Sulfinyl carbanions, syntheses with

This chapter, however, does not deal with above-mentioned reactions of sulfoxides. Rather it is limited to asymmetric synthesis using a-sulfinyl carbanions and -unsaturated sulfoxides, specifically in which the stereogenic sulfoxide sulfur atom is enantiomerically pure. Therefore reactions of racemic sulfoxides are for the most part excluded from this review. For more general discussions, the reader is referred to other chapters in this volume and to other reviews on the chemistry of sulfoxides. Especially useful are the reviews by Johnson and Sharp and by Mislow in the late 1960s and by Oae and by Nudelman as well as a book by Block . A review by Cinquini, Cozzi and Montanari" through mid-1983 summarizes the chemistry and stereochemistry of optically active sulfoxides. This chapter emphasizes results reported from 1984 through mid-1986. 

The choice of the base and the solvent is crucial for the yield of a-sulfinyl carbanion alkylation. A high diastereoselection (80%) was observed in the alkylation of an a-sulfinyl carbanion with a-bromoacrylate56. In this ease the choice of the base appears to be decisive the highest asymmetric induction is found when the metalation is carried out using highly hindered bases, e.g., lithium tetramethylpiperidide. An interesting asymmetric synthesis of chiral 5-alkyl(or phenyl)dihydro-3-methylene-2(3f/)-furanoncs is based on this reaction56. 

Many other uses of a-sulfinyl carbanions are found in the literature, and in the recent past the trend has been to take advantage of the chirality of the sulfoxide group in asymmetric synthesis. Various ways of preparation of enantiopure sulfoxides have been devised (see Section 2.6.2) the carbanions derived from these compounds were added to carbonyl compounds, nitriles, imines or Michael acceptors to yield, ultimately, with high e.e. values, optically active alcohols, amines, ethers, epoxides, lactones, after elimination at an appropriate stage of the sulfoxide group. Such an elimination could be achieved by pyrolysis, Raney nickel or nickel boride desulfurization, reduction, or displacement of the C-S bond, as in the lactone synthesis reported by Casey [388]. 

A novel synthesis of allyl sulfoxides has been developed by the reaction of primary o -lithiosulfinyl carbanions with group 6 Fischer carbene complexes (Scheme 9).49 The Fischer carbene complex experiments involve a 1,2-addition of two molecules of sulfinyl carbanion to give an intermediate that, after a -elimination, furnishes the mentioned product. 

This transannular addition of a-sulfinyl carbanions to nonactivated double bonds is utilized as the key step in a synthesis of trans-1-thiadecalin (70) in enantiomerically pure form. The required ( )-thiacy-clodec-4-ene 5-oxide (66a,b) was prepared via several steps frtm (/ ,/ )-l,6-dibromo-3,4-hexanediol. Upon treatment with butyllithium, a 4 1 mixture of isomeric sulfoxides (66a and 66b) undergoes smooth cyclizadon to give a mixture of isomeric bicyclic sulfoxides (67a and 67b) in the same 4 1 ratio as the starting material, suggesting that the cyclizadon is essentially stereospecific. The major isomer (67a) is reduced with PCb to a sulfide (68) from which the desired (70) is derived via a thiaoctaline (69 heme 16). 

Sulfonyl carbanions are even more stable than sulfinyl carbanions and are consequently of greater significance in synthesis. They can be alkylated and acylated at the a-carbon atom using organolithium bases, and cyclic sulfones can be formed by intramolecular alkylation, (see Chapter 10, p. 202). Sulfonyl carbanions (73) also react with terminal epoxides, and this reaction is applicable for the synthesis of unsaturated alcohols (74) (Scheme 33). 

In 1982, Solladi reported a highly efficient, asymmetric synthesis of both enantiomers of methyl carbinols based on the stereoselective reduction of an enantiomerically pure P-ketosulfoxide [1], Prior to this work, only low to moderate levels of enantiomeric purity had been observed by Cinquini [2] and Johnson [3] in similar studies. The P-ketosulfoxides used in Solladie s study were prepared by condensation of the a-sulfinyl carbanion of (J )-methyl p-tolyl sulfoxide with esters (Scheme 4.1). 

One of the crucial synthesis steps, the introduction of the side chain, was solved by Siddall and co-workers with the aid of a sulfinyl-stabilised carbanion alkylating agent, as shown is Scheme 1.26. 

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