Preparative isolation of (+)-beta-eudesmol fromAmyris balsamifera

Summary Optically pure (+)-beta-eudesmol is a possible starting material for the synthesis of several termite defense compounds. A two step procedure for the isolation of gram quantities of (+)-beta-eudesmol from commercially availableAmyris balsamifera oil (syn. West Indian sandalwood oil), containing 8% beta-eudesmol, was developed. Step one consisted of an efficient vacuum distillation of the total oil. Step two was a medium pressure LC separation with an AgNO₃ impregnated silica gel stationary phase. Several other separation procedures failed due to the presence of many closely related sesquiterpene alcohols (75% of the oil).     

Formation of the corannulene core by nickel-mediated intramolecular coupling of benzyl and benzylidene bromides: a versatile synthesis of dimethyl 1,2-corannulene dicarboxylate

[reaction: see text] A practical synthesis of dimethyl 1,2-corannulene dicarboxylate (5) is reported, with the final ring-forming step achieved by the double intramolecular nickel powder mediated coupling of benzyl and benzylidene bromide groups with 60% isolated yield.     

Pathways for ring inversion in 9, 10-dihydroanthracene, 9, 10-dihydrophenanthrene,and 7, 12-dihydropleiadene: Combined molecular orbital and molecular mechanics study

The combined semiempirical AM1 and MM2 study of the ring inversion processes in 9, 10-dihydroanthracene ( 1 ), 9, 10-dihydrophenanthrene ( 2 ), and 7, 12-dihydropleiadene ( 3 ) predicts planar transition states for 1 and 2 , in accord with earlier assumptions. In the case of 3 , however, a nonplanar, chair-like transition state is located on the potential energy surface in contrast with previous conclusions. Implications of the nonplanar TS on the rationalization of available experimental data are discussed. The calculated barriers for ring inversion in 1–3 are in reasonable agreement with both experiment and previous theoretical results. In general, AM1 is found to slightly underestimate the barriers as compared to the MM2 calculations.     

Spectroscopic study of a representative polar cap of buckminsterfullerene: Cyclopentacorannulene

The spectroscopic properties of corannulene and cyclopentacorannulene are examined by use of absorption and steady-state fluorescence measurements. A red shift in the emission maxima of cyclopentacorannulene is noted with respect to the emission maxima in the corannulene spectrum. Similar differences in the absorption spectra of both molecules are also observed. Reasons for the dissimilarities in the absorption and emission spectra of these molecules are discussed. The fluorescence quantum yields and lifetimes of the molecules measured in organic solvents are reported. The fluorescence quantum yield and lifetime of cyclopentacorannulene are lower than those of corannulene. This difference is attributed to the highly strained and nonplanar structure of cyclopentacorannulene. The effectiveness of nitromethane as a quencher of corannulene and cyclopentacorannulene fluorescence is examined. In contrast to previously reported results, the fluorescence of these molecules is quenched by nitromethane. Fluorescence quenching of the molecules has been attributed to complex formation in the ground state, i.e., static quenching.     

Metal-ammonia reduction: the ring reduction of aromatic carboxylic esters

Aryl benzoates are reduced to 1,4-dihydroaromatics with Na/NH₃ in the presence of water.     

A computational study of the hydrodynamically assisted organization of DNA-functionalized colloids in 2D

We study computationally the self-organization of DNA-functionalized colloidal particles confined to two dimensions and subjected to a linear shear force. We show that hydrodynamic forces allow a more thorough sampling of phase space than thermal or Brownian forces alone. Two particle types are present in each of our dynamic simulations each signifying its own specific oligonucleotide sequence grafted to the particle surface: A-type and B-type. Particles are modeled as interacting via a type-specific DNA attraction where unlike-types have affinities for each other while like-types do not. The particles are small enough to feel Brownian motion while the shear adds motion to the particles. We find the formation of lines of A-type and B-type particles in simulations with an imposed shear. Simulations without imposed shear form a frustrated network with little or no linear order. An orientational distribution function, g2(r), quantifies the degree of linear order. A phase diagram is constructed, finding a linear dependence of the minimum DNA force necessary for line formation on the dimensionless shear rate. A force analysis performed on the structures shows that the lines orient perpendicular to the axis of the elongation component of the shear because it is this orientation that allows the DNA attraction to resist the shear.     

Computational study of the self-organization of bidisperse nanoparticles

We study computationally the self-organization of bidisperse mixtures of thiol-stabilized gold particles in two dimensions through random sequential adsorption (RSA) coupled with the Metropolis algorithm for determining surface diffusion. It was previously shown [Doty et al. Phys. Rev. E 2002, 65, 061503] that ordered lattices of bidisperse particles cannot form with hard sphere interactions. Here we include the effects of interparticle forces. Osmotic and steric interactions provide a repulsive force at close distances, while at longer ranges the van der Waals interaction leads to attraction. Two size ratios (sigma) of 0.375 and 0.577, determined experimentally to form LS (the two-dimensional NaCl analogue) and LS2 (the two-dimensional AlB2 analogue) lattices, were studied. The calculated jamming limits for RSA fall well below the minimum surface coverage necessary for stable ordering as determined by melting simulations. Uniform compression of the particles' positions, as a model of the convection and lateral capillary forces that would be experienced during solvent evaporation, allowed this critical surface coverage to be achieved, and LS lattice formation was observed for sigma = 0.375. No LS2 lattice formation was observed for sigma = 0.577 with compression. The melting coverage of the LS2 lattice far exceeds the coverage observed experimentally and so is not observed.