Ketones saturated, formation

Unsubstituted 20-ketones undergo exchange dioxolanation nearly with the same ease as saturated 3-ketones although preferential ketalization at C-3 can be achieved under these conditions. " 20,20-Cycloethylenedioxy derivatives are readily prepared by acid-catalyzed reaction with ethylene glycol. The presence of a 12-ketone inhibits formation of 20-ketals. Selective removal of 20-ketals in the presence of a 3-ketal is effected with boron trifluoride at room temperature. Hemithioketals and thioketals " are obtained by conventional procedures. However, the 20-thioketal does not form under mild conditions (dilution technique). ... 

The saturated carbon atom can be functionalized by xenon difluoride in various ketones and diketones where, beside a-mono and a-difluoro ketones, the formation of rearranged products can also be observed however, the course of reaction depends strongly on the catalyst used22-24 (Scheme 5). 

Fig. 6.19. General scheme for the hydrogenation of a, P-nnsatnrated ketones. The formation of the satnrated ketone and of the nnsaturated alcohol occnrs through parallel reactions, whereas the saturated alcohol is mainly obtained by the further hydrogenation of the C = O bond of the saturated ketone (adapted from [326])...

In the reaction of Q,/3-unsaturated ketones and esters, sometimes simple Michael-type addition (insertion and hydrogenolysis, or hydroarylation, and hydroalkenylation) of alkenes is observed[53,54]. For example, a simple addition product 56 to methyl vinyl ketone was obtained by the reaction of the heteroaromatic iodide 55[S5]. The corresponding bromide affords the usual insertion-elimination product. Saturated ketones are obtained cleanly by hydroarylation of o,/3l-unsaturated ketones with aryl halides in the presence of sodium formate, which hydrogenolyses the R—Pd—I intermediate to R— Pd—H[56]. Intramolecular hydroarylation is a useful reaction. The diiodide 57 reacts smoothly with sodium formate to give a model compound for the afla-toxin 58. (see Section 1.1.6)[57]. Use of triethylammonium formate and BU4NCI gives better results. 

Ketones may be prepared from 5a- and 5/5-saturated-3-ketones, A -3-ketones, A -3-ketones and 5a-A -3-ketones. At one time it was the only direct chemical alternative to microbiological methods for the preparation of the prednisone type of corticoids. A -Double bond formation is not observed, although A -trienones can be prepared from A -3-ketones. In common with DDQ, selenium dioxide is not normally useful for the preparation of A -3-ketones from 5a-3-ketones or A -3-ketones from 5j5-3-ketones. 

The direct formation of a dimethyl ketal by reaction of the ketone with methanol is particularly sensitive to steric effects. Only cyclohexanones react under these conditions.In the steroid series only saturated 3-ketones form dimethyl ketals with methanol and acid although partial reaction of a 2-ketone has been observed in the presence of homogenous rhodium catalyst. ... 

The saturated 3-ketone can also be protected as the ethylene ketal, which is prepared directly by reaction with ethylene glycol or by exchange dioxo-lanation. Selective formation of 3-ethylenedioxy compounds is also possible, but the former method is not particularly effective in the presence of 6-, 17- or 20-ketones. However, the exchange dioxolanation technique is more sensitive to steric effects and good selectivity at C-3 can be achieved in the presence of a 17-ketone, provided the reagent does not contain glycol. ... 

The selective formation of the 3-monosemicarbazones of polyketonic steroids can be achieved only in the presence of 21-acetoxy-20-ketones by the use of nonbuffered conditions. Partial hydrolysis of 3,20-bissemicar-bazones can be achieved with acetic anhydride in pyridine "" to yield the 3-semicarbazone. Kinetic studies on the rates of formation of Girard hydra-zones showed that the A" -3-ketone is less reactive than the saturated 3- and 6-ketones, as reactive as the 7- and 17-ketones and more reactive than the 20- and 12-ketones. ... 

Olefin formation by reduction of a,/3-un-saturated ketones with diborane, 347 Olefin formation by reductive eliminations, 343... 

Ketones 2,4-Dinitrophenyl- hydrazine Formation of colored hydrazones or osazones. It is possible to distinguish between saturated and unsaturated hydrazones using potassium hexa-cyanoferrate(UI) [5]. [3,4]... 

Reduction of aryl diazonium ions by Ti(III) in the presence of a,(3-unsaturated ketones and aldehydes leads to (3-arylation and formation of the saturated ketone or aldehyde. The early steps in this reaction parallel the copper-catalyzed reaction. However, rather than being oxidized, the radical formed by the addition step is reduced by Ti(III).116... 

Mechanisms, exemplified by alcohol as donor (493, 496), usually invoke coordination of the substrate (olefins, saturated and unsaturated ketones, and aldehydes), then coordination of the alcohol and formation of a metal alkoxide, followed by /8-hydrogen transfer from the alkoxide and release of product via protonolysis ... 

---