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1.
We are interested in improving the Varshamov bound for finite values of length n and minimum distance d. We employ a counting lemma to this end which we find particularly useful in relation to Varshamov graphs. Since a Varshamov
graph consists of components corresponding to low weight vectors in the cosets of a code it is a useful tool when trying to
improve the estimates involved in the Varshamov bound. We consider how the graph can be iteratively constructed and using
our observations are able to achieve a reduction in the over-counting which occurs. This tightens the lower bound for any
choice of parameters n, k, d or q and is not dependent on information such as the weight distribution of a code.
This work is taken from the author’s thesis [10] 相似文献
2.
Liu JF Ye P Zhang B Bi G Sargent K Yu L Yohannes D Baldino CM 《The Journal of organic chemistry》2005,70(16):6339-6345
A microwave-promoted three-component one-pot reaction has been developed to provide access to the core pyrazino[2,1-b]quinazoline-3,6-dione (1) scaffold, which is common to several families of alkaloids with significant biological activities. By adapting this synthetic strategy through the use of selected Boc-amino acids and amino acid esters, we have accomplished highly efficient and concise total syntheses of glyantrypine (2), fumiquinazoline F (3), and fiscalin B (5), achieving overall yields of 55, 39, and 20%, respectively. 相似文献
3.
A procedure for the selective deprotection of p-methoxybenzyl ethers using catalytic amounts of DDQ and of sodium nitrite, with oxygen as the terminal oxidant, is reported. 相似文献
4.
Dr. Luke D. Elliott Dr. Jonathan P. Knowles Dr. Paul J. Koovits Katie G. Maskill Michael J. Ralph Dr. Guillaume Lejeune Lee J. Edwards Richard I. Robinson Ian R. Clemens Brian Cox David D. Pascoe Guido Koch Martin Eberle Malcolm B. Berry Prof. Kevin I. Booker‐Milburn 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(46):15226-15232
The use of flow photochemistry and its apparent superiority over batch has been reported by a number of groups in recent years. To rigorously determine whether flow does indeed have an advantage over batch, a broad range of synthetic photochemical transformations were optimized in both reactor modes and their yields and productivities compared. Surprisingly, yields were essentially identical in all comparative cases. Even more revealing was the observation that the productivity of flow reactors varied very little to that of their batch counterparts when the key reaction parameters were matched. Those with a single layer of fluorinated ethylene propylene (FEP) had an average productivity 20 % lower than that of batch, whereas three‐layer reactors were 20 % more productive. Finally, the utility of flow chemistry was demonstrated in the scale‐up of the ring‐opening reaction of a potentially explosive [1.1.1] propellane with butane‐2,3‐dione. 相似文献
5.
Gerea AL Branscum KM King JB You J Powell DR Miller AN Spear JR Cichewicz RH 《Tetrahedron letters》2012,53(32):4202-4205
A collection of fungal isolates was obtained from a complex microbial mat, which occupied an iron-rich freshwater spring that feeds into Clear Creek, Golden, Colorado, USA. Two of the fungal isolates, a Glomeromycete (possibly Entrophospora sp.) and a Dothideomycete (possibly Phaeosphaeria sp.), were investigated for bioactive secondary metabolites. In total, six new compounds consisting of clearanols A-E (5, 6, 10-12) and disulochrin (7) were purified and their structures were determined. Disulochrin exhibited modest antibacterial activity against methicillin-resistant Staphylococcus aureus, whereas clearanol C showed weak inhibitory activity against Candida albicans biofilm formation. 相似文献
6.
The purpose of this study was to investigate the effect of parameter changes that can potentially lead to unreliable measurements in fat quantification. Chemical shift imaging was performed using spoiled gradient echo sequences with systematic variations in the following: two-dimensional/three-dimensional sequence, number of echoes, delta echo time, fractional echo factor, slice thickness, repetition time, flip angle, bandwidth, matrix size, flow compensation and field strength. Results indicated no significant (or significant but small) changes in fat fraction with parameter. The significant changes can be attributed to the known effects of T1 bias and two forms of noise bias. 相似文献
7.
8.
The first examples of the directed, boron-mediated aldol reaction between different ketones are presented. Transformation of a variety of ketones to their corresponding boron enolates with Chx2BCl/Et3N, followed by reaction with acceptor ketones in diethyl ether, and oxidation of the resultant boron aldolate (H2O2, MeOH/pH 7 buffer), provided the aldol addition products. The reaction was most facile when cyclic ketones were used, with the highest yields obtained for the reaction of boron enolates with cyclohexanone as the acceptor. 相似文献
9.
10.
Sylvain Laclef Katie Anderson Andrew J.P. White Anthony G.M. Barrett 《Tetrahedron letters》2012,53(2):225-227
The total synthesis of amorfrutin A, a prenyl bibenzyl natural product has been achieved in five steps from 2,2,6-trimethyl-4H-1,3-dioxin-4-one. The key step of the synthesis is an efficient palladium(0)-catalyzed decarboxylative prenylation migration and aromatization sequence. 相似文献