Oximes carbonyl group derivatization

Derivatization of carbonyl groups (reduction or formation of N,N-dimethyl-hydrazones or oximes)... 

Two features about steroid oximes should be mentioned since they impact quantification firstly, an 11-carbonyl group is sterically hindered and not derivatized, so steroids containing this moiety have a mass 29 units less than expected. Secondly, each carbonyl can be derivatized two stereospecific ways, termed syn- and anti- . These may or may not be resolved by GC. If they are resolved it is preferable to determine the area of both peaks since the relative amount of syn- and anti- forms may vary between different derivatizations. 

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... 

Brp3 is also useful for transforming carbonyl groups to CF2 units. First, however, the carbonyl groups need to be derivatized to hydrazones or oxime ethers ... 

More importantly, some silylating reagents used for protection of functional groups with active hydrogen atoms can react with carbonyl fragments with the formation of trimethylsilyl (TMS) derivatives of enols. If such unpredictable mode of derivatization is undesirable, all carbonyl fragments should be protected before silylation, for example, by conversion into alkoxyimino groups (alkyl ethers of oximes, see below). 

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