Meerwein—Pondorff—Verley reduction

The mechanism of the oxidation is considered to proceed via the cyclic complex as shown below. The reverse of this process is known as the Meerwein, Pondorff, Verley reduction. 

Meerwein-Pondorff-Verley reduction 57, 59, 496 Meldrum s acid 258 menthol 343 ff. 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

In the general context of donor/acceptor formulation, the carbonyl derivatives (especially ketones) are utilized as electron acceptors in a wide variety of reactions such as additions with Grignard reagents, alkyl metals, enolates (aldol condensation), hydroxide (Cannizzaro reaction), alkoxides (Meerwein-Pondorff-Verley reduction), thiolates, phenolates, etc. reduction to alcohols with lithium aluminum hydride, sodium borohydride, trialkyltin hydrides, etc. and cyloadditions with electron-rich olefins (Paterno-Buchi reaction), acetylenes, and dienes.46... 

The factors leading to isoracemisation can lead to misinterpretation of isotope-ex-change experiments. The lack of exchange of the proton with solvent protons is classical evidence for hydride transfer in the Cannizzaro reaction and Meerwein-Pondorff-Verley reduction and in the rearrangements involved in steroid biosynthesis. For some time the lack of deuterium exchange in the glyoxalase reaction was attributed to a hydride-transfer mechanism (Eqn. 64). 

The Oppenauer oxidation with aluminium alkoxides provides an alternative method for the oxidation of secondary (and less commonly primary) alcohols. The reaction is the reverse of the Meerwein-Pondorff-Verley reduction (see Section 7.3). Typically aluminium triisopropoxide (or aluminium tri-tert-butoxide) is used, which serves to form the aluminium alkoxide of the alcohol. This is then oxidized through a cyclic transition state at the expense of acetone (or cyclohexanone or other carbonyl compound). By use of excess acetone, the equilibrium is forced to the right (6.45). 

Hydride transfer from the LDA can take place, similar to the Meerwein-Pondorff-Verley reduction (see Section 7.3). A mechanism is given below. 

Grignard additions are sensitive to steric effects, and with hindered ketones, a competing process involving reduction of the carbonyl group is observed. A cyclic transition state similar to that proposed for Meerwein-Pondorff-Verley reduction can account for this transformation. The extent of this reaction increases with... 

Meerwein-Pondorff-Verley reduction Additive reduction... 

Like the Meerwein-Pondorff-Verley reduction, these reactions are believed to proceed under thermodynamic control, and the more stable stereoisomer is the main product. 

Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process, and the reaction can be driven to completion if the ketone is removed from the system, by distillation, for example. This process is called the Meerwein-Pondorff-Verley reduction. ... 

The development of the CrOs-pyridine and DMSO-based methods has decreased the number of instances in which older oxidation techniques are used. One such method, the Oppenauer oxidation is the reverse of the Meerwein-Pondorff-Verley reduction (Chapter 5). It involves heating the alcohol to be oxidized with an aluminum alkoxide in the presence of a carbonyl compound, which acts as the hydrogen acceptor. The reaction is an equilibrium process and proceeds through a cyclic transition state. 

The Woodward synthetic route was initiated with a Diels-Alder reaction between 1,4-benzoquinone (14) and diene 15. The cycloadduct 16 formed in this way underwent Meerwein-Pondorff-Verley reduction to afford tricyclic lactone 17 which was converted to bromoether 18. Treatment of this substance with methoxide gave the methyl ether 19. Conversion of 19 to its halohydrin followed by chromium oxidation provided the a-bromo ketone 20 which upon treatment with zinc in glacial acetic acid afforded the bicyclic enone 21. This substance was transformed to the aldehyde-acid 23 by an osmylation-periodate cleavage sequence. The acid function in 23 was es-terified and the aldehyde moiety was condensed with 6-methoxytryptamine. The Schiff base intermediate obtained in this fashion was reduced to give an amine which provided the lactam 24 upon intramolecular acylation. Bischler-Napieralski cyclization of 24 gave the pentacyclic intermediate 25 in which... 

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