A NOVEL REARRANGEMENT OF THE STEROID NUCLEUS. SYNTHESIS OF 18-NOR-D-HOMOSTEROIDS¹

Chemistry of Acyl Radicals

“…The precise mechanism of thermodynamic equilibration of β-acylalkyl radicals has been the subject of considerable discussion in the literature. As early as 1961, a group of workers at Schering noted the rearrangement of a 17-ketosteroid, following hydrogen-atom abstraction from the C13 methyl group, to give a d -homo-18-norsteroid (Scheme ) 82 …”

“…As early as 1961, a group of workers at Schering noted the rearrangement of a 17-ketosteroid, following hydrogen-atom abstraction from the C13 methyl group, to give a D-homo-18-norsteroid (Scheme 82). 221 In collaboration with Barton, they suggested that the expansion might occur via β-scission to give an acyl radical followed by recombination to give the larger ring. Following the exploitation of this type of rearrangement some 25 years later by the Beckwith and Dowd groups, [222][223][224] the discussion has expanded to encompass the question of rearrangement via a cyclopropyloxyl radical either as a true intermediate or a transition state (Scheme 83).…”

Free radical-mediated ring expansion and related annulations

“…An early one-carbon ring expansion was reported by Barton and his co-workers. 15 The Barton nitrite ester photolysis was employed to abstract a hydrogen atom from the steroid 18-methyl group, generating the 18methylene radical, which then inserted into the adjacent 17-ketone (eq 6). Barton also synthesized a tropone from a cyclohexadienone by tri-n-butyltin hydride initiated free radical reaction (eq 7).16 Examples of ring expansion have also been reported by the Tada group17 during studies of 1,2-acyl migration of alicyclic 2-alkyl-2-(bromomethyl)alkanones using organocobaloximes (eqs 8 and 9).…”

Chemistry of β-(Acyloxy)alkyl and β-(Phosphatoxy)alkyl Radicals and Related Species: Radical and Radical Ionic Migrations and Fragmentations of Carbon−Oxygen Bonds

“…9 Despite this, the process attracted little attention until 1984, when it was employed in the expansion of a cyclohexadienone ring to a tropone. 10 This intramolecular radical chemistry of ketones has subsequently been much studied and employed in synthesis.…”