If R1 and R2 (note equation at top of page) are different substituents, there is a new stereocenter formed at the alpha position when an enol converts to its keto form. Depending on the nature of the three R groups, the resulting products in this situation would be diastereomers or enantiomers. Enediols Enediols are alkenes with a hydroxyl group on each carbon of the C=C double bond. Normally such compounds are disfavored components in equilibria with acyloins. One special case is catechol, where the C=C subunit is part of an aromatic ring. In some other cases however, enediols are stabilized by flanking carbonyl groups. These stabilized enediols are called reductones. Such species are important in glycochemistry, e.g., the Lobry de Bruyn-van Ekenstein transformation. Ribulose-1,5-bisphosphate is a key substrate in the Calvin cycle of photosynthesis. In the Calvin cycle, the ribulose equilibrates with the enediol, which then binds carbon dioxide. The same enediol is also susceptible to attack by oxygen (O2) in the (undesirable) process called photorespiration. Phenols Phenols represent a kind of enol. For some phenols and related compounds, the keto tautomer plays an important role. Many of the reactions of resorcinol involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione. Biochemistry Keto–enol tautomerism is important in several areas of biochemistry. The high phosphate-transfer potential of phosphoenolpyruvate results from the fact that the phosphorylated compound is "trapped" in the less thermodynamically favorable enol form, whereas after dephosphorylation it can assume the keto form. The enzyme enolase catalyzes the dehydration of 2-phosphoglyceric acid to the enol phosphate ester. Metabolism of PEP to pyruvic acid by pyruvate kinase (PK) generates adenosine triphosphate (ATP) via substrate-level phosphorylation. Reactivity Addition of electrophiles The terminus of the double bond in enols is nucleophilic. Its reactions with electrophilic organic compounds is important in biochemistry as well as synthetic organic chemistry. In the former area, the fixation of carbon dioxide involves addition of CO2 to an enol. Deprotonation: enolates Deprotonation of enolizable ketones, aldehydes, and esters gives enolates. Enolates can be trapped by the addition of electrophiles at oxygen. Silylation gives silyl enol ether. Acylation gives esters such as vinyl acetate. Stable enols In general, enols are less stable than their keto equivalents because of the favorability of the C=O double bond over C=C double bond. However, enols can be stabilized kinetically or thermodynamically. Some enols are sufficiently stabilized kinetically so