Elimination reactions biological

All three elimination reactions—E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particular common. 1he substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carboiwl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the correspotiding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

E2—how can you keep it all straight and predict what will happen in any given case Will substitution or elimination occur Will tile reaction be bimoJecular or unimolecular There are no rigid answ ers to 

Thomson Click Organic Interactive to use a web-based palette to design syntheses using substitution and elimination reactions. 

I Primary alkyl halides Snj2 substitution occurs if a good nucleophile is used, E2 elimination occurs if a strong base is used, and ElcB elimination occurs if the leaving group is two carbons away from a carbonyl group. 

WORKED EXAMPLE 12.6 Predicting the Product and Mechanism of a Reaction 

Tell whether each of the following reactions is likely to be 8 1, 8 2, El, ElcB, or E2, and predict the product of each  

Look carefuUy in each reaction at the substrate, leaving group, nucleophile, and solvent. Then decide from the preceding summary which kind of reaction is likely to be favored. 

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In the ElcB reaction, C-H bond-breaking occurs first. A base abstracts a proton to give an anion, followed by loss of the leaving group from the adjacent carbon in a second step. The reaction is favored when the leaving group is two carbons removed from a carbonyl, which stabilizes the intermediate anion by resonance. Biological elimination reactions typically occur by this ElcB mechanism. 

Chapter 11, Reactions of Alkyl Halides Nucleophilic Substitutions and Eliminations—A discussion of the El cB reaction has been added to Section 11.10, and a new Section 11.11 discusses biological elimination reactions. 

Biological Elimination Reactions Trichloroethylene (a solvent) Halothane (an inhaled anesthetic) Dichlorodifluoromethane (a refrigerant) Bromomethane (a fumigant)... 

The Hofmann elimination reaction is not often used today in the laboratory, but analogous biological eliminations occur frequently, although usually with protonated ammonium ions rather than quaternary ammonium salts. In the biosynthesis of nucleic acids, for instance, a substance called adenylosuccinate... 

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

Elimination reaction, 138. 383-393 biological examples of, 393 summary of, 393-394 Embden-Meyerhof pathway, 1143-1150 see also Glycolysis... 

Considerable attention has been directed to dehalogenation mediated by corrinoids and porphyrins in the presence of a chemical reductant (references in Gantzer and Wackett 1991 Glod et al. 1997 Workman et al. 1997). Illustrations are provided by the dechlorination and elimination reactions carried out by titanium(III) citrate and hydroxocobala-min (Bosma et al. 1988 Glod et al. 1997). The involvement of corrinoids and porphyrins is consistent with the occurrence of analogous mechanisms for biological reactions that... 

Unfortunately, this procedure did not permit preparation of the corresponding ketonucleosides postulated as key intermediates in many biosynthetic routes. In fact, -elimination reactions precluded the coupling of ketoses with nitrogen heterocycles. These difficulties may explain the scarcity of information concerning their biological properties or functions. 

Nucleophilic aromatic substitution reactions of nitro-substituted thiophenes have been utilized to prepare biologically active thiophenes including reverse transcriptase <02H(57)97> and nitric oxide synthase <02JHC857> inhibitors. The addition-elimination reaction of 2-chloro-3-nitrothiophene (56) with metallated indole 57 afforded 58 which was transformed into the corresponding thiophene-fused azepino[5,4,3-cd]indoles 59 <02H(57)1831>. 

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