Of a dicarbonyls

Analysis Our methods for making 1,2-dioxygenated compounds (frames 154-157) involve reductive linking of a dicarbonyl compound ... 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

De Bruijn et al.26 30 used chromatographic and spectroscopic techniques to analyze the effect of reaction variables (such as pH and monosaccharide concentration) on the product profile and developed a reaction model (see Fig. 9) that emphasized the role of a-dicarbonyl compounds. Some of the features of the model shown in Fig. 9 are ... 

The band at 1994 cm observed at low CO coverage, was assigned to a monocarbonyl Rh(CO) species. The nature of the species observed at 2117cm has been elucidated using mixtures of different CO isotopes. As shown in Fig. 4, the band splits into three bands after adsorption of CO/ CO mixtures proving the presence of a dicarbonyl species... 

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

At least in bromoform, not the slightest trace of a dicarbonyl formed according to Eq. (2), M = Fe, could be observed. Obviously, the d -electron density at the Fe11 ion is not sufficient to feed the 7r -orbitals of two axial carbon monoxide molecules with the necessary electron density for bonding. In other words, the first jr-acceptor molecule labilizes a second one tram to itself according to Case E (Fig. 1) which is the inserve of Case C (trans-jr-donor labilization, Sect. 5.2). 

A desymmetrizing reduction of a dicarbonyl has also been achieved as a route to flMfi-aldol adducts. Yamada and coworkers have shown that a chiral cobalt complex catalyzes the desymmetrization of diaryl-1,3-diketones in excellent yield and enantioselectivity, greatly favoring the anti isomer [Eq. (10.65)]. Anti selectivity is rationalized using a Felkin-Anh model ... 

Reaction of a-Dicarbonyl or a-Hydroxycarbonyl Compounds with Ammonia in the Presence of Formaldehyde... 

The photochemical reactions of a-dicarbonyl containing compounds have recently been the subject of a number of investigations. This review will consider reactions that take place in oxygen-free solution and will contrast them with reactions that have been observed in the vapor state. 

In order to understand the photoreactions of a-dicarbonyl compounds it is necessary to have some knowledge of their excited states. We will not exhaustively review the large amount of work that has been done on these compounds, particularly biacetyl, but merely touch upon those points of interest to photochemists. More detailed discussions are available in various review articles.14-19... 

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. 

Several mechanisms have been proposed for the formation of pyrazines in food flavours [18, 23, 25], but the major route is from a-aminoketones, which are products of the condensation of a dicarbonyl with an amino compound via Strecker degradation (Scheme 12.3). Self-condensation of the aminoketones, or condensation with other aminoketones, affords a dihydropyrazine that is oxidised to the pyrazine. 

A valuable synthetic route to 1-benzothiepin (46) (74AG(E)142) and a range of substituted 1-benzothiepins (241)-(244) (75TL2697) is based upon the availability of the corresponding valence isomers containing a bicyclo[1.1.0]butane moiety. Thus, isomerization to 1-benzothiepin (46) occurs under mild conditions ( 0 °C) in the presence of a dicarbonyl-2,4-pentanedionatorhodium(I) catalyst (equation 77). 

A general way of synthesizing heterocyclic compounds is by cyclization of a dicarbonyl or diketo compound using a nucleophilic reagent that introduces the desired hetero-atom. 

Monoacelais of a-dicarbonyl compounds. Frimer s epoxidation of enol ethers (8, 7) has been extended to a synthesis of monoacetals of a-dicarbonyl compounds.1 Examples ... 

In addition to preparation of arylhydrazones from the carbonyl compounds and an arylhydrazine, the Japp-Klingemann reaction of arenediazonium ions with enolates and enamines is an important method for preparation of arylhydrazones. This method provides a route to monoarylhydrazones of a-dicarbonyl compounds from /3-keto acids and to the hydrazones of pyruvate esters from / -keto esters. Enamines also give rise to monoarylhydrazones of a-diketones. Indolization of these arylhydrazones provides the expected 2-acyI-or 2-alkoxycarbonyl-indoles (equations 95-97). 

The lowest excited triplet states of a-dicarbonyl compounds are considerably less energetic than those of simple carbonyls. For instance the energy of the vibrationally relaxed triplet of glyoxal is 55 kcal,366 as compared to 72 kcal for formaldehyde. Irradiation of glyoxal at 4358 A populates the lowest vibrational levels of the first excited singlet, 30% of which fluoresce and 70% of which cross over to the triplet manifold.388 Almost all of the triplet molecules then decompose to formaldehyde and carbon monoxide, the phosphorescence yield being only 0.1%. 

An interesting observation regarding the structure of a-dicarbonyl triplets has been made with the naphthils, each of which seems to have two separate triplets, presumably geometric isomers.397 Such behavior would parallel that displayed by 1,3-dienes. 

Interestingly, 2,3-dicarbonyl derivatives formed by the condensation of a-dicarbonyl compounds and 2-aminophenols (80JHC1625) exist in the 2Z/-form (81 Scheme 36). However, when there is an additional carbon atom in the side-chain of the 3-substituent so that hydrogen bonding with the NH group may now occur, as in the benzoxazinone (82), the equilibrium is directed strongly in favour of the 4Z/-tautomer (Scheme 37) (76JHC681). 

Hatakeyama et al. [115] have also carried out detailed product studies in the HO-initiated oxidation of the alkynes both in the presence and in the absence of NOx. The major products consisted of a-dicarbonyl compounds, i.e., HC(0)CHO from acetylene, CH3C(0)CH0 from propyne and CH3Q0)C(0)CH3 from 2-butyne, as well as HC(0)OH from acetylene and propyne and CH3C(0)OH from 2-butyne. The formation of these products was attributed to 02-reactions of the hydroxyvinyl radicals resulting from the addition reaction of HO with the alkynes, e.g.,... 

Reaction (44a) is analogous to the 02-reaction of the vinyl radical leading to HCO and HCHO as reported by Gutman and co-workers [118,119], Evidence for the occurrence of reaction (44b) was based on the observation of a-dicarbonyl products in the absence of NO. Schmidt et al. [120] have recently observed the regeneration of HO from the HO + C2H2 reaction by means of a laser fluorescence technique. The branching ratios for the two unimolecular dissociation channels typified by reactions (44a) and (44b) were estimated to be 0.4 0.1 and 0.7 + 0.3 (acetylene), 0.12 + 0.02 and 0.53 0.03 (propyne), and 0.12 + 0.87 0.07 (2-butyne), respectively. In any case, the formation of acidic products as well as the a-dicarbonyl products from these reactions is of potential importance in the atmosphere. 

Reaction of a-dicarbonyl compounds and aldehydes with hydrogen... 

Fig. 17.55. Reductive coupling of a dicarbonyl compound to afford diastereomeric glycols or diastereomeric alkenes (McMurry reaction).

In the presence of alanine, all four a-dicarbonyls are formed in each system, glyoxal being produced in largest amounts by glucose and fructose and 2-oxopropanal by xylose and 3-deoxyglucosone. Fructosylalanine gave lower amounts of a-dicarbonyls (except 2,3-pentanedione) in the presence of alanine than in its absence. 

Knowledge of the Maillard reaction is being extended very actively in many different ways. The participation of free radicals has already been dealt with in Chapter 2 and work on colour and flavour aspects is being deferred to Chapters 4 and 5, respectively. This chapter deals with a number of relatively disparate topics, namely, the effects of pH, high pressure, 7g, and the use as reactants of amines other than amino acids, of lipids, and of oligo- and polysaccharides, as well as the determination of a-dicarbonyl intermediates, control of aldol/retroaldol reactions, fluorescence, kinetic aspects, and sites of protein glycation. 

M. A. Glomb and R. Tschimich, Detection of a-dicarbonyl compounds in Maillard reaction systems and in vivo, J. Agric. Food Chem., 2001, 49, 5543-5550. 

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