beta-oxidation

Examples of beta-oxidation in the following topics:

• Connecting Lipids to Glucose Metabolism

  • Fatty acids are catabolized in a process called beta-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups, while producing NADH and FADH2.

• Lipid Metabolism

  • Biological lipids, which are broken down and utilized though β-oxidation, represent a potent energy source.
  • In brief, the oxidation of lipids proceeds as follows: two-carbon fragments are removed sequentially from the carboxyl end of the fatty acid after dehydrogenation, hydration, and oxidation to form a keto acid, which is then cleaved by thiolysis.
  • β-oxidation can be broken down into a series of discrete steps:
  • Oxidation: The initial step of β-oxidation is catalyzed by acyl-CoA dehydrogenase, which oxidizes the fatty acyl-CoA molecule to yield enoyl-CoA.
  • Cleavage: A thiolase then cleaves off acetyl-CoA from the oxidized molecule, which also yields an acyl-CoA that is two carbons shorter than the original molecule that entered the β-oxidation pathway.

• Types of Catabolism

  • These include breaking down and oxidizing food molecules.
  • This oxidation releases carbon dioxide as a waste product.
  • The glycerol initiates glycolysis and the fatty acids are broken down by beta oxidation to release acetyl-CoA, which then is fed into the citric acid cycle.
  • Fatty acids release more energy upon oxidation than carbohydrates because carbohydrates contain more oxygen in their structures.
  • The oxidation pathway starts with the removal of the amino group by a transaminase.

• Oxidation States of Nitrogen

  • In comparing the chemistry of the amines with alcohols and ethers, we discover many classes of related compounds in which nitrogen assumes higher oxidation states, in contrast to limited oxidation states of oxygen.
  • Amine oxides are prepared by oxidizing 3º-amines or pyridines with hydrogen peroxide or peracids (e.g.
  • For such a mechanism, the beta-hydrogen and amine oxide moieties necessarily have a syn-relationship.
  • Cope elimination of diastereomeric amine oxides, such as those shown in examples #2 & 3 above, provide proof of the syn-relationship of the beta-hydrogen and amine oxide groups.
  • Nitroxides are oxidized to unstable oxammonium cations by halogens.

• Modification of Condensation Products

  • The aldol reaction produces beta-hydroxyaldehydes or ketones, and a number of subsequent reactions may be carried out with these products.
  • As shown in the following diagram, they may be (i) reduced to 1,3-diols, (ii) a 2º-hydroxyl group may be oxidized to a carbonyl group, (iii) acid or base catalyzed beta-dehydration may produce an unsaturated aldehyde or ketone, and (iv) organometallic reagents may be added to the carbonyl group (assuming the hydroxyl group is protected as an ether or a second equivalent of reagent is used).
  • The Claisen condensation produces beta-ketoesters.
  • The fourth reaction demonstrates that the conjugate base of the beta-ketoester products from Claisen or Dieckmann condensation may be alkylated directly.
  • Thermal decarboxylation of the resulting beta-ketoacid gives a mono-alkylated cyclic ketone.

• Applications of Transition Metals to Organic Chemistry

  • When an alkyl group having beta-hydrogens is covalently bonded to a metal, beta-elimination commonly occurs.
  • Equation # 1 describes a sequence of double bond insertion (intramolecular) followed by a beta-elimination.
  • The oxidation state of the metal remains unchanged throughout the process.
  • Finally, the last example illustrates an oxidative coupling reaction, similar to oxidative addition.
  • The addition of hydrogen in the first step is such a case, the "oxidative" adjective referring to the fact that Rh(I) is oxidized to Rh(III) in the event.

• Disaccharides

  • A beta-glycosidase has the opposite activity.
  • This leaves the anomeric carbon in ring B free, so cellobiose and maltose both may assume alpha and beta anomers at that site (the beta form is shown in the diagram).
  • Gentiobiose has a beta-glycoside link, originating at C-1 in ring A and terminating at C-6 in ring B.
  • Because cellobiose, maltose and gentiobiose are hemiacetals they are all reducing sugars (oxidized by Tollen's reagent).
  • Lactose, also known as milk sugar, is a galactose-glucose compound joined as a beta-glycoside.

• Substitution and Elimination Reactions of Amines

  • Since the counter anion in most 4º-ammonium salts is halide, this is often replaced by the more basic hydroxide ion through reaction with silver hydroxide (or silver oxide).
  • In example #2 above, two of the alkyl substituents on nitrogen have beta-hydrogens, all of which are on methyl groups (colored orange & magenta).
  • At the other extreme, if the acidity of the beta-hydrogens is enhanced, then substantial breaking of C–H may occur before the other bonds begin to be affected.
  • Furthermore, the 4º-ammonium substituent is much larger than a halide or hydroxyl group and may perturb the conformations available to substituted beta-carbons.
  • It seems that a combination of these factors acts to favor base attack at the least substituted (least hindered and most acidic) set of beta-hydrogens.

• Beta Decay

  • Beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted from an atomic nucleus.
  • There are two types of beta decay.
  • Beta minus (β) leads to an electron emission (e−); beta plus (β+) leads to a positron emission (e+).
  • Beta decay is mediated by the weak force.
  • The continuous energy spectra of beta particles occur because Q is shared between a beta particle and a neutrino.

• Fischer Carbene Reactions

  • Reversible loss of CO ligands frees coordination sites and facilitates the oxidative addition of alkenes or alkynes.
  • Since the cyclopropanation reaction on page three requires an electron withdrawing group on the alkene reactant, a bonding interaction of the electrophilic beta-carbon with the nucleophilic metal is expected.
  • Oxidative addition produces a metallocyclobutene in the initial step, which is then followed by ring-opening, insertion and ring closure steps.
  • The aromatic product is released from the stable cobalt tricarbonyl complex by treatment with mild oxidizing agents.
  • Schrock alkylidenes generally incorporate metals having higher oxidation states, and because of the electrophilic nature of the metal their reactivity is significantly different.