Hans J. Reich

Bio: Hans J. Reich is an academic researcher from University of Wisconsin-Madison. The author has contributed to research in topics: Lithium & Silylation. The author has an hindex of 51, co-authored 193 publications receiving 8378 citations. Previous affiliations of Hans J. Reich include University of California, Los Angeles.

Organoselenium chemistry. Conversion of ketones to enones by selenoxide syn elimination

Abstract: The scope and limitation of the transformation of ketones to enones by selenenylation followed by selenoxide elimi- nation have been examined. Several procedures for the preparation of a-phenylseleno ketones have been developed. The most useful are direct selenenylation of ketone enolates using PhSeBr and the reaction of enol acetates with electrophilic selenium species such as benzeneselenenyl trifluoroacetate. Several oxidants (ozone, hydrogen peroxide, sodium metaperiodate) and reaction conditions are described to allow optimization of the yield obtained in the transformation of a-phenylseleno ketones to enones. The reaction is quite general for acyclic carbonyl compounds and for tertiary selenides. Difficulties in achieving high yields may be anticipated when very strained double bonds are introduced, when the a-phenylseleno ketone is cyclic and has an a-hydrogen, or when the product is extremely reactive. Qualitative mechanistic studies have revealed two types of side reactions: (1) Pummerer-like transformations to give a-diketones and (2) reactions between the enolate or enol of a-phenyl- selenino ketones and selenenylating species formed during the disproportionation of benzeneselenenic acid. Reaction condi- tions which minimize these side reactions have been developed. The utility of benzeneseleninyl chloride as a seleninylating agent has been explored. One pot transformations of ketones to enones using this reagent can be achieved in satisfactory yield, but the procedure is prone to side reactions because of the sensitivity of the selenoxide function. The many synthetic transformations originating from a,P-unsaturated carbonyl compounds have made their prep- aration a long standing important synthetic problem. The most straightforward method is the dehydrogenation of car- bonyl compounds. There are a number of methods for per- forming this the most important of which is the a-bromination-dehydrobromination method.' Orienta- tional control is difficult to achieve in direct bromination of ketones, but Stotter and HillId have recently shown that bromination of cyclohexanone enolates can be carried out in high yield, and also that dehydrobromination can be per- formed without loss of regiospecificity. Isomerization of a- bromo ketones under conditions of the debrominations has been frequently reported,Ie however, particularly for bro- mides of 0-dicarbonyl compounds.If,g The vigorous reaction conditions (frequently temperatures in excess of 120') also severely limit this method because of the sensitivity of many enones. Direct dehydrogenations can be performed by a number of reagents, including selenium dioxide,2a%b dichlorodicyan- oquinone,2c periodic acid,2d oxygen in the presence of tran- sition metal catalysts,2e and pyridine N-oxide-acetic anhyd- ride.2f The first two methods have been studied in great de- tail, and some excellent procedures have been developed, but yields vary greatly, and effective control of regioselecti- vity is frequently a problem. The discovery by Jones, Mundy, and Whitehouse3 that selenoxides undergo clean syn elimination to form olefins at or below room temperature suggested that this reaction may offer a solution to the problem discussed above. We re- port here full details of our ~ork~~-~ on the conversion of ketones to enones using the selenoxide elimination (eq 1). have explored the reaction for the dehydrogenation of ketone^,^^^^^^^,^^ e~ters,~~,~~ lac- tone~,~~,'~ and nitriles.8 Selenium stabilized anions9 have been used for formation of new carbon-carbon bonds, with subsequent selenoxide elimination to give aJ-unsaturated e~ters,~~,"~ olefin^,^^,^' allyl alcohol^,^^^,^^^ and dienes.4d Sulfoxide eliminations have also been explored for the in- troduction of unsaturati~n.~'~ ~~~'~

Why Nature Chose Selenium

Abstract: The authors were asked by the Editors of ACS Chemical Biology to write an article titled “Why Nature Chose Selenium” for the occasion of the upcoming bicentennial of the discovery of selenium by the Swedish chemist Jons Jacob Berzelius in 1817 and styled after the famous work of Frank Westheimer on the biological chemistry of phosphate [Westheimer, F. H. (1987) Why Nature Chose Phosphates, Science 235, 1173–1178]. This work gives a history of the important discoveries of the biological processes that selenium participates in, and a point-by-point comparison of the chemistry of selenium with the atom it replaces in biology, sulfur. This analysis shows that redox chemistry is the largest chemical difference between the two chalcogens. This difference is very large for both one-electron and two-electron redox reactions. Much of this difference is due to the inability of selenium to form π bonds of all types. The outer valence electrons of selenium are also more loosely held than those of sulfur. As a result,...

Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Spectra of Steroids

Abstract: The natural abundance ^(13)C resonance spectra of a variety of sterols and steroidal hormones have been determined at 15.1 MHz. The chemical shifts of the carbons in these substances were found to span on the order of 200 ppm and for most steroids with the aid of complete proton decoupling it was possible to resolve all of the carbon resonances one from the other. It has also been possible by using specific single-frequency and off-resonance proton decoupling, hydroxyl acetylation effects on chemical shifts, deuteration, and substituent influences in analogous compounds to make self-consistent and unambiguous assignments of nearly all of the resonances encountered. The carbon resonances are in general far more informative than proton resonances for structural analysis of steroids.

Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Chemical Shifts in Acyclic and Alicyclic Alcohols

Abstract: The chemical shifts of ^(13)C in a variety of acyclic and alicyclic alcohols have been determined by high resolution nmr spectroscopy with the aid of proton decoupling It has been found that there are rather good linear relationships between carbon chemical shifts in alcohols and analogously constituted hydrocarbons, wherein a methyl group replaces the hydroxyl group The linear correlation coefficients for relationships of the general type δ_CROH = Aδ_CRCHa + B are better than 098 for shifts of corresponding ɑ (directly attached hydroxyl), β, y, and δ carbons for a variety of primary, secondary, and tertiary acyclic alcohols and cyclohexanols carrying both axial and equatorial substituents The shifts of the carbons of a number of cycloalkanols have been investigated in hope of providing information about conformations in medium-sized ring compounds

Organoselenium and Organotellurium Compounds: Toxicology and Pharmacology

Abstract: The organoselenium and organotellurium compounds have been described as promising pharmacological agents in view of their unique biological properties. Glutathione peroxidase mimic, antioxidant activity and thioredoxin reductase inhibition are some of the properties reviewed here. On the other hand, little is known about the molecular toxicological effects of organoselenium and organotellurium compounds. Most of our knowledge arose from research on inorganic selenium and tellurium. However, the ability to oxidize sulfhydryl groups from biological molecules can be involved both in their pharmacological properties and in their toxicological effects. In fact, exposition to high doses of organoselenium or to low doses of organotellurium causes the depletion of endogenous reduced glutathione in a variety of tissues. Thus, the design of compounds that cause low depletion of glutathione and react with specific targeted proteins, controlling specific metabolic pathways, will represent an important progress in understanding the field of organochalcogen compounds. Furthermore, the development of new organochalcogens with higher thiol-peroxidase activity that can use other non-toxic thiol reducing agents, such as N-acetylcysteine instead of glutathione, will permit the investigation of the co-administration of organochalcogens and thiols as a formulation for antioxidant therapy.

Aromatic rings in chemical and biological recognition: energetics and structures.

Abstract: This review describes a multidimensional treatment of molecular recognition phenomena involving aromatic rings in chemical and biological systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chemistry, biological affinity assays and biostructural analysis, data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S⋅⋅⋅aromatic, cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramolecular systems, and should inspire further utilization of interactions with aromatic rings to control the stereochemical outcome of synthetic transformations.