Reinvestigation of the234Pa gamma-ray spectrum

The decay of²³⁴Pa(UZ) was reinvestigated using high-resolution coaxial and planar HPGe detectors. Accurate energy and intensity values of 327 photons are given, of which 144 previously unobserved.     

Reinvestigation of the matrix i.r. spectrum of peroxyacetylnitrate (PAN)

The mid-i.r. spectrum of peroxyacetylnitrate (CH₃COO₂NO₂,PAN) isolated in an argon matrix is reported. It is shown that a number of matrix bands attributed earlier to PAN are to be assigned to decomposition products.     

A Reinvestigation of the Dichlorination of Thymidine

The reaction of thymidine with thionyl chloride in HMPA, reported¹ to give pure 1-(3,5-dichloro-2,3,5-trideoxy-β-D-threo-pentofuranosyl)thymine, has been found to give a mixture of erythro and threo isomers. Upon reacting thymidine with thionyl chloride, according to the literature procedure,¹ a product was isolated which melted 138–141°C [reported¹ 138–140°C] and gave one spot in CHCl₃/ETOH (4:1) on Eastman silica gel sheets at R_f=0.6 [reported¹ one spot at R_f=0.7 on Baker silica gel sheets]. However, the TLC of the product in neat ethyl acetate or CHCl₃/MeOH (9:1) gave two spots. A sample was separated into pure components by flash chromatography².     

Reinvestigation of the stereochemistry of kulokekahilide-2

An attempt to carry out a total synthesis of kulokekahilide-2 (1) by macrolactonization of a seco acid prepared from a suitably protected hexapeptide and a dioxy acid moiety unexpectedly resulted in the formation of the 43-epimer (1a) of the cytotoxic depsipeptide, for which structure 1b has previously been proposed. A second attempt involving macrolactamization of the corresponding amino acid gave the target product, 1b, but the spectral data of the product did not match those of natural 1. Furthermore, neither 1a nor 1b showed any cytotoxicity, from which it is concluded that the structure of natural 1 is incorrect and should be re-examined.     

Reinvestigation of Reported Nucleophilic Demercurations

Reaction of 8-mercurio-1-naphthalenecarboxylate with iodide or bromide ions in aqueous acetic acid yields the corresponding 8-halogenomercurio-l-naphthalenecarboxylic acid together with 1-naphthalenecarboxylic acid and not the 8-halogeno-l-naphthalenecarboxylic acid.     

Reinvestigation of the Structure of Protonated Lysine Dimer

To better understand inconsistencies between the predicted infrared (IR) spectra of previously suggested isomers of Lys₂H⁺ reported by Wu et al. (J. Am. Soc. Mass Spectrom. 22:1651–1659, 18) and the experimental IR photon dissociation (IRPD) spectrum obtained by Oh et al. (J. Am. Chem. Soc. 127:4076–4083, 4), the structure of Lys₂H⁺ was reinvestigated using IRPD spectroscopy in the extended region 2700–3700 cm^?1 and theoretical calculations. The new experimental IRPD spectrum is in good agreement with Oh’s spectrum in the corresponding wavelength range. Based on calculations at the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) and MP2/6-31?+?G(d,p)//MP2/6-31?+?G(d,p) levels, a new salt-bridged isomer, ZW1, was found to be the most stable isomer; it is more energetically favored than the previously suggested charge-solvated isomer LL-CS01 by 10 or 26 kJ mol^?1. Although the calculated IR spectrum of ZW1 is in good agreement with the experimental one in the range 2700–3700 cm^?1, it is in poor agreement with the previous IRPD spectrum in the range 1000–1900 cm^?1. This investigation shows that the intermolecular interactions inside the dimer are more complex than previously supposed. It is possible that both salt-bridged and charge-solvated isomers of Lys₂H⁺ are stable in the gas phase, and the isomers generated during ionization are sensitive to the experimental conditions.

Reinvestigation of the electronic spectroscopy of the Au-Ar complex

The Au-Ar complex is reinvestigated employing resonance-enhanced multiphoton ionization spectroscopy. Spectra are reported, corresponding to the atomic transition Au(6p<--6s). This electronic excitation yields (2)Pi and (2)Sigma(+) states of Au-Ar, which interact under the influence of spin-orbit coupling. The spectra are consistent with strong sigma-pi mixing induced by the large spin-orbit coupling of Au, leading to strong interaction of the two Omega=12 states, which arise from the Ar((1)S(0))+Au((2)P(12,32)) asymptotes, and the consequent formation of a "shelf" on the outer wall of the lowest Omega=12 state. In addition, high-level ab initio calculations are reported on the ground electronic state, X (2)Sigma(+), including extrapolation to the basis set limit.     

A Reinvestigation of the Reaction of Hmpa With Ketoximes

A series of ketoximes have been found to react with HMPA in the absence of acid catalysis to produce N,N-dimethyl-N′-arylamidines in reasonable yields.     

Reinvestigation of the rearrangement of 2-acetylbenzofuran oxime tosylate

The acetal, resulting from the rearrangement of anti-2-acetylbenzofuran oxime tosylate (I) was proved to be a mixture of trans- (IX) and cis- (X) 2,3-dihydro-2-acetyl-2,3-dimethoxybenzofuran. The structure of these isomers as well as that of their oximes XI and XII was elucidated by ir, nmr, and mass spectral investigation and by the ir and nmr spectra of the benzoyl (XIII and XIV) and acetyl (XV and XVI) derivatives of the oximes. A new reaction mechanism concerning the rearrangement process is discussed.     

Reinvestigation of the regioselectivity of deprotonation of ketone dimethylhydrazones

The lack of regioselectivity in the deprotonation of ketone dimethylhydrazones by lithium diethylamide has been determined by ¹³C NMR spectroscopy of deuterium labelled samples.     

Reinvestigation of the synthesis of 5-arylamino-2-picolines

Reaction of 5-bromo- or 3-bromo-2-methylpyridine with o-nitroaniline gave the corresponding N-[2-methyl-5(or 3)pyridyl]-o-nitroanilines. Reduction to the corresponding amino derivatives and ring closure to 1-[2-methyl-5(or 3)pyridyl]benzotriazole allowed the structures to be confirmed and an earlier literature report to be corrected. Displacement of bromide by anthranilic acid from 5-bromo-2-methylpyridine and decarboxylation gave N-(2-methyl-5-pyridyl)aniline.     

Reinvestigation of the Conformation of Cyclosporin A in Chloroform

New investigations of cyclosporin A in CDCl₃ have been performed to obtain additional and more accurate distance restraints than utilized in our previous studies of cyclosporin A. Build-up rates at 600 MHz using 6 different mixing times at low temperatures (252.5 K) were determined and transformed into distances using the two-spin approximation. With the new distance restraints in the MD simulations using the GROMOS package, we can unambiguously conclude the presence of a βII′-turn. The new structure resembles the X-ray structure more than the structure previously determined, especially regarding the orientation of the MeBmt side chain. In the new structure and in the solid state, the side chain is folded over the backbone (although there are substantial differences in the χ₁ torsion), in contrast to the old structure, where the side chain is extended away from the backbone.