Intramolecular oxymercuration of stereoisomeric cyclohexyl-belted poly(spirotetrahydrofuranyl) platforms

The four tetraspiro carbinols 28, 30, 41, and 43 and the three trispiro cyclohexenones 25, 38, and 53 have been synthesized and individually subjected to intramolecular oxymercuration. The three-dimensional structures of all 10 products have been unequivocally established by X-ray crystallographic analysis. In seven of these structures, the preferred solid- state conformation features an axially disposed C-Hg bond where the mercury atom is internally chelated in. a 1,3-diaxial relationship to at least one or, more often, two oxygen centers. An unusually strong preference has been observed for equatorial occupancy on the part of the C-O bonds. This bias can be attributed to the relief of torsional strain effects that arise when gauche CH₂- - -CH₂ interactions are skirted. An important mechanistic distinction separates the kinetically preferred trans mercuricyclization of the tetraspiro alcohols from the contrasting cis stereochemical ring closure exhibited predominantly by the unsaturated ketones. In the first instance, the approach of Hg²⁺ to the double bond is governed to a large extent by coordination to a proximal axially oriented ether oxygen. Where the ketones are concerned, precoordination to the Lewis basic carbonyl oxygen presumably initiates spirocyclization in a fashion controlled by the conformational preference of the resident tetrahydrofuranyl subunits.