A dicobalt hexacarbonyl (Co2(CO)6) cluster is essential for the unusually broad dipolarophile scope and for the sense and degree of diastereoselection in a catalytic, three-component synthesis of tetrahydrofurans and dihydrofurans. Likely involving a new class of carbonyl ylide, these cycloadditions are stereospecific with respect to the dipolarophile and exhibit high diastereoselectivity and regioselectivity in most cases. Differentiation of all four positions of the tetrahydrofuran can thus be accomplished in a triply convergent manner."