Apparent elimination

Some studies seeking preferred conditions for this reaction have been reported. Optimum yields of 1-ethoxy-1-propyne and 1-ethoxy-l-butyne are found when the product is worked up before allowing the ammonia solvent to evaporate, as the product evidently volatilizes with the ammonia. An experiment with 1-ethoxy-1-propyne showed a marked increase in yield when ammonia predried over calcium hydride was used instead of ammonia directly from the cylinder. A twofold excess of ethyl bromide is required to obtain a good yield of l-ethoxy-l-but5me, since elimination apparently competes with alkylation in this case. 

Hilado and Clark report difficult-to-find autoignition temperatures while eliminating apparent discrepancies in literature values. Table 1 shows their compilation. 

Effect of Cholesterol. Cholesterol inclusion into the lipid bilayers composed of DPPC or DSPC, eliminates apparent Tc and reduces permeability at and above the usual Tc. On the other hand, cholesterol inclusion increases packing of fluid bilayer composed of lipids with unsaturated fatty acyl chains. Since cholesterol rich liposomes are stable in plasma, cholesterol is commonly used as a liposomal component. 

Recent work shows that 2,2-dichloroenamines are available from 2,2,2-trichloro-ethylamines and t-butoxide. Using two equivalents of base, the elimination apparently proceeds further to give 2-chloroethynyl amines 22. These compounds cannot be isolated but their formation is suggested by a strong IR band at v = 2200 cm-1. Only l-(t-butoxy)-2-chlorovinylamines 23 are isolated with excess tert-butoxide (46) 87>. 

The internal salt (Me0-C0 N-S02-NEt3), derived from methyl(carboxy-sulphamoyl)triethylammonium hydroxide, is effective in dehydrating alcohols (36) to give olefins (38), in anhydrous benzene at room temperature. " The cis proton is eliminated, apparently through an ion-pair mechanism (37). Tertiary... 

The three most important indices in pharmacokinetics are clearance (related to the rate of elimination), apparent volume of distribution V the volume into which a drug distributes in the body) and bioavailability (the fraction of a drug absorbed into the systemic circulation). 

ABSORPTION, DISTRIBUTION, AND EXCRETION Gastrointestinal (Gl) absorption of amphotericin B is negligible and intravenous dehvery is used. Amphotericin B in plasma is >90% bound to proteins. Drag elimination apparently is unchanged in anephiic patients and those on hemodialysis. Hepatic or biliary disease has no known effect on drag metabolism in humans. The terminal phase of elimination has a tj, of 15 days. Concentrations of amphotericin B in fluids from inflamed pleura, peritoneum, synovium, and aqueous humor are approximately two-thirds of plasma trough concentrations. Little amphotericin B penetrates into cerebrospinal fluid (CSF), vitreous humor, or amniotic fluid. 

As an example of ion pairing, consider the elimination of meso-l,2-dichloro-l,2-diphe-nylethane in Eq. 10.87. Here, syn elimination occurs 13% of the time. Addition of 18-crown-6 completely wipes out any syn elimination. Apparently the small percent of syn elimination arises from an ion pair where the potassium cation bridges the base and the leaving group (see margin). Addition of the crown ether negates the ability of the K to act in this manner. 

It is obvious that the chirality will be eroded if the p-hydride elimination occurs at the chiral a-position of tosyUiydrazones. The selectivity of p-hydride elimination in this reaction can be interpreted as follows. Alkylpalladium complex, the intermediate generated after migratory insertion of palladium carbene, is favorable to afford 1,1-disubstituted chiral olefin (Fig. 18, path a) because of the less steric interactions in the transition state at p-hydride elimination. Apparently the alternative syn p-hydride elimination will lead to the erosion of chiral center. However, the latter pathway is not preferred because it leads to the eclipse of the bulky substituent with the methyl group of the substrate (Fig. 17, path b). 

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