A close and relevant parallel is the ionization of (the isoelectronic) primary amides. Here too calculations indicate that the 2 isomer (of the anion) is more stable in the gas phase by about 5 kcal/mol.'8 Since the isomeric anions are in equilibrium with the same conjugate acid, this is a direct measure of the difference (1 7) The calculated (>5 kcal/mol) preference of esters for the 2 conformation is also much reduced in solution, as indicated by a measured ("C NMR) value of 1.67 kcal/mol for ethyl formate. (Grindley, T. B. Terrahedron k r t . 1982, 1757-60.) Increasing the size of either the acyl or the alkyl substituent increases the preference for the Z conformation, presumably for steric reasons, so any stereoelectronic preference should be most clearly expressed in formate esters. (18) Zielinski, T. J.; Poirier, R. A.; Peterson, M. R.; Csizmadia, I. G. J . Comput. Chem. 1982, 3, 477-485. Nguyen, N. T.; Hegarty, A. F. J. Org. Chem. 1986, 51. 4703-6. in acidity between the E and 2 protons. In this case it is possible HE :N-W E-anionZ-anion to measure the equilibria, and the proton-transfer processes involved, in solution. In aqueous solvent, the base-catalyzed exchange of H E is never more than a few times faster than that of Hz; the E / Z ratio is strongly solvent-dependent, and under some conditions the solvated E isomer is actually preferred.19 (19) Perrin, C. L.; Johnston, E. R.; Lollo, C. P.; Kobrin, P. A. Abstract:The design, synthesis, and study of a molecular template, 1 la, that catalyzes its own formation from simpler components, 9 and lOd, is described. Autocatalysis is shown by the following: (1) the increase in rate of reaction of 9 and 1Od in the presence of Ila; (2) the reduction in rate of reaction in the presence of 2,6-bis(acetylamino)pyridine inhibitor; and (3) the reduction in rate of reaction when a component incapable of template behavior, 10h, is substituted for 1Od. Three kinetic pathways are elucidated: ( I ) the background bimolecular reaction (eq 6); (2) the preassociative mechanism (eq 7); and (3) the template termolecular process (eq 8). A kinetic model for the replication process is introduced, and equilibrium and rate constants are determined. Predictions for the shapes of the product growth curves are made.Helbing, J.; Mattzen, M.; Jordan, S. Abstract:The stereoselectivity of the intramolecular photocycloaddition of a series of bicyclic dioxenones has been examined.The selective formation of trans-fused photoadducts is observed in almost all cases, which upon fragmentation lead to the synthesis of bicycloalkanones with an 'inside-outside" or trans intrabridgehead stereochemical relationship. This methodology has been applied to the synthesis of several 'inside-outside" bicycloalkanones that cannot otherwise be prepared. The unusual reactivity of both the dioxanone photoadducts and the bicycloalkanone fragmentation products is described.
3215 phase to the system. As shown in Figure 1 B, plots of carboxyl chemical shift and pH values as a function of concentration exhibited abrupt and discontinuous changes at a unique concentration (29 mM). This unique concentration value was unequivocally defined by the intersection of the two least-squares straight lines observed for carboxyl carbon chemical shift values as a function of concentration (Figure IB, top). Above 29 mM, pH values remained constant and carboxyl chemical shift values decreased linearly from the value expected for monomeric laurate (184.25 ppm) toward a value expected for predominately micellar laurate (C183.1 ppm).'* To facilitate the interpretation of these data, the Gibbs phase rule was applied. The formation of micelles in the system shown in Figure 1 B resulted in thermodynamic invariance ( F = 0), as demonstrated by the invariance in pH values and the linear change in chemical shift values above 29 mM.20,21 Since the system in Figure 1 B contained three components (potassium laurate, water, and the phase rule indicated that, at constant temperature and pressure, two and three phases were present below and above 29 mM, respectively ( Figure IB). Therefore, the micelles formed in this system exhibited the characteristics of a true thermodynamic phase, and a unique cmc value was obtained.In contrast, the system shown in Figure IA contained one less phase (no acidsoap crystals) at all concentration values examined. This system contained three components and at least 1 degree of freedom at constant temperature and pressure. This degree of freedom, composition or concentration, had to be defined in order to fix the state of the system. Therefore, the composition and properties of the micellar phase, and hence the true cmc value, could change as a function of concentration, and a unique cmc value could be defined only at a particular concentration value. As a result, the measured parameters exhibited gradual and continuous changes as a function of concentration, and an approximate range of cmc values was obtained ( Figure IA).Because of thermodynamic invariance, the micellar phase in Figure 1 B had a fixed composition and, by deduction, was homogeneous and monodisperse. This deduction, which was based on the Gibbs phase rule and equilibrium thermodynamics, did not require any special knowledge of the atomic or molecular factors governing the size and shape of micelles. In contrast, in the system shown in Figure IA, the properties of the micellar phase could have changed as a function of concentration, giving rise to inhomogeneity and polydispersity.On the basis of the above comparisons, we propose that the gradual and continuous changes often observed near the cmc for ionic amphiphiles in water as a function of concentration6-I0 probably resulted from the presence of 1 or more degrees of freedom in the system. Because of this degree of freedom, such experimental measurements have provided only an approximate range of nonunique cmc values. As suggested by the results shown in Figure 1 B, ...
Several new synthetic agents show high affinity for binding adenine derivatives. The structures feature complementary hydrogen bonds that cause the molecular chelation of the purine nucleus. The high lipophilicity of the new agents permits the transport of adenosine and deoxyadenosine across organic liquid membranes. The use of synthetic receptors for small biological targets may have application in drug delivery.
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