Acetoxy sulfide

Sharpless and Masumune have applied the AE reaction on chiral allylic alcohols to prepare all 8 of the L-hexoses. ° AE reaction on allylic alcohol 52 provides the epoxy alcohol 53 in 92% yield and in >95% ee. Base catalyze Payne rearrangement followed by ring opening with phenyl thiolate provides diol 54. Protection of the diol is followed by oxidation of the sulfide to the sulfoxide via m-CPBA, Pummerer rearrangement to give the gm-acetoxy sulfide intermediate and finally reduction using Dibal to yield the desired aldehyde 56. Homer-Emmons olefination followed by reduction sets up the second substrate for the AE reaction. The AE reaction on optically active 57 is reagent... 

The carbenoid displacement reaction (see Section 1.4.5.2.1.4.) of the optically active acetoxy sulfide derivative 19 (or the corresponding methoxymethyl ether) with diazomalonate in the presence of a catalytic amount of rhodium acetate in refluxing benzene affords the tram-alkylation productl22. 

Recently, new examples of asymmetric induction in the Pummerer reaction of chiral sulfoxides have been described. Oae and Numata (301) reported that the optically active a-cyanomethyl p-tolyl sulfoxide 275 undergoes a typical Pummerer rearrangement upon heating with excess of acetic anhydride at 120°C, to give the optically active a-acetoxy sulfide 276. The optical purity at the chiral a-carbon center in 276, determined by means of H- NMR spectroscopy using a chiral shift reagent, was 29.8%. 

Arylsulfenyl trifluoroacetates have been generated in situ and used for the hydroxysulfenylation of al-kenes. These reagents are prepared from diaryl disulfides and LTA in trifluoroacetic acid. Yields for the addition are generally good (42-95%). The hydroxysulfenate intermediate can be oxidatively cleaved by LTA (provided the groups can achieve an antiperiplanar arrangement) to form an a-aldehyde-[Pg.518]

The Pummerer reaction of optically active R)- +)-a-(p-tolylsulfinyl)-A/,Al-dimethylacetamide with Acetic Anhydride in the presence of 1,3-Dicyclohexylcarbodiimide is highly stereoselective, affording the corresponding a-acetoxy sulfide in moderate yield but with nearly 70% ee (eq 2). The recovered starting sulfoxide is obtained in 63% yield. 

The previous reaction describes the synthesis of a novel class of heterocycles by the name of [l,2,3]thiadiazolo[4,5- /]-pyrimidines. Visibly, a redox process is involved, whereby the initial hydrazine derivative becomes an R-N=N-R system and, at the same time, the sulfur atom in thionyl chloride is converted to an azo-sulfide. The transfer of oxidaton level from sulfur—as sulfoxide—to the neighboring atom is a well documented process in sulfur chemistry that is called the Pummerer rearrangement. In essence, it involves the treatment of sulfoxides with an electrophile such as acetic anhydride to yield an a-acetoxy sulfide, according to the following sequence (see Scheme 26.1) ... 

In many respects the Pummeter reaction can be regarded as the sulfur version of the Polonovski reaction (and vice versa), and by analogy to the Polonovski reaction the central intermediate is a sulfur-stabilized carbocation (thionium ion). Although the existence of this species is only transient, it reacts to give a number of different products, e.g. a-acetoxy sulfides, vinyl sulfides, cationic cyclization products, etc., depending upon the sulfoxide structure and reaction conditions. Other reaction pathways ate specific to the Pummerer reaction as a result of sulfur s ability to expand its valence shell (additive Pum-merer reactions). A moderate degree of asymmetric induction is also observed in certain Pummerer reactions, where optically pure sulfoxides are substrates. 

Although the emphasis in this chapter is placed on the use of acid anhydrides in the Pummerer reaction, a number of other activating reagents, e.g. acids, in particular p-toluenesulfonic acid, and trial-kylsilyl halides, have been employed. Mention of these reagents is made in cases where their use leads to an improvement in yields or selectivity, or to a transformation which is not possible using an acid anhydride. Acetyl chloride is usually not employed in the Pummerer reaction because of the simultaneous presence of acetate ion and chloride ion in the reaction medium. The product of these reactions is predominantly the thioacetal derived from spontaneous decomposition of the initially formed a-acetoxy sulfide. 

Interestingly, the a-acetoxy sulfides (106) obtained from the reaction of 3-keto sulfoxides with acetic anhydride containing sodium acetate undergo in situ oxidation-reduction and acyl transfer, giving a-acetoxy thiol esters (107) as products (Scheme 23)J Yields in this process are generally high. 

Selective transformation of a-acetoxy sulfides to primary alcohols is achieved using LiAlH4, diisobutylaluminum hydride at 0 °C and sodium borohydride in ethanol at room temperature. Selective reduction of the Pummerer product (108) to the primary alcohol (109) is a key step in the elegant approach developed for the synthesis of monosaccharides (Scheme 24). ... 

A sulfoxide containing at least one a-hydrogen atom is reduced to a sulfide with concomitant oxidation at the a-carbon atom. The Pummerer rearrangement is generally effected by treatment of the sulfoxide (24) with acetic anhydride to yield the corresponding a-acetoxy sulfide (25) (Scheme 15). 

Sulfenyl carbanions can also be used in the formation of new carbon-carbon bonds under mild conditions. This is achieved by reaction of the carbanion (40) with a ketone and reductive desulfuration of the intermediate acetoxy sulfide (44) to produce the alkene (45) (Scheme 22). 

Reaction of 2,4-dinitrobenzenesulfenyl chloride with norbornadiene, used to increase the effective electrophilicity of weak electrophiles in electrophilic addition to 1,3-dienes, gave a mixture of chloronorbornenyl and chlorotricyclo[2.2.1.0 ]heptyl sulfides 20 and 21 with regioselec-tivity and stereochemistry depending on the solvent and temperature of the addition.Thus in acetic acid at 25°C two acetoxy sulfides trans- and cis-22 were also formed. 

Reaction of epichlorhydrin a-Acetoxy sulfide Enzymes and metals together Hydrogenation Parallel Kinetic Resolutions Regiodivergent resolutions Double Methods... 

The reaction is unusual in that sulfoxides processing an a-hydrogen atom normally undergo a Pummerer reaction under these conditions to form the acetoxy sulfide. The reaction can be compared with the Morin rearrangement of penicillin sulfoxide to cephalosporins.121... 

Pummerer reaction. Upon reaction with a-ethoxyvinyl acetate chiral sulfoxides in which the a-carbon is substituted with an electron-withdrawing group are transformed into a-acetoxy sulfides. In most cases the enantiomer excess of the products amount to 70-80%. 

Pummertr rearrangement (6, 2, 5-6). a-Acetoxy sulfides can be obtained by rearrangement of sulfoxides in TFAA-AcaO in the presence of 2,6-lutidine at 20°. The rearrangement is more rapid than in AcaO-NaOAc. ... 

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