Rotational spectra of bicyclic decanes: the trans conformation of (-)-lupinine

Molecular Rotation Spectrum of Tetracyclic Quinolizidines: Observation of trans-Matrine and the Elusive cis-Matrine

We characterized the bis-quinolizidine tetracyclic alkaloid (5S, 6S, 7R, 11R)-matrine in a supersonic jet expansion, using chirped-pulsed broadband microwave spectroscopy. Previous crystal diffraction analyses suggested 16 diastereoisomers associated with matrine's four carbon stereocenters but were inconclusive whether the lactamic nitrogen atom would additionally produce separated trans-/cis- diastereoisomers or if both species may interconvert through low potential barriers. Our experiment simultaneously detected trans- and cis-matrine through their rotational spectrum, confirming the possibility of conformational rearrangement in matrine alkaloids. The two matrine conformers mainly differ in the envelope or half-chair lactamic ring, as evidenced by the experimental rotational and nuclear quadrupole coupling parameters. Molecular orbital calculations with ab initio (MP2) and density functional methods (B3LYP-D3(BJ) and MN15) were tested against the experiment, additionally offering an estimation of the cis-/trans- barrier of 24.9-26.9 kJ mol^-1. The experiment illustrates the structural potential of chirped-pulsed broadband microwave spectroscopy for high-resolution rotational studies of biomolecules in the range of 20-40 atoms.

Structural and spectral properties of quinolizidine alkaloids: quantum chemical calculations

Structural and spectroscopic properties of quinolizidine alkaloids lupinine and epilupinine stereoisomers were studied theoretically. The influence of the calculation method and structural change in the molecule on the results of geometry and other properties of compounds was considered. The equilibrium geometry, harmonic vibrational frequencies and infrared intensities were obtained by means of density functional theory (DFT/B3LYP) calculations with the splitvalence medium-sized 6-31G(d) basis and Dunning’s correlation consistent basis set cc-pVDZ. From the optimized structure of the (+)-lupinine and (+)-epilupinine molecules, geometric parameters were compared with the literature X-ray experimental data. Structural and vibrational parameters for the (-)-lupinine and (-)-epilupinine molecules are predicted by theoretical calculations at B3LYP/6-31G(d) and B3LYP/cc-pVDZ levels of theory. Some physical characteristics for the title compounds, such as total electronic energy, zero-point energy, rotational constants and dipole moments were also defined by DFT methods. The thermodynamic functions of the title compounds were performed at the same theory levels. Stationary points are identified by the solution of the oscillatory problem.

Divergent Stereoselectivity in Phosphothreonine (pThr)-Catalyzed Reductive Aminations of 3-Amidocyclohexanones

Phosphothreonine (pThr)-embedded peptide catalysts are found to mediate the reductive amination of 3-amidocyclohexanones with divergent selectivity. The choice of peptide sequence can be used to alter the diastereoselectivity to favor either the cis-product or trans-product, which are obtained in up to 93:7 er. NMR studies and DFT calculations are reported and indicate that both pathways rely on secondary interactions between substrate and catalyst to achieve selectivity. Furthermore, catalysts appear to accomplish a parallel kinetic resolution of the substrates. The facility for phosphopeptides to tune reactivity and access multiple products in reductive aminations may translate to the diversification of complex substrates, such as natural products, at numerous reactive sites.

Rotational spectra of tetracyclic quinolizidine alkaloids: does a water molecule flip sparteine?

Sparteine is a quinolizidine alkaloid used as a chiral auxiliary in asymmetric synthesis. We examine whether hydration by a single molecule can flip sparteine from the most stable trans conformation to the bidentate cis arrangement observed in catalytic complexation to a metal center. Sparteine and the sparteine-water dimer were generated in a supersonic jet expansion with H₂¹⁶O and H₂¹⁸O, and characterized by broadband chirped-pulse microwave spectroscopy. Even though the bidentate water dimer was predicted with larger binding energy, a single isomer was observed for the monohydrated cluster, with sparteine retaining the trans conformation observed for the free molecule. The absence of the bidentate dimer is attributed to the kinetic control of cluster formation, favoring the pre-expansion most abundant monomer. The structural properties of the O-HN hydrogen bond in the dimer are compared with those of complexes of other secondary and tertiary amines.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.