The above structural relationships all involve a change at "one" position in the synephrine molecule, and numerous other similar changes, many of which have been explored, are possible. However, the structure of ephedrine differs from that of synephrine at "two" different positions: ephedrine has no substituent on the phenyl ring where synephrine has a 4-OH group, and ephedrine has a methyl group on the position α- to the N in the side-chain, where syneprine has only a H atom. Furthermore, "synephrine" exists as either of two enantiomers, while "ephedrine" exists as one of four different enantiomers; there are, in addition, racemic mixtures of these enantiomers.