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Mahiuddin Baidya Grygoriy Y. Remennikov Dr. Peter Mayer Dr. Herbert Mayr Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(4):1365-1371
TiCl4‐induced Baylis–Hillman reactions of α,β‐unsaturated carbonyl compounds with aldehydes yield the (Z)‐2‐(chloromethyl)vinyl carbonyl compounds 5 , which react with 1,4‐diazabicyclo[2.2.2]octane (DABCO), quinuclidine, and pyridines to give the allylammonium ions 6 . Their combination with less than one equivalent of the potassium salts of stabilized carbanions (e.g. malonate) yields methylene derivatives 8 under kinetically controlled conditions (SN2’ reactions). When more than one equivalent of the carbanions is used, a second SN2’ reaction converts 8 into their thermodynamically more stable allyl isomers 9 . The second‐order rate constants for the reactions of 6 with carbanions have been determined photometrically in DMSO. With these rate constants and the previously reported nucleophile‐specific parameters N and s for the stabilized carbanions, the correlation log k (20 °C)=s(N + E) allowed us to calculate the electrophilicity parameters E for the allylammonium ions 6 (?19<E <?18). The kinetic data indicate the SN2’ reactions to proceed via an addition–elimination mechanism with a rate‐determining addition step. 相似文献
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Susanta Kumar Manna Arabinda Mandal Suresh Kumar Mondal Jayanta K. Ray Mahiuddin Baidya 《合成通讯》2013,43(5):625-634
A novel synthesis of 3,4-fused furans (both tricyclic and bicyclic) through platinum-catalyzed cyclization of 3-(2-formylcycloalkenyl)-acrylic amides 2 in methanol is described (up to 90% yield). Tricyclic 3,4-fused dihydrofuran derivatives were also obtained via reductive cyclization of 2. The substrates 2 were obtained from β- bromovinyl aldehydes by a Pd-catalyzed Heck reaction. 相似文献
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Significant Structural Differences between Transient Amyloid‐β Oligomers and Less‐Toxic Fibrils in Regions Known To Harbor Familial Alzheimer′s Mutations
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Bidyut Sarkar Dr. Venus Singh Mithu Bappaditya Chandra Arghya Mandal Muralidharan Chandrakesan Debanjan Bhowmik Prof. Perunthiruthy K. Madhu Prof. Sudipta Maiti 《Angewandte Chemie (International ed. in English)》2014,53(27):6888-6892
Small oligomers of the amyloid β (Aβ) peptide, rather than the monomers or the fibrils, are suspected to initiate Alzheimer′s disease (AD). However, their low concentration and transient nature under physiological conditions have made structural investigations difficult. A method for addressing such problems has been developed by combining rapid fluorescence techniques with slower two‐dimensional solid‐state NMR methods. The smallest Aβ40 oligomers that demonstrate a potential sign of toxicity, namely, an enhanced affinity for cell membranes, were thus probed. The two hydrophobic regions (residues 10–21 and 30–40) have already attained the conformation that is observed in the fibrils. However, the turn region (residues 22–29) and the N‐terminal tail (residues 1–9) are strikingly different. Notably, ten of eleven known Aβ mutants that are linked to familial AD map to these two regions. Our results provide potential structural cues for AD therapeutics and also suggest a general method for determining transient protein structures. 相似文献