5.5- Dibromo-l ,3-dioxanes

Zinc-silver acetic acid Protection of carbonyl groups as 5,5-dibromo-l,3-dioxanes Reductive removal of tbe protective group... 

Aldehydes and ketones can be protected by conversion into the 5,5-dibromo-l,3-dioxan derivative. The diol required for the protection step, 2,2-dibromopropane-l, 3-diol, is readily prepared. The protecting group is stable to peracids and to hydride reducing agents, and the carbonyl group is readily regenerated by the action of a zinc-silver couple (Scheme 54). ... 

The enone 445 was then converted to ( )-oxocrinine (415) by a sequence that commenced with the bromination of 445 using excess 5,5-dibromo-2,2-di-methyl-4,6-dioxo-l,3-dioxane to provide a mixture of bromo ketones 446. Removal of the jV-carbobenzyloxy protecting group according to the protocol previously detailed gave 448 as a mixture (a-Br (3-Br = 3 1) of diastereomers, but only the a-bromo isomer underwent dehydrobromination on heating with lithium bromide and lithium carbonate in dry DMF to furnish 415. Interestingly, treatment of the (3-bromo derivative of 448 under similar conditions afforded the debrominated product 447 (200). 

Photo-catalytic bromination of epoxides derived from terminal alkenes results in the formation of ketones brominated exclusively on the less substituted a-carbon atom [equation (47)]. 5,5-Dibromo-2,2-dimethyl-4,6-dioxo-l,3-dioxan (32) has been shown to monobrominate saturated aldehydes and ketones... 

Aliphatic, aromatic, and alicyclic alkenes have been converted into the corresponding bromo fluorides 2 by reaction with l,3-dibromo-5,5-dimethylhydantoin (DBH) and silicon tetrafluoride in 1,4-dioxane at room temperature in a highly regio-, stereo- and chemoselective manner.9... 

---