On reactions of steroidal 23-oxo and 23,24-epoxysapogenins with Lewis acids

A Temperature-Controlled Switch between Fürst–Plattner Rule and Anti-Fürst–Plattner Rule Ring Opening of 2,3-Epoxy-steroids with Various Halide Sources in the Presence of Imidazolium Ionic Liquids

“…40 As another example, the formation of a diequatorial chlorohydrin from the 23α,24epoxy derivative of tigogenin acetate was supposed to be the result of the initial coordination of the reagent TiCl 4 to the pyranose oxygen in the epoxy-pyranose moiety followed by an intramolecular transfer of chloride from the titanium atom to C-24. 41 In the present case, there are no functional groups in the vicinity of the epoxide moiety that could alter the selectivity. The favored formation of diequatorial products at higher temperatures (Table 1, entries 13 and 14 and Table 2, entries 10 and 11) and longer reaction times (Table 1, entries 10 and 11 and Table 2, entries 7 and 8) might be explained by the operation of thermodynamic control in a reversible reaction.…”

“…For example, neighboring hydroxyl groups may modify the stereochemical outcome of the reaction due to competition between polar and steric effects that leads to the destabilization of the diaxial transition state . As another example, the formation of a diequatorial chlorohydrin from the 23α,24-epoxy derivative of tigogenin acetate was supposed to be the result of the initial coordination of the reagent TiCl 4 to the pyranose oxygen in the epoxy-pyranose moiety followed by an intramolecular transfer of chloride from the titanium atom to C-24 …”

A Convenient Synthesis of (16S,20S)-3β-Hydroxy-5α-pregnane-20,16-carbolactam and Its N-alkyl Derivatives

“…The latter are readily available from sapogenins using the Barton's procedure (NaNO 2 in the presence of BF 3 •Et 2 O in glacial acetic acid) [8,9]. Further oxidation of 23-oxo-sapogenins to bisnorcholanic lactones can be easily carried out using different oxidizing agents, e.g., MCPBA/BF 3 •Et 2 O [10][11][12], H 2 O 2 /H + [6], (PhSe) 2 /PhIO 2 [13], or even may proceed with TMSOTf in the absence of an oxidant [14]. However, direct transformation of sapogenins or spirosolanes to bisnorcholanic lactones is advantageous over two-step procedures.…”