STEREOELECTRONIC EFFECTS IN THE 6-EXO FREE-RADICAL CYCLIZATION OF ACYCLIC SUGAR-DERIVATIVES - SYNTHESIS OF BRANCHED-CHAIN CYCLITOLS

The synthesis and free radical carbocyclization (BU3SnH + AIBN) of acy clic sugar derivatives 1-12 is reported. The yields of these 6-exo cyc lization processes are good and the diastereomeric excesses are from m oderate to excellent. The resulting cyclohexanes are polyoxygenated, e nantiomerically pure building blocks for the synthesis of complex bran ched chain cyclitols. The results obtained in the cyclization of radic al precursors 1, 4, 6, and 8 are in sharp contrast with the results ob served in the ring closure of compounds 2, 3, 5, and 7. An electron-at tracting group (acetate or mesylate), located in a vicinal position to the carbon-centered radical, modifies the conformation of the reactin g species in the transition state and thus changes the stereochemical outcome of the cyclization. This allowed us to select the nature of th e absolute configuration at the newly formed stereocenter by simply ch anging the type of the substituents at the vicinal carbon where the ra dical is generated.