Unexpected Stereoselective 6-<i>exo versus </i>7<i>-endo</i> Aryl Radical Cyclisation Controlled by Positional Isomers in a Chiral Octahydro-1,3-benzoxazine Moiety
Chiral perhydro-1,3-benzoxazines which carry both olefinic and bromo-aromatic substituents undergo stereo- and regio-radical cyclisations mediated by Bu3SnH. Diastereomeric six-membered and seven-membered rings are formed in positional isomers through selective 6-exo and 7-endo attack respectively.
Diastereoselective Tandem 6-<i>e</i><i>xo</i> Carbolithiation Intramolecular Ring Opening in (−)-8-Aminomenthol-Derived Perhydrobenzoxazines. A New Synthesis of Enantiopure 4-Substituted Tetrahydro Isoquinolines and 2-Azabenzonorbornanes
作者:Rafael Pedrosa、Celia Andrés、Jesús M. Iglesias、Alfonso Pérez-Encabo
DOI:10.1021/ja002864q
日期:2001.3.1
perhydro-1,3-benzoxazines participate in the intramolecular 6-exo carbolithiation reaction with unactivated double bonds attached to the nitrogen substituent of the heterocycle. When the reaction time is extended or no TMEDA is used, the cyclized lithium intermediates react intramolecularly with the N,O-acetal system leading to 2-azabenzonorbornane derivatives. The reactions are highly stereoselective
Synthesis of enantiopure mono- and disubstituted tetrahydroisoquinolines by 6-exo radical cyclizations
作者:Rafael Pedrosa、Celia Andrés、Jesús M Iglesias、Manuel A Obeso
DOI:10.1016/s0040-4020(01)00274-5
日期:2001.4
menthol, readily cyclized stereoselectively by reaction with tributyltin hydride in the presence of AIBN. The cyclization compounds were transformed into enantiopure 4-alkyl tetrahydroisoquinolines by reductive ring opening of the N,O-acetal moiety with lithium aluminum hydride. Enantiopure 1,4- and 3,4-dialkyl-substituted tetrahydroisoquinolines were also prepared by reaction of the cyclized compounds