Rearrangement of vindoline and derivatives in superacidic media

In HF - SbF5 at O degrees C, vindoline 1a and its deacetyl derivative 1b yi eld the cathovaline like compounds 4 and 5, respectively, and the furanic d iastereoisomers 2 and 3. Fluoro derivatives (20S)-10b-d rearrange in supera cids to 4 (92%), 5 (60%) and to lactone 11 (58%), respectively, whereas the (20R) analogs are completely unreactive. This last result can be accounted for by intervening of a reactive conformation of the fluoroethyl group in the (20S) series, the corresponding one in the (20R) series being disfavore d for steric reasons. (C) 1999 Elsevier Science Ltd. All rights reserved.