Abstract

Tramadol, previously only known as a synthetic analgesic, has now been found in the bark and wood of roots of the African medicinal tree Nauclea latifolia. At present, no direct evidence is available as to the biosynthetic pathway of its unusual skeleton. To provide guidance as to possible biosynthetic precursors, we have adopted a novel approach of retro-biosynthesis based on the position-specific distribution of isotopes in the extracted compound. Relatively recent developments in isotope ratio monitoring by ¹³C NMR spectrometry make possible the measurement of the nonstatistical position-specific natural abundance distribution of ¹³C (δ¹³C_i) within the molecule with better than 1‰ precision. Very substantial variation in the ¹³C positional distribution is found: between δ¹³C_i = −11 and −53‰. Distribution is not random and it is argued that the pattern observed can substantially be interpreted in relation to known causes of isotope fractionation in natural products. Thus, a plausible biosynthetic scheme based on sound biosynthetic principals of precursor–substrate relationships can be proposed. In addition, data obtained from the ¹⁸O/¹⁶O ratios in the oxygen atoms of the compound add support to the deductions made from the carbon isotope analysis. This paper shows how the use of ¹³C NMR at natural abundance can help with proposing a biosynthetic route to compounds newly found in nature or those difficult to tackle by conventional means.