Photoinduced Isomerization of 23-Oxosapogenins: Conformational Analysis and Spectroscopic Characterization of 22-Isosapogenins

The first synthesis of 22-isospirostane derivatives is described. They were obtained by photochemical isomerization of 23-oxosapogenins. The structure of 23-oxo-22-isotigogenin acetate (<b>12</b>) was proved by a single crystal X-ray diffraction, while structures of 23-oxo-22-isodiosgenin acetate (<b>13</b>) and 23-oxo-22-isosarsasapogenin acetate (<b>14</b>) were elucidated by spectroscopic methods. 22-Isodiosgenin acetate (<b>17</b>) was obtained by NaBH₄ reduction of the 23-oxo derivative <b>13</b> followed by the two-step Barton–McCombie deoxygenation procedure. Conformational analysis of 22-iso compounds was carried out with CD and NMR, as well as DFT calculations

Oxidation of Olefins with Benzeneseleninic Anhydride in the Presence of TMSOTf

A new oxidizing system for olefins, consisting of benzeneseleninic anhydride and trimethylsilyl triflate, was studied. The highly reactive benzeneseleninyl cation is presumably formed under these conditions. It has been shown that different products are formed with this species depending on the specific structure of olefin. The 1,1-disubstituted olefins afforded mostly α,β-unsaturated carbonyl compounds. The sterically encumbered tri- or tetrasubstituted olefins yielded 1,2- or 1,4-dihydroxylated products, presumably via four-membered cyclic intermediates