Efficient measurement of the sign and the magnitude of long-range proton-carbon coupling constants from a spin-state-selective HSQMBC-COSY experiment

Challenges in the configuration assignment of natural products. A case-selective perspective

An overview by a case study approach on the currently available methods for the configurational analysis of natural products.

Accurate measurement of long range proton–carbon scalar coupling constants

The accuracy and ease-of-use of various experimental NMR methods for measuringⁿJ_CHvalues is assessed.

Monitoring organic reactions by UF-NMR spectroscopy

Standard 2D NMR experiments suffer from the many t1 increments needed for spectra with sufficient digital resolution in the indirect dimension. Despite the different methodological approaches to overcome this problem, these increments have prevented studies of fast reactions. The development of ultrafast NMR (UF-NMR) has decisively speeded up the time scale of standard NMR to allow the study of organic reactions as they happen in real time to reveal mechanistic details. This mini-review summarizes the results achieved in monitoring organic reactions through this exciting technique.

Extending long-range heteronuclear NMR connectivities by HSQMBC-COSY and HSQMBC-TOCSY experiments

The detection of long-range heteronuclear correlations presenting J(CH) coupling values smaller than 1-2Hz is a challenge in the structural analysis of small molecules and natural products. HSQMBC-COSY and HSQMBC-TOCSY pulse schemes are evaluated as complementary NMR methods to standard HMBC/HSQMBC experiments. Incorporation of an additional J(HH) transfer step in the basic HSQMBC pulse scheme can favor the sensitive observation of traditionally missing or very weak correlations and, in addition, facilitates the detection of a significant number of still longer-range connectivities to both protonated and non-protonated carbons under optimum sensitivity conditions. A comparative (1)H-(13)C study is performed using strychnine as a model compound and several examples are also provided including (1)H-(15)N applications.

Simultaneous determination of the magnitude and the sign of multiple heteronuclear coupling constants in 19F or 31P-containing compounds

The presence of a highly abundant passive nucleus (Z = 19F or 31P) allows the simultaneous determination of the magnitude and the sign of up to three different heteronuclear coupling constants from each individual cross-peak observed in a 2D 1H-X selHSQMBC spectrum. Whereas J(HZ) and J(XZ) coupling constants are measured from E.COSY multiplet patterns, J(XH) is independently extracted from the complementary IPAP pattern generated along the detected F2 dimension. The incorporation of an extended TOCSY transfer allows the extraction of a complete set of all these heteronuclear coupling constants and their signs for an entire 1H subspin system. 1H-X/1H-Y time-shared versions are also proposed for the simultaneous measurement of five different couplings (J(XH), J(YH), J(XZ), J(YZ), and J(ZH)) for multiple signals in a single NMR experiment.

Efficient and fast sign-sensitive determination of heteronuclear coupling constants

Two complementary 1D NMR approaches for the fast and easy determination of the magnitude and the sign of heteronuclear J(XH) coupling constants are proposed: The Up&Down technique relies on the direct analysis of anti-phase multiplets whereas the Left&Right technique is based on the relative displacement between separate IPAP components.

On the interference of J(HH) modulation in HSQMBC-IPAP and HMBC-IPAP experiments

The effects of phase modulation due to homonuclear proton-proton coupling constants in HSQMBC-IPAP and HMBC-IPAP experiments are experimentally evaluated. We show that accurate values of small proton-carbon coupling constants, (n)J(CH), can be extracted even for phase-distorted cross-peaks obtained from a selHSQMBC experiment applied simultaneously on two mutually J-coupled protons. On the other hand, an assessment of the reliability of (n)J(CH) measurement from distorted cross-peaks obtained in broadband IPAP versions of equivalent HMBC and HSQMBC experiments is also presented. Finally, we show that HMBC-COSY experiments could be an excellent complement to HMBC for the measurement of small (n)J(CH) values.

Long-range proton-carbon coupling constants: NMR methods and applications

A general review of novel NMR methods to measure heteronuclear long-range proton-carbon coupling constants ((n)JCH; n>1) in small molecules is made. NMR experiments are classified in terms of NMR pulse scheme and cross-peak nature. A discussion about simplicity, general applicability and accuracy for each particular NMR experiment is presented and exemplified. Important aspects such as the sign determination and measurement of very small coupling values involving protonated and non-protonated carbons as well as the complementarity between different experiments are also discussed. Finally, a compilation of applications in structural and conformational analysis of different types of molecules since 2000 is surveyed.

Simultaneous measurement of J(HH) and two different (n)J(CH) coupling constants from a single multiply edited 2D cross-peak

Three different J-editing methods (IPAP, E.COSY and J-resolved) are implemented in a single NMR experiment to provide spin-state-edited 2D cross-peaks from which a simultaneous measurement of different homonuclear and heteronuclear coupling constants can be performed. A new J-selHSQMBC-IPAP experiment is proposed for the independent measurement of two different (n)J(CH) coupling constants along the F2 and F1 dimensions of the same 2D cross-peak. In addition, the E.COSY pattern provides additional information about the magnitude and relative sign between J(HH) and (n)J(CH) coupling constants.

Using NMR to identify and characterize natural products

Over the past 28 years there have been several thousand publications describing the use of 2D NMR to identify and characterize natural products. During this time period, the amount of sample needed for this purpose has decreased from the 20-50 mg range to under 1 mg. This has been due to both improvements in NMR hardware and methodology. This review will focus on mainly methodology improvements, particularly in pulse sequences, acquisition and processing methods which are particularly relevant to natural product research, with lesser discussion of hardware improvements.