2- dimedone, formation

A new domino lithium acetylide addition/rearrangement procedure on trans-1,2-dibenzoyl-3,5-cyclohexadiene furnished 3-alkylidene-2,3-dihydrofurans via an intriguing mechanism involving three bond formations and two bond cleavages in one single operation <06SL1230>. The reaction of dimedone with meso-diacetoxycyclohexene in the presence of a palladium catalyst led to the formation of the tricyclic product as depicted below <06S865>. 

The early addition of Dimedon is reported234 to depress the formation of hexamethylenetetramine from formaldehyde in the presence of ammonia. O Dea282 found that the activated methylene center, sometimes formed in periodate oxidations of carbohydrates, reacts appreciably with the formaldehyde formed, thus giving low yields of apparent formaldehyde. He was able to depress this side reaction by the use of lowered temperatures and by the addition of benzaldehyde or of p-hydroxybenzaldehyde. The analyses for formaldehyde have often been more successful at a pH of 7.5 than at lower pH values.57, 68 59a 60 264... 

Lipson et al. in several publications [91-93] in the reactions of dimedone and aromatic aldehydes with 2-aminobenzimidazole, 3,5-diamino-l,2,4-triazole, and 3-amino-5-methyltio-l,2,4-triazole described a formation of angular tri- and four-cyclic heterocycles as minor reaction products. However, aldehydes did not participate in these MCRs and the solvent (DMF) acted as a carbonyl component... 

Very interesting transformations were observed with dimedone 103b and arylidenethioacetamides 110 (88ZOR460). When the reaction was carried out at ambient temperature, Michael adducts 111 were formed. Compounds 111 can be converted on heating into hydrogenated pyridi-nethiones 112 (in ethanol) or to 2-aminopyran-3-thioamides 113 (in benzene). The formation of the latter ones was found to be reversible. Thus, 113 convert into 112 on heating in ethanol, and yield 2-amino-3-cyanopyr-ans 104 when heated with MN in benzene (Scheme 39). 

The only known example of 2-monoalkylaminopyran 123 in the series of tetrahydrochromenes was obtained by the less common three-component reaction of alkylisonitriles 124, acetylenedicarbonic esters 125, and dimedone or 1,3-cyclohexanedione 103 (03M1585) (Scheme 42). This example illustrates how the isonitrile —N =C group can be involved in the formation of a 2-aminopyran ring, similar to the other groups with triple bonds - nitrile C=N and alkynyl C=C. 

Scheme 4 Possible mechanism involved in the formation of 18 from D-ribose (9) and dimedone (14)...

The efficient formation of diaryliodo-nium salts during the electrolysis of arylio-dides has been reported by Peacock and Fletcher [166]. The electroiodination of a 3D-aromatic molecule, dodecahydro-7,8-dicarba-nido-undecaborate has also been reported [167]. The iodination (and bromi-nation) of dimedone has been reported to yield 2-iododimedone, which formally is an electrophilic substitution reaction [123]. In a similar process, the indirect electrochemical oxidation of aliphatic ketones in an alkaline Nal/NaOH solution environment has been shown to yield a,a-diiodoketones, which rapidly rearrange to give unsaturated conjugated esters [168]. Dibenzoylmethane has been converted into dibenzoyliodomethane [169]. Terminal acetylenes have been iodinated in the presence of Nal. However, this process was proposed to proceed via oxidation of the acetylene [170]. 

The unexpected 8-hydroxymethylhexahydropyrido[2,3-, pyrimidine-6-spiro-l -cyclohexane-2, 4,6 -triones 520 were obtained from microwave-assisted cyclocondensation of equimolar amounts of 6-aminopyrimidin-4-ones 460 and dime-done with a large excess of formaldehyde (37% in water) in the presence of EtsN as a catalyst <2006TL27>. The reaction proceeded via an initial formation of the 2 1 dimedone/formaldehyde adduct 521 that gave intermediate 522 and 523 which could undergo cyclocondensation with excess formaldehyde to give 520 (Equation 42) <2006TL27>. 

The analogous reaction to that described in Equation (42) using paraformaldehyde in equimolar amount to dimedone (instead of a large excess) was irradiated for 3 min to yield pyrimido[4,5- ]quinoline-4,6-dione 591 via formation of intermediate adduct 590 <2006TL27>. The 5-aryl-5,6,7,8,9,10-hexahydropyrimido[4,5-/ ]quinolines 592 were also prepared by the cyclocondensation reaction of 6-aminopyrimidine 460 with dimedone and 4-sub-stituted benzaldehydes by boiling in absolute ethanol for 30 min <1998JHC231>. 

The Niki reaction474 (alkylation of dimedone by l,4-dibromo-2-methyl-2-butene, with formation of compound 218), applied to phloro-acetophenone and to 3-isopentyl-2,4,6-trihydroxyisobutyrophenone, leads respectively to 2,3-dihydrobenzofurans 219a and 219b. Furan 219b with toluene- >-sulfonic acid rearranges to the benzofuran 220b.475... 

When one of the components of such multicomponent reactions was a molecule of DMF, the formation of angular dihydropyrimidines was also observed. For example, amines 197,198 and 208 in boiling DMF reacted with dimedone 200, leading to azolopyrimidines 217-219 [184, 186, 187]. These treatments, in the opinion of Lipson et al. [184], resulted in the reaction passing through an enamine intermediate 216 formed at the endocyclic nitrogen atom (Scheme 3.60). 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

Competition between C=0 and C=C was observed in diphenylketene from which a lactone and a ketene acetal were obtained with the ylide of dimedone [23]. A different type of cycloaddition occurred with PhI=C(C02Me)2 this reacted catalytically with several alkenes (1-hexene, styrene, methyl methacrylate, etc.) with formation of lactones [24] ... 

In one pot and under aqueous conditions, a proline-catalysed Knoevenagel -cyclocondensation sequence involving an aromatic aldehyde, an activated methylene compound and dimedone results in the rapid formation of fused 477-pyrans (Scheme 2) <06SL263>. 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

Dimethylcyclohexane-l,3-dione (dimedone) and Ai,Ai-dimethylbarbituric acid are relatively acidic (3-dicarbonyl compounds 5.3 and 4.7, respectively). The deprotection reaction involves (Scheme 16) a prototropic equihbrium between the P-dicarbonyl compounds and the deprotected carbamate species (shifted further to right by the formation of carbon dioxide and the respective amine). 

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. 

Reaction of acylylides with a 2-(cyclohexadienylidene)dimedone derivative resulted in the formation of a dihydrofuran ring . Furans have been made from reactions of 3-alkylsiloxyallylylides with aldehydes followed by acid hydrolysis of the resultant epoxides . 

In a very similar manner, tandem 1,6- and 1,4-additions of -dicarbonyl compounds to methyl 2,4-pentadienoate were utilized by Danishefsky and coworkers- foi the formation of several bi- and tricyclic ring systems. For example, reaction of the enolate of dimedone with this ester gave the expected 1,6-addition product protonation/deprotonation set the stage for a subsequent intramolecular 1,4-addition (equation 9). Likewise, a ketodiester was used to transform the pentadienoate in a one-pot procedure by consecutive 1,6- and 1,4-additions into a richly functionalized tricyclic product which was then converted into the natural product ( )-cpiclovane (equation 10). According to this principle, Irie and coworkers obtained several decalin-2,7-diones by treatment of 2-methylen-2-cyclohexenones with dimethyl 3-oxoglutarate. 

The one-portion addition of the reagents in the model reaction between benzaldehyde, ethyl cyanoacetate and dimedone resulted in exclusive formation of the unwanted dimeric product 78 (Figure 3.3), as a consequence of the preferential condensation of benzaldehyde and dimedone due to the higher acidity of dimedone (p/y, 12.0) than that of ethyl cyanoacetate (pKa 13.1). 

There are several theories of the pathway of the formation of melanoidins. Because dimedone, which is an established analytical reagent for carbonyl... 

The addition of water (see Section II,B) and other nucleophiles across the 3,4-double bond of quinazoline is well established. More recently other nucleophiles, such as dimedone, have given isolable 3,4-adducts. ° Similarly, malononitrile and other compounds with active methylene groups added across the 3,4-double bond of quinazoline, but in this case the reaction proceeded further with ring opening and formation of 2-amino-3-cyano-quinoline. The dimedone adduct was also made to react further in the presence of alkali to form the acridone 38. ... 

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