Trimethylphenylammonium tribromide

Direct halogenation of a 20-ketopregnane (lacking an hydroxyl at C-17) generally does not give satisfactory yields of the 21-bromo derivative, unless the presence of a C-16 substituent e.g., methoxyl or methyl) inhibits formation of the usual 17-bromo primary product. However, an indirect method has been devised. If the 20-ketaI is first prepared, it can be brominated satisfactorily at C-21 using trimethylphenylammonium tribromide as halo-... 

Scheme 18 Preparation of keto sugars by regioselective, organotin-catalyzed oxidation. [TMPhA] Br3 denotes trimethylphenylammonium tribromide...

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