Conformations assignment

In view of the possible ambiguities which can attend conformational assignments based on vicinal coupling constants, it is fortunate that both long-range and geminal couplings each exhibit stereospecific dependences. 

The first three carba-sugars were synthesized by McCasIand and coworkers. Two other carba-sugars were prepared from myoinositol, and the remaining eleven carba-sugars have been synthesized from the Diels-Alder adduct of furan and acrylic acid. Conformational assignments of the carba-sugars were established with the aid of H-n.m.r. spectroscopy. 

However, the unambiguous conformational assignment of the UV bands of (Si- -Oct2) , (Si-/z-Dec2) , and PDHS at 350 and 365 nm is still lacking. Based on the Michl-West conformer classification,10 since these bands fall between the 375 nm all-A and 320 nm all-D, it is reasonable that they could be due to intermediate conformations such as all-T or a... 

In order to obtain robust conformational assignments from vibrational spectra without rotational resolution, it is important to predict reliable monomer frequency shifts between conformations. Harmonic B3LYP predictions were shown to correlate reasonably well with experiment [69], and simple mles based on repulsive and attractive intra-monomer interactions were developed. However, the predicting power of the B3LYP method for the energy sequence... 

The Hartree-Fock approximation also provided the basis for what are now commonly referred to as semi-empirical models. These introduce additional approximations as well as empirical parameters to greatly simplify the calculations, with minimal adverse effect on the results. While this goal has yet to be fully realized, several useful schemes have resulted, including the popular AMI and PM3 models. Semi-empirical models have proven to be successful for the calculation of equilibrium geometries, including the geometries of transition-metal compounds. They are, however, not satisfactory for thermochemical calculations or for conformational assignments. Discussion is provided in Section n. 

A conformational assignment to benzoylaziridines from IR spectra concluded (74SA(A)1471)that the c/sderivative(63, R = Ar) should have an s-cis conformation in order to be able to minimize steric interactions, while the s-trans conformation better applies to the trans derivative 64 in order to... 

All of the stereoisomerie methyl 2,3-anhvdro-6-deoxy-DL-hexo-pyranosides have been prepared by epoxidation of 2,3-unsaturated pyranosides (328-331, R = Me) with hydrogen peroxide-benzo-nitrile. The H-n.m.r. data of their 4-O-acetyl derivatives were reported, and discussed in connection with configurational and conformational assignments.223 The1 regio- and stereo-selectivity of the... 

Conformational Assignment of the Red Shift. The ubiquitous appearance of strikingly similar red-shifted Cotton effects in the ORD of membrane systems suggests some structural feature common to all. There are three obvious possibilities in terms of molecular association protein-protein interaction, protein-lipid interaction, and lipid-lipid interaction. All three have been suggested in the literature. 

Although the magnitudes of Aae for the C-8 methylene protons in dibenzo-[a,g]quinolizidines differ from those in quinolizidines, use may be made of Aae in conformational assignments.48 For example, in tetrahydropalmatine (trans-fused) (24) Aae is 0.70 ppm, whereas in O-methylcapaurine (cis-fused) (25) Aae is 0.38 ppm. 

In the anancomeric thiazine derivatives (shown in Table V), however, the trans-fused compound shows two equal couplings of 145 Hz and the cis-fused compound two equal couplings of 149 Hz.101 These results and others102 suggest caution in the use of this parameter in conformational assignments. 

Such relationships have permitted conformational assignments to be made to a variety of hexahydropyridazines. Thus 1,2-dimethylhexahydropyrida-zine (104) shows a pair of peaks (AE —2.32) corresponding (in the vapor phase) to 105 (0 = 170°) and a pair (A 0.77 eV) corresponding to 106 or 107 (0 = 65°) with 105 predominating.147,149,152... 

IR spectra of indolizidines are extremely useful for conformational assignments (71CR109), as shown in Section 3.08.2.4.2. 

Configurational and conformational assignment of cis/trans-2-tBu-5-MQ-sulfonyl-l,3-dithiane, based on lH NMR analysis (esp. 3/h-5,h-4,6 = 6.0 Hz), revealed that the trans isomer adopts the chair, and the cis isomer a twist-boat conformation in order to minimize steric and electrostatic repulsive interactions of the axial S02Me group (cf. Scheme 23) (02TL 9369). Chemical equilibration shows the trans isomer to be more stable by AG°323k = — 1.50 0.04 kcal/mol. 

Many overlapping lH NMR signals of /7-(y-aminopropyl)piperidine, which prevent conformational assignments, were converted to well resolved... 

Co(EDTA)]- has been said (to yield, on Gaussian analysis, (69) more components than are given in reference (68). However, the validity of such analyses of circular dichroism spectra is questionable (70). The ORD spectra of (—)s46-[Co(- -CHXTA)]- also compared (67) with those of the derivatives of EDTA and (+)PDTA giving a further assignment to the S(C2) series. A similar conformation assignment on complexes of L-(—)-... 

The conformational assignments may be applied in the same way to the pXr+ values of benzhydrols, but unfortunately no quantitative information was obtained because of limited substituent data in the respective Y sets. The substituent effects on the solvolyses of benzhydryl chlorides (Table 13) are treated even more precisely with equation (2) than those on the pXr values and they can be interpreted in the same way based on the conformational assignment. 

Perry NB, Aiyaz M, Kerr DS, Lake RJ, Leach MT (2001) NOESY on Neurotoxins NMR and Conformational Assignment of Picrotoxins. Phytochem Anal 12 69... 

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