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971.
A C35 pentacyclic triterpenoid of the hopane series containing a thiophene ring, 30-(2′-methylene thienyl)-17β(H),21β(H)-hopane 1, has been confirmed in immature sediments by synthesis, suggesting incorporation of bacterial sulfur into geological organic matter at early stages of sedimentation. 相似文献
972.
Rudolf Albrecht 《Monatshefte für Mathematik》1958,62(2):146-162
Ohne Zusammenfassung 相似文献
973.
Ohne ZusammenfassungHerrnJosef Lense zum 70. Geburtstag am 28. Oktober 1960 gewidmet 相似文献
974.
975.
Albrecht Pietsch 《Mathematische Annalen》1960,140(2):153-164
Ohne Zusammenfassung 相似文献
976.
977.
D. Albrecht M. Csatlós J. Erö Z. Fodor I. Hernyes B.A. Khomenko N.N. Khovanskij P. Koncz Z.V. Krumstein Yu.P. Merekov V.I. Petrukhin Z. Seres L. Végh 《Nuclear Physics A》1980,338(2):477-494
The 6Li(p, pd)4He reaction was investigated at 670 MeV by a coincidence experiment with a large-angle scattering geometry. The energy sharing and angular correlation of the reaction products were measured and the momentum distribution of the recoil nucleus was determined for transitions leading to residual nuclei in the ground and excited states. Results were analysed in terms of a simplified distorted-wave impulse approximation using cluster-model and three-body wave functions. The observed momentum distribution of the p-n pair in the p-shell of 6Li is in agreement with three-body calculations, while the spectroscopic factor is larger than predicted by theory. Transitions to breakup states of the α-particle also have the characteristics of quasi-free scattering on deuteron clusters. 相似文献
978.
Melzig L Metzger A Knochel P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(10):2948-2956
A variety of unsaturated thioethers have been subjected to cross‐coupling reactions with functionalized zinc reagents in the presence of a transition‐metal catalyst. Three different catalytic systems based on Pd(OAc)2 or [Ni(acac)2] and the ligands S‐Phos or DPE‐Phos gave the best results. N‐Heterocyclic thioethers based on a pyridine, pyrimidine, pyrazine, pyridazine, triazine, benzothiazole, benzoxazole, pyrrole, or quinazoline ring, as well as thiomethylacetylenes, serve as electrophiles in this cross‐coupling reaction. Aryl‐, heteroaryl‐, benzylic, and alkylzinc halides with sensitive functionalities, such as ester, nitrile, or ketone groups react at ambient temperature with unsaturated thioethers using a Ni catalyst. The corresponding Pd‐catalyzed reactions require slightly higher temperatures. Large‐scale cross‐coupling experiments (10–20 mmol) with N‐heterocycles are also reported. 相似文献
979.
A general methodology for the synthesis of trisubstituted, optically active thiophenes by an organocatalytic one-pot reaction cascade is presented. The target products are synthesized in good yields (up to 92%) and with excellent enantioselectivities (up to 98% ee). Importantly, based on practical and easily available starting materials, the presented methodology can be conducted under mild reaction conditions. To further elucidate the generality, the synthesis of optically active thienoindoles, as well as selenophenes, is also demonstrated. 相似文献
980.
Duangdee N Harnying W Rulli G Neudörfl JM Gröger H Berkessel A 《Journal of the American Chemical Society》2012,134(27):11196-11205
Aldol reactions with trifluoroacetophenones as acceptors yield chiral α-aryl, α-trifluoromethyl tertiary alcohols, valuable intermediates in organic synthesis. Of the various organocatalysts examined, Singh's catalyst [(2S)-N-[(1S)-1-hydroxydiphenylmethyl-3-methylbutyl]-2-pyrrolidinecarboxamide] was found to efficiently promote this organocatalytic transformation in a highly enantioselective manner. Detailed reaction monitoring ((19)F-NMR, HPLC) showed that, up to full conversion, the catalytic transformation proceeds under kinetic control and affords up to 95% ee in a time-independent manner. At longer reaction times, the catalyst effects racemization. For the product aldols, even weak acids (such as ammonium chloride) or protic solvents, can induce racemization, too. Thus, acid-free workup, at carefully chosen reaction time, is crucial for the isolation of the aldols in high (and stable) enantiomeric purity. As evidenced by (19)F-NMR, X-ray structural analysis, and independent synthesis of a stable intramolecular variant, Singh's catalyst reversibly forms a catalytically inactive ("parasitic") intermediate, namely a N,O-hemiacetal with trifluoroacetophenones. X-ray crystallography also allowed the determination of the product aldols' absolute configuration (S). 相似文献