Monoprotection of Dicarbonyl Compounds

MONOPROTECTION OF DICARBONYL COMPOUNDS Selective Protection of a- and /3-Diketones 

R2NH = piperidine, TsOH, benzene, reflux, 92% yield/ 

R2NH = morpholine, TsOH, PhCH3, reflux, 4-5 h, 72-80% yield/ 

Trimethylsilyl enol ethers can be used to protect ketones, but in general are not used for this purpose because they are reactive under both acidic and basic conditions. More highly hindered silyl enol ethers are much less susceptible to acid and base. A less hindered silyl enol can be hydrolyzed in the presence of a more hindered one.  

The preparation of silyl enol ethers has been reviewed. 

Methyl Enol Ether Ethyl Enol Ether i-Butyl Enol Ether 

R = (CH3)2CHCH2 (i-BuOH, benzene, reflux, TsOH, 16 h, 100% yield). In this case, 2-methyl-1,3-cyclopentanedione was mono-protected. 

Rechsteiner, F. Texier-Boullet, and J. Hamelin, Tetrahedron Lett., 34, 5071 

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Methyiaiuminum Bis(2,6-di-f-butyi-4-methyiphenoxide) Compiex, 364 MONOPROTECTION OF DICARBONYL COMPOUNDS... 

Acetals and Ketals, 178 Miscellaneous Derivatives, 210 Monoprotection of Dicarbonyl Compounds, 220... 

Substituted Hydrazones and Oximes 3.1.63 Monoprotection of Dicarbonyl Compounds... 

Scheme 40. Phenylselenyl Sulfate Catalyzed One-Pot Conversions of Dicarbonyl Compounds into Vicinal Monoprotected Tricarbonyl and Diprotected Tetracarbonyl Compounds...

The use of supported KMn04 is disavantaged by the removal of the dicarbonyl compound from the surface of the silica gel and the yields are not convenient for large scale experiments. 44-146 .iodylben-zoic acid ii is a mild oxidative reagent which does not affect the dithiolane moiety of the monoprotected 1,4-diketone (219 Scheme 53). 

Dicarbonyl Compounds.—Ozonolysis of 2-cyclopentylidenecyclopentanone provides a convenient synthesis of cyclopentane-l,2-dione on the ca. 0.4 mole scale.The by-product, cyclopentanone, is recycled via an aldol condensation to regenerate the enone. The method should be applicable to other alicyclic 1,2-diones. Various methods for the preparation of monoprotected 1,2-diones have appeared. Monoacetals of a-dicarbonyl compounds are prepared regio-specifically from enol ethers [equation (32)], following epoxidation in methanol and oxidation of the resulting acyloin acetal. 

Dicarbonyl Compounds.— 2,3-Dihydroxy-l,4-dioxan functions as a stable synthetic equivalent to glyoxal, particularly in the synthesis of a variety of heterocycles. The dioxan overcomes the prerequisite preparation of pure glyoxal immediately prior to its use (because of its tendency to polymerize) and offers an alternative available for reaction under non-aqueous conditions. Monoprotected a-keto-aldehydes are seldom available by selective derivatization of the parent dicarbonyl compound. However, 1,1,2,2-tetramethoxy-alkanes, readily prepared from a,a-dichloro-aldehydes, undergo regioselective hydrolysis to give l,l-dimethoxyalkan-2-ones. The sequence from the dichloro-aldehyde may be carried out without isolation of the tetramethoxylated intermediate. a,a-Bis(phenylseleneno)-aldehydes may be prepared from aldehydes or the intermediate a-(phenylseleno)-aldehyde by treatment with morpholinophenyl-selenenamide. a-(Arylseleno)-ap-unsaturated aldehydes result from the electrochemical oxidation of 3-hydroxyalkynes in the presence of a diaryl diselenide [equation (42)]. Treatment of a,a -dibromo-ketones with primary... 

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