A conjugated diene, such as 1,4 butadiene, combines with an alkene in the Diels-Alder reaction. Especially favored are those alkenes which are dienophiles (having electron withdrawing substituents). The Diels-Alder reaction is a pericyclic process. In other words, it occurs by means of a particular type of transition state in which electrons simultaneously redistribute in a cyclic manner. The process starts with the bonding overlap of the terminal π-lobes of the conjugated diene with those of the alkene. The stereochemistry of the Diels-Alder reaction is informed by concerns particular to pericyclic reactions. The overlap occurs between the HOMO of one of the species (Highest Occupied Molecular Orbital) and the LUMO of the other (lowest unoccupied molecular orbital). Cycloaddition reactions can be antarafacial or, suprafacial, which means that the reagents must rotate for orbital overlap or not, respectively. Diels-Alder is of the suprafacial type, so it takes place more easily, i.e. at lower temperatures. Note that if the alkene reagent is trans, the derived ring substituents will also be trans.