DIASTEREOSELECTIVE SYNTHESIS OF CYCLOALKYLAMINES BY SAMARIUM DIIODIDE-PROMOTED CYCLIZATIONS OF ALPHA-AMINO RADICALS DERIVED FROM ALPHA-BENZOTRIAZOLYLALKENYLAMINES

The condensation of omega-unsaturated aldehydes with benzotriazole and secondary amines affords alpha-benzotriazolylalkenylamines that exist in solution as mixtures of the corresponding benzotriazol-1-yl and be nzotriazol-2-yl isomers resulting from their rapid dissociation into i minium cations and the benzotriazolyl anion. The reduction of these ad ducts with samarium diiodide (SmI2) takes place with formation of the benzotriazolyl anion and alpha-amino alkenyl radicals that undergo 5- or 6-exo-trig cyclizations leading to substituted cycloalkyl- or cyclo heteroalkylamines. The presence of an electron-withdrawing substituent in the alkene subunit is required for efficient cyclizations. The for mation of cyclopentylamines takes place with unusually high 1,5-cis se lectivity (hex-5-enyl radical numbering), and the presence of a 2- or 4-Me substituent also imparts high 1,2- or 1,4-trans stereoinduction, respectively. The corresponding six-membered rings, however, are forme d with low diastereoselectivity. Semiempirical calculations performed on model systems suggest that a stabilizing secondary orbital interact ion between the amino group and the electron-deficient alkene might in part account for the enhanced cis-selectivity encountered.