Energetics of the loss of H2 from [C3H7]+

The internal energies of [C₃H₇]⁺ ions contributing to the metastable peak [C₃H₇]⁺ → [C₃H₅]⁺ + H₂ are higher (by perhaps > 100 kJ mol^?1) than those of the ion contributing to the threshold current in appearance energy measurements on [C₃H₅]⁺. The measured appearance energy may lead to an underestimation of the activation energy, i.e. negative ‘kinetic shift’, due to quantum, mechanical tunnelling. The distribution of energy released in the decomposition can be explained on the basis that much of the reverse activation energy and a statistical proportion of the excess energy is released as translation.     

Preparation of Protected β2‐ and β3‐Homocysteine, β2‐ and β3‐Homohistidine,and β2‐Homoserine for Solid‐Phase Syntheses

The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β³‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β²‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→O^tBu etherification (for β²hSer; Schemes 5 and 6), OH→STrt replacement (for β²hCys; Scheme 7), and CH₂OH→CH₂N₃→CH₂NH₂ transformations (for β²hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (t_R), melting points, optical rotations, HPLC on chiral columns, IR, ¹H‐ and ¹³C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis.     

PERSONAL DOSIMETRY OF SOLAR UV RADIATION FOR DIFFERENT OUTDOOR ACTIVITIES

Abstract Quantifying individual exposure to ultraviolet radiation (UVR) is critical to understanding the etiology of a number of diseases including nonmelanotic and melanotic skin cancers. Measurements of personal exposure to solar UVR were made in Hobart, Tasmania in February (summer) 1991 for six different outdoor activities using UVR-sensitive polysulfone (PS) film attached at seven anatomical sites. Concurrent behavioral and environmental observations were also made. To date many studies have relied on subject recall to quantify past solar UVR exposures. To gain insight into the accuracy of subject recall the measured UVR exposures received by different subjects using the PS film were compared to those calculated from personal diaries and ambient solar UVB levels from a monitoring station. In general, when UVR exposure activities took place under close supervision, good correlations were obtained between the PS badges and the ambient measurements/diaries approach. Ultraviolet radiation exposures for the field study involving 94 subjects engaged in a number of outdoor activities are presented.     

Synthese von enantiomerenreinen Violaxanthinen und verwandten Verbindungen

Syntheses of Enantiomerically Pure Violaxanthins and Related Compounds The epoxides 16 and ent- 16 , prepared by Sharpless-Katsuki oxidation of 15 in excellent yield and very high enantiomeric purity, were used as synthons for the preparation of (+)-(S)-didehydrovomifoliol (45) , (+)-(6S, 7E, 9E)-abscisic ester 46 , (+)-(6S, 7E, 9Z)-abscsic ester 47 , (?)-(3S, 7E, 9E)-xanthoxin (49) , (?)-(3R, 7E, 9E)-xanthoxin (50) , (3S, 5R, 6S, 3′S,5′R, 6′S, all-E)-violaxanthin (1) (3R, 5R,6S,3′R,5′R,6′S, all-E)-violaxanthin (55) and their (9Z) (see 53 , 57 ), (13Z) (see 54 , 58 ), and (15Z) (see 60 ) isomers. The novel violadione ( 61 ) was prepared from 1 by oxidation with DMSO/Ac₂O. By base treatment, 61 was converted into violadienedione (62) , a potential precursor of carotenoids with phenolic end groups.     

Studies on the Biosynthesis of Spirostaphylotrichin A

Incorporation of ¹⁴C-labelled acetate and amino acids as well as of [1-¹³C]-, [2-¹³C]-, and [1,2-¹³C₂] acetate, L -[methyl¹³C] methionine, [2,3-¹³C₂] succinate, and L -[2,3-¹³C₂] aspartate into spirostaphylotrichin A ( 1 ) by Staphylotrichum coccosporum demonstrates that the building blocks of 1 are 5 units of acetate/malonate, 1 unit of methionine, and a C₄-dicarboxylic acid. The latter is most likely aspartate and derived from the citric-acid cycle. Using [2-¹³C, 2-²H₃] acetate as a precursor, the starter unit of the polyketide chain was identified.     

On the stabilizer subgroup of a fair of matrices

Let (F,G) be a pair of matrices defined over an arbitrary field, Fn × n, Gn × m. Consider the natural action of GL_n x GL_m on this pair given by (F,G) ? (gFg^-1,gGh^-1), where (g,h) ∈ GL_n × GL_m. This action is of interest in system theory as well as the representation theory of quivers. In this paper we study the stabilizer subgroup of this action stab(F,G), i.e.

$\text{{(g,h) ∈ GL}{}_{\mathrm{n}}\text{x GL}{}_{\mathrm{m}}\text{|gFg}{}^{-1}\text{= F,gGh}{}^{-1}\text{= G}}$

.     

On the sphericity and cubicity of graphs

The sphericity of a graph G is the smallest n such that the vertices in G can be mapped into unit-diameter spheres in n-space in such a way that two vertices are adjacent in G if and only if their assigned spheres intersect. The cubicity of G is defined similarly with unit cubes (edges parallel to the axes) used instead of unitdiameter spheres. In his study of molecular conformation, Havel (Ph.D. dissertation, Univ. of Calif., Berkeley, 1982) showed that there are finite graphs of sphericity 2 that have arbitrarily large cubicity, but he left open the question of whether there are graphs with sphericity greater than cubicity. It is shown here that such graphs exist for cubicity 2 or 3. The construction used to prove this generalizes to larger values for cubicity, but the proof technique does not. Hence it remains open whether there are graphs with large cubicities whose sphericities exceed their cubicities.     

Diterpenoide Chinomethane,vinyloge Chinone und ein Phyllocladan-Derivat aus Plectranthus purpuratus HARV. (Labiatae)

Diterpenoids from Leaf Glands of Plectranthus purpuratus: p-Quinomethanes, Extended Quinones, p-Acylcatechols and a Novel Phyllocladanon Derivative From the complex mixture of terpenoids from the title plant, the following novel diterpenoids have been isolated: 11-hydroxy-19-(3-methyl-2-butenoyloxy)- and 11-hydroxy-19-(3-methylbutanoyloxy)-5,7,9 (11), 13-abietatetraen-12-one ( 1a / 1b ), 11-hydroxy-19-(3-methyl-2-butenoyloxy)- and 11-hydroxy-19-(3-methylbutanoyl-oxy)-7,9(11), 13-abietatrien-6,12-dione ( 2a / 2b ), 6α, 11-dihydroxy-19-(3-methyl-2-butenoyloxy)- and 6α, 11 -dihydroxy-19-(3-methylbutanoyloxy)-7,9 (11), 13-abieta-trien-12-one ( 3a / 3b ), 11,12-dihydroxy-19-(3-methyl-2-butenoyloxy)- and 11,12-di-hydroxy-19-(3-methylbutanoyloxy)-8,11,13-abietatrien-7-one ( 4a / 4b ), and (16R)-17,19-diacetoxy-16-hydroxy-13β-kauran-3-one (=(16R)-17,19-diacetoxy-16-hydro-xyphyllocladan-3-one; 10 ). Compounds 2 and 3 are derivates of taxodione and taxodone, respectively, 4 is a derivative of cryptojaponol. The structure of 10 is Wised on a single-crystal- X -ray analysis and CD . data.     

The molecular structure of [dimethyl(phenyl)silyl]tris(trimethylsilyl)methane

The structure of the crowded molecule (Me₃Si)₃C(SiMe₂Ph) has been determined by single crystal X-ray diffraction. The steric strain manifest itself mainly in lengthening of the Me₃SiC and Me₂PhSiC bonds (average length 1.920(6) $\text{,ac>A}\text{?}$) and closing up of the CSiC angles within the Me₃Si and Me₂PhSi groups (average 105.2(10)°), with correspondingly large C(1)SiC angles (113.5(13)°; C(1) is the central carbon atom).     

A small molecule with osteogenesis-inducing activity in multipotent mesenchymal progenitor cells

Purmorphamine, which is a 2,6,9-trisubstituted purine compound, was discovered through cell-based high-throughput screening from a heterocycle combinatorial library. It differentiates multipotent mesenchymal progenitor cells into an osteoblast lineage. It will serve as a unique chemical tool to study the molecular mechanisms of osteogenesis of stem cells and bone development.