Dicarbonyl compounds, 1,4-, synthesis

Bensa, D., Constantieux, T. and Rodriguez, J. (2004) P-BEMP a new efficient and commercially available user-friendly and recyclable heterogeneous organocatalyst for the Michael addition of 1,3-dicarbonyl compounds. Synthesis, 923-927. 

The synthesis of pyrazolcs starting from a hydrazine and a 1,3-dicarbonyl compound is a well established reaction in organic synthesis. If a mono-suhstituted hydrazine is reacted with an unsymmctrically substituted 1,3-dicarbonyl compound, two different pyrazole products which arc regioisomers could he formed (sec figure 10.3-2). 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

Enolate ions of p dicarbonyl compounds are useful intermediates m organic synthesis We shall see some examples of how they are employed m this way later m the chapter... 

The addition of 1,3-dicarbonyl compounds to /3-chloroazoalkenes is the basis of a pyrrole synthesis (Scheme 70a) 81TL1059). Pyrroles are also obtained by the reaction of enamines with azoalkenes (Scheme 70b) (79TL2969,81TL1475), and the copper(II) chloride catalyzed addition of 1,3-dicarbonyl compounds to arylazoalkenes (Scheme 70c) (82JOC684). 

Isoxazoles are susceptible to attack by nucleophiles, the reactions involving displacement of a substituent, addition to the ring, or proton abstraction with subsequent ring-opening. Isoxazolium salts are even more susceptible to attack by a variety of nucleophiles, providing useful applications of the isoxazole nucleus in organic synthesis. Especially useful is the reductive cleavage of isoxazoles, which may be considered as masked 1,3-dicarbonyl compounds or enaminoketones. 

Dicarbonyl compounds pyridazine synthesis from, 3, 45 pyrrole synthesis from, 4, 329 reactions... 

Dicarbonyl compounds pyrylium salt synthesis from, 3, 861... 

Synthesis of fused cyciopentanones (bicyciotS 3 0] octachones) or of propeltanes from a-dicarbonyl compounds wa the double aktol condensation with etoesters 2... 

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