7- -triones, formation

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

An unusual reaction was been observed in the reaction of old yellow enzyme with a,(3-unsat-urated ketones. A dismutation took place under aerobic or anaerobic conditions, with the formation from cyclohex-l-keto-2-ene of the corresponding phenol and cyclohexanone, and an analogous reaction from representative cyclodec-3-keto-4-enes—putatively by hydride-ion transfer (Vaz et al. 1995). Reduction of the double bond in a,p-unsaturated ketones has been observed, and the enone reductases from Saccharomyces cerevisiae have been purified and characterized. They are able to carry out reduction of the C=C bonds in aliphatic aldehydes and ketones, and ring double bonds in cyclohexenones (Wanner and Tressel 1998). Reductions of steroid l,4-diene-3-ones can be mediated by the related old yellow enzyme and pentaerythritol tetranitrate reductase, for example, androsta-A -3,17-dione to androsta-A -3,17-dione (Vaz etal. 1995) and prednisone to pregna-A -17a, 20-diol-3,ll,20-trione (Barna et al. 2001) respectively. 

Pyridine-4-carboxylate is hydroxylated by Mycobacterium sp. strain INAl to 2,6-dihydroxypyridine-4-carboxylate. Two different hydroxylation enzymes were involved and were apparently Mo-dependent (Kretzer and Andreesen 1991). The formation of 2-ketoglutarate can, however, be rationalized equally as (3-oxidation to hexahydropyridine-2,3,6-trione-4-carboxy-CoA ester followed by hydrolysis. 

Formation of sodium l,3-dichloro-l,3,5-triazine-2,4-dione-6-oxide ( sodium di-chloroisocyanurate ) from an aqueous slurry of the trione, cyanuric acid and sodium hydroxide is accompanied by evolution of nitrogen trichloride. Nitrogen purging will keep the concentration of the explosive gas below the lower explosive limit of 5-6%. 

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>. 

A biogenetic flavour is apparent in the formation of dihydroxybenzophenones by the reaction of o-hydroxybenzoate esters with the trianion of heptane-2,4,6-trione (81JOC2260). Acylation of the trianion occurs at 0-25 °C to give a tetraketone which cyclizes under mildly basic conditions to the corresponding benzophenone. A second ring closure takes place when the benzophenone is treated with sodium methoxide, leading to the xanthone (Scheme 188). 

The photolysis of cyclic ketones, in general, results in homolytic cleavage of the carbon-carbon bond adjacent to the carbonyl function. For details of this process, the reader is referred to the review by Srinivasan.425 Included among the photoproducts are certain oxygen heterocycles, formed by cyclization of reactive intermediates. Thus, cleavage in a cyclic /3-diketone, such as tetramethylcyclobutane-1,3-dione or hexamethylcyclohexane-l,3,5-trione [Eq. (110)] is followed by cyclization on the oxygen atom to form a lactone.426,427 y-Lactone formation is also observed on photolysis of 2,2-diphenylindane-l,3-dione [Eq. (Ill)] in ether, and the process is reversible.428... 

Reaction of 4-benzylidene-l-phenylpyrrolidine-2,3,5-trione 56 with thiourea 28 occurs with hydrolysis of the amino group and leads to the formation ofl,4-dihydropyrrolo[3,4-J][l,3]thiazine-2,5,7-trione 57 [65] (Scheme 3.17). There is just one reference to the formation of a dihydrothiazine derivative 59 in the reaction of thiourea with an unsaturated ketone 58 under basic catalysis [68] (Scheme 3.18). 

Reaction of arylidenecycloalkanones with derivatives of 6-aminouracil is not described in the literature but there are examples of multicomponent synthesis involving cyclic 1,3-diketones and aldehydes. In [244] the treatment of 1,3-dimethyl-6-aminouracil 305 with 1,3-diketones 320 and aliphatic aldehydes in boiling acetic acid was reported to lead to the formation of 1,3-dimethyl-8,9-dihydropyrimido[4,5-/ ]quinoline-2,4,6-trione 321 in yields of 50 60% (Scheme 3.87). 

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