Sequential- and tandem-oxidation-epoxidation reactions employing guanidine bases

1,5,7-Triazabicyclo[4.4.0]dec-1-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-1-ene (MTBD) promote the efficient formation of sulfonium ylides from sulfonium salts in the Corey-Chaykovsky epoxidation of aldehydes and provide excellent yields of the corresponding epoxides. This reaction protocol can be further adapted to allow for the development of both sequential and tandem-oxidation-epoxidation protocols from aldehydes generated in situ by manganese dioxide (MnO2) or barium manganate (BaMnO4) mediated oxidation reactions.