Cleavage aldol

Keto acids are obtained by acylation of cyclopentanone enamines (see p. I3f.) with acid chlorides and subsequent base-catalyzed mro-aldol cleavage (S. Hdnig, 1960). 

This cleavage is a retro aldol reaction It is the reverse of the process by which d fruc tose 1 6 diphosphate would be formed by aldol addition of the enolate of dihydroxy acetone phosphate to d glyceraldehyde 3 phosphate The enzyme aldolase catalyzes both the aldol addition of the two components and m glycolysis the retro aldol cleavage of D fructose 1 6 diphosphate... 

The reactions that accomplished the conversion of intermediate 16 into intermediate 23 have taken place very smoothly. It is worth acknowledging that the //-hydroxy lactam moiety did not, at any stage, participate in any undesirable side reaction processes. The stability of the //-hydroxy lactam substructure in the presence of basic reagents is particularly noteworthy since a destructive retro-aldol cleavage reaction could have conceivably occurred on several occasions. The stability of this potentially labile moiety permits all of the desired transformations leading from 16 to 23 to be conducted without prior protection of the C-8 hydroxyl group. 

The most convenient method is the formation of two equivalents of (25) by retro-aldol cleavage from commercially available (26) by the combined action of FruA and triose phosphate isomerase (Figure 10.18 inset) [84]. This scheme has been extended into a highly integrated, artificial metabolism for the efficacious in situ preparation of (25) from inexpensive feedstock such as glucose and fructose (two equivalents of... 

Originally, the term aldol condensation referred specifically to the reaction of an aldehyde (having an a-hydrogen) with an aldehyde/ketone to form a j8-hydroxy aldehyde (the aldol). The reverse reaction is often referred to as a retrograde aldol reaction, a retro-aldol condensation (or reaction), or an aldol cleavage. March categorizes aldol condensations into five classes. The first is condensation between two identical aldehydes... 

This enzyme [EC 4.1.2.17] catalyzes the aldol cleavage of L-fuculose 1-phosphate to yield glycerone phosphate and (5 )-lactaldehyde. 

PLP-dependent enzymes catalyze the following types of reactions (1) loss of the ce-hydrogen as a proton, resulting in racemization (example alanine racemase), cyclization (example aminocyclopropane carboxylate synthase), or j8-elimation/replacement (example serine dehydratase) (2) loss of the a-carboxylate as carbon dioxide (example glutamate decarboxylase) (3) removal/replacement of a group by aldol cleavage (example threonine aldolase and (4) action via ketimine intermediates (example selenocysteine lyase). 

Synthesis of Macrocycles Employing Intramolecular Nitrile Oxide Cycloaddition and Aldol Cleavage... 

Transfer of hydrogen from C-4" of uridine 5 -(a-D-glucopyranosylur-onic acid pyrophosphate) to C-3 of the apiosyl group in the ester 117 has been demonstrated.4458 The conversion of 116a into 118 was hypothesized446 to involve aldol cleavage, isomerization of the resulting a-hydroxy aldehyde, and intramolecular, aldol reaction as shown. 

A major group of C-C-bonding enzymes consists of aldolases which serve in various metabolic pathways, mainly in the direction of aldol cleavage. Aldol con-... 

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

---