Allyl sulphones, alkylation

Sulphones (coni.) allenic - see Allenic sulphones allyl - see Allyl sulphones aryl unsaturated alkyl - see Aryl unsaturated alkyl sulphones aryl vinyl - see Aryl vinyl sulphones as radiolytic products 907 bicyclic - see Bicyclic sulphones bis(/ -hydroxyalkyl) - see Bis... 

Allyl sulphones can be converted to dienes by alkylation and elimination of sulphinic acid under basic conditions (equation 64)105. Several vitamin A related polyenes have been synthesized following this two-step protocol (Table 10)106. The poor leaving-group ability of the arylsulphonyl group requires treatment with strong base for elimination. However, elimination of the allylsulphonyl group takes place readily under palladium catalysis (equation 65)107. Vinyl sulphones can be converted to dienes via Michael addition, alkylation with allyl halides and elimination of sulphinic acid sequence (equation 66)108. 

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]. 

Combined with the efficient and regiospecific alkylation of allylic sulphones shown in the Scheme (contrast the corresponding alkylation of allylic sulphox-ides), this method can give allylic alcohols trisubstituted on the carbon-carbon double bond. The sequence can also be adapted to open up two new routes to exocyclic a-methylene-carbinols (32) (Scheme 15)/" either by overall [1,3] transposition of oxygen from (33), or from ketones via the vinyl sulphones (34), which are isomerized to the thermodynamically favoured allylic isomers (35). 

The reduction of /Sy-epoxy-sulphones has been used as a new synthesis of a-methylene-carbinols. "" The epoxy-sulphone (153), prepared from 1-cyclo-hexenylmethanol, was reductively eliminated by Na(Hg) in THF-MeOH to methylenecyclohexanol (154). The same reduction/elimination procedure comprises part of that for a general synthetic sequence of allylic alcohols (156 R ,R, R" = H or Me, R = hexyl), via )3y-epoxy-sulphones (155), in 60—90% yields. "" Any group R" may be introduced by alkylation of (155 R" = H). 

Methods reported this year for the reduction of alkyl halides to alkanes include the potassium-dicyclohexyl-18-crown-6 reduction of alkyl fluorides, sodium borohydride reduction of alkyl chlorides, bromides, and iodides (or sulphonate esters) under liquid-liquid phase-transfer conditions, and the selective reduction of tertiary alkyl, benzyl, and allyl halides with the borate (61). Continuing... 

The sulphone (97) converts alcohols, ROH, into (98). 7V-Quaternization or protonation of (98) gives a species so susceptible to nucleophilic attack that alkylated derivatives of such poor nucleophiles as CIO4 or FSOs may be prepared. Sulphur dioxide undergoes a reversible ene reaction with alkenes, such as methylenecyclohexene as in (99). In the absence of water, overall allylic shift (to methylcylohexene) merely results, but exhaustive allylic deuteriation of the starting material occurs in the presence of 02 . ... 

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