ChemInform Abstract: Formal Asymmetric Synthesis of Pentalenolactone E and Pentalenolactone F. Part 1. Retrosynthesis and π‐Facial Differentiation in Palladium‐Catalyzed and Dipolar [3 + 2]‐Cycloaddition Reactions of Bicyclic Alkenes: Evidence for Electrostatic Control of Stereoselectivity.

Abstract

Formal Asymmetric Synthesis of Pentalenolactone E and Pentalenolactone F. Part 1. Retrosynthesis and π-Facial Differentiation in Palladium-Catalyzed and Dipolar [3 + 2]-Cycloaddition Reactions of Bicyclic Alkenes: Evidence for Electrostatic Control of Stereoselectivity.-A new strategy for the formal asymmetric synthesis of the title sesquiterpenoids (I) and (II) with a Pd-catalyzed [3 + 2]-cycloaddition for the construction of the diquinane skeleton as the key step is developed. Several [3 + 2]-cycloadditions of the diquinene (V) with e.g. (VI), (IX), or (XI) lead to the formation of the adducts with unexpected facial selectivities. Michael reactions of (V) take an expected steric course, too. The practically exclusive formation of the cisoid triquinane (XV) in the 1,3-dipolar cycloaddition of (V) with CH 2 N 2 represents an exceptional example of the electrostatic control of the stereoselectivity by a functional group. The linear triquinane (X) would be a suitable precursor for the key intermediate (III). -(ROSENSTOCK, B.; GAIS, H.-J.; HERRMANN, E.; RAABE, G.; BINGER, P.; FREUND, A.; WEDEMANN, P.; KRUEGER, C.; LINDNER, H. J.; Eur.