Allylic anions 1,4-addition reaction with conjugated enones

The butyllithium-generated anion of an allylic sulfone was reported to add to conjugate enones, but different regioselectivity was observed for 2-cyclohexenone (1,4-y) and 3-penten-2-one (1,4-a), as shown in equation (31). Clean 1,4-a additions to both acyclic and cyclic enones can be realized when the lithio carbanions are allowed to react in the presence of HMPA (2 mol equiv.) at -78 C. Under these conditions, allyl phenyl sulfone reacts with 2-cyclohexenone. 2-methyl-2-cyclopentenone and 3-methyl-2-cyclohexenone, giving the corresponding 1,4-a adducts as a 75 25 mixture of two dia-stereoisomers. In the reaction of allylic phenyl sulfone with an acyclic enone, 4-methyl-3-penten-2-one,... 

Conjugate addition of the complete allylic alcohol fragment is possible with the mixed cuprate reagents 33 prepared by asymmetric reduction (chapter 26) of acetylenic ketones 29 to give the alcohols 30, protection as a silyl ether 31 and hydroboration-iodination. Lithiation and reaction with hexynyl copper (I) gives the mixed cuprate 33 from which the less stable anion is transferred selectively to an enone.3 This approach has been widely used in the synthesis of prostaglandins. 

In order to prepare conjugated olefins, either an allylsulfone or an a,) -unsaturated carbonyl compound is needed. In the case of allylic sulfones, the delocalized carbanion can undergo either a- or y-addition, and the use of enals or enones can result in 1,2- or 1,4-addition products. In general, allylsulfonyl anions, derived from non-stabUized sulfones, undergo additions to carbonyls with excellent a-selectivities (reaction 3.17 [59] Scheme 3.16). The situation is more complex with a, -unsaturated compounds, but conditions have been defined that lead with high preference to adducts resulting from 1,2-addition (reaction 3.18) [60]. 

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