The aldol reaction unites two relatively simple molecules into a more complex one. Increased complexity arises because up to two new stereogenic centers (on the α- and β-carbon of the aldol adduct, marked with asterisks in the scheme below) are formed. Modern methodology is capable of not only allowing aldol reactions to proceed in high yield but also controlling both the relative and absolute configuration of these stereocenters. This ability to selectively synthesize a particular stereoisomer is significant because different stereoisomers can have very different chemical and biological properties.