Carbon-13 autocorrelation NMR using double-quantum coherence

Advanced NMR approaches for a detailed structure analysis of natural products

Some new nuclear magnetic resonance (NMR) approaches to elucidate chemical structures, which have not been determined by routine NMR methods, are presented. Selective detection of methine (CH), methylene (CH(2)), or methyl (CH(3)) signals in each subspectrum by editing NMR methods was utilized to reduce the complexity in crowded spectra. It also increased the peak separation and enhanced the sensitivity by limiting the measuring area of the 2D spectra. Several 2D methods to measure (2,3)J(CH) values, which are useful for stereochemical assignment are then introduced. To determine the structure of a highly hydrogen-deficient molecule, efficient correlation methods for long-range (13)C-(13)C coupling and (1)H-(15)N HMBC are also described.

Analysis and elimination of artifacts in indirect covariance NMR spectra via unsymmetrical processing

Indirect covariance NMR offers an alternative method of extracting spin-spin connectivity information via the conversion of an indirect-detection heteronuclear shift-correlation data matrix to a homonuclear data matrix. Using an IDR (inverted direct response)-HSQC-TOCSY spectrum as a starting point for the indirect covariance processing, a spectrum that can be described as a carbon-carbon COSY experiment is obtained. These data are analogous to the autocorrelated 13C-13C double quantum INADEQUATE experiment except that the indirect covariance NMR spectrum establishes carbon-carbon connectivities only between contiguous protonated carbons. Cyclopentafuranone and the complex polynuclear heteroaromatic naphtho[2',1':5,6]-naphtho[2',1':4,5]thieno[2,3-c]quinoline are used as model compounds. The former is a straightforward example because of its well-resolved proton spectrum, while the latter, which has considerable resonance overlap in its congested proton spectrum, gives rise to two types of artifact responses that must be considered when using the indirect covariance NMR method.

1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid. A novel cyclic amino acid from halophilic phototrophic bacteria of the genus Ectothiorhodospira

A novel cyclic amino acid was detected in and subsequently isolated from extremely halophilic species of the bacterial genus Ectothiorhodospira. The structure of this new compound was elucidated by a combination of nuclear magnetic resonance (NMR) techniques and mass spectrometry. 1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid is only accumulated within the cytoplasm under certain growth conditions and seems to serve an osmoregulatory function. There is no previous reference to this molecule in the chemical literature and we, therefore, propose to use the trivial name 'ectoine', due to its discovery in members of the bacterial genus Ectothiorhodospira. (formula: see text).