A characterization of linear spaces based on the number of transversals

We characterize a class of linear spaces by the property that through any point outside two disjoint, but non-parallel lines there is at most one transversal.     

How the Co-C bond is cleaved in coenzyme B12 enzymes: a theoretical study

The homolytic cleavage of the organometallic Co-C bond in vitamin B12-dependent enzymes is accelerated by a factor of approximately 10(12) in the protein compared to that of the isolated cofactor in aqueous solution. To understand this much debated effect, we have studied the Co-C bond cleavage in the enzyme glutamate mutase with combined quantum and molecular mechanics methods. We show that the calculated bond dissociation energy (BDE) of the Co-C bond in adenosyl cobalamin is reduced by 135 kJ/mol in the enzyme. This catalytic effect can be divided into four terms. First, the adenosine radical is kept within 4.2 angstroms of the Co ion in the enzyme, which decreases the BDE by 20 kJ/mol. Second, the surrounding enzyme stabilizes the dissociated state by 42 kJ/mol using electrostatic and van der Waals interactions. Third, the protein itself is stabilized by 11 kJ/mol in the dissociated state. Finally, the coenzyme is geometrically distorted by the protein, and this distortion is 61 kJ/mol larger in the Co(III) state. This deformation of the coenzyme is caused mainly by steric interactions, and it is especially the ribose moiety and the Co-C5'-C4' angle that are distorted. Without the polar ribose group, the catalytic effect is much smaller, e.g. only 42 kJ/mol for methyl cobalamin. The deformation of the coenzyme is caused mainly by the substrate, a side chain of the coenzyme itself, and a few residues around the adenosine part of the coenzyme.     

Steric controls in the preparation of 1-benzyl-1,4-dihydro-3-(2H)isoquinolones and 1-benzyl-1,2,4,5-tetrahydro(3H)-2-benzazepin-3-ones via Beckmann rearrangement

The effect of substituents in the 1-position of 2-indanones and 2-tetralones on the course of Beckmann rearrangement reactions was studied. Nmr spectral anaylses of the intermediate oximes and final products demonstrated that steric controls were operative both before and during rearrangement. Use of the appropriate substituents and reaction conditions makes this reaction scheme attractive for preparation of the title compounds.     

General protocols for the synthesis of C(2)-symmetric and asymmetric 2,8-disubstituted analogues of Tröger's base via efficient bromine-lithium exchanges of 2,8-dibromo-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine

Methods for facile synthesis of symmetric and unsymmetric functionalized analogues of Tr?ger's base were developed with use of 2,8-dibromo-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (2) as the starting material. C(2)-symmetric 2,8-disubstituted analogues of Tr?ger's base (4a-f) were synthesized via double bromine-lithium exchange of 2 followed by quench with electrophiles. Desymmetrization via single bromine-lithium exchange of 2, followed by quench with electrophiles, afforded asymmetric analogues of Tr?ger's base (6a-g). Further reaction of 2-bromo-8-(trimethylsilyl)-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (6b) produced 7a-c via single bromine-lithium exchange and subsequent quench with electrophiles.     

Drei neue furanogermacrene aus myrrhe

From the essential oil of myrrh (Commiphora molmol ENGLER) three new furanogermacrens have been isolated and their structures elucidated to $\text{1}$,$\text{2}$ and $\text{3}$.     

Polyethers: Structural analysis of the 1,4,5,8-tetraoxadecalins and 2,2'-bis(1,3-dioxolane) by photoelectron spectroscopy,molecular mechanics and molecular orbital calculations

The photoelectron (PE) spectrum of cis-1,4,5,8-tetraoxadecalin exhibits in contrast to most polyoxa compounds an extremely well resolved low energy pan due to large ‘through-bond’ interactions between the oxygen lone pairs and the ideally oriented C-C bonds. The ‘through-bond’ interactions of the cis- as well as the unknown trans-1,4,5,8-tetraoxadecalin are discussed based on simple perturbation molecular orbital theory and PRDDO molecular orbital calculations. Additionally the PE spectrum of 2,2'-bis (1,3-dioxolane) is reported. Molecular mechanics (MM1 and MM2) and molecular orbital (PRDDO) calculations of the different conformations of the 1,4,5,8-telraoxadecalin and 2,2'-bis(1,3-dioxolane) systems allow the absence of the trans-1, 4,5,8-tetraoxadecalin, and the cis-trans energy difference in the series decalin, 1,8-dioxadecalin and 1,4,5,8-tetraoxadecalin systems to be explained.