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.