title: A combined NMR, DFT, and X-ray investigation of some cinchona alkaloid O-ethers.
abstract: Structures and conformational behavior of several cinchona alkaloid O-ethers in the solid state (X-ray), in solution (NMR and DFT), and in the gas phase (DFT) were investigated. In the crystal, O-phenylcinchonidine adopts the Open(3) conformation similar to cinchonidine, whereas the O-methyl ether derivatives of both cinchonidine and cinchonine are packed in the Closed(1) conformation. Dynamic equilibria in solutions of the alkaloids were revealed by combined experimental-theoretical spin simulation/iteration techniques for the first time. In the (1)H NMR spectra in CDCl3 and toluene-d8 at room temperature, Closed(1) conformation was observed for the O-silyl ethers as a separate set of signals. For O-methyl ether derivatives Closed(1) could be separated only at -30 degrees C in CDCl3 or toluene-d8 and for O-phenylcinchonidine at -70 degrees C in CDCl3/CD2Cl2. The ratio between the Closed(2) and Open(3) conformers was estimated by analyzing the vicinal coupling constant (3)J(H9,H8) at ambient and low temperatures. The observed conformational equilibria of O-(tert-butyldimethylsilyl)cinchonidine in CDCl 3 and toluene-d8 are in good agreement with the theoretically estimated equilibrium populations of the conformations according to Boltzmann statistics. The conformational equilibria of four cinchona alkaloid O-ether solutes in CDCl3 and toluene-d8 are discussed in the light of their relevance to the mechanism of 1-phenyl-1,2-propanedione (PPD) hydrogenation over cinchona alkaloid modified heterogeneous platinum catalysts. It was demonstrated that the conformation found to be abundant in the liquid phase has no direct correlation with the enantioselectivity of the PPD hydrogenation reaction.
authors.affiliation: Laboratory of Organic Chemistry, Abo Akademi University, FI-20500 Turku, Finland.
authors.forename: Igor
authors.initials: I
authors.lastname: Busygin
authors.forename: Ville
authors.initials: V
authors.lastname: Nieminen
authors.forename: Antti
authors.initials: A
authors.lastname: Taskinen
authors.forename: Jari
authors.initials: J
authors.lastname: Sinkkonen
authors.forename: Esa
authors.initials: E
authors.lastname: Toukoniitty
authors.forename: Reijo
authors.initials: R
authors.lastname: Sillanpää
authors.forename: Dmitry Yu
authors.initials: DY
authors.lastname: Murzin
authors.forename: Reko
authors.initials: R
authors.lastname: Leino
mesh_terms: D000465:Algorithms; D002384:Catalysis; D047188:Chalcones / Q000737:chemistry; D002930:Cinchona Alkaloids / Q000737:chemistry*; D018360:Crystallography, X-Ray / Q000379:methods*; D004987:Ethers / Q000737:chemistry*; D005740:Gases; D006865:Hydrogenation; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D008968:Molecular Conformation; D010984:Platinum / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D047188:Chalcones; D002930:Cinchona Alkaloids; D004987:Ethers; D005740:Gases; D012997:Solvents; D010984:Platinum; C100349:1-phenyl-1,2-propanedione