Generation of trimethylsulfonium cation from dimethyl sulfoxide and dimethyl sulfate: implications for the synthesis of epoxides from aldehydes and ketones

The reaction between dimethyl sulfoxide (DMSO) and dimethyl sulfate has been monitored qualitatively by 1H NMR spectroscopy. At reaction temperatures of 20, 50 and 90 °C, the major product is methoxydimethylsulfonium methyl sulfate 5 whereas at 100 °C the major product is trimethylsulfonium methyl sulfate 6 together with minor amounts of methoxydimethylsulfonium methyl sulfate; formaldehyde was qualitatively identified as a product by derivatisation (2,4-dinitrophenylhydrazone). Treatment of the product ensuing from the latter reaction with potassium hydroxide, followed by addition of (separately) benzaldehyde and 1-(2,4-dichlorophenyl)pentan-1-one gave the expected epoxides 4a and 4b, respectively.