Establishment of Highly Selective Radical Cyclization and Application to the Synthesis of Biologically Active Compounds

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Bu_3SnH-mediated regio- and stereoselective radical cyclizations of alpha-holo amides have been examined. The results are summarized below.1. The 4-exo radical cyclization of N-vinylic alpha-halo amides, bearing a chiral auxiliary on the nitrogen atom or at the alpha-position of the carbonyl group, was found to proceed with high degree of diastereoselectivy to give optically active beta-lactams. The methods were applied to the synthesis of optically active carbapenem antibiotics such as (+) -PS-5, (+) -thienamycin and 1beta-methylcarbapenem.2. The 5-endo radical cyclization of N- (1-cyclohexenen-1-yl) -alpha-haloacetamides bearing a chiral auxiliary on the nitrogen atom was found to proceed with moderate degree of diastereoselectivy t give optically active octahydroindol-2-ones. The method was applied to the total synthesis of (-) -gamma-lycorane.3. The radical cyclization of N- (2-phenylthio-1-cyclohexenen-1-yl) -alpha-haloacetamies has been examined, and it was found that the course of the cyclizations was atrikingly affected by the reaction temperature employed. Thus, at room temperature, 4-exo cyclization predominated, and at higher temperature (in boiling toluene), 5-endo cyclization predominated.4. The kN-vinyl-2-bromobenzamides or N-vinyl-2- (2-bromophenyl) acetamides bearing a phenylthio group at the terminus of their N-vinylic bond were found to undergo radical cyclization in a 5-exo-trig or in a 6-exo-trig manner effectively to give isoindolone and isoquinolinones, respectively. The methods were applied to the synthesis of some alkaloids such as chilenine, lennoxamine, tetrahydropalmatine and saulatine.5. The N- [o- (2-propenyl) phenyl] -2,2-bis (phenylthio) acetamides were found to undergo radical cyclization in an 8-endo-trig manner to give benzoazocinone derivatives.