Dimedone reaction with

Acetaldehyde can be isolated and identified by the characteristic melting points of the crystalline compounds formed with hydrazines, semicarbazides, etc these derivatives of aldehydes can be separated by paper and column chromatography (104,113). Acetaldehyde has been separated quantitatively from other carbonyl compounds on an ion-exchange resin in the bisulfite form the aldehyde is then eluted from the column with a solution of sodium chloride (114). In larger quantities, acetaldehyde may be isolated by passing the vapor into ether, then saturating with dry ammonia acetaldehyde—ammonia crystallizes from the solution. Reactions with bisulfite, hydrazines, oximes, semicarb azides, and 5,5-dimethyl-1,3-cyclohexanedione [126-81 -8] (dimedone) have also been used to isolate acetaldehyde from various solutions. 

In the reaction with enamino ketones derived from dimedone (e.g., 49) p-toluenesulfonyl chloride gives the chloroiminium cation (138) isolated as the perchlorate. This indicates that initial O sulfonation is followed by addition of chloride ion and subsequent expulsion of tosylate (42) in a manner similar to the trichloroacetyl chloride reaction with 49 (Section IV.A). 

Dimedone reacts with carbonyl compounds with the elimination of water yielding the condensation product [1]. The reaction is specific for ketoses aldoses do not react or only weakly [6],... 

Amines, thiols, eOH (p. 226), etc., will also add to the 0-carbon atom of 0-unsaturated carbonyl compounds and esters, but the most important reactions of C=C—C=0 systems are in Michael reactions with carbanions reactions in which carbon-carbon bonds are formed. A good example is the synthesis of l,l-dimethylcyclohexan-3,5-dione (dimedone, 100) starting from 2-methylpent-2-ene-4-one (mesityl oxide, 101) and the carbanion 0CH(CO2Et)2 ... 

Furopyrrolizidine, antitumour antibiotic 46 can be brominated by reacting with NBS in TF1F at rt to give the 7-bromo derivative 229 in 64% yield. The reaction can be reversed by refluxing 229 with dimedone in toluene to give back 46 in 80% yield. Compound 229 on further reaction with diethylamine in toluene afforded thermodynamically more stable epimer 231 (Scheme 37) <2002OL4387>. 

The reaction of dimedone with 3-alkyl-5-ercapto-1,2,4-triazoles 315 in the presence of NBS gives intermediates, which after reaction with a solution of aqueous sodium carbonate afford 2-alkyl-5a-hydroxy-6,6-dimethyl-8-oxo-5a, 6,7,8,8a-hexahydro[l,2,4]triazolo[3,2-3]benzothiazoles 49. Finally, reaction with PPA provides the dehydrated heterocyclic derivatives 316 (Scheme 30) <1999IJH127>. 

Cycloaddition Reactions with Other Nucleophiles The anodic two-electron oxidation of catechol affords o-quinone that may react with the enolates of 4-hydroxycoumarine or 5,5-dimethyl-1,3-cyclohexanedione (dimedone). The resulting adducts undergo a second anodic oxidation leading to benzofuran derivatives in good yields (90-95%) (Scheme 53) [75, 76]. 

Simple a-(phenyl-A3-iodanyl) ketones have never been isolated however, a-A3-iodanyl jS-diketones are known. 2-Dimedonyl(phenyl)-A3-iodane has been prepared from dimedone by the reaction with iodane 17 in a good yield as a white solid, which on heating in acetonitrile affords 2-tosyloxydimedone [Eq. (54)] [93]. 

The cyclic /J-dicarbonyl iodonium ylides can undergo [3 + 2] cycloaddition reactions with various substrates under catalytic or photochemical conditions, presumably via a stepwise mechanism [153-156]. In a recent example, iodonium ylide 211, derived from dimedone, undergoes dirhodium(II) catalyzed thermal [3+ 2]-cycloaddition with acetylenes 212 to form the corresponding furans 213 (Scheme 75). Under photochemical conditions ylide 211 reacts with various alkenes 214 to form dihydrofuran derivatives 215 [156]. 

Approaches that represent a type (ii) synthesis of 277-pyran-2-ones include the self-condensation of 1,3-dicarbonyl compounds, the reaction of cyclopropanones with pyridinium enolbetaines and the reaction of activated methylene groups with acetylenic esters <1984CHEC, 1996CHEC-II>. 4-Perfluoroalkyl-6-aryl-pyran-2-ones are formed by the reaction of the phosphonium salts 631 with 2-perfluoroalkynoates (Equation 254) <1999JFC(95)135, 1998JFC(91)99>. Dimedone reacts with dimethyl acetylenedicarboxylate to afford the pyran-2-one 632 in excellent yield (Equation 255) <2003PS2627>. 

The reaction of dimedone enamines with aldehydes is complex and the products depend on the structure of the aldehyde and the conditions. Acetaldehyde leads to benzoxazine as was independently reported by Greenhill27 and Akhrem and cow-orkers30. The same reaction proceeds according to Hammouda and coworkers with formaldehyde when the Af-phenylenaminone of dimedone31 or indanedione32 is used. 

Condensation of primary dimedone enaminones with formaldehyde according to Greenhill and coworkers26,27 give, in neutral medium, methylenbisenaminones, whereas acidic conditions produce spirocompounds. Structure A is given by Greenhill27 for the spiro-product, but structure B (equation 20) also seems reasonable. Modern spectroscopic methods should be used to solve this problem. Similar products are obtained in the Mannich reaction of enaminones derived from indan-l,3-dione33. 

Reaction of dimedone enamines with a-ketoacids has recently been shown to lead mainly to indole-2,4-dione derivatives64 (equation 47). Glyoxylic acid, however, reacts similarly to other aldehydes in a Hantzsch-type reaction yielding iV-phenylacridinedione in 30% yield64. 

Dimedone enol acetate, reaction with TeCl4, 58, 96... 

Due to the relative rapidity of the reactions and to the protonation of the free amine by the excess scavenger during the deprotection of allyl carbamates, the risk of formation of allylamine seems very limited and has not been reported. In the reactions with dimedone, a possible complication is the formation of substantial amounts of hydrolytically stable oxo-enamine by condensation with the liberated amine. For this reason, the use of N,N-di-methylbarbituric acid is probably a safer solution. 

This type of transformation forms the basis of the approach to polyoxa-zoles elaborated by Yoo (92TL2159). A good yield of oxazole 541 was obtained by using phenyliodonium dimedonate 542 as a carbene precursor in a reaction with acetonitrile (87TL4449). 

Reaction with aldehydes. Dimedone is particularly useful for isolation and identification of lower aldehydes. It reacts with gaseous formaldehyde without catalyst to give high-melting (188°) methylenebisdimedone with a 10-fold increase in mass. 

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