Modification of Chrysotile Surface by Organosilanes: An IR-Photoacoustic Spectroscopy Study

Chrysotile and its dimethylsilyl (DMS) and dimethylphenylsilyl (DMPS) derivatives were studied by Fourier transform infrared-photoacoustic spectroscopy. In the Si-O stretching region of chrysotile a new band was revealed at 985 cm(-1), besides absorptions at 1083, 1028, and 947 cm(-1). The Si-O stretching frequencies did not undergo major changes in the DMS derivative, but the 985- and 1028-cm(-1) peaks were undetected in DMPS due to the HCl attack on chrysotile tetrahedral sheets. Similar effects were observed in the region 900-400 cm(-1), by a decrease in intensities of the 600- and 642-cm(-1) Mg-OH libration modes in the DMPS spectrum, indicating also a HCl attack on the octahedral sheet. The Si-C band at 800 cm(-1) in the spectra of both DMS and DMPS was accompanied by minor components. DMPS showed a strong peak at 813 cm(-1) assigned to a Si-phenyl vibration. A sharp peak at 1263 cm(-1) in the DMS spectrum was ascribed to a diagnostic C-H bending mode of the dimethylsilyl groups in DMS. The complex bands around 1413 cm(-1) in DMS were attributed to CH(3) deformation vibrations and that at 1466 cm(-1) in DMPS to phenyl groups. In DMPS a distinct peak at 1593 cm(-1) was attributed to a Si-phenyl vibration. In the region 3700-2500 cm(-1) absorptions at 2964, 2931, and 2907 cm(-1) in DMS were ascribed to C-H-stretching vibrations of dimethylsilyl groups, while a strong peak at 2919 cm(-1) in the DMPS spectrum was attributed to a Si-C(6)H(5) mode. Copyright 2001 Academic Press.     

Distribution of Primary and Specialized Metabolites in <em>Nigella sativa</em> Seeds, a Spice with Vast Traditional and Historical Uses

Black cumin (Nigella sativa L., Ranunculaceae) is an annual herb commonly used in the Middle East, India and nowadays gaining worldwide acceptance. Historical and traditional uses are extensively documented in ancient texts and historical documents. Black cumin seeds and oil are commonly used as a traditional tonic and remedy for many ailments as well as in confectionery and bakery. Little is known however about the mechanisms that allow the accumulation and localization of its active components in the seed. Chemical and anatomical evidence indicates the presence of active compounds in seed coats. Seed volatiles consist largely of olefinic and oxygenated monoterpenes, mainly p-cymene, thymohydroquinone, thymoquinone, γ-terpinene and α-thujene, with lower levels of sesquiterpenes, mainly longifolene. Monoterpene composition changes during seed maturation. γ-Terpinene and α-thujene are the major monoterpenes accumulated in immature seeds, and the former is gradually replaced by p-cymene, carvacrol, thymo-hydroquinone and thymoquinone upon seed development. These compounds, as well as the indazole alkaloids nigellidine and nigellicine, are almost exclusively accumulated in the seed coat. In contrast, organic and amino acids are primarily accumulated in the inner seed tissues. Sugars and sugar alcohols, as well as the amino alkaloid dopamine and the saponin α-hederin accumulate both in the seed coats and the inner seed tissues at different ratios. Chemical analyses shed light to the ample traditional and historical uses of this plant.     

(S,S)-(+)-Pseudoephedrine as chiral auxiliary in asymmetric conjugate addition and tandem conjugate addition/alpha-alkylation reactions

Organolithium reagents undergo highly regio- and diastereoselective 1,4-addition to (S,S)-(+)-pseudoephedrine enamides furnishing the corresponding beta-alkyl-substituted adducts in excellent yields and diastereoselectivities. In addition, the intermediate lithium enolates generated after the conjugate addition step undergo a highly diastereoselective alkylation reaction, furnishing alpha,beta-dialkyl-substituted amides in high yields. The obtained adducts have been converted into chiral nonracemic beta-alkyl- and alpha,beta-dialkyl-substituted carboxylic acids and gamma-alkyl- and beta,gamma-dialkyl-substituted alcohols using very simple and high-yielding procedures.     

Direct asymmetric intermolecular aldol reactions catalyzed by amino acids and small peptides

In nature there are at least nineteen different acyclic amino acids that act as the building blocks of polypeptides and proteins with different functions. Here we report that alpha-amino acids, beta-amino acids, and chiral amines containing primary amine functions catalyze direct asymmetric intermolecular aldol reactions with high enantioselectivities. Moreover, the amino acids can be combined into highly modular natural and unusual small peptides that also catalyze direct asymmetric intermolecular aldol reactions with high stereoselectivities, to furnish the corresponding aldol products with up to >99 % ee. Simple amino acids and small peptides can thus catalyze asymmetric aldol reactions with stereoselectivities matching those of natural enzymes that have evolved over billions of years. A small amount of water accelerates the asymmetric aldol reactions catalyzed by amino acids and small peptides, and also increases their stereoselectivities. Notably, small peptides and amino acid tetrazoles were able to catalyze direct asymmetric aldol reactions with high enantioselectivities in water, while the parent amino acids, in stark contrast, furnished nearly racemic products. These results suggest that the prebiotic oligomerization of amino acids to peptides may plausibly have been a link in the evolution of the homochirality of sugars. The mechanism and stereochemistry of the reactions are also discussed.     

Tandem asymmetric conjugate addition/alpha-alkylation using (S,S)-(+)-pseudoephedrine as chiral auxiliary

Alpha,beta-unsaturated amides derived from the chiral amino alcohol (S,S)-(+)-pseudoephedrine undergo a very clean and diastereoselective tandem conjugate addition/alpha-alkylation reaction. Excellent results have been achieved using a wide range of differently substituted conjugate acceptors, organolithium reagents, and alkyl halides. The chiral auxiliary could be easily removed from the obtained adducts by reduction, furnishing chiral nonracemic alpha,beta-branched alcohols in a very easy and efficient way. [reaction: see text]     

Enantioselective Synthesis of Tertiary Propargylic Alcohols under N‐Heterocyclic Carbene Catalysis

A straightforward procedure to carry out the enantioselective benzoin reaction between aldehydes and ynones by employing a chiral N‐heterocyclic carbene (NHC) as catalyst was developed. Under the optimized reaction conditions, these ynones undergo a clean and selective 1,2‐addition with the catalytically generated Breslow intermediate, not observing any byproduct arising from competitive Stetter‐type reactivity. This procedure allows the preparation of tertiary alkynyl carbinols as highly enantioenriched materials, which have the remarkable potential to be used as chiral building blocks in organic synthesis.     

Ideal intrinsic extensions with connections to PI-rings

In this paper, we extend various classical results by Armendariz and Steinberg, Fisher, Kaplansky, Martindale, Posner, and Rowen on semiprime PI-rings. We do this by introducing several new generalizations of the class of semiprime PI-rings. For these new classes, some structure theorems are obtained, and connections to arbitrary semiprime rings are made (e.g., a semiprime ring has a largest essentially closed ideal from some of these classes). Numerous examples are provided to illustrate and delimit our results.     

Analog modeling of Worm-Like Chain molecules using macroscopic beads-on-a-string

This paper describes an empirical model of polymer dynamics, based on the agitation of millimeter-sized polymeric beads. Although the interactions between the particles in the macroscopic model and those between the monomers of molecular-scale polymers are fundamentally different, both systems follow the Worm-Like Chain theory.