Dithioacetals, alkylation hydrolysis

The dithiane-derived anion can be generated by the action of Bu"Li in THF at -78 C or with complex bases NaNH2-RONa at room temperature. Lithiated dithiane can also be prepared in situ by sonica-tion of n-butyl chloride with lithium in the presence of dithiane. Dithioacetals or ketals are resistant to acidic or basic hydrolysis. Regeneration of the carbonyl group from the dithioketal sometimes presents difficulties but can be carried out by hydrolysis in polar solvents (acetone, alcohols, acetonitrile) in the presence of metallic ions such as Hg, Cu, Ag, Ti or Tl. Alternatively, alkylative hydrolysis... 

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

Aminolysis of phenyl dithioacetates,8 pyridinolysis of O-ethyl dithiocarbonates,9 reaction of pyrrolidine with O-ethyl 5-aryl dithiocarbonates,10 aminolysis of chlorothionformates,11 pyridinolysis of alkyl aryl thioncarbonates,12 reaction of anionic nucleophiles with nitrophenyl benzoate and its sulfur analogues,36 hydrolysis of methyl benzoate and phenyl acetate containing SMe, SOMe and S02Me substituents,42 solvolysis of phenyl chlorothioformate,79 synthesis of new thiadiazoles,124 examination of a neighbouring sulfonium group in ester hydrolysis,136 hydrolysis of V-type nerve agents,250 and the reactions of peroxymonosulfate ion with phosphorus(V) esters have all been looked at previously in this review. 

Dithioacetals of aldehydes are sources of carbanions and hence may be used to form new C-C bonds in reactions in which the formerly electron-deficient character of the aldehydic carbon has been reversed. The 1,3-dithianes derived from formaldehyde or a higher aldehyde may be metallated and then alkylated (Scheme 2.27). Hydrolysis of the dithioac-etal is usually carried out in the presence of a thiophilic (sulfur seeking) metal salt such as a mercury salt. The insoluble sulfides cause the equilibrium to move in favour of the parent carbonyl compound. 

Scheme 21). Some important a-sulfinyl carbanions (42) are derived from dithioacetal S-oxides (43), and on subsequent alkylation and hydrolysis they yield the aldehydes (40) (Scheme 22).

Recent developments have tried to remedy the difficult hydrolysis of the dithian. The monosulfoxides of dithioacetals are better at stabilising anions and more easily hydrolysed. They can be prepared by alkylation of the parent compound 50 with a suitable alkyl halide, e.g. to give 51.9... 

A small number of papers that cover mechanistic aspects deal with the factors that stabilize the developing carbonium ion in the hydrolysis of 0,S-thioacetaIs. An electron-transfer, mild reduction system (iron polyphthalocyanine) has been shown to reduce benzil dithioacetal to the / -keto-sulphide [PhCOCPh(SPh)2 PhCOCHPhSPh]. Homolysis of dithioacetals on heating with BuKDOBu in PhCI followed by a 1,2-shift of an alkylthio-group leads to l,2-bis-(alkylthio)alkyl compounds/ ... 

A novel synthesis of cr-diketones from aldehydes proceeds by the formation of the anion of the ethylene dithioacetal of the aldehyde, reaction with Fe(CO)j, alkylation, and hydrolysis. 

Lithio-2-trimethylsilyl-l,3-dithiane is the most widely utilized reagent for the conversion of ketones and aldehydes to the corresponding ketene dithioacetals (Scheme 2.49) [126-128]. It is used for the synthesis of functionalized 2-alkylidene-1,3-dithianes 79 [129-135]. The 2-alkylidene-l,3-dithianes 79 thus synthesized are useful synthetic intermediates, which are conveniently accessible by means of Peterson reactions, and they can be transformed into various compounds [136, 137]. For example, compounds 79 are converted to the corresponding carboxylic acids, aldehydes, and enones by hydrolysis, hydrogenation followed by hydrolysis, and deprotonation followed by alkylation and hydrolysis, respectively (Scheme 2.49) [138-140]. 

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