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Rate constants for the thermal isomerizations of vinylcyclopropane to cyclopentene have been measured over a wide temperature range, 577−1054 K, in a static reactor and a single-pulse shock tube; they are well represented by log(A, s^-1) = 14.3 and Ea = 51.7 kcal/mol. This activation energy is higher than two previously reported values by some 2 kcal/mol; rate constants at high temperatures are about a factor of 2 larger than calculated from the Arrhenius parameters derived from the earlier low-temperature data. The thermal decomposition and isomerization reactions of vinylcyclobutane to give ethene plus 1,3-butadiene and cyclohexene have also been followed in shock-tube kinetic studies at 839−965 K. Combining the new rate constants with those from two lower-temperature studies gives the following:  for the total consumption of vinylcyclobutane, log(A, s^-1) = 14.5 and Ea = 49.3 kcal/mol; for production of ethene and butadiene, log(A, s^-1) = 14.5 and Ea = 49.8 kcal/mol; and for isomerization to cyclohexene, log(A, s^-1) = 13.4 and Ea = 47.5 kcal/mol. These values are close to previously reported Arrhenius parameters based on lower temperature static-reactor kinetic investigations. The diradical transition structure for the vinylcyclobutane to cyclohexene isomerization appears to be strain free, while the transition structure for the vinylcyclopropane to cyclopentene conversion retains some 4.6 ± 0.9 kcal/mol of ring strain and torsional strain energy.

2 beta-Carbomethoxy-3 beta-(4-iodophenyl)tropane (beta-CIT; also designated RTI-55) is an analog of cocaine that has been developed as a single photon emission computed tomography radiotracer that labels dopamine and serotonin transporters. We have prepared the 125I- and 123I-labeled ([1R] "active" and [1S] "inactive") enantiomers of beta-CIT. Total homogenate binding of the 125I-labeled inactive isomer to baboon caudate and cortex was approximately equal to nonspecific binding of the active isomer in cortex and much lower than total binding of the active isomer in caudate. However, inactive isomer homogenate binding in caudate was somewhat higher than in cortex, and during single photon emission computed tomography scanning in vivo striatal (1S)-[123I]beta-CIT uptake was also slightly greater than in cortex. Following intravenous administration of the 123I-labeled enantiomers, the plasma clearances of the active and inactive enantiomers were not significantly different. Single photon emission computed tomography imaging demonstrated that a bolus dose of nonradioactive (1R)-beta-CIT rapidly displaced the uptake of (1R)-[123I]beta-CIT. In contrast, the brain uptake of (1S)-[123I]beta-CIT was not displaced by nonradioactive (1R)-beta-CIT using either a bolus ("kinetic") or bolus plus constant infusion ("equilibrium") paradigm for administration of the radiotracer. In scans with bolus administration of radiotracer, peak striatal uptake of the active isomer was approximately twice that of the inactive isomer. In comparison to the 123I-labeled active tracer, the inactive tracer showed earlier times to peak activity and faster washouts of activity in all brain regions. These studies demonstrate beta-CIT stereoselectivity using both homogenate binding and in vivo imaging and suggest that the inactive enantiomer may be a useful measure of the kinetics of both blood-brain barrier transport and nonspecific binding.