Diketone rearrangement

When treated with base, a-diketones rearrange to give the salts of a-hydroxy acids, a reaction known as the benzil-benzilic acid rearrangement (benzil is... 

The MT0/H202 system also catalyzes the oxidation of both internal and terminal alkynes. It provides a significantly cleaner and safer method of alkyne epoxidation than the oxidation by organic peracids [61], Internal alkynes yield carboxylic acids and a-diketones. Rearrangement products were observed only for aliphatic alkynes. Terminal alkynes give carboxylic acids, derivatives thereof, and acids as the major products. 

Irradiation of the 5 -methyl-9-en-6-one (496 Westphalen structure) in acetone gave the 5a-isomer (497), presumably via the biradical (498) in a triplet reaction. The 5a-methyl-3,6-diketone rearranged on silica gel, or with acid, to give the C-lO-spirocyclopentanone (500). A probable mechanism involving the enol (499) is illustrated. Further reaction with acid gave a compound formulated as the 4,10-bridged structure (501). ... 

Aliphatic diketones rearrange similarly. Thus, 1,2-cyclohexanedione is converted to 1-hydroxy-l-cyclopentanecarboxylic acid (Equation 9.70). 

Diketones rearrange in base to 2-hydroxycarboxylic acids (benzylic acid rearrangement). Although possible in principle for all 1,2-diketones, this reaction is only common for nonenolizable species. 

This reaction applies to many i,2 diketones, and is termed the Benzilic Acid Rearrangement. It provides a ready method for the preparation of disubstituted a4iydroxy-carboxylic acids. When applied to a cyclic 1,2-diketone, the ring system is necessarily reduced by one carbon atom for example, cyclohexan-i,2 ... 

Benzil (and other a-diketones Ar—CO—CO—Ar) upon refluxing with aqueous-alcoholic potassium hydroxide undergo the beozilic acid rearrangement. Thus benzil is converted into a salt of benzilic acid ... 

The high nucleophilicity of sulfur atoms is preserved, even if it is bound to electron withdrawing carbonyl groups. Thiocarboxylales, for example, substitute bromine, e.g. of a-bromo ketones. In the presence of bases the or-acylthio ketones deprotonate and rearrange to episulfides. After desulfurization with triphenylphosphine, 1,3-diketones are formed in good yield. Thiolactams react in the same way, and A. Eschenmoser (1970) has used this sequence in his vitamin B]2 synthesis (p. 261). 

Conjugate addition of vinyllithium or a vinyl Grignard reagent to enones and subsequent oxidation afford the 1.4-diketone 16[25]. 4-Oxopentanals are synthesized from allylic alcohols by [3,3]sigmatropic rearrangement of their vinyl ethers and subsequent oxidation of the terminal double bond. Dihydrojasmone (18) was synthesized from allyl 2-octenyl ether (17) based on Claisen rearrangement and oxidation[25] (page 26). 

Diketones and tetraketones derived from aromatic compounds by conversion of two or four SCH groups into keto groups, with any necessary rearrangement of double bonds to a quinonoid structure, are named by adding the suffix -quinone and any necessary affixes. 

Similarly, enol esters undergo rearrangement to give the corresponding 1,3-diketones. This reaction can be accompHshed thermally (500°C) or with a catalyst (58,59) ... 

LIEBIG BenzyBc Aad Rearrangement Benzylic a<5ds by rearrangement of diketones (also a-ketol rearrangement)... 

Selective hydroxylation with osmium tetroxide (one equivalent in ether-pyridine at 0 ) converts (27) to a solid mixture of stereoisomeric diols (28a) which can be converted to the corresponding secondary monotoluene-sulfonate (28b) by treatment with /7-toluenesulfonyl chloride in methylene dichloride-pyridine and then by pinacol rearrangement in tetrahydrofuran-lithium perchlorate -calcium carbonate into the unconjugated cyclohepte-none (29) in 41-48 % over-all yield from (27). Mild acid-catalyzed hydrolysis of the ketal-ketone (29) removes the ketal more drastic conditions by heating at 100° in 2 hydrochloric acid for 24 hr gives the conjugated diketone (30). 

The benzilic acid rearrangement of steroid a-diketones is often the preferred method for ring contraction. Yields are generally excellent, and in several instances the reaction has been shown to be stereospecific. Preparation of the starting a-diketones is discussed below and illustrated with several excellent experimental procedures. 

In practice, the scope of the benzilic rearrangement as a tool for the ring contraction of steroids is defined by the availability of a-diketones, or the corresponding diosphenols. 

Kubota and co-workers describe a novel oxidative rearrangement of the diosphenol (58) of 17iS-hydroxyandrost-4-ene-2,3-dione to the A-nor-A -1,2-diketone (59) in 33 % yield by the action of specially prep d manganese dioxide in boiling acetone. The rate of ring contraction is very sensitive to the source of the oxidant, and a trace of dilute sulfuric acid in the reaction mixture causes oxidative fission of ring A. 

In systems which preclude retro-aldol condensations, benzilic acid rearrangement of 11,12-diketones affords normal C-norsteroids in fair yields. For example, 11,12-diketotigogenin (82) is converted to the C-nor-(5oc,9(, 22a)-spirostane (83) in 65 % yield by barium oxide in boiling aqueous methyl-cellosolve. ... 

Ring contraction by benzilic acid rearrangement of a-diketones... 

Ring contraction by oxidative rearrangements of A-ring a-diketones by manganese dioxide... 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

Syntheses from Phthalimidines Syntheses from Ortho-Disubstituted Benzenes Condensation of 1,4-Diketones with Amines and Pyrroles Other Reactions and Rearrangements Leading to Isoindoles IV. Properties of the Isoindole System A. Tautomerism... 

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