| Research Abstract |
Mechanistic aspects in the 5-endo radical cyclization of N-vinylic alpha-halo amides and its application to the synthesis of biologically active alkaloids were investigated, The results obtained were as follows :
1, The radical cyclization of N-benzyl-2,2,2-trichloro-N-[2-(phenylthio) cyclohex-1-en-1-yl] acetamide in boiling toluene gave the 5-e.ndo-trig cyclization product, whereas, at room temperature, it gave the 4-exo-trig cyclization product. The results may be explained in terms of the reversibility of the 4-exo cyclization.
2, The regioselectivity in Bu_3SnH-mediated radical cyclizations (4-exo-trig vs 5-endo-trig) of a range of 2-halo-N-(3,4-dihydro-2-naphthyl)acetamides has been examined from the standpoint of the effects of substituents on the radical center and on the nitrogenatom as well as the reaction temperature. When the substituent on the radical center is a hydrogen or chlorine atom, the 4-exo-trig cyclization (the beta-lactam formation) is favored in boiling toluene, w
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hile the radical stabilizing substituents such as alkyl, phenyl, and phenylthio groups bring about the 5-endo-trig cyclization (the gamma-lactam formation) predominantly or exclusiyely. In boiling benzene, however, the predominant formation of the beta-lactams is observed for the methyl substituent, On the other hand, no remarkable difference in the product distributions between the methyl and benzyl substituents on the nitrogen atom is observed. These results are discussed in terms of kinetic or thermodynamic considerations.
3, The radical cyclization of 2-iodo-N-[(S)-1-phenylethyl]-N-(6-oxocyclohex-1-en-1-yl)acetamide proceeded in a 5-endo-trig manner with moderate diastereoselectivity to give the (3aS,7aR)-octahydroindole-2,7-dione as the major product, which was converted into (-)-gamma-lycorane.
4, The radical cyclization of N-(cyclohex-1-en-1-yl)-N-(p-methoxybenzyl) -2- (3, 4-methylenedioxyphenyl) -2- (phenylthio) acetamide gave (3R*, 3aS*, 7aS*) -3-aryloctahydroindol-2-one, which was transformed into the key intermediate for the synthesis of (*)-pancracine. Less
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