The correspondence fedorov-schoenflies: The groups Pm,Pc, Cm,Cc

In the discovery of the crystallographic space groups there occur some differences between Fedorov and Schoenflies. We try to clarify this point on the basis of their correspondence.     

Zum mechanismus Massenspektrometrischer Fragmentierungsreaktionen–III: Stereospezifische Reaktionen in den Massenspektren der Methyläther cyclischer Polyalkohole

The mass spectra of stereoisomers of polymethoxy cycloalkanes depend on the geometry of the molecular ions. The magnitude of the stereochemical effect is influenced by the stability of the cyclic molecular ions. Due to energetically favourable ring-fragmentations the effect is cancelled by vicinal methoxy substituents and diminished by a methyl group next to a methoxy substituent. Stereochemically controlled fragmentations are the eliminations of a methoxy group in the form of a methanol or formaldehyde molecule from the molecular ions. By an investigation of di- and trimethoxy cyclohexanes, specifically labelled with deuterium, it is shown that both reactions are initiated by a transfer of an H-atom from a carbinol-C-atom to an O-atom of a methoxy group. Whether or not these energetically favourable reactions will occur depends on how close the H- and O-atoms involved can approach each other in the possible conformations of the molecular ion. The stereochemical control of the fragmentation of 1,3-dimethoxy cyclopentane, containing a more or less fixed five membered ring is small, that of dimethoxy cycloheptanes with a flexible seven membered ring is of comparable magnitude as the steric effect in the mass spectra of cyclohexane derivatives.     

Lessons learned from macrolide synthesis

The highlights of three macrolide syntheses recently completed in our laboratory are described. In the epothilone B synthesis we developed the "early epoxide approach," which resulted in the first completely stereocontrolled synthesis of this natural product. In the laulimalide synthesis our contribution was the auxiliary controlled ene-macrocyclization and Sharpless' kinetic resolution for achieving a regioselective epoxidation of two pseudo-enantiomorphic allylic alcohol subunits. The tartrolon B synthesis was the first to be completed. In this case, a substrate controlled aldol addition was used to assemble the entire carbon skeleton of the compound.     

Steroid binding by antibodies and artificial receptors: Exploration of theoretical methods to determine the origins of binding affinities and specificities

Binding mode calculations for complexes between an artificial paracyclophane receptor and digoxins, cholic acids as well as cortisone steroids show encapsulation of different ring combinations. Docking experiments were performed between the 26-10 antibody and digoxins. Coordination affinity arises from hydrophobic desolvation and van der Waals interactions rather than from hydrogen bonds. The specificity and affinity arises mainly from shape complementarity. Computed binding free energies and Kohonen neural network computations both point to physicochemical and structural similarities of natural antibodies and artificial receptors.     

Formation of mixed aryl–, alkyl–lithium aggregates in the heteroatom assisted lithiation of α,α′-dialkyl substituted 1,3-bis[(dimethylamino)methyl]benzene

The heteroatom assisted lithiation of 1,3-bis[1-(dimethylamino)ethyl]benzene with n-BuLi afforded 2,6-bis[1-(dimethylamino)ethyl]phenyllithium. An X-ray crystal structure determination revealed a dimeric aggregate in which the four benzylic chiral centers are identical, pointing to stereoselective crystallization. In contrast, reaction of 1,3-bis[1-(dimethylamino)propyl]benzene with n-BuLi afforded a dimeric aggregate comprising the parent lithiated compound and n-BuLi in a 1:1 molar ratio. The four Li atoms and the four bridging carbon atoms are arranged in a unique ladder-type C–Li₂–C₂′–Li₂–C framework.     

New Pyrrolo[1,2-a]azepine Type Alkaloids from Stemona and Stichoneuron (Stemonaceae)

Summary. Three new pyrroloazepine type alkaloids, stichoneurines A and B and 6-hydroxycroomine were isolated from the lipophilic root extracts of Stichoneuron caudatum and Stemona tuberosa collected in Thailand together with the already known croomine, tuberostemonine, and tuberostemonine A. The structures were elucidated by spectroscopic methods including H/H-COSY, HMQC, and HMBC. Information on the relative stereochemistries and conformational behaviour was obtained by analysis of the NOESY spectra. The formation of pyrroloazepine alkaloids in the genus Stichoneuron is reported for the first time and supports its affiliation to the family Stemonaceae. The occurrence of two different types of alkaloids, of the tuberostemonine and croomine series, in different geographical provenances of Stemona tuberosa is of special chemosystematic interest and may contribute to a more natural species delimitation within that complex group.     

The decarboxyla tive dehydration of 3-hydroxycarboxylic acids with dimethyl-formamide-dimethylacetal - evidence for a zwitterionic intermediate

Dimethylformamide dimethylacetal ($\text{1}$) and 3-hydroxycarboxylic acids ($\text{2}$) react with formation of esters ($\text{4}$) and olefins ($\text{5}$). Evidence is provided that $\text{5}$ is generated via an E1/E2-type fragmentation of a zwitterionic intermediate

. (Scheme 1).     

Monodentate substitution reactions of [MO2(CN)4]n− type complexes: The crystal structure of the sodium thiocyanate adduct of tetramethylammonium tetracyanooxothiocyanatotungstate(IV)

The crystal structure of (Me₄N)₃[WO(CN)₄(NCS)]·NaNCS was determined from three-dimensional x-ray diffraction data. The dark blue crystals are monoclinic, space group C2/m, with cell dimensions a=13.105(4), b=12.688(2), c=18.871(3) Å, =100.4(8)°, z=4 and D=1.46 g cm^–3. Anisotropic refinement of 1333 observed reflections converged to R=0.068. The [WO(CN)₄(NCS)]^3– ion is a distorted octahedron with the tungsten atom displaced by 0.35 Å out of the plane formed by the four cyano ligands, towards the oxo ligand. The coordinated thiocyanate ligand is bonded to the tungsten atomvia the nitrogen atomtrans to the oxo ligand. Bond distances found: W–CN_av=2.14(3), W–O=1.61(2) and W–NCS=2.23(2) Å. The compound crystallizes as an adduct with an additional thiocyanate anion, ionically bonded to a severely disordered sodium cation. The results correlate with available data from similar complexes.