Allene Sulfoxide Rearrangement

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. 

Elucidate the reaction sequence and give a mechanism for the formation of 2. 

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Synthetic Utility of the Propargylic Sulfenate-to-Allene Sulfoxide Rearrangement... 

The synthetically most useful oxygen-sulfur transpositions (equation 16) are the allyl sulfenate-allyl sulfoxide and the propargyl sulfenate-allene sulfoxide rearrangements, the driving force of both being the formation of the strong S=0 bond at the cost of the weak O—S bond (Scheme 12). ... 

When adding malonic ester anion to an allenic sulfoxide, rearrangement of the derived addition product leads to an allylic alcohol which cyclizes to (18) and isomerizes to the isolated product (19). If one alkylates the intermediate carbanion one gets an entry to a -methylenelactone system (18 Scheme 23). ... 

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... 

Cutting and Parsons described the transformation of acetylenic alcohols 314 into allenyl phenyl thioethers 316 by a two-step procedure (Scheme 8.85) [174], Deprotonation of alkynes 314 with n-butyllithium is followed by addition of phenylsulfenyl chloride, forming sulfenyloxy intermediates which subsequently rearrange to allenic sulfoxides 315. Treatment of allenes 315 with methyllithium results in loss of the sulfoxide moiety to form allenyl sulfides 316 in reasonable yields. 

The sulfenate-sulfoxide and sulfinate-sulfone rearrangements are very reliable and proceed with complete syn stereoselectivity17, ls. The allenic sulfoxides can be used for the synthesis of chiral alkylallenes with retention of configuration (see Section 1.1.3.). The relative configuration at sulfur in the allenic sulfoxides is not important for further synthetic purposes and racemization at sulfur is often observed without affecting the allenic axial dissymmetry. 

Scheme 12 [2,3]Sigmatropic rearrangement of propargylic trichloromethanesulfinates to allenic sulfoxides...

Scheme 13 [2,3]Sigmatropic rearrangements of nitroaryl propargyl sulfenates to allene sulfoxides and their cycloaddition reactions...

A somewhat related [2,3]sigmatropic rearrangement of 6-allenic sulfoxides to vinyl sulfenates has been reported by Padwa and coworkers [65]. 

Clean conjugate addition of appropriate nucleophiles to allenic sulfoxides has been used to produce allylic sulfoxide systems. Homer has described the formation of functionalized allylic alcohols when adding nucleophiles like amines, alcohols or thiols to allenic sulfoxides in excess. If the addition is performed with equimolar amounts at lower temperature, so that the 2,3-sigmatropic rearrangement is avoided, the intermediate addition products (enamines or enol ethers) can be hydrolyzed to the synthetically valuable 3-keto sulfoxides (Scheme 20). ... 

Under the influence of the angular methyl group the addition of acetylene occurs from the a-side, giving the opposite configuration at C-17 to that required for the corticosteroids, but the rearrangement of the diastereomeric methanol adducts of the derived allenic sulfoxide is also forced to proceed at the a-side, leading finally to the correct configuration. The elaboration of the required side chain in (17) can easily be accomplished. 

Certain sulfoxides can be prepared by rearrangement of the sulfenate esters thus, allyl arenesulfenate esters (7), obtained by condensation of the sulfenyl chloride with allyl alcohol (8), spontaneously rearrange to the allyl sulfoxides (9) (Scheme 6). The rearrangement also occurs with alkynic alcohols for instance, trichloromethanesulfenyl chloride (10) reacts with propargyl alcohol (11) to form the allenic sulfoxide (12) (Scheme 7). 

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