Construction of Enantiomerically-Pure Heterocycles
As most pharmaceuticals are heterocyclic, there is continuing interest in methods for the direct enantioselective construction of heterocycles. Greg Fu of MIT reports (J. Am. Chem. Soc. 2003, 125, 10778. ) that the addition of the dipole 1 to alkynes is catalyzed by CuI, and that in the presence of the planar-chiral ligand 2 the reaction proceeds in high enantiomeric excess. The ee is maintained with aryl-substituted alkynes, and is higher when there are alkyl substituents on the heterocyclic ring of 1.
Many methods have been developed for the enantioselective synthesis of unnatural α-amino acids. Jeff Johnston of Indiana University reports (J. Am. Chem. Soc. 2003, 125, 163. ) coupling the asymmetric alkylation of O'Donnell with intramolecular radical cyclization, leading to what appears to be a general method for the enantioselective construction of indolines.
Takeo Kawabata of the Institute for Chemical Research associated with Kyoto University reports (J. Am. Chem. Soc. 2003, 125, 13012. ) that unnatural amino acids can also be used to assemble four-, five-, six-, and seven-membered cyclic amines having quaternary stereogenic centers. Given the conventional wisdom that ester enolates are sp2-hybridized, this memory effect is remarkable.