Of symmetrical diketones

The diastereoselectivity of reduction of a series of symmetrical diketones to the corresponding diols revealed an intriguing dependance on the separation of the carbonyl groups, but the selectivity was not generally useful. However, in the case of 1,3-diketone (71) lithium borohydride alone produced the anti isomer (70) with 91% diastereoselectivity, but prior addition of titanium tetrachloride gave the syn-diol (72) with 96% diastereoselectivity via a chelated intermediate analogous to the crystalline complex between the diketone and TiCU (Scheme 11). ... 

The two carbonyl groups of symmetrical diketones are distinguishable, with the carbonyl group undergoing reduction doing so with enantiotopic specificity. Some acyclic and monocyclic examples are shown in Scheme 9.- 2,44.49.50 Once more, enantiomeric products can be selected by the use of organisms with opposite enantiotopic face specificities, as shown for the reduction of (19) to (/ )- or (S)-(20) (Scheme 10). ... 

Cyclisation of symmetrical diketone (13) could occur in two ways, depending on which side of the ketone enolises. In practice, in acid or base, only route (a) is followed to give stable six-membered (14) rather than less stable eight-membered (15). In acid solution, 85% of (14) is formed. ... 

Scheme 7.4 Asymmetric intramolecular cross-benzoin reaction of symmetrical diketones reported by Ema.

Xul983 Xu, Z.H., McArthur, C.R. and Leznoff, C.C., The Use of Polymer Supports in Organic Synthesis. XXVIII. The Monoblocking of Symmetrical Diketones on Insoluble Polymer Supports, Can. J. Chem., 61 (1983) 1405-1409. 

This strategy has been effectively used to prepare a number of 1,4-symmetrical diketones which have served as precursors to thiophenes such as 30, 31, and 32 via the Paal synthesis. 

Compound A has the formula C K)Hi6. On catalytic hydrogenation over palladium, it reacts with only 1 molar equivalent of H2. Compound A also undergoes reachun with ozone, followed by zinc treatment, to yield a symmetrical diketone, B (CjoH C )-... 

Following a similar protocol, novel hexacoordinated phosphate anions 19 to 22 bearing two different dioxo ligands could be simply prepared as their di-methylammonium salts from the reaction of tetrachlorocatechol derived phos-phoramidite 23 with a variety of symmetrical diones other than o-chloranil (a-diketones or orf/zo-quinones) and subsequent addition of tetrachlorocatechol to the corresponding phosphorane 24 (Scheme 3) [42]. 

Although electroreduction of a mixture of esters with an Mg electrode gives a mixture of symmetrical and unsymmet-rical 1,2-diketones (Scheme 19), the reduction of diesters affords unsymmetrical 1,2-diketones as the single product (Scheme 20) [38, 39]. The selectivity of the reaction shown in Scheme 20 is highly dependent on the value of n and the best result is obtained when n = 2. 

A range of symmetrical bicyclic /3-diketones can be converted to 2,3-disubstituted cycloalkanones in high yield with high diastereomeric and enantiomeric excess using a cell-free preparation of a retro-Claisenase enzyme, or /3-diketone hydrolase, the gene for which has been heterologously expressed in Escherichia coli. ... 

In order to prepare carbohydrate-based amphiphiles by this efficient methodol-ogy, symmetrical p-diketones 63a, b were used [111]. The reactions of these diketones with o-glucose (10) in EtOH/H20 as the solvent and NaHC03 as the base were complete and the C-glycolipids were isolated in 75% (64a) and 52% yield (64b) using the less soluble diketone 63b. The condensation was also applied to D-maltose (49) to obtain 65a or 65b (Fig. 11) [111, 121]. 

The synthetic methods which are illustrated in this section are (a) the formation of symmetrical 1,4-diketones from 1,3- (or / -)keto esters (Expt 5.104), and (b) a Michael addition reaction involving nitroalkanes and a, /f-unsaturated ketones (Expt 5.105). The synthesis of symmetrical 1,4-diketones from the sodio derivatives of /f-keto esters, or their mono-alkyl derivatives, by treatment with iodine [Method (a)], may be formulated in the following general manner. 

The simplicity of the spectrum can be understood if we are dealing with a symmetric diketone. 

Step 2b Utilization of 0.5 equivalent of ethyl formate HC(0)OEt ensures that the SE A and B are formed in a 2 1 ratio, a prerequisite for the formation of the symmetrical diketone. 

The spectra fit the ester structure well, but not the more symmetrical diketone structure at all. There are three types of proton (cyclobuta-l,3-dione would have just one) with allylic coupling between one of the protons on the double bond and the CH2 group in the ring. The carbonyl group has the shift (185 p.p.m.) of an acid derivative (not that of a ketone which would be about 200 p.p.m.) and all four carbons are different. 

Acid-catalysed cyclization of the symmetrical diketone nona-2,8-dione could give two enols. 

Simple symmetrical isoxazoles are easily made by the hydroxylamine route. If R1 = R3, we have a symmetrical and easily prepared 1,3-diketone as starting material. The central R2 group can be inserted by alkylation of the stable enolate of the diketone (Chapter 26). 

The term keto enol tautomerism implies two isomers and for most purposes this simple view is adequate. However for a symmetric /3-diketone, RCOCH2COR, or for MDA, HCOCH2COH, there are three diketo conformers and six enol ones, assuming that is, that the skeletal framework of oxygens and carbon atoms is planar Fig. 2. For an asymmetric /3-diketone or /3-ketoaldehyde this increases to four and eleven respectively since it is possible to distinguish the various conformers of RC(OH)=CHCOR from those of RCOCH=C(OH)R. The same is true of symmetric /J-thioketones, RCSCH2COR. 

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