Steroids rearranged

CI(L)109) Similar to reviews (2) and (3) above, but more concerned with steroidal rearrangements. 

Several instructive illustrations taken from the field of natural products will serve to conclude the present discussion of acid-catalyzed epoxide isomerization. Much has been said about the occurrence of simple i,2-hydride transfers in boron trifhioiide-catajyzed epoxy steroid rearrangements. Although initiated by a mineral acid rather than a Lewis acid, an instance of traneatmular hydride transfer has boon reported far a 9/3,110-epoxy steroid,1824 as shown in Eq. (484). Such migrations can in principle occur elsewhere. 

Acetoxy-17a-ethynyl steroids rearrange to 21-acetoxy-17,20,21-allenes under the influence of Ag+ catalysis, with equal amounts of both possible isomers being formed.19 Acid or base hydrolysis then gives the trans-21-aldehyde in high overall yield. 

With dihalogenated analogs of these steroids, rearrangement yields the anticipated products in high yield (Scheme 22), and these unsaturated acids are appropriately substituted for elaboration of the steroid side chain. 

The products of addition of bromine to unsaturated steroids play a part, as intermediates, in steroid rearrangements. The reaction is carried out in glacial acetic acid, ether, or chloroform at temperatures below 20° and leads... 

Dibromo-3-keto steroids rearrange to 2,4-dibromo-3-keto steroids.670... 

---