A-Sulfonyl carbanion

Intramolecular cyclization of 2-phenysulfonylmethyl lactam 3 took place upon reaction with lithium hexamethyldisilazan via generating its a-sulfonyl carbanion to give a cyclized postulated intermediate that can be quenched with trimethylchlorosilane to afford the stable silyl ketal 4. The later ketal was desulfonylated by Raney-Ni and desilylated through treatment with tetrabutyl ammonium fluoride (BU4NF) to afford the carbacephem 5 (94M71) (Scheme 1). 

The nucleophilic attack of strong bases (e.g. hydroxide ion, alkoxide ions and carbanions) on either the a-carbon111 or the sulfur atom of the sulfone group99,113 of the thiirane dioxides is the initial key step that is responsible for the subsequent ring opening and further reaction. The formation of a three-membered a-sulfonyl carbanion is not observed in these cases (equation 11). 

Significantly, (a) a-sulfonyl carbanions of thiirane dioxides, generated from the latter in the presence of strong bases such as potassium t-butoxide19 and alkoxide ions99, do epimerize to relieve steric repulsion between substituents as in 42 above and (b) the a-hydrogen in aryl-substituted three-membered sulfoxides (e.g. 46c) are sufficiently acidic to... 

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

This chapter describes the chemical behavior of a-sulfinyl and a-sulfonyl carbanions. The stereoelectronic effects of these sulfur-containing groups have been the subject of much controversy for more than a decade which has now gradually settled down. Meanwhile, the special features of the chemical behavior of these groups have been utilized for syntheses of thousands of useful organic substances. This chapter deals with the... 

Behavior of a-sulfinyl and a-sulfonyl carbanions TABLE 3. pK, values of substituted phenols in water at 25 °C... 

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