Reverse condensations

Continuing the process we may now change the pressure on the gas to the vapor pressure of the glass as given by (12) and then carry out a reversible condensation. These steps will be seen to involve the entropy change... 

Movement across a boundary line corresponds to a phase change. The arrows on the figure show six different phase changes sublimation and its reverse, deposition melting and its reverse, freezing and vaporization and its reverse, condensation. 

The highlighted section reacts with water in a reverse condensation reaction to give the monomer that polymerizes with glycerol. 

The intramolecular version of ester condensation is called the Dieckmann condensation.217 It is an important method for the formation of five- and six-membered rings and has occasionally been used for formation of larger rings. As ester condensation is reversible, product structure is governed by thermodynamic control, and in situations where more than one product can be formed, the product is derived from the most stable enolate. An example of this effect is the cyclization of the diester 25.218 Only 27 is formed, because 26 cannot be converted to a stable enolate. If 26, synthesized by another method, is subjected to the conditions of the cyclization, it is isomerized to 27 by the reversible condensation mechanism. 

Consider the reversible condensation polymerization reaction of a hydroxyacid A to form a polyester polymer,... 

The KDO aldolase (EC 4.1.2.23), which catalyzes the reversible condensation of pyruvate with D-arabinose to form KDO 2 (Scheme 6), was first isolated from, Enterobacter cloacae... 

Aldolases such as fructose-1,6-bisphosphate aldolase (FBP-aldolase), a crucial enzyme in glycolysis, catalyze the formation of carbon-carbon bonds, a critical process for the synthesis of complex biological molecules. FBP-aldolase catalyzes the reversible condensation of dihydroxyacetone phosphate (DHAP) and glyceralde-hyde-3-phosphate (G3P) to form fructose-1,6-bisphosphate. There are two classes of aldolases the first, such as the mammalian FBP-aldolase, uses an active-site lysine to form a Schiff base, whereas the second class features an active-site zinc ion to perform the same reaction. Acetoacetate decarboxylase, an example of the second class, catalyzes the decarboxylation of /3-keto acids. A lysine residue is required for good activity of the enzyme the -amine of lysine activates the substrate carbonyl group by forming a Schiff base. 

Setting (AA — BB) = Mx, we can write the general reversible condensation reaction... 

In our selected example, Lehn and coworkers [80] reported the synthesis of a dynamic 12-member, template-directed imine library 1, obtained from the reversible condensation of three aldehydes (monomer set M, Figure 7.11), with four primary amines (monomer set M2, Figure 7.11) in buffered aqueous conditions, followed by irreversible reduction to amines 2 with sodium cyanoborohydride. The library was prepared in the presence of a large excess of M2, to prevent further condensation of an aldehyde onto the secondary amine product. A template-driven imine library 1 was prepared in the presence of the metalloenzyme carbonic anhydrase II (CAII). After the template-assisted, reversible dynamic reaction was complete, the reducing agent was added and the amine library 2 was produced (Figure 7.11). Without any... 

The second intramitochondrial step of the urea cycle is the reversible condensation of carbamoyl phosphate and ornithine, catalyzed by OTC. The high-energy phosphate bond in carbamoyl phosphate is cleaved during this... 

And cleavage then reverses condensation. Acetyl CoA formation Outside ready for ligation Now forms by carboxylation Malonyl CoA. 

Eslertt, like aldehydes and hetones. nre weakly acidic. When an ester Mritli an ide, a reversible condensation reaction occurs to yield a /J-keto este-r. Par example, ethyl acetate yields ethyl acetoacetate on treatment with base. This reaction between two ester molecules is known an the Cliusen condensation reaction, f Well use ethyl esters for coitsistency, but other esters will also work. I... 

Lrthreonine aldolase (L-threonine acetaldehyde-lyase) catalyzes the reversible condensation of acetaldehyde and glycine to form L-threonine. The enzyme has been shown to be an activity distinct from serine hydroxy-methyltransferase that also catalyzes the above reaction (85,86). The substrate specifically of the adolase has been demonstrated to be flexible with respect to the aldehyde involved. The enzyme has been shown to form phenylserine derivatives from substituted benzaldehydes and glycine (86). 

The mechanism presented for gastric acid production did not reveal the source of protons and chloride ions. Continued production of gastric acid by this mechanism would result in depletion of the chloride ions in the cell and alkalinization of the cell because of the loss of protons. Figure 2.27 shows that the source of chloride ions is the bloodstream and the source of protons is carbonic add, H COj. As detailed in a later section, carbonic acid is produced by the action of carbonic anhydrase, the enzyme that catalyzes the reversible condensation of a molecule of water with a molecule of carbon dioxide. Carbonic acid ionizes to produce a... 

When acetaldehyde is treated with a base, such as sodium ethoxide or sodium hydroxide, a rapid and reversible condensation reaction occurs. The product is a /3-hydroxy aldehyde, or aldol (oidehyde -l- alcohoZ). 

In recent years, the in situ generation of chiral iminium ions from a chiral secondary amine and the Michael acceptor, mostly a,P-unsaturated aldehydes and ketones, is used as a powerful strategy for a range of asymmetric cycloaddition reactions [24]. However, the chiral a,P-unsaturated iminium ions, provided by reversible condensation from a,P-unsaturated ketones with secondary amine, could not always work well in this asymmetric approach, probably because of poor generation of the corresponding iminium cations [25],... 

---