INTRAMOLECULAR OXYMERCURATION OF STEREOISOMERIC CYCLOHEXYL-BELTED POLY(SPIROTETRAHYDROFURANYL) PLATFORMS

The four tetraspiro carbinols 28, 30, 41, and 43 and the three trispir o cyclohexenones 25, 38, and 53 have been synthesized and individually subjected to intramolecular oxymercuration. The three-dimensional str uctures of all 10 products have been unequivocally established by X-ra y crystallographic analysis. In seven of these structures, the preferr ed solid-state conformation features an axially disposed C-Hg bond whe re the mercury atom is internally chelated ina 1,3-diaxial relationshi p to at least one or, more often, two oxygen centers. An unusually str ong preference has been observed for equatorial occupancy on the part of the C-O bonds. This bias can be attributed to the relief of torsion al strain effects that arise when gauche CH2- - -CH2 interactions are skirted. An important mechanistic distinction separates the kineticall y preferred trans mercuricyclization of the tetraspiro alcohols from t he contrasting cia stereochemical ring closure exhibited predominantly by the unsaturated ketones. Ln the first instance, the approach of Hg 2+ to the double bond is governed to a large extent by coordination to a proximal axially oriented ether oxygen. Where the ketones are conce rned, precoordination to the Lewis basic carbonyl oxygen presumably in itiates spirocyclization in a fashion controlled by the conformational preference of the resident tetrahydrofuranyl subunits.