Many forms of the mechanism are known. In 1957, the Zimmerman and Traxler suggested that some aldol reactions have "six membered transition states having a chair confirmation." This is known as the Zimmerman-Traxler model. E-enolates gives rise to anti products, whereas Z-enolates give rise to syn products. Factors that control selectively are the preference for placing substituents equatorially in six membered transition states and the avoidance of syn-pentane interactions, respectively. E and Z refer to the cis-trans stereochemical relationship between the enololate oxygen bearing the positve counterion and the highest priority group on the alpha carbon. In reality, only some metals such as lithium and boron reliably follow the Zimmerman-Taxler