Oximes to protect carbonyl groups

For decades oximes and hydrazones have been used for the derivatization of carbonyl compounds and are mainly applied for carbonyl group characterization in analytical procedures. However, they can also be applied as viable protecting groups in complex reaction schemes. For instance in the synthesis of ery-thronolide A a carbonyl group had to be converted to an oxime to allow the selective removal of the remaining isopropylidene protecting group without side reactions. The reconversion of the oxime to the carbonyl compound proceeds by A -nitrosation (Scheme 92). The previously attempted removal of the... 

The carbonyl group forms a number of other very stable derivatives. They are less used as protective groups because of the greater difficulty involved in their removal. Such derivatives include cyanohydrins, hydrazones, imines, oximes, and semicarbazones. Enol ethers are used to protect one carbonyl group in a 1,2- or 1,3-dicarbonyl compound. 

In a prostaglandin synthesis a carbonyl group was protected as an oxime in which the hydroxyl group was protected against Collins oxidation by the phenylthiome-thyl-group. The phenylthiomethyl group is readily removed to give an oxime that is then cleaved to the carbonyl compound. ... 

NaN02, 1 N HCl, CH3OH, H2O, 0°, 3 h, 76% yield. In the last step of a synthesis of erythronolide A, acid-catalyzed hydrolysis of an acetonide failed because the carbonyl-containing precursor was unstable to acidic hydrolysis (3% MeOH, HCl, 0°, 30 min, conditions developed for the synthesis of erythronolide B). Consequently, the carbonyl group was protected as an oxime, the acetonide was cleaved, and the carbonyl group was regenerated. 

Dave and co-workers have reported a successful synthesis of 2,2,4,4-tetranitroadamantane (117) which uses the mono-protected diketone (113) as a key intermediate. In this synthesis (113) is converted to the oxime (114) and then treated with ammonium nitrate and nitric acid in methylene chloride to yield the em-dinitro derivative (115). This nitration-oxidation step also removes the acetal-protecting group to leave the second ketone group free. Formation of the oxime (116) from ketone (115), followed by a similar nitration-oxidation with nitric acid and ammonium nitrate, yields 2,2,4,4-tetranitroadamantane (117). In this synthesis the protection strategy enables each carbonyl group to be treated separately and thus prevents the problem of internal nitroso dimer formation. 

In the case of 4-oestrene-3,17-dione, protection of the 3-carbonyl group as its enol ether and subsequent oximation, rearrangement and deprotection gives the lactam with a functionality at C-3 which allows further elaboration to a neuromuscular blocker (R. J. Marshall et al.y Eur. J. med. Chem.-Chim. Ther., 1984, 19, 43). 

Oximes have been used as protecting groups for carbonyl compounds owing to their hydrolytic stability. Consequently, the development of newer deoximation reagents... 

---