Nazarov Cyclization Entry to Chiral Bicyclo­[5.3.0]­decanoid Building Blocks and Its Application to Formal Synthesis of (−)-Englerin A

A divergent entry to the chiral bicyclo[5.3.0]­decane skeletons relevant to sesqui- and higher terpenoids has been achieved. Its usefulness was demonstrated by formal synthesis of a guaiane sesquiterpenoid (−)-englerin A. The key reactions are (i) diastereoselective Nazarov cyclization for stereoselective construction of the bicyclo[5.3.0]­decane skeleton, (ii) intramolecular C–H amination for tuning an oxidation state, and (iii) introduction of an alkyl group to a β-alkoxy ketone with a zinc­(II) ate complex