Mild generation of o-quinodimethanes via fluoride induced 1,4-elimination of α-(o-trimethylsilylmethyl)benzylesters: stereoselective synthesis of 19-nor steroids and RU486 precursors

19-Nor steroids and RU486 tricyclic synthetic intermediates were stereoselectively prepared by an intramolecular Diels–Alder reaction involving an o-quinodimethane possessing a pro-17 unique chiral stereocenter, substituted by a protected hydroxyl group. The o-quinodimethane was generated in mild conditions by fluoride induced 1,4-elimination of α-(o-trimethylsilylmethyl)benzylesters and the present methodology allows a flexible access to α,α′-disubstituted o-quinodimethanes, as shown by the 11β-substituted steroid approach. The IMDA diastereoselections reported herein were highly dependent on the nature of the hydroxyl protective group and the diastereoselectivities superior to those observed with the thermolysis of the corresponding benzocyclobutenes.