1 ,3- Dicarbonyl compounds cleavage

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. 

The initially formed ]5-dicarbonyl compounds are subject to further photo-transformations. One example is provided in the case of epoxy ketone (88), where the resulting /5-diketone (89) undergoes partly a-cleavage and acyl-alkyl... 

Alkenes with at least one vinjdic hydrogen undergo oxidative cleavage when treated with ozone, yielding aldehydes (Section 7.9). If the ozonolysis reaction is carried out on a cyclic alkene, a dicarbonyl compound results. 

The obvious Vfittig disconnection gives stabilised ylid (5fi) and keto-aldehyde (57). We have used many such long-chain dicarbonyl compounds in this Chapter and they are mostly produced from available alkenes by oxidative cleavage (e.g. ozonolysis). In this case, cyclic alkene (58) is the right starting material, and this can be made from alcohol (59) by elimination,... 

Highly substituted pyrrolo[l,2- ][l,2,4]triazines were synthesized from pyrrole derivatives, by closure of the triazine ring. Thus, hydrolytic cleavage of some 1,2-diaminopyrroles having a 1-NH-BOC-protected amino function 43 followed by reaction with 1,2-dicarbonyl compounds afforded a one-pot access to the corresponding bicyclic heterocycles 44 (BOC = f-butoxycarbonyl Equation 6) < 1997J(P 1)1829>. 

Utilization of fluoroalkyl-substituted dicarbonyl compounds is accompanied with side reactions. Trifluoro diketone 49 undergoes an acidic Claisen cleavage in ethanol with formation of dicyanoketone 50, which... 

Despite all of the activity in pyrimidine-based synthesis, only one study has emerged of solid-phase versions of these reactions <2003TL1267, 20030BC1909>. This chemistry was based upon condensation of dicarbonyl compounds with resin-bound pyrimidine-5,6-diamines through a 2-alkylthio link and oxidative cleavage as described in Section 10.18.7.2. The value of alkylthio substituents in the synthesis of complex substituted pterins has also been demonstrated in the synthesis of nucleic acid conjugates <2004OBC3588> (see Section 10.18.12.4). 

The photoreactions of a-dicarbonyl compounds are quite different in the vapor and condensed phases. In the vapor phase, carbon-carbon bond cleavage is the preferred mode of reaction but in the condensed phase, many of the observed reactions can be rationalized by a mechanism involving hydrogen abstraction. Internal hydrogen abstraction, when possible, is generally preferred over abstraction from the solvent. With the exception of diethyl oxalate, which undergoes photoreactions typical of an ester, only those compounds that are reasonably strained or can yield reasonably stable free radicals give decarbonylation products. In the presence of suitable substrates, cycloaddition reactions have also been observed. 

Ring opening by base cleavage of [3-dicarbonyl compounds. 

The two major characteristic oxidation processes of alkynes are their transformation to 1,2-dicarbonyl compounds and their cleavage reaction to carboxylic acids.710 The structure of the starting compounds has a decisive effect on the selectivity of oxidation. Since 1,2-dicarbonyl compounds proved to be intermediates in further oxidations, carefully controlled reaction conditions are often necessary to achieve selective synthesis. Certain oxidizing agents such as peroxyacids and ozone are nonselective oxidants. 

Polyaza rings suffer complete hydrolytic ring cleavage. Monocyclic 1,2,3-triazines are hydrolyzed by acid to yield 1,3-dicarbonyl compounds (Scheme 15). 1,2,3-Benzotriazines are easily converted into derivatives of 2-aminobenzaldehyde. 1,2,4,5-Tetrazines (180) are hydrolyzed with a base to give aldehyde hydrazones, RCH = NNHCOR. 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

Pyridinium chlorochromate in dichloromethane reacts with furans to give a range of products, but they are all formed by 1,4-electrophilic attack of chlorochromate on the furan ring the fate of the resultant intermediate (57) by heterolytic cleavage of the Cr—O bond depends on the substituents at the a-positions of the substrate. 2,5-Dialkylfurans yield a, /8-unsaturated-y-dicarbonyl compounds, 5-methyl-2-furylcarbinols yield pyran derivatives, and 5-bromo-2-furylcarbinols yield 5-hydroxyfuran-2(5//)-ones (Scheme 25) (80T661). 

Numerous examples of the preparation of tetramic acids from N-acylated amino acid esters by a Dieckmann-type cyclocondensation have been reported (Entries 7-9, Table 15.4). Deprotonated 1,3-dicarbonyl compounds and unactivated amide enolates can be used as carbon nucleophiles. In most of these examples, the ester that acts as electrophile also links the substrate to the support, so that cyclization and cleavage from the support occur simultaneously. The preparation of five-membered cyclic imi-des is discussed in Section 13.8. 

The cyclobutane dione (491) was converted quantitatively to the diketone (492) by treatment with zinc in HOAC. The reaction involves also the reductive cleavage of 1,4-dicarbonyl compounds 167 168). 

Agapie, T. Diaconescu, P. L. Mindiola, D. J. Cummins, C. C. Radical sdssion of symmetrical 1,4-dicarbonyl compounds C-C bond cleavage with titanium(IV) enolate formation and related reactions. Organometallics 2002,... 

Retrosynthetic cleavage of a carbon-heteroatom bond in 2.13 gives enol 2.14 which is equivalent to ketone 2.15. Repeating the process gives us a 1,4-dicarbonyl compound and the heteroatom-containing fragment such as a primary amine or hydrogen sulphide. 

As early as 1938 Rambaud reported the first synthesis of a donor-acceptor-substituted cyclopropane — obtained by the addition/elimination path (0 (Scheme 1) — and he also recognized that these cyclopropanes are prone to ring cleavage providing 1,4-dicarbonyl compounds. After saponification 19 opens to 20 which is oxidized by air to the isolated succinic acid 219). 

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