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.