Cholestanone bromination

Since the rate of enolisation generally determines the rate of bromination of a ketone [i2g], it should be possible to correlate bromination rates for steroid ketones with their rates of enolisation. These in turn should be related to the thermodynamic stabilities of the respective olefinic bonds, insofar as transition states for enolisation under acidic conditions resemble the structures of the enols (p. 154). The very limited kinetic data available [133,134] confirm this relationship for three 3a-cholestanones whose reactivity falls in the order C(3) > C(e> > C(7). This would reasonably be predicted from consideration of strain associated with the A -, A -, and A -double bonds, coupled with steric hindrance to the transition... 

Ruzicka, Plattner, and Furrer used the reaction for the conversion of cholestanone into the two enol forms of cholestane-2,3-dione bromination to the 2-bromo derivative, conversion into the 2-pyridinium bromide, and condensation of this with... 

Bromination of ketones. Djerassi and Scholz demonstrated use of the reagent for efficient a-bromination of steroid ketones. For example, a solution of 38 mg. of cholestanone (1) in 1 ml. of acetic acid was treated at 40-60° with 31 mg. (I equiv.)... 

A soln. of 3 -cholestanoI and bis(tributyltin) oxide in tetrahydrofuran warmed 1 hr. at 50-60° in the presence of molecular sieve, then a soln. of bromine in tetrahydrofuran added dropwise at room temp, during 0.5 hr. 3-cholestanone. Y 89%. F. e. s. K. Saigo, A. Morikawa, and T. Mukaiyama, Bull. Chem. Soc. Japan 49, 1656 (1976) also preferential oxidation of sec. with retention of prim, alcohol groups s. Y. Ueno and M. Okawara, Tetrah. Let. 1976, 4597. 

---