Sulfone-stabilized anion

While the chemistry of alkyl and allylic sulfoxide anions is similar to that of phosphine oxides, phosphinates and sulfone stabilized anions (Sections 1.5.2.2.1 -2), the situation is further complicated by the additional stereogenic center at sulfur. Therefore in all cases, asymmetric induction may arise from the stereocenter at sulfur. 

The sulfone-stabilized anions 7 reacted in much the same way with ethyl bromocrotonate but more cleanly, giving the cyclopropane carboxylates 8 exclusively in high yields98. The only observed products were the truws-substituted cyclopropanes. 

The utility of the creation of a y-lactone enolate through 1,4-addition of a carbanion and its interception by an electrophile has also been demonstrated in other classes of natural products, e.g., in the enantioselective synthesis of 10-oxa-l 1-methyl PGE2 analogues22. This synthesis starts with 1,4-addition of the sulfone-stabilized anion from 27 to ( + )-(S )-4-methyl-2-buteno-lide which has been prepared in three steps from (—)-(S)-l,2-epoxypropane. The intermediate enolate 28 is reacted with the acetylenic iodide to give the trisubstituted diastereomeric mixture of lactones 29, which is eventually converted into the pure compound 30, both reactions occurring with high diastereoselectivity. 

There are many examples of BF3-OEt2 promoted openings of (1) by carbanions, including sulfone-stabilized anions, vinylic anions, allylic anions, and phosphonate-stabilized anions. For example, the lithium anion of trans-1,2-Bis(tributylstannyl)ethylene opens (5)-(l) in the presence of BF3-OEt2 in THF at —78°C, affording fra/w-l-(tributylstannyl)-5-tosyl-4-hydroxypent-l-ene in 50% yield the latter is converted into oxirane (3) in 76% yield on treatment with powdered Sodium Hydroxide in monoglyme. ... 

Common leaving groups are carboxylates such as acetate or benzoate, and the starting materials are very easily made. The sulfone-stabilized anion adds to aldehydes and a simple esterification step, which can be done in the same reaction vessel, introduces the acetate or benzoate group. This is how the starting material for the eUmination above was made. 

A new sulfone-stabilized anion is formed and produces the opening of the epoxide by an intramolecular attack, which results in the formation of a cyclopentane. 

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