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121.
122.
Polarography and chronopotentiometry are used to study the primary processes of the reduction of 2,2′-bipyridine at mercury electrodes in aqueous solutions (12 < pH < 14). Bipyridine and its first reduction product are strongly adsorbed. The reduction is preceded by a complex equilibrium between bipyridine and the cation of the supporting electrolyte (Li, Na, K, Ba). Due to the great excess of the cations, the equilibrium is shifted completely to the side of the complex, which exists as cis- and trans-isomer. The cis-form is more stable, forming a chelate, the trans-form however is more easily reducible. The kinetic and thermodynamic data of the isomerization are determined. The first reduction product reacts irreversibly giving a substance which can be reoxidized to bipyridine via a radical intermediate. 相似文献
123.
Silvia E. Braslavsky Alfred R. Holzwarth Harald Lehner Kurt Schaffner 《Helvetica chimica acta》1978,61(6):2219-2222
Freshly prepared solutions of biliverdin dimethyl ester ( 2 ) in ethanol showed fluorescence maxima at 710 and 770 nm [ΦF = 1.1. 10?4 (room temperature) and 5.0 10?4 (77 K)]. The maxima of monoprotonated 2 at 77 K were shifted to 725 and 806 nm and the quantum yield was increased to 2.6. 10?2. This acid effect was reversible by neutralization with base. When a neutral solution was kept standing in the dark at room temperature, or when an acidic solution was neutralized by base, an additional fluorescence maximum at 500 nm with a mirror image excitation spectrum with λmax = 470 nm developed, which disappeared on addition of acid and which is attributed to a chemical change of 2 . 相似文献
124.
Aldol/elimination reactions of ß-ketoamides with methyl glyoxylate result in highly selective production of Z-α,ß-unsaturated amides. An intramolecular Diels-Alder reaction of a triply activated dienophile derived from chiral dienylamine 7ZE stereospecifically affords chiral bicyclic lactam 11 at room temperature. 相似文献
125.
126.
Kasher R Balass M Scherf T Fridkin M Fuchs S Katchalski-Katzir E 《Chemistry & biology》2001,8(2):147-155
BACKGROUND: Alpha-bungarotoxin (alpha-BTX) is a highly toxic snake venom alpha-neurotoxin that binds to acetylcholine receptor (AChR) at the neuromuscular junction, and is a potent inhibitor of this receptor. We describe the design and synthesis of peptides that bind alpha-BTX with high affinity, and inhibit its interaction with AChR with an IC(50) of 2 nM. The design of these peptides was based on a lead peptide with an IC(50) of 3x10(-7) M, previously identified by us [M. Balass et al., Proc. Natl. Acad. Sci. USA 94 (1997) 6054] using a phage-display peptide library. RESULTS: Employing nuclear magnetic resonance-derived structural information [T. Scherf et al., Proc. Natl. Acad. Sci. USA 94 (1997) 6059] of the complex of alpha-BTX with the lead peptide, as well as structure-function analysis of the ligand-binding site of AChR, a systematic residue replacement of the lead peptide, one position at a time, yielded 45 different 13-mer peptides. Of these, two peptides exhibited a one order of magnitude increase in inhibitory potency in comparison to the lead peptide. The design of additional peptides, with two or three replacements, resulted in peptides that exhibited a further increase in inhibitory potency (IC(50) values of 2 nM), that is more than two orders of magnitude better than that of the original lead peptide, and better than that of any known peptide derived from AChR sequence. The high affinity peptides had a protective effect on mice against alpha-BTX lethality. CONCLUSIONS: Synthetic peptides with high affinity to alpha-BTX may be used as potential lead compounds for developing effective antidotes against alpha-BTX poisoning. Moreover, the procedure employed in this study may serve as a general approach for the design and synthesis of peptides that interact with high affinity with any desired biological target. 相似文献
127.
[reaction: see text]. A variety of heterocyclic ring systems can be prepared by subjecting N-aryl-substituted 5-amido-1,3-dioxins to Lewis acids. The reactions proceed via catalyzed retrocycloadditions to afford 2-amidoacroleins and concomitant regioselective electrophilic aromatic substitution reactions. The transformation is also successful using dioxins with amides that are within the incipient ring to afford the analogous lactams. 相似文献
128.
Ewald Sattler Harald Krautscheid Eberhard Matern Gerhard Fritz Ilona Kovcs 《无机化学与普通化学杂志》2001,627(2):186-193
Formation and Reactions of the CH2Li‐Derivatives of tBu2P–P=P(CH3)tBu2 and (Me3Si)tBuP–P=P(CH3)tBu2 With nBuLi, (Me3Si)tBuP–P=P(CH3)tBu2 ( 1 ) and tBu2P–P=P(CH3)tBu2 ( 2 ) yield (Me3Si)tBuP–P=P(CH2Li)tBu2 ( 3 ) and tBu2P–P=P(CH2Li)tBu2 ( 4 ), wich react with Me3SiCl to give (Me3Si)tBuP–P=P(CH2–SiMe3)tBu2 ( 5 ) and tBu2P–P=P(CH2–SiMe3)tBu2 ( 6 ), respectively. With tBu2P–P(SiMe3)–PtBuCl ( 7 ), compound 3 forms 5 as well as the cyclic products [H2C–P(tBu)2=P–P(tBu)–PtBu] ( 8 ) and [H2C–P(tBu)2=P–P(PtBu2)–P(tBu)] ( 9 ). Also 3 forms 8 with tBuPCl2. The cleavage of the Me3Si–P‐bond in 1 by means of C2Cl6 or N‐bromo‐succinimide yields (Cl)tBuP–P=P(CH3)tBu2 ( 10 ) or (Br)tBuP–P=P(CH3)tBu2 ( 11 ), resp. With LiP(SiMe3)2, 10 forms (Me3Si)2P–P(tBu)–P=P(CH3)tBu2 ( 12 ), and Et2P–P(tBu)–P=P(CH3)tBu2 ( 13 ) with LiPEt2. All compounds are characterized by 31P NMR Data and mass spectra; the ylide 5 and the THF adduct of 4 additionally by X‐ray structure analyses. 相似文献
129.
Reaction of the chiral lithium stannate [HC{SiMe2N[(S)–CH(Me)Ph]}3SnLi(thf)] ( 1 ) with Me3SnCl gave the corresponding distannane [HC{SiMe2N[(S)–CH(Me)Ph]}3Sn–SnMe3] ( 2 ) in good yield. Its [2,2,2]bicyclooctane‐related cage structure, comprising the trisilylsilane unit and the triamido‐tin fragment, as well as the Sn–Sn bond (2.7978(15)–2.8020(15) Å in the three crystallographically independent molecules) were established by a single crystal X‐ray structure analysis: Space proup P3, Z = 3, lattice dimensions at 293(2) K: a = 17.724(3), c = 10.597(2) Å, R = 0.0374. 相似文献
130.
Jrg Schrder Herbert Wenzel Hans‐Georg Stammler Anja Stammler Beate Neumann Harald Tschesche 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(5):593-596
The title compounds, (2S)‐N‐[5‐(4‐chlorophenyl)‐2,3‐dihydro‐6H‐1,3,4‐thiadiazin‐2‐ylidene]‐2‐[(phenylsulfonyl)amino]propanamide, C18H17ClN4O3S2, (I), (2R)‐N‐[5‐(4‐fluorophenyl)‐6H‐1,3,4‐thiadiazin‐2‐yl]‐2‐[(phenylsulfonyl)amino]propanamide, C18H17FN4O3S2, (II), and (2S)‐N‐[5‐(5‐chloro‐2‐thienyl)‐6H‐1,3,4‐thiadiazin‐2‐yl]‐2‐[(phenylsulfonyl)amino]propanamide, C16H15ClN4O3S3, (III), are potent inhibitors of matrix metalloproteinases. In all three compounds, the thiadiazine ring adopts a screw‐boat conformation. The molecules of compound (I) show a short intramolecular NAla—H?Nexo hydrogen bond [N?N 2.661 (3) Å] and are linked into a chain along the c axis by Nendo—H?Sendo and Nendo—H?OAla hydrogen bonds [N?S 3.236 (3) and N?O 3.375 (3) Å] between neighbouring molecules. In compound (II), the molecules are connected antiparallel into a chain along the a axis by Nexo—H?OAla and NAla—H?Nendo hydrogen bonds [N?O 2.907 (6) and N?N 2.911 (6) Å]. The molecules of compound (III) are dimerized antiparallel through Nexo—H?Nendo hydrogen bonds [N?N 2.956 (7) and 2.983 (7) Å]. The different hydrogen‐bonding patterns can be explained by an amido–imino tautomerism (prototropic shift) shown by different bond lengths within the 6H‐1,3,4‐thiadiazine moiety. 相似文献