Abstract: 1, 3-Thiazolidines react with carbon monoxide, in the presence of catalytic quantities of chloro (1, s-cyclooctadiene) rhodium (I) dimer and potassium iodide, to give thiazolidinones in 56-8896 yields. Reaction in the absence of KI afforded the six-membered ring thiazin-3-one. The rhodium (1) complex can catalyze the quantitative conversion of the thiazin-3-one to the thiazolidinone under carbon monoxide, with ketene as the reaction by ...