Abstract The one-pot reactions of N-acetylserine methyl ester and N-acetyl-α-methylserine methyl ester with Lawesson's or Woollins’ reagent directly produced the corresponding cysteine or selenocysteine derivatives in moderate yields. The transformation would be initiated by phosphorylation of the hydroxy group, rather than chalcogenation of the amide group, and involve the oxazoline as a key intermediate