Intramolecular chiral relay at stereogenic nitrogen: oxazolidine catalysts derived from Ephedra alkaloids

Direct asymmetric syntheses of chiral aldehydes and ketones via N-acyl chiral auxiliary derivatives including chiral Weinreb amide equivalents

This article covers N-acyl chiral auxiliary-based approaches to the asymmetric synthesis of enantiopure aldehydes and ketones. The use of diastereoisomerically pure N-acyl derivatives of chiral auxiliaries (including chiral Weinreb amide equivalents) and their conversion to the corresponding enantiopure aldehydes and ketones in a single synthetic operation by treatment with a hydride reducing agent or an organometallic reagent, respectively, are highlighted.

Formation of specific configurations at stereogenic nitrogen centers upon their coordination to zinc and mercury

The coordination of (R,R)-tetramethylcyclohexane-1,2-diamine derivatives with stereogenic nitrogen centers to zinc and mercury halides is investigated. It is shown that the resulting complexes display one specific configuration at the stereogenic nitrogen centers. This fact is unusual due to the fast inversion of nitrogen centers but highly desirable as the stereoinformation of the ligands is brought closer to the metal centers of the potential catalysts. A combination of NMR studies and quantum chemical calculations gives insight into the selective formation of one specific configuration at the stereogenic nitrogen centers of the zinc complexes.

(6S)-2-tert-Butyl-6-[(4S,5R)-3,4-dimethyl-5-phenyloxazolidin-2-yl]phenol

The title compound, C₂₁H₂₇NO₂, exhibits hydrogen bonding between the phenolic H atom and the heterocyclic N atom. The absolute configuration of the mol­ecule is known from the synthetic procedure.

(6R)-2-tert-Butyl-6-[(4R,5S)-3-isopropyl-4-methyl-5-phenyloxazolidin-2-yl]phenol

In the title compound, C₂₃H₃₁NO₂, the lone pair on the nitro­gen atom is oriented to facilitate intra­molecular hydrogen bonding with the hydr­oxy group residing on the phenyl substituent. The five-membered ring adopts an envelope confornmation with the O atom at the flap. The absolute stereochemistry was verified by measurement of optical activity using a digital polarimeter.

(6S)-2,4-Di-tert-butyl-6-[(4S,5R)-3-isopropyl-4-methyl-5-phenyloxazolidin-2-yl]phenol

The title oxazolidine compound, C₂₇H₃₉NO₂, was synthesized from N-isopropyl­norephedrine. The dihedral angle between the aromatic rings is 70.33 (5)°. The N atom of the heterocycle is oriented to allow intra­molecular O—H⋯N hydrogen bonding with the hydr­oxy substituent.