The first total synthesis of (+)-chaetocin has been accomplished in nine steps starting from known N-Cbz-N-Me-serine using radical alpha-bromination reaction of diketopiperazine 10 and Co(I)-mediated reductive dimerization reaction of 12 as key reactions. The enantiomers show comparable inhibitory activity toward histone methyltransferase (HMT) G9a, but analogues without the sulfur functionality are inactive.