Kinetically Controlled, Stereoselective Formation of Vinylic Sulfones by Conjugate Addition of Lithiated 3-Alkylallylic Sulfones to Cyclic Enones

Article abstract of DOI:10.1021/jo00269a040

Like the corresponding lithiated sulfoxides, lithiated but-2-enyl and oct-2-enyl sulfones undergo kinetically controlled, highly diastereoselective aprotic conjugate addition to five-membered cyclic enones in tetrahydrofuran to deliver vinylic sulfones whose formation and stereochemistry are rationalized in terms of planar or near-planar lithiated reagents reacting through an extended trans-decalyl or trans-fused chair-chair transition state.In contrast to cyclopentenone, 4-tert-butoxycyclopent-2-enone gives mixtures of conjugate and carbonyl adducts with the lithiatedsulfones at -70 deg C.This is ascribed to a steric effect involving the tert-butoxy group at C4 of the enone destabilizing the extended transition state.Reactions with cyclohexenone are less stereoselective and are temperature dependent, with lower temperatures (-85 deg C) favoring carbonyl addition to generate allylic sulfones as mixtures of diastereomers.At 0 deg C rapid conjugate addition takes place to give the vinylic sulfone.The lithiated alkoxides of the carbonyl adducts rearrange to the conjugate vinylic sulfones at 0 deg C at a considerably slower rate than that of direct conjugate addition of the lithiated sulfone to the cyclohexenone at 0 deg C.The stereoconvergence in the rearrangement excludes an intramolecular Cope rearrangement.Overall the conjugate addition reactions are more sensitive to temperature and steric effects than are the reactions involving the lithiated allylic sulfoxides and, unlike those reactions, are sensitive to the presence of hexamethylphosphoric triamide, which includes formation of allylic sulfones.