pro-E/pro-Z convention

if the two chemically-like hydrogen atoms of abC=CH₂ are separately replaced by the achiral, d, an achirotopic stereogenic element is generated and two (diastereoisomeric) alkenes, abC=CHd are fomed that are E and Z stereoisomers (see E/Z convention). The replacement has converted a prostereogenic element into which one which is stereogenic; the situation is comparable to the change, prochiral → chiral, although a chiral element is not involved. The two chemically-like hydrogen atoms are designated as pro-E and pro-Z, and are abbreviated as H_e and H_z. Replacement of a pro-E hydrogen with ²H yields an E diastereoisomer and the similar replacement of a pro-Z hydrogen yields the Z form. Example: the two hydrogen atoms of phosphoenolpyruvate; that on the same side of the double bond as the COOH group is H_E, that on the same side as the OPO₃H₂ group is H_Z. Replacement of ¹H_E by ²H yields (E)-[3-²H₁]phosphoenolpyruvate; the Z diastereoisomer results if ¹H_Z is replaced by ²H. A similar situation is found in the side chain of chorismic acid.

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