Unsaturated dicarbonyl

This ring system has been covered in several reviews <1984CHEC(5)305, 1984CHEC(5)607, 1987AHC(41)319, 1996CHEC-II(8)345, B-2002SOS(12)613>. The methods used for synthesis are quite diverse, and include the reactions of thiosemicarbazide with 1,3,5-tricarbonyl compounds and unsaturated dicarbonyl compounds, or their equivalents. 

Some steps of the sequence proposed, of course, may be others, but key stages including formation of ot,p-unsaturated dicarbonyl derivative 16, cyclization into tetrahydropyrimidine 17, and dehydration are common in the most cases. 

Unsaturated dicarbonyls DCB Unsaturated acyl peroxy radicals TCO,... 

Oxidation of 2,5-dialkylfurans with pyridinium chlorochromate results in high yields of, -unsaturated -dicarbonyl compounds. Similar results are obtained by peracid oxidation of furans, most frequently CPBA. It is assumed that the first step involves epoxidation as shown in Scheme 67. Mono-, di-, and trisubstituted furans are oxidized to (Z)-enediones by methyltrioxorheniumureaH202 <1998TL5651>. Mo(CO)6-catalyzed oxidation of 2,5-dialkyl furans by cumyl hydroperoxide provides (A)-enediones selectively. In the presence of Na2C03, the corresponding (Z)-isomers are obtained <2003TL835>. 

The Knoevenagel condensation is a base-catalyzed aldol-type reaction, and the exact mechanism depends on the substrates and the type of catalyst used. The first proposal for the mechanism was set forth by A.C.O. Hann and A. Lapworth Hann-Lapworth mechanism) In 1904." When tertiary amines are used as catalysts, the formation of a p-hydroxydlcarbonyl Intermediate is expected, which undergoes dehydration to afford the product. On the other hand, when secondary or primary amines are used as catalyst, the aldehyde and the amine condense to form an Imlnlum salt that then reacts with the enolate. Finally, a 1,2-ellmlnatlon gives rise to the desired a,p-unsaturated dicarbonyl or related compounds. The final product may undergo a Michael addition with the excess enolate to give a bis adduct. 

Knoevenagel condensation Condensation of aldehydes and ketones with active methylene compounds to afford a,P-unsaturated dicarbonyl or related compounds. 242... 

Photolysis rates and subsequent product branching ratios of unsaturated -dicarbonyls (an important source of radicals) have been adjusted according to Thuener et al. (2003) for butenedial and 4-oxo-2-pentenal, and Graedler and Barnes (1997) for 3-hexene-2,5-dione. The photolysis rates of the epoxydicarbonylene products have also been increased as they were originally estimated by analogy with the unsaturated -dicarbonyls. 

Figures. Relative sensitivities of ozone towards the rate coefficients of selected elementary reactions of the chemical mechanism used at t = 30 min. The results for all diesel exhaust experiments are shown in comparison with those for the reference experiment with base mixture I. DCB = unsaturated dicarbonyls. For the calculation of the total fraction for the other reactions, the signs of the single sensitivities were not considered.

If an unsaturated dicarbonyl precursor is available, no oxidant needs to be added a synthesis of the perchlorate of pyrylium itself, shown below, falls into this category careful perchloric acid treatment of either glutaconaldehyde, or of its sodium salt, produces the parent salt. (CAUTION potentially explosive). 

The Knoevenagel condensation is the method of choice for the preparation of a,p-unsaturated dicarbonyl compounds and related compounds and only a few alternative methods have been developed. However, with the traditional Knoevenagel condensation there are problems with the reactivity of ketones, with the competitive Michael addition occuring in the reaction of some active methylene compounds. There is also a problem with steieocontrol in the synthesis of Knoevenagel products from unsymmetrical 1,3-dicarbonyl compounds. An alternative method is the addition of Grignard reagents to vinylogous carbamates (see Section 11.2.6). Another possibility is the reaction of a metal ketimate with malonodini-trile to yield ylidenemalonodinitriles (see Section 11.3.1.7). ... 

The product from the addition of an enolate of a 1,3-dicarbonyl-type compound with an aldehyde (or ketone) dehydrates readily to give the a,(3-unsaturated dicarbonyl product. This is known as the Knoevenagel condensation reaction, typically carried out under mild conditions with an amine base, and is a useful method for the formation of alkenes bearing electron-withdrawing substituents (see Section 1.1.3). 

These data show that under atmospheric conditions the adducts will react predominantly with O2 to form hydroxycyclohexadienylperoxy radicals. The subsequent reactions of these peroxy radicals leading to ring-opening are still unclear. Several possible reaction pathways which result in formation of dicarbonyl and unsaturated dicarbonyl compounds are outlined in Fig. 10. Experiments on radical cycling (Zetzsch) indicate that the reaction of the aromatic-OH adducts with O2 produce HO2 radicals with high yield on a short timescale. The mechanism leading to this so-called "prompt HO2" formation in the aromatic compound oxidation systems is presently unclear. 

The results presented in this study allow for the elucidation of the reaction mechanism and kinetics of the major products of alkylated aromatic compounds in the troposphere. The results show that the peroxy radical chemistry achieves the conversion of dimethylphenol to unsaturated dicarbonyls and aldehydes, thereby aiding oxidation and combustion processes that either releases large amount of energy or form reactive free radicals. 

As one would expect, oximes and nitroso-compounds are frequently encountered starting materials throughout the chemistry of this group of heterocycles. Further examples of their reactions have appeared. Reaction with enol ethers produces dihydro-oxazines (260), which may be converted to pyridine N-oxides upon treatment with HCl. Dihydro-oxazines are also obtained upon treatment of 7,6-unsaturated dicarbonyl compounds with nitrous acid. Thermolysis of these products leads to nitrones (261). 

Probably the most popular carbon-carbon double-bond-forming reaction involving sulphur proceeds via the elimination of a sulphur-oxygen species. This is illustrated by a synthesis of 1,5-unsaturated dicarbonyl compounds (236) which proceeds by phenylthioalkylation of enolates (236a), using the phenyl-thioalkene (237), followed by ozonolysis and elimination of the sulphoxide moiety to provide the double bond. An alternative method for double-bond formation is shown in the preparation of alkenes R CH=CHR by reductive cleavage of the sulphide (238) with titanium salts, and demonstrates the versatility of sulphur in such double-bond formations. In the latter example... 

Dicarbonyl compounds condense with aldehydes and ketones that do not undergo self-aldol reaction. The products are afi-unsaturated dicarbonyl compounds, and the process goes by the colorful name of Knoevenagel condensation, (a) An example of a Knoevenagel condensation is given below. Propose a mechanism. 

The Knoevenagel reaction is the condensation of an active methylene compound with an aldehyde or ketone, to give an a,p-unsaturated dicarbonyl compound.(l + 2 —> 3). ... 

Indeed, when the pre-formed aminal 12 was treated with ethyl acetoacetate in ethanol, the bis-adduct 7 was isolated in good yield. Knoevenagel s proposal for the mechanism of this reaction therefore was first, condensation of the amine and aldehyde to give the aminal 12, followed by attack of this by the acetoacetate to produce a presumed 3-amino dicarbonyl intermediate 13, which would then eliminate piperidine to give the a,p-unsaturated dicarbonyl compound 14, which in this case underwent a final Michael addition with an additional equivalent of ethyl acetoacetate to provide the bis-adduct 7. 

The hydridotetracarbonylferrate anion effects both double bond reduction and deacylation of a,/ -unsaturated dicarbonyls to alkyl methyl ketones (Yamashita et al, 1975). 

The Painter group developed another approach to tetramates (e.g., 320) that involved the reductive amination of y-oxobutenoates (e.g., 319) in the presence of primary amines (Scheme 81 2002S869). Unsaturated dicarbonyl 319 was prepared in three steps by the phosphine-catalyzed Michael addition of benzyl alcohol to butynoate 318, desilylation, and oxidation of the resulting primary alcohol with 2-iodoxybenzoic acid (IBX). Treatment of 319 with tert-butylamine followed by sodium borohydride gives the O-benzyl tetramate 320 via a reductive amination—lactamization. 

Liu, X. (1999), Atmospheric photochemisiry study of 1,4-unsaturated dicarbonyls and dienes, Ph.D. Thesis, University of North Carolina Chapel Hill, Chapel HiU, NC. 

---