TAXANE DITERPENES .3. FORMATION OF THE 8-MEMBERED B-RING BY SEMI-PINACOL REARRANGEMENT

The substituted furan carboxaldehyde 15 was converted into enone 20 vi a a stereoselective intramolecular pyrylium ylide-alkene cyclization. Subsequent elaboration of 20 into the triflate 23, followed by solvoly sis in acidic trifluoroethanol resulted in quantitative rearrangement to the taxane B/C core 24. At a higher oxidation level 27 was ring exp anded to give 28. The ring expansion strategy could also be initiated by beta-elimination of the 3 alpha,10 alpha-oxido bridge. Treatment of 50 with MeOH at reflux gave 52. Similar transformations in the 7-oxy series resulted in ring B expansion of 67 to give 68, and 75 to give 7 6. Copyright (C) 1996 Elsevier Science Ltd