L isomerization

Their studies revealed that acylation occurs preferentially at N(l) leading to a 40% yield of l-(ethoxycarbonyl)-7-methyl-l/7-pyrrolotetrazole 38 along with traces (4.6% yield) of l,5-bis(ethoxycarbonyl)-7-methyl-l//-pyrrolotetra-zole 39. The 1/7-pyrrolotetrazole structures of 38 and 39 were established by comparison of their 13C NMR spectra with a model 1//-isomeric system 40. Additional support for the l//-isomeric system came from 13C NMR spectral comparisons with pyrrolotetrazoles 41 and 42 that are representatives of the 2H- and 3//-systems. These studies showed that the 13C NMR spectral data presented earlier by Dulcere and co-workers <1990JOC571> in a previous acylation study of the monoanion 37 was not compatible with what was believed to be the 5//-pyrrolotetrazole 43. 

The structural regularity in such polymers gives rise to optical (D-L isomerism) and tacticity of polymers and geometrical cross-trans isomerism. 

Optical Isomerism of (D-L-Isomerism) and Tacticity of Polymers Optical isomerism has its origin in the way different substituents occupy positions on an asymmetric carbon atom in a polymer molecule. For example, polyethylene molecule has fully saturated carbon atoms as shown in the following chemical formula ... 

EC50 (48-h) for Dapbniapulex680 pg/L (isomeric mixture, Johnon and Finley, 1980)... 

ECso (48-h) for Daphnia pulexQSO pg/L (isomeric mixture, Johnon and Finley, 1980 LCso (96-h) values of 1.66, 1.52, and 1.10 mg/L for Poecilia reticulata, Brachydanio rerio, and Paracheirodon axelrodi, respectively (Oliverira-Filho and Paumgarten, 1997) for isomeric mixture cutthroat trout 9 pg/L, rainbow trout 18 pg/L, goldfish 348 pg/L, fathead minnow 125 pg/L, channel catfish 105 pg/L, bluegill sunfish 67 pg/L, largemouth bass 41 pg/L (Johnson and Finley). 

Isomerization with no incorporation of L Isomerization with incorporation of L Retention with incorporation of L Retention without incorporation of L... 

Amphetamine and methamphetamine occur as structural isomers and stereoisomers. Structural isomers are compounds with the same empirical formula but a different atomic arrangement, e.g., methamphetamine and phentermine. Stereoisomers differ in the three-dimensional arrangement of the atoms attached to at least one asymmetric carbon and are nonsuperimposable mirror images. Therefore, amphetamine and methamphetamine occur as both d- and L-isomeric forms. The two isomers together form a racemic mixture. The D-amphetamine form has significant stimulant activity, and possesses approximately three to four times the central activity of the L-form. It is also important to note that the d- and L-enantiomers may have not only different pharmacological activity but also varying pharmacokinetic characteristics. 

Pure activated aluminas are also capable of catalyzing the skeletal isomerization of olefins (104, 105), but at considerably higher temperatures (350°-400°C) than those required for double-bond isomerization. The results obtained by Pines and Haag (105,106) leave little doubt that this type of isomerization is acid catalyzed. They found that (a) skeletal isomerization of cyclohexane or 3,3-dimethylbutene-l over pure alumina was poisoned upon ammonia addition and (b) the order of appearance of products from 3,3-dimethylbutene-l isomerization as contact time is increased was that predicted from carbonium ion theory. They also used indicator tests to show that the seat of acid activity in -y-alumina consists of Lewis, not Br0nsted, acidity. Independent infrared studies of pyridine chemisorbed on pure alumina have verified the existence of Lewis acidity and the absence of Brpnsted acidity in pure alumina (23, 107). 

Materials 27 and 28 decomposed above ca. 200 °C. Regarding the high thermal stability of the model compounds, this result was unexpected and was attributed to the presence of unreacted end-groups. None of the l,l -isomeric structures 28 exhibited liquid crystal properties before decomposition. Interestingly, oligomers 27 clearly showed nematic phases between ca. 180—200 °C. 

When the hydrogen and the bromine are anti-periplanar to each other, and the attacking species is also in that same plane, then the alkene isomer with the phenyl groups cis to each other would be formed. This type of elimination is called a /-elimination. If the d,l isomeric pair of the same compound reacted in the same way, what would be the stereochemistry of the resultant alkene from each optical isomer ... 

If instead of eliminating HBr from this compound, a bromine molecule is eliminated by the action of an iodide ion, what would be the geometry of the resultant alkene from the meso isomer and from the d,l isomeric pair ... 

A rule of thumb for determining the D/L isomeric form of an amino acid is the "CORN" rule. The... 

The isomerization of 1-hexene to cis- and tra 5-2-hexene was performed with [RuAu( M-H)2(CO)(PPh3)4](PF6)J l Isomerization of 1-pentene was studied in the presence of [H3Ru4 Au(PPh3) (CO)i2] and [H2Ru4 Au(PPh3) 2(CO)i2]. These two clusters are clearly more active for 1-pentene isomerization than the parent [H4Ru4(CO)i2], in marked contrast to the behavior of the ruthenium-copper analogs. 

All metabolic reactions in an organism are catalyzed by enzymes that are generally specific for either the D or the L isomeric form of a substrate. If an animal (an herbivore in this case) is to be able to digest the protein from a plant and build its own protein from the resulting amino acids, both the animal and the plant must make their proteins from amino acids having the same configuration. 

Certain L-isomeric phospholipids with their gel transition temperature near body temperature can produce neurotoxic side effects as reported in the early literature on Iysopho.spholipids. which suggests that the choice of lipids for the formulation of emulsions intended for drug delivery should be carefully considered (50). 

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