ... 

Another useful reaction of these Diels-Alder adducts is shown in equation (54). Dihydrothiazine oxide (123) from ( . )-2,4-hexadiene can be opened with a Grignard reagent to allylic sulfoxide (124) which undergoes a stereoselective 2,3-sigmatropic rearrangement via the envelope-like transition state conformation shown, having a quasi-equatorial methyl group to afford sulfenate ester (125). Desulfurization of (125) provides E)-threo smino alcohol derivative (126) in excellent overall yield. If ( ,Z)-2,4-hexadiene is used, the E)-erythro epimer of (126) is formed cleanly. 

Weinreb and coworkers have examined some reactions of dihydrothiazine imines and have developed a new approach to vicinal diamines using these intermediates. Their method is outlined in Scheme 18. Cycloaddition of ( -2,4-hexadiene with the tosyl bis-imine gives a 1.1 1 mixture of epimeric dihydrothiazine imines (140) and (144). Subsequent transformations of these adducts took two different courses. In one, adduct (140) could be opened to allylic sulfilimine (141) which underwent a stereoselective 2,3-sigmatiopic rearrangement to sulfenamide (142) (cf. equation 54). Desulfurization of (142) yielded E)-threo vicinal sulfonamide (143). Adduct (144), which presumably exists in conformation... 

They then turned to the less hindered reagent trimethyloxonium fluoroborate in nitromethane and obtained the desired salt (3). The product is rearranged to (4) on treatment with n-butyllithium, and reduction with sodium in liquid ammonia gives the squalenelike hydrocarbon (5). Desulfurization with Raney nickel gives complex mixtures in the case of allylic sulfides. 

Other Alkylations. NCS and allylic sulfides react with 3-unsubstituted indoles at —20 °C to give initially 3-sulfonium salts which on warming to 20 °C rearrange to 2-allyl-3-thiomethyl-indoles (eq 13). These are readily desulfurized, either with or without concomitant reduction of the allylic double bond, to give 3-allyl or 3-alkyl substituted indoles. ... 

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