Cyclopentane ring formation in the cycloaddition reaction of 3-alkenyl radicals to radicophilic olefins

Regioselective additions of 3-alkenyl radicals (10 and 20) to electron deficient olefinic bonds (7 and 17), followed by intramolecular 5- exo-cyclization and repeated addition to radicophilic double bonds were investigated and combined into a sequence of reactions for the preparation of cyclopentane derivatives (9 and 19). High regioselectivity of 3-alkenyl radical (10) addition to the electron deficient olefinic bond (7) as well as of 5-exo -cyclization of intermediary 5-hexenyl type radical (11), was achieved. The fate of intermediary cyclopentylmethyl radicals (22 and 27) depends on the nature of substituents attached to radical carbon. In case of alkyl groups (22), the nucleophilic reactivity of radical intermediates was increased and an intermolecular addition reaction occurred with the formation of a new C-C bond. However, in case of aryl groups, mesomeric stabilization of the radical (27) suppress the addition reaction to the radicophilic olefinic bond, even when the latter was used in excess, in these cases only hydrogen abstraction takes place. © 1990

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

Radical annulation methodology. 2-Vinylcyclopentane derivative formation by a 3 + 2 cycloaddition reaction

By thermally or photolytically induced decomposition of O-acyl derivatives of N-hydroxypyridine-2-thione or other thiohydroxamic esters, in the presence of an excess of electron deficient olefins, 2-vinylcyclopentane derivatives were obtained. This sequence of addition/cyclization/elimination reaction is mediated by a phenylthio radical. © 1990

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