Ketals ketones, preferential

Cation exchanger Cyclic ketals from ketones Preferential 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). ... 

When unsymmetrical ketones were used in this reaction (with BF3 as catalyst), the less highly substituted carbon preferentially migrated. The reaction can be made regioselective by applying this method to the a-halo ketone, in which case only the other carbon migrates. The ethyl diazoacetate procedure has also been applied to the acetals or ketals of a, P-unsaturated aldehydes and ketones. ... 

Carbonyls and acetals are converted to 1,3-dithianes and -dithiolanes upon treatment with 2-stanna-l,3-dithianes and -dithiolanes under catalysis by organotin tri-flates [147]. In these competition reactions, various types of carbonyl and acetal are differentiated. Aldehydes react preferentially over ketones (Eq. 104), but the preference is completely reversed in the competition reactions between the corresponding acetals and ketals (Eq. 105). The reactivity of aliphatic aldehydes is greater than that of the acetals of aliphatic aldehydes and ketones. Conversely, an aromatic acetal is... 

Of the many examples that illustrate the ability to easily select between the two forms, we highlight that of methyl ketone 47a and the corresponding dimethyl ketal 47b (Scheme 6). While the ketone 47a preferentially affords linear cycloadduct 48, the dimethyl ketal 47b leads to the bridged adduct 49 with high selectivity. A useful... 

The corresponding dimethyl ketal is too labile to be useful, particularly in large scale operations. Its relatively facile hydrolysis complicates the diyl cycloaddition since the diazene-ketone leads preferentially to the linearly fused rather than the desired bridged adduct. The ethylene glycol ketal provided an ideal solution to the problem. 

Extending this technique to the reduction of the dione, L, demonstrated an intramolecular selectivity attributable to conversion of the more reactive C-3 carbonyl to a monoketal intermediate (Gemal and Luche, 1979). This allows the preferential reduction at C-17, and also demonstrates the sharp contrast between aldehyde and ketone ketalization using lanthanide chlorides as catalysts. 

A convenient new one-pot procedure for the selective reduction of ketones in the presence of aldehydes (the less usual chemoselectivity) is outlined in Scheme 9. ° The aldehyde is protected as an imine and the ketone is then reduced in situ with a hindred hydride reagent the aldehyde is regenerated on hydrolytic work-up. Conjugated and aromatic aldehydes are protected satisfactorily by this sequence, and moderate discrimination between aliphatic and aromatic aldehydes, with preferential reduction of the aromatic aldehyde, can also be achieved. These authors claim better selectivity than previous methods based on ketalization or hydration of aldehydes with lanthanoid cation catalysts (4,141). 

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