Studies on the asymmetric synthesis of huperzine A. Part 2: Highly enantioselective palladium-catalyzed bicycloannulation of the β-keto-ester using new chiral ferrocenylphosphine ligands
作者:Xu-Chang He、Bin Wang、Gengli Yu、Donglu Bai
DOI:10.1016/s0957-4166(02)00006-x
日期:2001.12
Newchiralferrocenylphosphineligands were designed and tested in the enantioselective bicycloannulation of β-keto-ester 2 with bifunctional allylic agent 4. A range of e.e. values from 80 to 90% of the bicyclic intermediate 6 was achieved. Subsequently, enantiopure (−)-huperzine A was prepared in ca. 40% yield from β-keto-ester 2.
The key intermediate (+)-5 of (-)-huperzine A 1 was prepared in an enantiomerically pure form via the asymmetric palladium-catalyzed bicycloannulation of the beta-keto ester 2 with 2-methylene-1,3-propanediol diacetate 3. The chiral ferrocenylphosphine ligand 7d gave the highest enantioselectivity of 64% ee in the asymmetric bicycloannulation. (C) 1997 Elsevier Science Ltd.