The unexpected formation of bicyclo[2.2.1]heptane derivatives by Lewis acid-catalyzed transannular double cyclization

Abstract

Attempted synthesis of the cyclooctanoid skeleton of taxane molecules, by an intramolecular Lewis acid catalyzed Hosomi–Sakurai cyclization of the allylsilane with the aldehyde moiety in 17, unexpectedly led to the formation of bicyclo[2.2.1]heptane derivatives 18 and 19a. Since the formation of spiro compounds by allylsilane terminated domino reaction has no precedent in literature, we extended the reaction conditions to unactivated olefins. Spontaneous aromatization accompanied by bicyclic ring formation was observed in these cases. Bicyclo[2.2.1]heptane derivative 11 was also formed in high yield by an intramolecular SmI₂ mediated ketyl–olefin coupling of aldehyde in the hydroxysulfone 10.