The optimized radical system showed that L-cysteine (1) was the best molecular catalyst among the selected 14 molecules (Figures. 4 and S8). Almost no catalytic reaction took place with the non-thiol analogs L-serine (11) and L-alanine (12), supporting the critical role of thiol groups in the catalysis. Low reactivity with butanethiol (10) will indicate the importance of hydrophilic adjacent groups, while the combination of amines and esters in the structure of the thiol catalysts may impair the catalytic reactions (3 and 5) under these conditions. In general, the other thiols also showed a fairly good activity in molecular catalysis, but the conversion was not satisfactory due to the competing side reactions.