Synthesis of des-n-tetramethyltriostin a from C-terminal Z-d-serine tetra-and octadepsipeptide intermediates

Des-N-tetramethyltriostin A (1), a known DNA-intercalation agent, has been synthesized from tetra- and octapeptide intermediates that have Z-d-serine at the C-terminal position. The procedure thus allows the fragment coupling and cyclization reactions leading to the synthesis of the title compound to occur without racemization at the C-terminal amino acid. Esterification of Boc-Val-OH with the p-bromophenacyl ester of Z-d-serine provided didepsipeptide Z-d-Ser(Boc-Val)-OBpa (4). Stepwise addition of the requisite amino acids provided tetradepsipeptide Z-d-Ser[Boc-Ala-Cys(Acm)-Val]-OBpa (6). Fragment coupling of the respective C- and N-deprotected tetradepsipeptides 7 and 8, derived from 6, furnished linear octadepsipeptide 9, which upon cyclization and disulfide formation gave the bicyclic octadepsipeptide 11, a known synthetic precursor to 1. The degree of racemization incurred in the alanine and valine residues of selected depsipeptides was measured and the results compared with those obtained in previous studies. It was concluded that alanine, perhaps because of sequence effects, undergoes a degree of racemization (4–10%) during hydrolysis of tetradepsipeptide 6 and octadepsipeptide 9.     

High-resolution laser excitation spectra of linear triatomic molecules: Analysis of the B2Σ+-X2Σ+ system of SrOH and SrOD

Single-mode cw dye laser excitation spectra of the (0, 0⁰, 0)-(0, 0⁰, 0), (1, 0⁰, 0)-(1, 0⁰, 0), (0, 1¹, 0)-(0, 1¹, 0) bands of the 611- to 607-nm system of SrOH and SrOD were observed and assigned. The spectrum is consistent with a ²Σ-²Σ system where the molecule is linear in both electronic states. The rotational analysis is obtained by measuring P-R separations in the excitation spectrum. A technique which greatly aided in the spectral assignment that utilizes a monochromator as a bandpass filter in obtaining excitation spectra is described. A new spin-rotation constant, γ⁽⁺⁾ - γ^(?), was found necessary to fit the l-type doubling of the (0, 1¹, 0) mode in B²Σ⁺ of SrOH. This new constant is assumed to arise from two ${}^2\text{Σ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ vibronic states, split by Renner-Teller interaction, of the (0, 1¹, 0) mode of A²Π. From a perturbation treatment of this electronic interaction, ?ω₂ of the (0, 1¹, 0) mode of A²Π was found to be 14 cm^?1. The linear structure of SrOH and SrOD in both electronic states is consistent with the model, in the one-electron approximation, in that the transition involves the promotion of the unpaired 5s to the 5p orbital localized on the strontium.     

Ab initio and semiempirical estimates of PN valence state interactions

Hartree-Fock wavefunctions for the valence states of PN arising from the lowest energy open shell configurations, 2π⁴7σ3π and 2π³7σ²3π, are generated and used in ab initio calculations of diagonal spin-orbit, off-diagonal spin-orbit, and rotation-electronic matrix elements. These results are compared with those from two semiempirical methods, one based on atomic orbital populations and the other dependent solely on atomic spin-orbit splittings. The latter method is found to be surprisingly successful in predicting the ³Δ spin-orbit constant for a series of isovalent molecules. Semiempirical estimates of the ³Δ and ³Π spin-orbit constants of AsN are given. The Hartree-Fock values of the a and b perturbation parameters are found to be 50 cm⁻¹ and 0.60 (unitless), respectively. A previously reported perturbation in the A¹Π-X¹Σ⁺ band system is shown to arise from a ³Σ^{− 1}Π spin-orbit interaction.     

Ab initio calculation of the spin-rotation constant for 2Π diatomics: Test of the Van Vleck approximation

Ab initio calculations are carried out to determine γ^Π (true), the first-order contribution to the spin-rotation constant, γ^Π, for the X²Π ground states of OH, HF⁺, and HCl⁺ and their deuterated analogs. These calculations demonstrate that the contribution of this term to the experimentally determined value of γ^Π is small, as has been previously conjectured. Furthermore, it is found in the cases under study that the Van Vleck approximation to γ^Π (true) significantly overestimates its importance, and a more realistic approximation is presented.     

Gas and liquid phase kinetics of the unimolecular isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane: The radical stabilization energy of an α-bromine atom

The kinetics of the gas phase isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane to 2-chloro-5-bromohexa-1,5-diene have been measured in a static system over the temperature range of 135–215°C, with a variation in the total pressure from 0.6 to 400 torr. For these conditions the rate constants are well represented by the Arrhenius equation: where θ = 2.303RT kcal/mole. Transition state estimates for the biradical mechanism for the isomerization of bicyclo[2.2.0]hexanes are shown to be in good agreement with these Arrhenius parameters. By comparison of the activation energy with that for the isomerization of bicyclo[2.2.0]hexane and 1,4-dichlorobicyclo[2.2.0]hexane, the radical stabilization energy of an α-bromine atom is shown to be 1.0 ± 1.8 kcal/mole. Rates are also reported in the liquid phase at temperatures of 155°C and 175°C with diphenyl ether, nitrobenzene, and dimethylsulfoxide as solvents. The observed rate constants are all faster (by a factor of 1.1–1.7) than those measured in the gas phase and display no correlation between rate and solvent polarity.