Bis(2-ethyl-8-quinolinolato)dioxomolybdenum(VI) (9) (1 mol%) shows remarkable catalytic activity for the dehydrative cyclization of cysteine-containing dipeptides 1 to give the corresponding thiazolines 2 with less than 6% epimerization at the C2-exomethine position. For the dehydrative cyclization of threonine-containing dipeptides 4, 1 mol% of bis(2-phenyl-8-quinolinolato)dioxomolybdenum(VI) (10) gives the corresponding oxazolines 5 with retention of configuration at the 5-position.