Aluminum amalgam sulfoxides

C. Reduction of )3-Keto Sulfoxides with Aluminum Amalgam (//)... 

In addition to deoxygenation sulfoxides undergo reductive cleavage at the carbon-sulfur bond when heated in tetrahydrofuran with aluminum amalgam. Keto sulfoxides were thus converted to ketones in usually quantitative yields (the keto group remained intact) [141]. 

As outlined by Roush and Walts,62 sulfoxide 21 was converted to the corresponding dianion with lithium diisopropylamide (LDA) and alkylated with n-butyl iodide to provide a diastereomerically complex mixture that was then converted directly to 2-butyl-3R-methylcyclohex-2-en-l-one (50% yield, 6 1 (P oc) mixture at C-2) upon thermolysis. We found that sulfoxide 21 could be alkylated with 2-(2-bro-moethyl)-2,5,5-trimethyl-1,3-dioxane63 and that the resultant complex mixture could be desulfurized with aluminum amalgam to furnish desired ketone 17 in 40-50% yield as 9 1 (P a) mixture at C-2. Surprisingly, alkylation of this sulfoxide dianion was not improved using 2-(2-iodoethyl)-2,5,5-trimethyl-l,3-dioxane17 in place of the bromide. 

Vinylic sulfoxide (30) is desulfurized to (31) with f-butyllithium (equation 80), and sulfone (32) affords triphenylethylene on reduction with aluminum amalgam or lithium aluminum hydride (equation 81). ... 

Corey and Chaykovsky described the reduction of carboxylic esters to ketones by way of / -keto sulfoxides. The ester is treated in tetrahydrofuran with a solution of methylsulfinyl carbanions in a mixture of dimethyl sulfoxide and tetrahydrofuran at 0°. This yields a j8-keto sulfoxide which is reduced with aluminum amalgam in 90% aqueous tetrahydrofuran.527 The methylsulfinyl carbanions are formed by the action of sodium hydride on dimethyl sulfoxide under nitrogen at 65-70°. 

Aluminum amalgam (Al/Hg) is also a useful reagent that is most commonly employed for hydrogenolysis of sulfur compounds.533 Reduction of sulfoxides is an important application,534 as is reduction of sulfones. Aluminum amalgam reduces the a-phenylsulfonyl moiety in 554 to give lactone 555 in 68% yield.535 Aluminum amalgam has also been used for reduction of nitro compounds to the amine,536 and reduction of azides to the amine.532... 

In contrast, the latter. ryw-isomer, ethyl (TS,2R)-2-(1 -hydroxymethyl)-4-pentenoate. was recently prepared as the only product of a reduction with an enzyme fraction obtained from baker s yeast176. Introduction of a sulfur-containing functional group into the substrate increases stereocontrol in baker s yeast reduction of many ketonesI2e>. 2-Methylthio-3-keto esters are selectively reduced to the (3S)-3-hydroxy esters (Table 5)123,127. The 2-methylthio group is easily removed by 3-chloroperbenzoic acid oxidation to the corresponding sulfoxide followed by subsequent reduction with aluminum-mercury amalgam. Thus, these compounds can also be used for the preparation of optically pure 2-unsubstituted 3-hydroxy esters. 

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