Phenylsulfonate anion

The examples presented in the previous two sections represent early approaches to the preparation of C-disaccharides. Building upon these early studies, new techniques became apparent and interesting reports surfaced in 1985. Aside from additional studies involving Diels-Alder methodology, Beau, et al.,7 reported the use of addition reactions between phenylsulfone anions and sugar-derived aldehydes as a viable method for the formation of C-disaccharides. As shown in Scheme 8.3.1, the sulfone associated with the addition product was cleaved on treatment with lithium naphthalide thus giving the final product. 

This paved the way for an Sn2 displacement with potassium thiophen-oxide, and sulfide oxidation to obtain phenylsulfone 5. Metallation of 5 was accomplished at —78°C with M-butyllithium in tetrahydrofuran (THF). The resulting a-phenylsulfonyl anion added readily to aldehyde 8 to afford a mixture of P-alkoxysulfones that underwent smooth O-acylation with acetic anhydride. The mixture of p-acetoxysulfones so formed was then reacted with 6% Na/Hg amalgam in methanol and ethyl... 

Fig. 17.85. Mechanistic analysis of the second part of the reaction process where the treatment of the acetoxy sul-fones syn- and anti-A with sodium amalgam completes the Julia-Lythgoe olefination. Series of a first electron transfer (—> alkenyl phenylsulfone radical anion E), homolysis (—> alkenyl radical G + sodium benzene sulfinate), second electron transfer (—> alkenyl anion trans"-D) and in-situ protonation.

Mulzer (Scheme 8 upper left) obtained the a,/(-unsaturated ester 33 with Z configuration from aldehyde 26a via a Still-Gennari olefination with phosphonate ester 34. Reduction of the ester with DIBAH and application of L-imidazole-PPhj gives allylic iodide 35. This acts as electrophile on the -anion of sulfone 36. After reductive removal of the phenylsulfone, group 28b is obtained [23]. 

The Julia reaction is remarkably versatile but it does need three steps to make the alkene addition, acylation, and reduction. A more recent version of the reaction cuts this down to one by using not a phenylsulfone but instead a sulfone carrying an electron-deficient heterocycle, for example a tetrazole. The anion of the sulfone is made with a strong base (here potassium hexamethyldisilazide, KHMDS—see p. 635) and is added to an aldehyde to give an alkene directly. 

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