Correlation information strychnine

Fig. 8.17 ROESY spectrum of the aliphatic region of strychnine (2) recorded with a 250 ms mixing time. The correlations observed and the assignment information which can be derived from them is the same as for the NOESY experiment presented in Fig. 8.15.

The progress in NMR hardware to support the structure elucidation of limited quantity samples allowed to exploit the ADEQUATE experiment to a greater extent [2,93,94]. The advantage of the ADEQUATE pulse sequence originates from C-C magnetization step transfers that allow to specifically differentiate between Jch and Jch bonds which is not possible from HMBC-based experiments. It also includes connectivity information to non-protonated carbon atoms. Thus, multiplicity-edited H-C HSQC and 1,1-ADEQUATE experiments were co-processed to yield a C—C correlation plot [28]. The map was diagonally symmetric in case of adjacent Jcc-coupled protonated carbons and asymmetric in case of Jcc between protonated and quaternary carbons. It was emphasized that the latter responses were observed at the C shift of the protonated carbon in the FI direction and the correlation at the C shift of the quaternary carbon in the F2 dimension, cf. exemplarily for the methylene Cl 1 and the carbonyl CIO in Fig. 5.12. The well-known compound strychnine served as proof-of-principle for the HSQC-1,1-ADEQUATE. 

After the feasibility study on strychnine, the bis-alkaloid vinblastine 19 was investigated by C-N HSQC-HMBC obtained again from H-C HSQC and H-N HMBC [27]. For the prediction and recognition of artefacts from spectra overlap, Martin et al. applied the approach based on indirect covariance of the parent HSQC, and aromatic solvent induced shift, which proved successful. The authors stated that no new information was created by UIC but a favourable visualization scheme was used. Figure 5.16 presents an example of a C-N correlation. 

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