Cadmium chloride mediated regiocontrol of dienolates and ketene thioacetals: γ condensation with aldehydes

The use of cadmium chloride to control the regioselectivity of vinylogous anions upon condensation with aldehydes is described. Addition occurred preferentially at the γ position with substituted crotonates and 2-ethylidene-1,3-dithiane (6). Experiments demonstrated that this selectivity arose as a consequence of isomerization of the initially formed kinetic α product to the thermodynamic γ product (e.g., 2 to 4). The optimum results were achieved with 2 equivalents of cadmium chloride and quenching at 0 °C.