Application of biosynthetic 13C‐enrichment using [1‐13C]‐, [2‐13C]‐ and [1,2‐13C2]‐acetate as precursor for 13C NMR spectral assignment of higher plant metabolites. The assignments of some bryonolic acid derivatives

The 13C NMR spectra of some derivatives of bryonolic acid (1) (D:C-friedoolean-8-en-3β-ol-29-oic acid) were assigned by means of 13C-enrichment, lanthanide-induced shifts (LIS) and comparison of chemical shift data between derivatives. The 13C-enriched species of 1, i.e., 1a, 1b and 1c were biosynthesized by Luffa cylindrica (Cucurbitaceae) callus fed with [1-13C]-, [2-13C]- or [1,2-13C2]-acetate, respectively. Methyl acetylbryonolates 2, 2a, 2b and 2c, methyl bryonolates 3, 3a, 3b and 3c, methyl bryononates 4 and 4a, diacetyl-3β,29-diols (3,29-diacetyl-D:C-friedoolean-8-en-β,29-diol) 5, 5a, 5b and 5c, and 3-acetyl-3β,29-diols 6, 6a and 6b were prepared from 1, 1a, 1b and 1c, and their 13C NMR spectra were recorded. The 13C concentration of the 13C-enriched species was high enough to exhibit the satellite peaks clearly, and the analysed data were very useful for this study. Thus, total assignments for 2, 3, 4, 5 and 6 were established. It was found that conversion of the methoxycarbonyl group at C-29 into an acetoxymethyl group caused complex changes in the chemical shifts of the C, D- and E-ring carbons and those of the methyl carbons linked to these rings.