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Constantine KL Mueller L Huang S Abid S Lam KS Li W Leet JE 《Journal of the American Chemical Society》2002,124(25):7284-7285
Nocathiacin I (BMS-249524) is a highly cross-linked thiazolyl peptide that displays potent activity against Gram-positive bacteria, including a number of antibiotic-resistant strains. This natural product contains 10 chiral centers. NMR studies have been performed to characterize the solution structure of nocathiacin I. A uniformly 13C,15N-labeled sample was used to obtain NMR assignments. Restrained simulated annealing calculations were performed by using accurately determined NOE distance restraints. All of the chiral centers were allowed to float during the simulated annealing protocol. Two clusters of structures were obtained that satisfy the NOE restraints very well and that are reasonably consistent with vicinal J-coupling constants. Within each cluster, all 10 chiral centers are uniquely defined. The two clusters are effectively mirror images of each other: all chiral centers that have the R(S) configuration in one cluster have the S(R) configuration in the other. The single threonine residue in nocathiacin I was subsequently determined to be l-threonine by chiral capillary electrophoresis, allowing the absolute configurations of all 10 chiral centers to be defined. 相似文献
996.
5-(2-Acetamidoaryl)thianthreniumyl perchlorates reacted with potassium hydroxide in methanol at reflux giving 2,3,8,9-dibenzo-5,6-(substituted)benzo-1,4-dithio-7-azacyclonona-2,5,8-trienes 4 in 43 to 75% yields, whereas the reactions of the same compounds with sodium hydride in the absence or in the presence of dimethyl sulfate in refluxing tetrahydrofuran gave N-acetylated and N-methylated 4 in 68 to 96% and 27 to 56% yields, respectively. The mechanism of the formation of the products might be explained by a nucleophilic attack of amide ions 10, 12, and 14 at the ipso-position of the thianthrene ring. A sulfuranyl radical mechanism might be involved in these reactions. 相似文献
997.
Wai?Chee?ShiuEmail author Peter?Che?Bor?Lam Heping?Zhang 《Journal of mathematical chemistry》2005,38(4):451-466
This paper considers the k -resonance of a toroidal polyhex (or toroidal graphitoid) with a string (p, q, t) of three integers (p ≥ 2, q ≥ 2, 0 ≤ t ≤ p − 1). A toroidal polyhex G is said to be k-resonant if, for 1≤ i ≤ k, any i disjoint hexagons are mutually resonant, that is, G has a Kekulé structure (perfect matching) M such that these hexagons are M-alternating (in and off M). Characterizations for 1, 2 and 3-resonant toroidal polyhexes are given respectively in this paper.
*This work is supported by FRG, Hong Kong Baptist University; NSFC and TRAPOYT. 相似文献
998.
Man LU Jian Guo FANG Zhong Li LIU Long Min WU* National Laboratory of Applied Organic Chemistry Lanzhou University Lanzhou 《中国化学快报》2002,13(10)
Nitroxides have been found to be the intermediate in the in vivo metabolism of hydroxamic acid, a well-known biological molecule1. Structural modification of some antitumor drugs by introducing nitroxides can lower toxicity remarkably with retention or even increase of the antitumor activity2. Furthermore, the use of nitroxides as spin probes makes it possible to get information regarding the microenvironment of the reaction. 4-Substituted Hantzsch 1,4-dihydropyridines (DHP) are analogs … 相似文献
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1000.
Donald L. Barton Arthur C. Fabian Margaret M. Lam Jeffrey T. Sterbenz 《Journal of heterocyclic chemistry》1995,32(4):1397-1399
This paper reports the large scale preparation of 7-ethoxy-3-hydroxy-6-methoxy-1-methyl-4-nitroiso-quinoline (RWJ 19959, 2 ) from an available starting material, 4-ethoxy-3-methoxyphenylacetic acid ( 9 ). Previously 2 was prepared via a perchlorate salt which was found to decompose exothermically. 相似文献