Regio- and Stereoselective 5-exo Radical Cyclizations on a Chiral Perhydro-1,3-benzoxazine Moiety. An Access to Enantiopure 3-Alkylpyrrolidines
摘要:
Both enantiomers of chiral, nonracemic S-alkyl-substituted pyrolidines are prepared by diastereoselective 5-exo-trig cyclization on (-)-8-aminomenthol-derived perhydro-1,3-benzoxazines used as chiral auxiliaries, followed by elimination of the menthol appendage. The diastereoselective radical cyclization is promoted by tributyltin hydride and occurs on a 3-aza-5-hexenyl-type radical, leading to five-membered rings in high yield. The stereocontrol of the cyclization is strongly influenced by 1,3-allylic strain so that an appropriate substitution pattern on the olefin-acceptor and the presence of a vicinal stereocenter are crucial for achieving good diastereoselectivity. The enantiopure pyrrolidines are obtained in three steps with concomitant recovering of the starting (+)-pulegone auxiliary.
Regio- and Stereoselective 5-exo Radical Cyclizations on a Chiral Perhydro-1,3-benzoxazine Moiety. An Access to Enantiopure 3-Alkylpyrrolidines
摘要:
Both enantiomers of chiral, nonracemic S-alkyl-substituted pyrolidines are prepared by diastereoselective 5-exo-trig cyclization on (-)-8-aminomenthol-derived perhydro-1,3-benzoxazines used as chiral auxiliaries, followed by elimination of the menthol appendage. The diastereoselective radical cyclization is promoted by tributyltin hydride and occurs on a 3-aza-5-hexenyl-type radical, leading to five-membered rings in high yield. The stereocontrol of the cyclization is strongly influenced by 1,3-allylic strain so that an appropriate substitution pattern on the olefin-acceptor and the presence of a vicinal stereocenter are crucial for achieving good diastereoselectivity. The enantiopure pyrrolidines are obtained in three steps with concomitant recovering of the starting (+)-pulegone auxiliary.
A Novel Approach to Chiral, Nonracemic Pyrrolidines by 5-exo-trig Diastereoselective Radical Cyclization on Acrylamides Derived from (−)-8-Aminomenthol
作者:Celia Andrés、Juan P. Duque-Soladana、Rafael Pedrosa
DOI:10.1021/jo981693t
日期:1999.6.1
alpha,beta-Unsaturated amides supported on perhydro-1,3-benzoxazines derived from (-)-8-aminomenthol as chiral auxiliaries undergo regio- and stereoselective 5-exo-trig radical cyclization leading to diastereomeric five-membered lactams. These cyclization products are transformed into enantiopure 3,4-disubstituted pyrrolidines by reduction with aluminum hydride followed by removal of the menthol appendage.