Synthesis,characterization, and X-ray crystallographic structure of 1,3-dimethyl-2(3H)-imidazoleselone

The reaction of elemental Se with 1,3-dimethylimidazolium iodide in methanolic K₂CO₃ yields 1,3-dimethyl-2(3H)-imidazoleselone for which three addition products, two with bromine and one with iodomethane, have been synthesized and for which X-ray crystallographic analysis shows the structure to consist of a selenium-substituted planar heterocyclic ring with bond distances and angles significantly different from those noted for the previously reported sulfur analog [1,3-dimethyl-2(3H)-imidazolethione, dmit]. Crystal data: C₅H₈N₂Se, space group C mcm, M = 175.03, a = 8.625(3), b = 11.447(6), c = 6.900(4) Å, V = 681.24 ų, Z = 4, D_c = 1.707 g cm⁻³, D₀ = 1.68 g cm⁻³, λ = 0.71073 Å (Mo-K_α), μ = 5.35 mm⁻¹, R = 0.034, and R_w = 0.031.     

Synthesis of Enantiopure N-tert-Butoxycarbonyl-2-aminocycloalkanones

A route to enantiomerically pure N-tert-butoxycarbonyl-2-aminocycloalkanones (ring size: 5-8 membered) from the corresponding cycloalkene oxides is described. The procedure involves (1) aminolysis with (S)-α-methylbenzylamine/Me₃A1 and chromatographic separation of diastereomers, (2) hydrogenolysis to afford the trans-2-aminocycloalkanols, (3) tert-butoxycarbonyl (Boc) protection, and (4) PCC oxidation.     

Kinetics and mechanistic studies of anticarcinogenic bisperoxovanadium(V) compounds: ligand substitution reactions at physiological pH and relevance to DNA interactions

Bisperoxovanadium(V) compounds with bidentate ligands have shown tumor growth inhibition by cleaving DNA. The kinetics and mechanisms of ligand substitution reactions of two bisperoxovanadium(V) compounds [VO(O(2))(2)(bpy)](-) (bpVbpy) and [VO(O(2))(2)(phen)](-) (bpVphen) with entering ligands picolinic acid (pic) and dipicolinic acid (dipic) at physiological pH are reported, and its relevance to their DNA-cleavage activities are discussed. The products of the ligand substitution reactions with pic and dipic are the monoperoxo complexes [VO(O(2))(pic)(2)](-) and [VO(O(2))(dipic)(H(2)O)](-), respectively. (51)V NMR experiments indicate that bpVphen is substantially more inert in aqueous solution than bpVbpy. As a result, bpVbpy is more prone to ligand substitution and subsequent conversion to monoperoxo species. The rate of reaction for bpVbpy was faster than that of bpVphen by an order of magnitude, indicating that the ancillary ligand plays an important role in ligand substitution reactions. The ligand substitution reactions of bpVbpy feature first-order dependence on both [pic](T) and [dipic](T) whereas the substitution kinetics of bpVphen feature saturation behavior with dipic. The substitution reactions of both bpVbpy and bpVphen with pic showed first-order dependence on [H(+)] whereas no acid dependence was observed for the reactions with dipic. Hydrogen peroxide was determined to be a competitive inhibitor with respect to dipic. The ligand substitution reaction mechanisms and the rate laws consistent with these results are presented. The substitution reactions with pic and dipic proceed through different mechanisms; the substitution reactions with dipic proceed via solvolysis as the first step in the mechanisms whereas the reactions with pic bypass solvolysis to go through a mixed ligand monoperoxo vanadium intermediate.     

Strobovideolaryngoscopy in professional voice users: Results and clinical value

Evaluation of the vibratory margin of the vocal fold is essential for accurate diagnosis and treatment of voice patients. Traditionally, physicians have performed this evaluation using their subjective impressions of the sound of the voice in conjunction with laryngoscopy under continuous light. Strobovideolaryngoscopy is a valuable addition to the diagnostic armementarium. It allows the physician to perform a proper physical examination of the vibratory margin. A review of 486 strobovideolaryngoscopy procedures in 343 voice patients during a 2-year period reveals that stroboscopic information influenced diagnosis or treatment in approximately one-third of the entities diagnosed.     

On the order of tandem queues

We consider the optimal order of servers in a tandem queueing system withm stages, an unlimited supply of customers in front of the first stage, and a service buffer of size 1 but no intermediate storage buffers between the first and second stages. Service times depend on the servers but not the customers, and the blocking mechanism at the first two stages is manufacturing blocking. Using a new characterization of reversed hazard rate order, we show that if the service times for two servers are comparable in the reversed hazard rate sense, then the departure process is stochastically earlier if the slower server is first and the faster server is second than if the reverse is true. This strengthens earlier results that considered individual departure times marginally. We show similar results for the last two stages and for other blocking mechanisms. We also show that although individual departure times for a system with servers in a given order are stochastically identical to those when the order of servers is reversed, this reversibility property does not hold for the entire departure process.     

Bayesian Ensemble Trees (BET) for Clustering and Prediction in Heterogeneous Data

We propose a novel “tree-averaging” model that uses the ensemble of classification and regression trees (CART). Each constituent tree is estimated with a subset of similar data. We treat this grouping of subsets as Bayesian ensemble trees (BET) and model them as a Dirichlet process. We show that BET determines the optimal number of trees by adapting to the data heterogeneity. Compared with the other ensemble methods, BET requires much fewer trees and shows equivalent prediction accuracy using weighted averaging. Moreover, each tree in BET provides variable selection criterion and interpretation for each subset. We developed an efficient estimating procedure with improved estimation strategies in both CART and mixture models. We demonstrate these advantages of BET with simulations and illustrate the approach with a real-world data example involving regression of lung function measurements obtained from patients with cystic fibrosis. Supplementary materials for this article are available online.     

Molecular structures of M(Bu(t))3 (M = Al, Ga, In) using gas-phase electron diffraction and ab initio calculations: experimental and computational evidence for charge-transfer processes leading to photodissociation

The gas-phase structures of AI(Bu')3 and Ga(Bu')3 have been investigated by electron diffraction and are shown to consist of monomeric units with very slightly pyramidal geometries. Salient structural parameters (r(hl)) include r(A1-C) = 2.008(2) A and r(Ga-C) = 2.032(2) A. For both compounds the ligand orientations and geometries are controlled by interligand interactions. The structures of M(Bu(t))3 (M = Al, Ga, In) have been calculated ab initio and those for the aluminium and gallium derivatives are in good agreement with the electron-diffraction structures. Comparison of the ab initio calculated structure of In(Bu')3 with those of Al(Bu(t))3 and Ga(Bu(t))3 suggests that the significantly different photochemistry exhibited by the former does not result from structural factors. In fact the compounds undergo a charge-transfer process in the UV region, with the wavelength required calculated to be slightly longer for the indium compound than for the other two.