Sulfonium salt protocol

Finally, chiral epoxides can be prepared from a,p-unsaturated carbonyl compounds through an entirely different approach, in which the epoxide oxygen is derived from the carbonyl moiety. For example, trans-aryl-vinyl epoxides 52 can be synthesized from conjugated aldehydes 50 and chiral sulfonium salts 51, with excellent ee s. The protocol is especially effective for substrates which bear a p-mcthoxy group on the aryl substituent <00TL7309>. 

Figure 10.2 illustrates selected examples of these epoxide products. Aromatic and heteroaromatic aldehydes proved to be excellent substrates, regardless of steric or electronic effects, with the exception of pyridine carboxaldehydes. Yields of aliphatic and a,/ -unsaturated aldehydes were more varied, though the enantio-selectivities were always excellent. The scope of tosylhydrazone salts that could be reacted with benzaldehyde was also tested (Fig. 10.3) [29]. Electron-rich aromatic tosylhydrazones gave epoxides in excellent selectivity and good yield, except for the mesitaldehyde-derived hydrazone. Heteroaromatic, electron-poor aromatic and a,/ -unsaturated-derived hydrazones gave more varied results, and some substrates were not compatible with the catalytic conditions described. The use of stoichiometric amounts of preformed sulfonium salt derived from 4 has been shown to be suitable for a wider range of substrates, including those that are incompatible with the catalytic cycle, and the sulfide can be recovered quantitatively afterwards [31]. Overall, the demonstrated scope of this in situ protocol is wider than that of the alkylation/deprotonation protocol, and the extensive substrate...

Aziridination of trans- and cis- 8-methylstyrene with benzy-lainine gave two diastereomeric aziridines with almost the same yields and diastereomeric ratio (1 1), suggesting that the stereochemistry of aziridines is not influenced by the olefin configuration. A one step protocol can also be appUed in case of unstable 2-arylethenyl(diphenyl)sulfonium salts. 

Hydrolysis of sulfonium salts serves as a useful protocol for removal of a protecting group or hydrolysis of a carboxylic acid derivative. For example, thioamides are converted into the corresponding methyl esters by methylation with methanolic methyl iodide followed by treatment with aqueous potassium carbonate (eq 25). Methyl thiomethyl ethers are readily hydrolyzed using an excess of methyl iodide in aqueous acetone (eq 26). Under similar reaction conditions, thioacetals are hydrolyzed to the corresponding carbonyl compounds (eq 21) ... 

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