Addressing the stereochemistry of complex organic molecules by density functional theory-NMR: vannusal B in retrospective - PubMed (original) (raw)

. 2011 Apr 20;133(15):6072-7.

doi: 10.1021/ja201108a. Epub 2011 Mar 25.

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Addressing the stereochemistry of complex organic molecules by density functional theory-NMR: vannusal B in retrospective

Giacomo Saielli et al. J Am Chem Soc. 2011.

Abstract

We have employed density functional theory (DFT) protocols to calculate the NMR properties of the vannusals, a class of natural products whose structures have been the subject of recent investigations. The originally assigned structure of vannusal B was revised after a long synthetic journey which generated a series of closely related diastereomers. In this work we show how DFT calculations based on density functionals and basis sets designed for the prediction of NMR spectra (M06/pcS-2 level of theory) can be used to reproduce the observed parameters, thereby offering to the synthetic chemist a useful tool to discard or accept putative structures of unknown organic molecules.

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Figures

Figure 1

Figure 1

The eight diastereomeric structures of the vannusals investigated in Ref. .

Figure 2

Figure 2

Correlation between calculated and experimental 13C chemical shifts of strychnine. Calculated data with the B3LYP/cc-pVTZ protocol are displaced by 30 ppm along the y axis for clarity. Fitting parameters of δcalc = a + _b_δexp are: a = 5.3 ppm, b = 1.0127 (B3LYP/cc-pVTZ); a = 0.7 ppm, b = 1.1117 (M06/pcS-2). The structure of strychnine is shown in the inset.

Figure 3

Figure 3

Calculated 13C chemical shifts of the originally proposed structure 2-1 plotted against (a) experimental values of natural vannusal B, 5-2; (b) experimental values of structure 2-1. The data point for C21 (a major outlier) is labelled.

Figure 4

Figure 4

Correlation between experimental and calculated 13C chemical shifts of vannusal B, 5-2.

Figure 5

Figure 5

Model systems of the “northeast” region of vannusals. TIPS, triisopropylsilyl; Bz, benzyl. For the sake of comparison, numbering is the same as in vannusals. Configuration labels refer to C21 and C25.

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