Semisynthesis of Taxol (R): an improved procedure far the isolation of 10-deacetylbaccatin III

From the needles of domestic yew, (Taxus baccata), 10-deacetylbaccatin III (10-DAB) call be isolated in quantities of up to 297 mg per kg of fresh needles. Additional quantities of 10-DAB call be obtained from the extract by NaBH4 mediated reductive hydrolysis of baccatin esters. A four-step procedure converts 10-DAB into taxol in 58% overall yield

Electrochemical Reduction of Acetonylacetone in Hydrochloric Acid

The electrochemical reduction of acetonylacetone on a mercury cathode in aqueous hydrochloric acid medium has been studied. It has been shown that the electroactive species is the protonated form of acetonylacetone. The experimental results indicate -that the rate of electrochemical reduction is dependent upon the preceding chemical protonation step and/or adsorption of the electroactive species. The electrolysis at controlled potential (- 1.2 V vs. S. C. E .) yielded the following products : acetaldehyde; acetone, acetic acid, 2,5-hexanediol and 2,5-dimethyltetrahydrofuran. Possible mechanistic routes leading to the formation of these products are discussed

Stereoselektivno slobodnoradikalsko fenilsulfenilovanje neaktiviranog δ-ugljenikovog atoma

A stereoselective free radical introduction of a phenylthio group onto a nonactivated methyl group in the 8-position, adjacent to a prochiral carbon atom, was achieved by photolysis of (-)-menthyl benzenesulfenate in the presence of hexabutylditin and (1R, 3R, 4S, 8S)-9-phenylthiomenthot (4) was obtained with 91 % optical purity. High stereoselectivity of the reaction was calculated (ab initio MP2/6-3 1G**) to be the consequence of the difference in the transition state eneregies (DeltaDeltaG(#) = 5.08 kJ/mol) favouring 4 relative to (1R,3R,4S,8R)-9-phenylthiomenthoI (5). The absolute configuration of a the new chiral carbon atom was confirmed by its correlation with the corresponding menthane-3,9-diol of known stereochemistry.Fotolizom (–)-mentil-benzensulfenata u prisustvu heksabutil-dikalaja izvršeno je stereoselektivno uvođenje feniltio grupe na neaktiviranu metil grupu u δ-položaju koja je susedna prohiralnom ugljenikovom atomu i dobiven je (1R 3R, 4S, 8S)-9-feniltio-mentol (4) sa 91 % optičke čistoće. Visoka stereoselektivnost reakcije, potvrđena računom (ab initio MP2/6-21G**) posledica je razlike u energijama prelaznih stanja ΔΔG# = 5.08 kJ/mol) koja favorizuje nastajanje 4 u odnosu na (1R, 3R, 4S, 8R)-9-feniltio-mentol (5). Apsolutna konfiguracija novog hiralnog ugljenikovog atoma potvrđena je korelacijom s odgovarajućim mentan-3,9-diolom poznate stereohemije

Sequential radical addition/cyclization/β-elimination reactions. 3-exo- and 5-exo-cycloaddition reactions of 5-phenylthio-3-pentenyl and 5-phenylthio-3-pentynyl radicals

Sequential radical addition/5-exo-cyclization/β-elimination reactions, accomplished by thermally or photolytically induced decompositions of O-acyl derivatives of N-hydroxypyridine-2-thione or other thiohydroxamic esters of 6-phenylthio-4-hexenoic acid 11 in the presence of an excess of radicophilic olefins, afforded the 2-vinylcyclopentane derivatives 13 in 50-90% yields, while decompositions of thiohydroxami esters or 6-phenylthio-4-hexynoic acids 24, under the same experimental conditions and in the presence of electron deficient olefins, afforded the corresponding 2-vinylidenecyclopentane (2-exo-allenic cyclopentane) derivatives 2 (61-72%). However, when decompositions of thiohydroxamic esters 39 were carried out in boiling toluene solution without radicophilic olefins, 3-exo-cyclization took place and the corresponding 2-vinylcyclopropane derivatives 40 were obtained in 43-60% yields. © 1992

Free radical annulation of cyclopentane ring

Homoallyl radical, e.g. 2, reacts with an electron-deficient olefinic bond 1 with a new CC bond forming and arising of a 5-hexenyl radical 3 which further undergoes to 5-exo-trigonal cyclization with a cyclopentane ring (4) annulation. © 1986

Free radical carbocyclic ring reconstruction

Alkenyl radical generated by β-fragmentation of tertiary cyclohexyloxy radical with carbocyclic ring opening, possessing a suitably located olefinic double bond, undergoes to the intramolecular 5-exo-trigonal cyclization and a new carbocyclic ring was formed. © 1986

Radical cyclization reactions. Cyclopropane ring formation by 3-exo-cyclization of 5-phenylthio-3-pentenyl radicals

Free radical cyclopropane ring closure was accomplished by thermal decomposition of thiohydroxamic esters of 6-phenylthio-4-hexencic acids. When the reaction was performed in the presence of acrylonitrile; an addition/cyclization/elimination process took place and the corresponding cyclopentanecarbonitriles were obtained. © 1990

История развития математических знаний

Isosorbide is a platform chemical of considerable importance for the future replacement of fossil resource-based products. Applications as monomers and building blocks for new polymers and functional materials, new organic solvents, for medical and pharmaceutical applications, and even as fuels or fuel additives are conceivable. The conversion of isosorbide to valuable derivatives by functionalization or substitution of the hydroxyl groups is difficult because of the different configurations of the 2- and 5-positions and the resulting different reactivity and steric hindrance of the two hydroxyl groups. Although a substantial amount of work has been published using exclusively the endo or exo derivatives isomannide and isoidide, respectively, as starting material, a considerable effort is still necessary to transfer and adapt these methods for the efficient conversion of isosorbide. This Minireview deals with all aspects of isosorbide chemistry, which includes its production by catalytic processes, special properties, and chemical transformations for its utilization in biogenic polymers and other applications of interest

Challenges for chemical sciences in the 21st century

Chemistry and chemical engineering have changed very significantly in the last half century. From classical sciences they have broadened their scope into biology, medicine, physics, material science, nanotechnology, computation and advanced methods of process engineering and control. The applications of chemical compounds, materials and knowledge have also dramatically increased. The development of chemical sciences in the scientifically most advanced countries, at the end of the last century was extrapolated to the next several decades in this review and challenges for chemists and chemical engineers are described. Research, discovery and invention across the entire spectrum of activities in the chemical sciences, from fundamental molecular-level chemistry to large-scale chemical processing technology are summarized. The strong integration of chemical science and engineering into all other natural sciences, agriculture, environmental science, medicine, as well as into physics, material science and information technology is discussed. Some challenges for chemists and chemical engineers are reviewed in the following fields: i) synthesis and manufacturing of chemical products, ii) chemistry for medicine and biology, iii) new materials, iv) chemical and physical transformations of materials, v) chemistry in the solving of energy problems (generation and savings), vi) environmental chemistry: fundamental and practical challenges