Solid state conformational studies

L. patella from Fiji contained patellins 1-5 (50-54) [82] and earlier, solution- and solid-state conformational studies were carried out on patellin 2 (51), and the structure was determined by X-ray analysis [83]. A Lissoclinum sp. from the Great Barrier Reef yielded patellins 3 (52), 5 (54) and 6 (55) and the heptapeptide trunkamide A (56) [82]. Compounds 50-56 were all identified by interpretation of spectral data and through use of Marfey s method to determine the absolute stereochemistry of the constituent amino acids [82]. A total synthesis of the proposed structure of trunkamide A (56) revealed that the structure... 

In order to obtain more information about the receptor-active conformation of muscimol, X-ray structure analysis, and ab initio quantum chemical studies have been applied to this compound (14). The flexibility of muscimol exclusively resides in the side chain (O-C-C-N bond) and a calculated potential energy curve for the rotation about this bond of muscimol in its zwitterionic form is shown in Fig. 3. These studies have demonstrated that in order for muscimol to obtain the conformation displayed by THIP, a conformational energy of 8.9 kcal/mol is required, which makes it less likely that the receptor-active conformation of muscimol corresponds to that of THIP. On the contrary, the conformation of THIP seems to mimic a high-energy conformation of muscimol as shown on the energy curve in Fig. 3, where the dihedral angles corresponding to the solid-state conformations of the muscimol zwitterion and the THIP cation are denoted. 

The conformation of two 4-phenylsulfonyl-tetrahydropyran-2-one derivatives both in solution and in the solid state was studied (95JP0747) the tetrahydropyran-2-one ring proved to adopt a skew-boat conformation. 

Wawer, I. and Zielinska, A., C CP-MAS NMR studies of flavonoids. I. Solid-state conformation of quercetin, quercetin 5 -sulphonic acid and some simple polyphenols, Solid State Nucl. Mag., 10, 33, 1997. 

The situation for 172 was quite similar, but trends between conformers were reversed the solid-state conformer (chair-chair-boat) corresponded to the minor solution isomer of 184, whereas the minor solution isomer of 172 corresponded to the major all-chair conformer of 184. Imakura s desethano artemisinin 171 was found to exist in the all-chair arrangement by X-ray and exhibited a nontempera-ture-dependent proton NMR spectrum. MM2 studies of 171 showed that the all-chair conformer was 4-5 kcal/M more stable than the next lowest energy conformer, a chair-chair-boat, and were thus consistent with experimental observations. Studies to correlate conformer populations and energetics with antimalarial activities have not provided clear answers.141... 

The structures of synthetic pyran-based ionophores have been investigated. m-2-Alkyl-3-oxytetrahydropyrans form useful subunits for the preparation of new types of ionophores with C2 symmetry <2005T8177>. X-Ray crystallography of some of these compounds provided useful information on solid-state conformational preferences that can be related to the cation-complexation properties in solution. In a related study, the synthesis of 18-32-membered cyclic pyran-based compounds of the type shown (50 and 51) was described <1996TL343, 1995JA12649>. Studies of these compounds focused on structural elements important for control of the shape and cation-binding ability and the structures of several of the compounds were determined by X-ray crystallography. 

Another application of solid-state NMR is in the study of solid-state conformation. In solution molecules tumble rapidly, and rotations about single bonds are fast recall that in solution spectra the three methyl protons are normally equivalent (Section 4.2). In the solid state many of these motions are restricted or suppressed entirely, and many of the chemical shift degeneracies to which we have become accustomed are now split. For example, pnra-dimethoxybenzene, structure 15-1, exhibits a solution l3C spectrum in which the two methoxy carbons are equivalent to each other, as are the two substituted aromatic carbons, and each set gives rise to a single resonance ... 

Figure 1.29 Solid-state NMR studies on the microtubule-bound conformation of taxoids using fluoro-taxoid probes.

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