Ariadne’s thread NMR challenge

Analytical and Bioanalytical Chemistry, Oct 2014

Reinhard Meusinger

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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 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]. - 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. In this challenge the usual one-dimensional 1H-NMR (Fig. 1), 13C-NMR and 135-DEPT spectra (Fig. 2), and the 2DFig. 3 The 13C heteronuclear single bond correlation spectra (HSQC). Attend the application of the DEPT spectrum in F1 projection to relieve the interpretation Fig. 4 The two-dimensional INADEQUATE spectrum of about 200 mg substance solved in CDCl3, optimized for a carbon-carbon coupling of 55 Hz and a repetition time of about five seconds. One hundred and twenty-eight scans were accumulated in 128 experiments, respectively, in a total measuring time of 23:15 h Fig. 5 Electron impact mass spectrum of the pure compound HSQC spectrum (Fig. 3) are given to show the connectivity between the hydrogen and carbon atoms in the substance. The 2D-INADEQUATE spectrum of the unknown substance is given in Fig. 4. While the carbon-carbon connectivity is traced out in the INADEQUATE spectrum like a COSY for carbon atoms, the spectrum shows no diagonal symmetry unlike the H,HCOSY spectrum. As usual, in the f2 dimension (horizontal axis) the regular carbon chemical shift is shown. On the other hand, in the f1 dimension (vertical axis) the double quantum frequency is shown, which is the sum of the frequencies of the two carbon atoms being connected. Figure 4 shows the 2DINADEQUATE spectrum, measured on a 500 MHz spectrometer for about 23 h. Each pair of connected 13C nuclei forms a spin system which is found in the same row of the spectrum. If a carbon atom is connected to more than one other carbon atom, the corresponding cross peaks are found at the same chemical shift f2, but at different double quantum frequency f1. Now, the carbon skeleton can be readily obtained by a criss-cross progression through the spectrum to string the carbon signals together much like using the Ariadnes thread. This is illustrated in Fig. 4 by dashed subsidiary lines and circles. Start with a terminal structural group like a =CH2 group, advantageously. The given numbering of the carbon atoms was carried out here in order of 13C chemical shift values beginning with the least shielded signal at 145 ppm, independently from their sequencing in the chemical structure. Last but not least, the electron impact mass spectrum is given in Fig. 5. Can you identify the substance in question? We invite our readers to participate in the Analytical Challenge by solving the puzzle above. Please send the correct solution to by December 1, 2014. Make sure you enter Ariadnes thread NMR challenge in the subject line of your e-mail. The winner will be notified by e-mail and his/her name will be published on the Analytical and Bioanalytical Chemistry homepage at and in the journal (volume 407/issue 7) where readers will find the solution and a short explanation. The next Analytical Challenge will be published in 407/1, January 2015. If you have enjoyed solving this Analytical Challenge you are invited to try the previous puzzles on the ABC homepage.

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Reinhard Meusinger. Ariadne’s thread NMR challenge, Analytical and Bioanalytical Chemistry, 2014, 6757-6761, DOI: 10.1007/s00216-014-8115-y