Solvolysis of (Z)-5-trimethylstannyl 2-adamantyl p-bromobenzenesulfonate: mechanistic implications of a record-breaking secondary alpha-deuterium kinetic isotope effect for an SN1 substrate

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.

Chemistry of the β-thiolactones: substituent and solvent effects on thermal decomposition and comparison with the β-lactones

The synthesis of a series of di-, tri-, and tetraalkyl β-thiolactones and β-lactones is described as well as their thermal decomposition with extrusion of carbon oxysulfide and carbon dioxide in two solvents of opposite polarities. The β-thiolactones are considerably more thermally stable than the β-lactones and require higher temperatures for efficient decomposition in both solvents, whatever the degree of substitution. The results are interpreted in terms of a zwitterionic mechanism for fragmentation with a change in the rate-determining step between the two series.

Cone-beam computed tomography analysis of curved root canals after mechanical preparation with three nickel-titanium rotary instruments

Cone beam computed tomography is a 3-dimensional high resolution imaging method. The purpose of this study was to compare the effects of 3 different NiTi rotary instruments used to prepare curved root canals on the final shape of the curved canals and total amount of root canal transportation by using cone-beam computed tomography. A total of 81 mesial root canals from 42 extracted human mandibular molars, with a curvature ranging from 15 to 45 degrees, were selected. Canals were randomly divided into 3 groups of 27 each. After preparation with Protaper, Revo-S and Hero Shaper, the amount of transportation and centering ability that occurred were assessed by using cone beam computed tomography. Utilizing pre- and post-instrumentation radiographs, straightening of the canal curvatures was determined with a computer image analysis program. Canals were metrically assessed for changes (surface area, changes in curvature and transportation) during canal preparation by using software SimPlant; instrument failures were also recorded. Mean total widths and outer and inner width measurements were determined on each central canal path and differences were statistically analyzed. The results showed that all instruments maintained the original canal curvature well with no significant differences between the different files (P = 0.226). During preparation there was failure of only one file (the protaper group). In conclusion, under the conditions of this study, all instruments maintained the original canal curvature well and were safe to use. Areas of uninstrumented root canal wall were left in all regions using the various systems.

Gas-Phase Diastereoselectivity of Secondary 5-Substituted (X)-Adamant-2-yl (X = F, Si(CH3)3) Cations

Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) have been generated in the gas phase (total pressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X and their kinetic diastereoselectivity toward CH318OH investigated in the 40-160 degrees C range. The experimental results indicate that the facial selectivity of IX is insensitive to the composition of the starting 1X epimers as well as to the presence and the concentration of a powerful base (N(C2H5)3). This kinetic picture, supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure for IX, that is, (Z)-5-F-adamant-2-yl (I(Z)F) and (E)-5-Si(CH3)3-adamant-2-yl cations (I(E)Si). The temperature dependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts [IX*CH318OH], characterized by a specific C2-H+...O18(H)CH3 hydrogen bonding interaction. Their conversion to the covalently bonded O-methylated (Z)- (II(Z)X) and (E)-5-X-adamantan-2-ols (II(E)X; X = F, Si(CH3)3) is governed by activation parameters, whose magnitude depends on the specific IX face accommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to that exhibited in related gas-phase and solution processes. The emerging picture indicates that the factors determining the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed media are completely different.

A Solvolysis Model for 2-Chloro-2-methyladamantane Based on the Linear Solvation Energy Approach

Solvolysis/dehydrohalogenation rates of 2-chloro-2-methyladamantane (CMA) in 15 hydrogen-bond acidic and/or basic solvents are studied. The rates of reaction in these solvents have been correlated with the solvation equation developed by Kamlet, Abraham, and Taft. The linear solvation energy relationship (LSER) derived from this study is given by the following equation: log k = -5.409 + 2.219 + 2.505alpha(1) - 1.823beta(1) where , alpha(1), and beta(1) are the solvation parameters that measure the solvent dipolarity/polarizability, hydrogen-bond acidity (electrophilicity), and hydrogen-bond basicity (nucleophilicity). A high correlation coefficient (r = 0.996, SD = 0.191) was achieved. The cavity term, which includes the Hildebrand parameter for solvent cohesive energy density, delta(H), was not found to be statistically significant for this reaction substrate. The resulting equation allows calculated rates of reaction in other solvents and provides insight into the reaction pathway. In a previously reported correlation for another tertiary chloride, tert-butyl chloride (TBC), the coefficients for alpha(1) and are significantly larger and the coefficient for is statistically significant. In addition, the coefficient for beta(1) in the TBC correlation is positive, rather than negative, indicating that the transition states for TBC and CMA are significantly different. These results demonstrate why the uses of simple solvolytic correlation methods may be invalid even for comparisons of similar type substrates, e.g., tertiary chlorides. Also, these results provide confidence in the use of multiple linear regression analysis for predicting solvolytic rates in additional solvents.

Transition state analysis for human and Plasmodium falciparum purine nucleoside phosphorylases

Recent studies have shown that Plasmodium falciparum is sensitive to a purine salvage block at purine nucleoside phosphorylase (PNP) and that human PNP is a target for T-cell proliferative diseases. Specific tight-binding inhibitors might be designed on the basis of specific PNP transition state structures. Kinetic isotope effects (KIEs) were measured for arsenolysis of inosine catalyzed by P. falciparum and human purine nucleoside phosphorylases. Intrinsic KIEs from [1'-(3)H]-, [2'-(3)H]-, [1'-(14)C]-, [9-(15)N]-, and [5'-(3)H]inosines were 1.184 +/- 0.004, 1.031 +/- 0.004, 1.002 +/- 0.006, 1.029 +/- 0.006, and 1.062 +/- 0.002 for the human enzyme and 1.116 +/- 0.007, 1.036 +/- 0.003, 0.996 +/- 0.006, 1.019 +/- 0.005, and 1.064 +/- 0.003 for P. falciparum PNPs, respectively. Analysis of KIEs indicated a highly dissociative D(N)A(N) (S(N)1) stepwise mechanism with very little leaving group involvement. The near-unity 1'-(14)C KIEs for both human and P. falciparum PNP agree with the theoretical value for a 1'-(14)C equilibrium isotope effect for oxacarbenium ion formation when computed at the B1LYP/6-31G(d) level of theory. The 9-(15)N KIE for human PNP is also in agreement with theory for equilibrium formation of hypoxanthine and oxacarbenium ion at this level of theory. The 9-(15)N KIE for P. falciparum PNP shows a constrained vibrational environment around N9 at the transition state. A relatively small beta-secondary 2'-(3)H KIE for both enzymes indicates a 3'-endo conformation for ribose and relatively weak hyperconjugation at the transition state. The large 5'-(3)H KIE reveals substantial distortion at the 5'-hydroxymethyl group which causes loosening of the C5'-H5' bonds during the reaction coordinate.

Secondary α-Deuterium Kinetic Isotope Effects: Assumptions Simplifying Interpretations of Mechanisms of Solvolyses of Secondary Alkyl Sulfonates

For solvolyses of 2-propyl and cyclopentyl sulfonates, logarithms of alpha-deuterium kinetic isotope effects (alpha-KIE) correlate linearly with logarithms of nucleophilic solvent assistance (NSA); correlations have the same slopes, but different intercepts, consistent with both solvent and structural effects on alpha-KIEs for heterolysis, further supported by recent theoretical and experimental data. It is argued that alpha- and beta-KIEs cannot yet distinguish between mechanisms proceeding via one or more transition states of similar energies. Structural, solvent, and isotope effects can be rationalized by heterolysis accompanied by NSA.

β-Deuterium Isotope Effects on Amine Basicity, “Inductive” and Stereochemical

Secondary beta deuterium isotope effects on acidity constants of ammonium ions are measured using a remarkably precise NMR titration method. Deuteration is found to increase the basicity of methylamine, dimethylamine, benzylamine, and N,N-dimethylaniline. The effect is attributed to a lowered zero-point energy of a CH bond adjacent to an amine nitrogen. The method permits a determination of the stereochemical dependence of the isotope effect in a locked piperidine, and it is found that deuteration is more effective when antiperiplanar to a lone pair. The values are consistent with a cos(2) dependence on dihedral angle, with no detectable angle-independent inductive effect.