Ariadne’s thread NMR challenge
Reinhard Meusinger
0
) Institute of Organic Chemistry and Biochemistry, University of Technology Darmstadt
, Alarich-Weiss-Str. 4, 64287 Darmstadt,
Germany
We would like to invite you to participate in the Analytical Challenge, a series of puzzles to entertain and challenge our readers. This special feature of Analytical and Bioanalytical Chemistry has established itself as a truly unique quiz series, with a new scientific puzzle published every other month. Readers can access the complete collection of published problems with their solutions on the ABC homepage at http://www.springer.com/abc. Test your knowledge and tease your wits in diverse areas of analytical and bioanalytical chemistry by viewing this collection. In the present challenge, nuclear magnetic resonance (NMR) is the topic. And please note that there is a prize to be won (a Springer book of your choice up to a value of 100). Please read on Meet the Ariadne's thread NMR challenge In the challenge presented today we are looking for the structure of one of the major floral scents in nature. The small biochemical compound is found in essential oils of over 200 plant species growing around the world from tropical areas up to boreal regions. In particular, many plants produce this substance in significant amounts and it can contribute up to 90 % of the essential oils of these plants. The trivial name, given at the end of the 19th century, was derived from the botanical name of the plant from which it was first isolated, an often used practice in the case of natural compounds. The analytical methods at that time were labour-intensive and this becomes apparent from the 1912 study of cocoa aroma [1].
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Here, our substance was identified among the main volatile
constituents in an extract prepared from 2 t of roasted cocoa
beans. Today, in addition to natural sources, this substance is
obtained from total organic synthesis and by synthesis from
other natural precursors. Often used as an ingredient in
fragrances, it can also be found in most cosmetic products,
detergents, insecticides, furniture waxes, even in foods and
beverages. Overall, the annual worldwide consumption of this
substance exceeds 1,000 t. Owing to its chiral properties, two
enantiomers of this substance occur in nature each having a
distinct scent. Of the two enantiomers, one is named after a
well-known herb whose seeds are used as a spice. The earliest
known form of the name of this herb is from Mycenaean
Greek and is similar to the name of the Greek goddess
Ariadne. Ariadne is known for her help to Theseus who had
to kill the Minotaur living in a labyrinth. She gave him a ball
of thread to navigate the labyrinth and then safely retrace his
path out of it. In this vein, the thread of Ariadne refers to a
central guiding idea, whether physical or conceptual, used to
solve a maze or a logic puzzle. Thus, Ariadnes thread can be
the guide line used for cave diving, or a logical backtracking
algorithm for solving Sudoku puzzles. The approach in
structural analysis using spectroscopy methods is often similar to
solving a puzzle and an Ariadnes thread is most welcome
here.
The Incredible Natural Abundance Double Quantum Transfer
Experiment, which receives the acronym INADEQUATE, is
the ultimate form of structure elucidation of organic substances
in solution. In this two-dimensional nuclear magnetic
resonance experiment all carbon-carbon connectivities can be
obtained and the carbon skeleton of the molecule can therefore be
established unequivocally. So, the INADEQUATE spectrum
Fig. 1 Five hundred megahertz 1H-NMR spectrum of a solution in CDCl3 with respect to tetramethylsilane (top). The spreads of the multiplets are given
in equal extension after apodization of the FID with Exponential and Gaussian functions (bottom)
shows which carbon atoms are attached to each other in a
molecule. If we know from a DEPT spectrum what type of
carbon it is (quaternary carbon, CH, CH2 or CH3), we can
almost write down the entire skeleton structure from these two
Fig. 2 One hundred and twenty-five megahertz 13C-NMR (bottom) and
the distortionless enhanced polarization transfer DEPT-135 (top) spectra
with respect to CDCl3 at 77.2 ppm. Note the phase orientation of carbon
signals in DEPT spectrum with odd numbered bonded hydrogen atoms
(positive) opposite to those with an even number of hydrogen atoms
(negative)
NMR experiments. The main drawback of this most useful
method is its very poor sensitivity. Sadly, the 13C-13C coupling
constants are difficult to determine at the 1.1 % natural
abundance of carbon-13 isotopes because only every 10.000th
molecule (0.0110.011) contains the necessary two 13C nuclei
and the experiment that provides the best structural information
is also the least sensitive of all the common NMR experiments.
This drawback may only be compensated for by a highly
concentrated solution of the sample and by abnormally long
measurement times (up to a few days) using a direct detection
probe (...truncated)