The Challenge of Non-Standard Spectral Responses and the Role of Fuzzy Structure Generation

Abstract

Examples discussed in Chapters 6 and 9 lead us to conclude that the CASE 2D NMR methodology can provide solutions for computer-assisted structure elucidation tasks in a reasonable time if the 2D NMR and MS data are true, consistent and complete (i.e. the number of observed NMR correlations is large enough to sufficiently define the connectivities within a structure). If at least one of these conditions is violated the possibility to somehow find a correct solution to the problem decreases significantly. As was shown earlier in Chapter 8, methods to overcome the presence of contradictions in experimental data have been suggested and the corresponding algorithms have been developed and implemented in the StrucEluc system. In this Chapter we will consider the different StrucEluc-based strategies for molecular structure elucidation in those situations when 2D NMR spectra contain non-standard correlations (NSCs). These strategies were developed and tested using a series of more than 60 separate structure elucidations with deliberately contradictory 2D NMR data. 2D NMR literature data, as well as raw 2D NMR spectra, were used in this study. The spectral data were entered into the program and the structures were assumed to be unknown. Later, an attempt was made to elucidate the structures on the assumption that no information existed regarding the presence of contradictions in the 2D NMR data. These problems were used to perform numerous experiments to determine and remove the contradictions. This was feasible, since the 2D NMR data related to these problems were known to contain COSY and/or HMBC connectivities of non-standard length.It was shown that the program was capable of determining the presence of connectivities of non-standard length in 90% of all cases using the MCD New Developments in NMR No. 1