Solution to the Ariadne’s thread NMR challenge
Anal Bioanal Chem
Solution to the Ariadne's thread NMR challenge
Reinhard Meusinger 0
0 The winner of the Ariadne's thread NMR challenge (published in volume 406, issue 27) is: D. Fatta-Kassinos, Dept. of Civil and Environmental Engineering, School of Engineering, University of Cyprus , Nicosia , Cyprus. The award entitles the winner to select a Springer book of their choice up to a value of 100. Our congratulations!
1 ) Institute of Organic Chemistry and Biochemistry, University of Technology , Darmstadt, Alarich-Weiss-Str. 4, 64287 Darmstadt , Germany
The substance of the Ariadnes thread NMR challenge 
is linalool (Fig. 1).
For the structural determination we should first consider
the 13C NMR spectrum (Fig. 2 in Ref. ). Ten carbon signals
were detected here. In addition, the DEPT_135 NMR
spectrum reveals that two of them are quaternary carbon atoms and
three are methylene (CH2) carbon atoms. The other carbon
atoms are part of two CH groups and three CH3 groups. With
this knowledge, 17 hydrogen atoms are expected for the
structure, with an empirical formula C10H17. This is
inconsistent with the 18 hydrogen atoms observed in the 1H NMR
spectrum (Fig. 1 in Ref. ). Therefore, the hydrogen atom
with the broad signal at 2.6 ppm must bond to a heteroatom
and the empirical formula becomes C10H18X. Without the
molecular mass given, the determination of this heteroatom
is difficult. We can only use the chemical shifts of the carbon
atoms. Fortunately an INADEQUATE spectrum was given in
this challenge. Here, by means of the chemical shifts
(horizontal auxiliary lines) and double quantum coherences
(vertical auxiliary lines), the whole carbon skeleton of a compound
may be easily traced out. However, beginning with the least
shielded carbon signal (1) at 145 ppm, the following skeleton
will be obtained (Fig. 2).
Here, the carbon atoms are numbered consecutively in
descending order of their 13C NMR chemical shifts. Not all
connectivities were revealed as clearly as the connectivity of
the carbon atoms at chemical-shift-position 5 to the carbon
Fig. 1 Linalool (C10H18O, M=154.25 g mol1). IUPAC name:
3,7dimethylocta-1,6-dien-3-ol, other names: -linalool, linalyl alcohol,
linaloyl oxide, allo-ocimenol
atoms at positions 1, 6, and 7. So, the assignment of the
lowerintensity carbon signal at shift position 2 to carbon atoms 3
and 10 provides only slight cross peaks in the INADEQUATE
spectrum after approximately one day of measuring time.
By consideration of the chemical shift of carbon 5 at
73.5 ppm, the heteroatom X is presumably oxygen. This is
in agreement with the weak molecular-ion peak in the mass
spectrum at m/z 154; (1012+181+116=154). Because of
this asymmetric quaternary carbon atom, two stereoisomers of
linalool exist: the (R)-()-linalool known as licareol and the
(S)-(+)-linalool known as coriandrol. Both isomers are found
in nature, for example in lavender and in coriander.
The word coriander comes from the Latin coriandrum
which, in turn, originates from the ancient Greek koriannon.
This is similar to the name of King Minos daughter Ariadne,
which later evolved to koriannon or koriandron. The
synthetically produced linalool analyzed here is a racemic mixture of
both isomers. In the year 2000 worldwide linalool production
was estimated at 12,000 tonnes. Over half of this amount is
believed to be made by chemical synthesis, and the rest is
produced from natural plant terpenes .
Fig. 2 Working structure of the substance 1. Meusinger R ( 2014 ) Anal Biol Chem 406 : 6757 - 6761 2. Screening Information Dataset (SIDS) ( 2002 ) Initial assessment report for SIAM 14 . UNEP, Paris