Sulfoxides, propargyl

Note 2. Traces of unreacted triethylamine might cause partial isomerization of the allenyl sulfoxide into the propargyl sulfoxide. The methyl iodide is added to ensure that no triethylamine remains. 

Following studies on the rearrangement of allylic arenesulfinates, Braverman and coworkers have investigated a number of natural extensions of this unique transformation, including the predictable [2,3] sigma tropic rearrangements of allylic sulfenates to sulfoxides and of propargylic sulfenates and sulfinates to allenic sulfoxides and sulfones respectively. The last reaction is described below, while the other two are described in Chapter 14. 

C. [2,3]-Sigmatropic Rearrangements of Propargylic Sulfenates to Allenic Sulfoxides... 

In one of its earliest applications, Horner and Binder143 prepared a variety of allenyl aryl sulfoxides by the rearrangement of propargylic arenesulfenates, and noticed that accumulation of electron-withdrawing groups in the aromatic ring retard the rate of rearrangement. These authors have also performed a detailed study of the chemistry of... 

Analogous with the rearrangement of allylic sulfoxides is the [2,3]-sigmatropic rearrangement of propargylic sulfoxides to allenic sulfenates. This process, which has been relatively little studied so far, appears to be the first step in the facile and quantitative rearrangement of sulfoxide 98 to the hemithioacetal 101 (equation 45)167. This reaction,... 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

Rearrangement of dienynols to vinylallene sulfoxides. A few years ago, Oka-mura et al. (11, 39) reported the rearrangement of a dienynol to an allenyldiene with transfer of chirality of the propargylic alcohol. This rearrangement has now been used for an enantioselective synthesis of a sesquiterpene, (+ )-sterpurene (3).Thus reaction of the optically active propargylic alcohol 1 with C6H,SC1 at 25° results in a vinylallene (a) that cyclizes to the optically active sulfoxide 2. Nickel-... 

The reactions of the corresponding propargyl sulfoxides and sulfones now resemble the chemistry of the other acceptor-substituted derivatives such as ketones and aldehydes (see Section 1.2.4). Compared with the thioethers, here much milder bases are sufficient apart from aluminum oxide, often triethylamine or potassium carbonate are used. Sometimes even a spontaneous isomerization takes place. The selective isomerization of one triple bond in the presence of a second triple bond in 126 [313] (Scheme 1.56) or an allyl sulfone in 129 [314] (Scheme 1.57) are just two examples out of a whole series [178, 304, 313, 315-331]. When, on the other hand, the in situ oxidation of 126 was carried out in an aprotic solvent, no isomerization at all was observed. 

The acidity of the propargylic proton of the starting compound 18 allows the equilibration with the allene 19 induced by bases such as tertiary amines or alcoholates (Scheme 7.4). Such prototropic rearrangements furnish the title compounds 19 with at least one proton at the terminal carbon atom, often in good yields. The EWG group involves carboxylic acids [33], esters [34], ketones [35, 36], isonitriles [37], sul-fones [38], sulfoxides [39, 40] and phosphonates [41], The oxidation of easily accessi-... 

Acceptor-substituted allenes can be prepared from the corresponding propargyl precursors by prototropic isomerization (see Section 7.2.2). Conversely, such allenes can also be used to synthesize propargyl compounds. For example, treatment of the sulfoxides 417 with 1 equivalent of a lithiation reagent leads to the intermediates 418, which furnish propargyl sulfoxides 419 by hydrolysis (Scheme 7.55) [101]. If the electrophiles used are not protons but primary alkyl halides or carbonyl compounds, the products 420 or 421, respectively, are formed by C,C linkage. 

In addition to the sulfur-substituted enyne-allenes depicted in Schemes 20.18-20.20, the sulfoxide 141 was prepared by treatment of the enediynyl propargylic alcohol 108 with benzenesulfenyl chloride to induce a [2,3]-sigmatropic rearrangement (Scheme 20.29) [10]. The Myers-Saito cyclization of 141 occurs at 37 °C with a half-life of only 16 min. 

The present method is practical and efficient as it employs readily available enantioenriched propargylic alcohols as precursors to the allenylindium reagents. With achiral aldehydes the diastereoselectivity is high for branched aldehydes, moderate for unbranched aldehydes, and low for benzaldehyde (Table I). With cHral a-methyl aldehydes the additions proceed under effective reagent control to afford anti adducts of high ee and with excellent diastereoselectivity (eq. 1 and 2). Comparable results were obtained with 3 1 dimethyl sulfoxide-tetrahydrofuran (DMSO-THF) as the solvent. 

Ethynylation of the ketone 1 and treatment of the propargyl alcohol thus obtained with phenylsulfenyl chloride in the presence of triethylamine gave the expected allene sulfoxide. Treatment of this sulfoxide with aluminium trichloride in benzene gave the sulfide 2 in 15% yield. 

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