Chiral formamidines. The total asymmetric synthesis of (-)-8-azaestrone and related (-)-8-aza-12-oxo-17-desoxoestrone
作者:A. I. Meyers、Todd R. Elworthy
DOI:10.1021/jo00043a036
日期:1992.8
Attachment of the chiral formamidine moiety to 6-methoxy-1,2,3,4-tetrahydroisoquinoline afforded the key chiral, nonracemic precursor 8, upon which the azasteroid skeleton was constructed. Asymmetric alkylation with alpha-halo esters or beta-halo ethers gave 15 and 22, respectively, in high ee's. Cyclization, following enamine formation with cyclopentanedione or cyclopentanone, led to the chiral steroidal skeletons 6 and 5, respectively. The final stereocenters, leading to 8-azaestrone 4 with unnatural absolute configuration (antipodal), were accomplished by intramolecular alkylation of (+)-6b and subsequent reduction and ether cleavage. For the 12-oxosteroid 3, the methyl at C-13 was inserted by initial conjupte reduction of the enone 5 with a copper hydride reagent method) requiring the presence of a silyl chloride affording 21. The addition of methyl iodide to C-13 occurred after transforming the triethylsilyl enol ether, 21, to its lithium enolate. Stereochemical assignments for both azasteroids 3 and 4 were confirmed by spectroscopic means including circular dichroism curves.