The cleavage of the lactone group of epothilone B (I) by means of Na-N3 and tetrakis triphenylphosphine palladium (0) in THF gives the azido carboxylic acid (II), which is reduced with polymer-supported PPH3 (or in solution) or with H2 over PtO2 in ethanol to yield the amino acid (III). Finally, this compound is cyclized by means of diphenylphosphoryl azide (DPPA) in DMF.