Frank B. Mallory

Phenacenes: a family of graphite ribbons. Part 3: Iterative strategies for the synthesis of large phenacenes

Abstract Iterative strategies are reported for the synthesis of some large monodisperse [n]phenacenes, which are polycyclic aromatic hydrocarbon derivatives having n benzene rings fused in an extended phenanthrene-like structural motif. The key carbon–carbon bond-forming steps involve either Wittig or McMurry coupling reactions to give diarylethylenes, and oxidative photocyclizations of those diarylethylenes. tert-Butyl substituents on the phenacene framework serve as solubilizing groups.

Phenacenes: A family of graphite ribbons. 1. Syntheses of some [7]phenacenes by stilbene-like photocyclizations

Abstract The largest previously reported phenacene, the name we propose for the family of polycyclic aromatic compounds having fused benzene rings in an extended phenanthrene-like structural motif, contains only six rings ([6]phenacene). We have employed stilbene-like photocyclizations to synthesize the unsubstituted [7]phenacene, an extremely insoluble compound, as well as 2,13-di- n -pentyl[7]phenacene and 2,13-di- tert -butyl[7]phenacene, two alkyl-substituted derivatives with greatly improved solubilities.

Dehydrogenation and dealkylation of various sterols byTetrahymena pyriformis

Sterols are not found inTetrahymena pyriformis when this protozoan is grown in a medium free from exogenous sterols; instead, the principal solid alcohol that can be isolated from the organism is tetrahymanol. a pentacyclic triterpenoid alcohol with an unusual structure. The biosynthesis of tetrahymanol has been shown by appropriate labeling studies to involve a direct, nonoxidative, proton-initiated cyclization of squalene rather than the more commonly found type of mechanism involving squalene 2,3-oxide as an intermediate. In contrast, whenT. pyriformis is incubated with any one of a wide variety of added sterols, the biosynthesis of tetrahymanol is inhibited and the added sterol is accumulated by the organism and, in most cases, is converted metabolically into one or more other sterols. Four different types of transformation have been observed: the introduction of Δ5, Δ7 and Δ22 double bonds, and the removal of ethyl groups, but not methyl groups, from C-24.

Methyl reorientation in methylphenanthrenes. I. Solid state proton spin–lattice relaxation in the 3‐methyl, 9‐methyl, and 3,9‐dimethyl systems

We have investigated the dynamics of methyl group reorientation in solid methyl‐substituted phenanthrenes. The temperature dependence of the proton spin–lattice relaxation rates has been measured in polycrystalline 3‐methylphenanthrene (3‐MP), 9‐methylphenanthrene (9‐MP), and 3,9‐dimethylphenanthrene (3,9‐DMP) at Larmor frequencies of 8.50, 22.5, and 53.0 MHz. The data are interpreted using a Davidson–Cole spectral density which implies either that the correlation functions for intramolecular reorientation are nonexponential or that there is a distribution of exponential correlation times. Comparing the fitted parameters that characterize the relaxation data for the three molecules shows that the individual contributions to the relaxation rate from the 3‐ and 9‐methyls in 3,9‐DMP can be separated and that the parameters specifying each are similar to the equivalent group in the two single methylphenanthrenes. The 9‐methyl group is characterized by effective activation energies of 10.6±0.6 and 12.5±0.9 kJ/...

α-Hydroxy fatty acids in sphingolipids of tetrahymena

Abstract Sphingolipids of Tetrahymena pyriformis W were found to contain six major fatty acids: the saturated fatty acids 16:0 (n), 17:0 (i), and 18:0 (n), as well as the α-hydroxy fatty acids which have the same three carbon skeletons and presumably are derived from the saturated fatty acids. The structures of the α-hydroxy fatty acids were proven by mass spectrometry, reductive deoxygenation to the corresponding saturated fatty acid, and a reduction-oxidation sequence to give the saturated fatty acid with one less carbon.

Phenyl groups versus tert-butyl groups as solubilizing substituents for some [5]phenacenes and [7]phenacenes.

In recent years, we have used the photocyclizations of diarylethylenes to synthesize a number of [n]phenacenes in the hope that they might be useful as the bridging groups for electron transfer processes in donor-bridge-acceptor molecules. Because [n]phenacenes with n > 5 are very insoluble, their synthesis and characterization has required the attachment of solubilizing substituents such as tert-butyl. The studies of Pascal and co-workers of some large polynuclear aromatic compounds having multiple phenyl substituents prompted us to explore the use of phenyls as alternative solubilizing groups for [n]phenacenes. Although phenyl groups turned out to provide significantly less solubilization than tert-butyl groups in these compounds, we found some interesting structural comparisons of the phenyl-substituted and tert-butyl-substituted [n]phenacenes.

Dealkylation of 24-ethylsterols byTetrahymena pyriformis

WhenTetrahymena pyriformis was incubated with sitosterol ([24R]-24-ethylcholest-5-en-3β-ol]) or itstrans-Δ22-derivative (stigmasterol), the C-24-dealkylated product, cholesta-5,7,trans-22-trien-3β-ol, was obtained in both cases. 24(S)-24-Ethylcholesta-5,7,trans-22-trien-3β-ol also was found to be a metabolite. When sitosterol was the substrate, 24(R)-24-ethylcholesta-5,7-dien-3β-ol was obtained as a third product. Identifications were made by mass spectroscopy, quantitative chromatography, labeling with14C, and by other means. The dealkylated product (cholestratrienol) represented 30% of the sterols isolable after incubation. The administration of sterols to this organism did not induce sterol biosynthesis, since 2-14C-mevalonate failed to yield radioactive sterol in the presence of added stigmasterol.

Intramolecular and intermolecular contributions to the barriers for rotation of methyl groups in crystalline solids: electronic structure calculations and solid-state NMR relaxation measurements.

The rotation barriers for 10 different methyl groups in five methyl-substituted phenanthrenes and three methyl-substituted naphthalenes were determined by ab initio electronic structure calculations, both for the isolated molecules and for the central molecules in clusters containing 8-13 molecules. These clusters were constructed computationally using the carbon positions obtained from the crystal structures of the eight compounds and the hydrogen positions obtained from electronic structure calculations. The calculated methyl rotation barriers in the clusters (E(clust)) range from 0.6 to 3.4 kcal/mol. Solid-state (1)H NMR spin-lattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (E(NMR)) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences E(clust) - E(NMR) for each of the ten methyl groups range from -0.2 kcal/mol to +0.7 kcal/mol, with a mean value of +0.2 kcal/mol and a standard deviation of 0.3 kcal/mol. The differences between each of the computed barriers in the clusters (E(clust)) and the corresponding computed barriers in the isolated molecules (E(isol)) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of E(clust) - E(isol) range from 0.0 to 1.0 kcal/mol.

Methyl and T-Butyl Group Reorientation in Planar Aromatic Solids: Low-Frequency Nuclear Magnetic Resonance Relaxometry and X-Ray Diffraction

We have synthesized 3-t-butylchrysene and measured the Larmor frequency ω/2π (= 8.50, 22.5, and 53.0 MHz) and temperature T (110–310 K) dependence of the proton spin–lattice relaxation rate R in the polycrystalline solid [low-frequency solid state nuclear magnetic resonance (NMR) relaxometry]. We have also determined the molecular and crystal structure in a single crystal of 3-t-butylchrysene using x-ray diffraction, which indicates the presence of a unique t-butyl group environment. The spin-1/2 protons relax as a result of the spin–spin dipolar interactions being modulated by the superimposed reorientation of the t-butyl groups and their constituent methyl groups. The reorientation is successfully modeled by the simplest motion; that of random hopping describable by Poisson statistics. The x-ray data indicate near mirror-plane symmetry that places one methyl group nearly in the aromatic plane and the other two almost equally above and below the plane. The NMR relaxometry data indicate that the nearly in...

The Quenching of Isopropyl Group Rotation in Van Der Waals Molecular Solids

X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol(-1) and the isopropyl group barrier is found to be about 240 kJ mol(-1), meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol(-1) for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR (1)H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11+/-1 kJ mol(-1) and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale.