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.