A novel ring enlargement methodology and its application to taxane A,B-ring synthesis is introduced. In a one pot-procedure an elimination-epoxidation sequence starting from 5 leads to molecules of type 6 which can be transformed into taxane A,B-ring systems via a tandemreaction. A retroaldol-epoxidation sequence starting from 15 yields taxane A,B-ring systems suitably functionalized for Diels-Alder