Fig. 2.

Regioselective ring expansions of 3-oxosteroids. a Steroids and hydroxyalkyl azides used in this study. The blue ketones indicate modification sites. b Ring expansions obtained by treating 1 with 3-azidopropanol 6, 3-azido-1-phenylpropanol (R)-7 and (S)-7. c Mechanistic rationale for ring expansion chemistry differentiating between C-2 (blue bond) and C-4 (red bond) migration; key antiperiplanar relationship between the migrating group and the N₂⁺ leaving group is indicated by bold bonds. d Dependence of migration outcome on C-5 stereochemistry. ^aInseparable mixture by normal phase chromatography. ^bSeparable mixture by normal phase chromatography, but for this control experiment the isomers were not separated. ^cRatio determined by ¹H nuclear magnetic resonance (NMR). ^dIsolated yield after chromatography on silica. See Supplementary Methods for full synthetic details. A and B series are 5α-cholestan-3-one derivatives; D2 is a 5α-dihydrotestosterone derivative; E1 is a 5β-dihydrotestosterone derivative; X and Y are proposed spirocyclic intermediates (including both chair conformers as designated); 15% KOH, 15% aqueous solution of potassium hydroxide; *, signifies site of stereochemistry