Synthesis of Enantioenriched and Diastereomerically Pure <i>cis</i>-Fused Bicyclic α-Oxy-Substituted γ-Lactones via Epoxidation of Optically Active Homoaldol Products
作者:Dieter Hoppe、Seda Ünaldi、Roland Fröhlich
DOI:10.1055/s-2005-872080
日期:——
By applying the (-)-sparteine-mediated asymmetric deprotonation to 1-O-(2-alkylcycloalk-1-enyl)methyl N,N-diisopropylcarbamates, combined with a lithium-titanium exchange followed by addition to achiral aldehydes, enantioenriched homoaldol products 6 were prepared. Hydroxyl-directed epoxidation resulted in the cleavage of the oxirane ring by the migration of the N,N-diisopropylcarbamoyloxy group to form bicyclic γ-lactols 10 via the corresponding γ-hydroxy aldehydes. After oxidation, diastereomerically pure, penta-substituted γ-lactones 11 were obtained.
trifluoride etherate to form cyclohexano- or cyclopentano-annulated tetrahydrofuran-3-carboxaldehydes, bearing two quaternary carbon atoms and four adjacent stereogenic centers, with complete diastereoselectivity.
Highly enantioenriched 4-hydroxy-1-alkenyl N,N-diisopropylcarbamates, easily available by asymmetric homoaldol reaction, cyclize by treatment with sodium hydride to form (1r,2t,3t)-configured 1-acylcyclopropanes with high diastereoselectivity. The decisive steps are the migration of the N,N-diisopropylcarbamoyl group onto the alkoxide oxygen atom, followed by an intramolecular homoallylic substitution