Kemp acid

NH2 an increase of the yield of a reaction of the Kemp acid derivative 117 owed to template action ofthe product 118a [24] (Figure 5.4). The... 

Y. Ikeura, K. Kurihara, T. Kunitake, Molecular Recognition at the Air-Water Interface. Specific Binding of Nitrogen Aromatics and Amino Acids by Monolayers of Long-Chain Derivatives of Kemp Acid , J. Am. Chem. Soc., 113, 7342 (1991)... 

In another approach, the catalyst performs different functions. As an example, different functions are combined in the catalyst, thus having a chiral catalyst covalently bound to a light-absorbing sensitizer [39, 40]. Derivatives of Kemp acids have proven to be excellent complexing agents, and when a sensitizer moiety is incorporated, the cycloaddition occurs with excellent stereoselectivity at —25 °C, which does not diminish much by shifting to room temperature. These characteristics make the reaction suitable for the application of solar light (Scheme 8.22). 

Scheme 8.22 A Kemp acid derivative functions both as photosensitizer (through the thioxanthone moiety) and as complexing agent...

A large variety of receptor molecules and catalytic sites is based on the idea that the inner cavity of macrocycles entraps metal ions or guest molecules if several binding groups are present. Protonation or thermal movement of these binding sites may then cause release of the metal ion or guest molecule [4]. Cyclic molecules usually only bind ions in water, e.g. metal ions, amines or phosphates, tubular molecules, in particular cyclodextrin, also recognize hydrophobic surfaces. Kemp acid derivatives finally provide a concave cavity with sizes up to about 10 A, in which dipole-dipole interactions are sufficient to bind molecules [5]. 

Kemp s Acid Enzyme-Cleft and Self-Replication Models... 

Kemp s Acid Enzyme-Cleft and Self-Replication Models 347 5.2.1 Enzyme-Cleft Models with Convergent Functional Croups... 

J. Rebek, Jr., (1987) first developed a new synthesis of Kemp s acid and then extensively explored its application in model studies. The synthesis involves the straightforward hydrogenation (A. Steitz, 1968), esterification and methylation of inexpensive 1,3,5-benzenetricar-boxylic acid (trimesic acid 30/100 g). The methylation of the trimethyl ester with dimethyl sulfate, mediated by lithium diisopropylamide (V. J. Shiner, 1981), produced mainly the desired aff-cis-1,3,5-trimethyl isomer, which was saponified to give Kemp s acid. 

Heating Kemp s acid with appropriate aromatic diamines yields bis-imides with two convergently oriented carboxylic acid groups on the edges of a hydrophobic pocket. Dozens of interesting molecular complexes have been obtained from such compounds and can be traced in the Journal of the American Chemical Society under the authorship of J. Rebek, Jr., (1985 and later e.g. T. Tjivikua, 1990 B). 

Amino-5 -deoxy-2, 3 -0-isopropylideneadenosine was acylated at N-5 with an activated derivative of the 6-carboxy-2-naphthyl ester of Kemp s acid imide. The resulting molecule possesses self-complementary binding sites, the key feature of replicating molecules that act as templates for their own reproduction. The dimer of this molecule is, however, not very stable K = 630 L mol ). When the two initially mentioned educts are added, a small proportion of the ternary complex is also formed and undergoes a fast, template-catalysed... 

The treatment of 2-hydroxyacetophenone with hydroxylamine-O-sulfonic acid in dilute aqueous base produced 3-methyl-1,2-benzisoxazole. The mechanism was reported to be a C(2)—C(3) ring closure via intermediate (560) (Scheme 171). Salicylaldehyde failed to cyclize with dilute base, but with 20% KOH and hydroxylamine-O-sulfonic acid the transformation to 1,2-benzisoxazole succeeded (76MI41600). Kemp and Woodward isolated an oxime sulfonate (561) from salicylaldehyde and hydroxylamine-O-sulfonic acid and the subsequent decomposition gave 1,2-benzisoxazole in 95% yield (65T3019). 

Formic acid is rather atypical and should be considered separately. The chromic acid oxidation has been examined in some detail by Kemp and Waters S, following the earlier work of Snethlage . At acidities of up to 3.6 M (HCIO4) the reaction follows kinetics... 

Oxidation of malonic acid by Cr(VI) has been noted briefly by Snethlage and later by Kemp and Waters . The kinetics are simple second-order but the acidity dependence is complex. Heckner et a/. find the alkaline permanganate oxidation of malonic acid (and also of o- and p-toluic acids and of p-toluene-sulphonic acid) to be retarded by added Mn(VI), viz. 

Kemp and Waters found a primary kinetic isotope effect of 8.7 for oxidation of C-deuterated mandelic acid and noted a large difference in rate between the oxidations of mandelic acid k at 24.4 °C = 1.7 l.mole . sec ) and a-hydroxy-isobutyric acid ( 2 at 24.4 °C = 5.6 x 10 l.mole . sec ) — a difference not reproduced for the oxidation of these compounds by the one-equivalent reagent, manganic sulphate. The various data are fully in accord with a Westheimer-type mechanism, viz. 

Kemp and Waters also found the oxidations of cyclohexanone and of mandelic, malonic and a-hydroxyisobutyric acids by Cr(VI) to be Mn(II)-catalysed. In these cases, as with oxalic acid, the [Cr(VI)] versus time plots are almost linear and the reaction becomes first order in substrate (or involves Michaelis-Menten kinetics), and, except at lowest catalyst concentrations, approximately first order in [Mn(II)]. Detailed examination of the initial rate of oxidation of a-hydroxyrobutyric acid as a function of oxidant concentration revealed, however, that the dependence is... 

Although Ce(IV) oxidation of carboxylic acids is slow and incomplete under similar reaction conditions , the rate is greatly enhanced on addition of perchloric acid. No kinetics were obtained but product analysis of the oxidations of -butyric, isobutyric, pivalic and acetic acids indicates an identical oxidative decarboxylation to take place. Photochemical decomposition of Ce(IV) carbo-xylates is highly efficient unity) and Cu(ll) diverts the course of reaction in the same way as in the thermal oxidation by Co(IIl). Direct spectroscopic evidence for the intermediate formation of alkyl radicals was obtained by Greatorex and Kemp ° who photoirradiated several Ce(IV) carboxylates in a degassed perchloric acid glass at 77 °K in the cavity of an electron spin resonance spectro-... 

C-C fission to give IV for both V(V) and Mn(III). Kemp and Waters have established two main features of these oxidations, namely, (i) for the oxidations of QH5CD(0H)C02H and the light compound are V(V), 2.0 Mn(III) sulphate, 1.2 Ce(lV) sulphate, 1.1 and (i7), that the trend of rates of oxidation of mandelic, a-hydroxyisobutyric, lactic and glycollic acids is as expected for Ce(IV) and Mn(III) if stabilisation of the radical RCHOH is important, but is altered for V(V) (Fig. 4). It appears from the latter observations that the presence of a-hydrogen atoms causes a drop in rate by a factor of almost 10 per a-hydrogen... 

Sengupta and Aditya ° find the usual reactivity sequence for Ce(IV) salts, viz. CIO4" > NOj" > S04 , but note that plots of log (Ce(lV)] versus time are linear even when equal molar concentrations of Ce(IV) and malonic acid are taken. This implies a first-order dependence on [Ce(IV)] and zero-order dependence on malonic acid concentration. Kemp , however, has found a clear first-order dependence on malonic acid concentration for the Ce(IV) sulphate oxidation, using an excess of reductant and making a four-fold variation in reductant concentration. Moreover, consumption of Ce(lV) was intermediate between first- and second-order. Further work is needed to resolve this discrepancy. 

The geometric nature of the problem is posed by the very structure of a carboxylic acid 4. If two, or even more, of such functions are to converge on a single basic site, the least one can anticipate is that a U-turn must be engineered into the system. This arises from the direction of the carbonyl-C, bond which points in a direction roughly opposite to the direction indicated by the OH bond. From this perspective, the tricarboxylic acid 5 a first described by Kemp 8) offers an unusually favorable architecture. In this, three methyls force the smaller carboxyl groups to assume a triaxial... 

Metges, C. C., Kempe, K. and Wolfram, G. (1994) Enrichment of selected serum fatty acids after a small oral dosage of (1 C 13)triolein and (8 C 13)triolein in human volunteers analyzed hy gas chromatography combustion isotope ratio mass spectrometry. Biological Mass Spectrometry 23,295 301. 

M. Wei wer, C.-C. Chen, M. M. Kemp, and R. J. Linhardt, Synthesis and biological evaluation of non-hydrolyzable 1,2,3-triazole-linked sialic acid derivatives as neuramidase inhibitors, Eur. J. Org. Chem. (2009) 2611-2620. 

F.S. Kemp, IFC, "Status of Development of Water - Cooled Phosphoric Acid Fuel Cells," in Proceedings of the Second Annual Fuel Cell Contractors Review Meeting, U.S. DOE/METC, 1990. 

Figure 15 Examples of artificial collagen model peptide knots, (a) Regioselective artificial cystine knot, (b) dilysine scaffold,(c) c/s,c/s-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid Kemp tri acid , (d) TREN-(suc-OH)3, (Tris(2-aminoethyl)amine) succinic acid, (e) Fe + [2,2 -bipyridyl-peptide]3, " (f) monoalkyl chains, " (g) Boc-/3-Ala-TRIS-(OH)3 knot, and (h) N-terminal GFGEEG link. ...

Aicagan ite. can incorporate up to 0.06 mol mol Cu and much smaller amounts of Cr, Mn, Co, Ni and Zn in the structure (Inouye et al., 1974 Holm, 1985 Buch-wald Clarke, 1989). Incorporation of Al, Cr and Ga has also been reported (Lorenz Kempe, 1987). Cornell (1992) produced akaganeite from acid Si-containing Fe " solutions and found by congruent dissolution that up to 0.04 mol mol Si could be incorporated. The Si species were probably located in the tunnels (0.5 nm ) of the akaganeite structure. 

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