TOCSY-HSQC

Haliclonacyclamine E (13) and arenosclerins A (14), B (15), and C (16) have been isolated from the marine sponge Arenosclera brasiliensis, endemic in Brazil. Crude extracts of this sponge displayed potent cytotoxic and antibiotic activities, and were subjected to fractionation by sihca-gel flash chromatography, medium pressure chromatography on a SiOH cyanopropyl-bonded column, and reversed-phase Cis column chromatography to give compounds 13-16 [18]. The structure elucidation was based on spectroscopic analysis, including HRFABMS, COSY, HSQC, HSQC-TOCSY, and HMBC NMR... 

COSY, HMQC, HMBC, and HSQC-TOCSY NMR spectra. The relative stereochemistry of each of the piperidine rings in 21 was estabhshed by analysis of NMR and NOESY spectra. However, the authors have not obtained experimental data to relate the relative stereochemistry of each piperidine moiety to the other. They suggested the relative stereochemistry indicated in 21, based on the Baldwin and Whitehead biogenetic proposal [24] (see below) and also by comparison with data for haliclonacyclamine E (5) and arenosclerin A (14). 

Nolls, P., Parella, T. Spin-edited 2D HSQC-TOCSY experiments for the measurement of homonuclear and heteronuclear coupling constants application to carbohydrates and peptides. /. Magn. Reson. 2005, 176, 15-26. 

If 2-D NMR techniques are really useful then 3-D ones must be even more so... shouldn t they A number of 3-D experiments have been devised which are in fact, produced by merging two, 2-D experiments together. The results could never be plotted in true 3-D format since etching them into an A3-sized block of glass would not be practical and viewing them as some sort of holographic projection, would probably not be feasible In essence, 3-D spectra have to be viewed as slices through the block which effectively yield a series of 2-D experiments. It is possible to combine techniques to yield experiments such as the HMQC-COSY and the HSQC-TOCSY. 

The resulting spectrum, Cindirect/ is a symmetric matrix whose axes are defined by the indirect dimension, F1. Applying indirect covariance processing to a non-symmetric data matrix such as heteronuclear 1H-13C HSQC-TOCSY spectrum affords a symmetric homonuclear 13C-13C TOCSY spectrum analogous to that described originally by Turner.22... 

The Inverted Direct Response (IDR)-HSQC-TOCSY pulse sequence is shown in Fig. 18 [58]. The experiment begins with an HSQC segment... 

Fig. 10.18. IDR (Inverted Direct Response)—HSQC-TOCSY pulse sequence. The experiment first uses an HSQC sequence to label protons with the chemical shift of their directly bound carbons, followed by an isotropic mixing period that propagates magnetization to vicinal neighbor and more distant protons. The extent to which magnetization is propagated in the experiment is a function of both the size of the intervening vicinal coupling constants and the duration of the mixing period. After isotropic mixing, direct responses are inverted by the experiment and proton detection begins.

As an example, consider the complex polyether marine toxin breve-toxin-2 (7) [59]. The proton NMR spectrum of this molecule, even at 600MHz, has considerable overlap making the establishment of proton-proton connectivity information from a COSY or TOCSY spectrum difficult at best. In contrast, the IDR-HSQC-TOCSY spectrum presented in Fig. 10.19, in conjunction with an HMBC spectrum, allows the total assignment of the proton and carbon resonances of the molecule. 

Fig. 10.19. IDR-HSQC-TOCSY spectrum of the complex marine polyether toxin brevetoxin-2 (7). The data were recorded overnight using a 500 pg sample of the toxin (MW = 895) dissolved in 30 pi of d6-benzene. The data were recorded at 600 MHz using an instrument equipped with a Nalorac 1.7 mm SMIDG probe. Direct responses are inverted and identified by red contours relayed responses are plotted in black. The IDR-HSQC-TOCSY data shown allows large contiguous protonated segments of the brevetoxin-2 structure to be assembled, with ether linkages established from either long-range connectivities in the HMBC spectrum and/or a homonuclear ROESY spectrum.

The selective excitation of the proton signal can be achieved through a heteronuclear spin, to which the proton is bonded, by the HMQC or HSQC type of heteronuclear polarization transfer. Many versions of the ID HMQC-TOCSY or HSQC-TOCSY have been proposed. The selective exeitation of the desired heteronucleus can be accomplished by using a selective pulse on the heteronuclear signal [28, 42, 55], or by using either a proton or a C CSSF [52-54]. 

Gradient enhanced versions of the ID HMQC-TOCSY and ID HSQC-TOCSY have also been published recently [31, 55]. 

Two- and Three-Dimensional HSQC-TOCSY, HSQC-NOESY,... 

FIGURE 2.5 Region of the 2D gs-HSQC-TOCSY spectrum of malvidin 3-0-(6"-0-a-rhamno-... 

Check it in 2D WIN-NMR C/H-COSY (HSQC-TOCSY, phase sensitive)... 

Load the spectrum of the gradient-assisted, inverse detected, 2D CH-HSQC-TOCSY experiment acquired with the echo-antiecho technique, D NMRDATA GLUCOSE 2D CH GCHICOTO 001999.RR. Check and if necessary correct its calibration in both dimensions. Set up a layout as for the basic HSQC spectrum. Compare the spectrum with the spectra of the basic HSQC and HMQC experiments. Use the same rows or columns to identify the additional TOCSY-peaks. 

Can be a full 3D technique, or a 2D technique. Provides information on 3H-13C framework of a molecule. Provides information complementary to that from HMBC. However, quaternary carbon are not detected using HSQC-TOCSY. 

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