Through the looking-glass challenge

Analytical and Bioanalytical Chemistry, Sep 2017

Reinhard Meusinger

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Through the looking-glass challenge

Chemistry” (ABC) homepage at abc and in the journal (volume 410/issue 10) Through the looking-glass challenge Reinhard Meusinger 0 0 Institute of Organic Chemistry and Biochemistry, Darmstadt University of Technology , Alarich-Weiss-Str. 4, 64287 Darmstadt , Germany 1 Reinhard Meusinger 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” (ABC) 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 your knowledge and tease your wits in diverse areas of analytical and bioanalytical chemistry by viewing this collection. In the present challenge, spectroscopy 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… scientific explanation. In 1849 Louis Pasteur noticed that the crystals of tartaric acid come in two asymmetric forms that are mirror images of one another. He deduced that the molecule in question is asymmetric and could exist in two different forms that resemble one another as would left- and right-hand gloves. The theoretical explanation for this phenomenon was given by van 't Hoff [1] and Le Bel [2] 3 years later in 1874. - Meet the challenge “How would you like to live in Looking-Glass House, Kitty?” Alice asked her cat in the novel Through the Looking-Glass, and What Alice Found There (1871). Only 6 years after Alice had experienced her adventures in Wonderland, the author Lewis Carroll (Charles Lutwidge Dodgson) lets Alice enter another fantastical world, this time by her climbing through a mirror into another world. “I wonder if they’d give you milk in there? Perhaps Looking-Glass milk isn’t good to drink…” Alice says questioningly to her cat. Specular optical activity had been known for a while then, yet it was still lacking The compound we are looking for in this challenge exists in two mirror-image forms as well. One of these forms rotates the plane of polarization of linearly polarized light clockwise (we will call it the (+)-compound) and the other rotates it counterclockwise (the (−)-compound). An interesting feature of these two forms is that they differ significantly in their smell and taste. Hence, it should not come as a surprise that both compounds are found separately in different plants, although they also exist as a racemic mixture. The name of this substance derives from one of the oldest known medicinal plants, which contains a particularly high proportion of it in its seeds. Seeds of this plant have been found at excavations of 3000-year-old pile dwellings. Today, this plant is widely cultivated throughout practically all of Europe. However, the global leader in the seed oil export of this plant is a Scandinavian country with long hours of sunlight during the summer, which leads to fruits with higher levels of essential oil. The fruits and the essential oil are used in many ways in cooking and in the preparation of certain medicines and liqueurs. The fruits are used in breads, cheeses, and even desserts, whereas the fruit oil is used as a fragrance in soaps, lotions, and breath fresheners, and has a long tradition of use in folk medicine. The etymology of the name of this miracle-working plant is complex and poorly understood, because it has been called by Fig. 1 The 500-MHz 1H-NMR spectra of both optical isomers, measured in dimethyl-d6 sulfoxide. The spectrum of the (+)-compound is colored blue and the mirrored spectrum of the (−)compound is colored red many names in different regions, with names deriving from Arabic, Greek, which was adapted into Latin, and Sanskrit. The English use of the name dates back to the middle of the fifteenth century and is considered to be of Arabic origin, probably. To the best of our knowledge, the compound we are looking for was first isolated in 1840 from its essential oil, and Berzelius named it by suffixing “ol” to the plant’s Latin name. The difference between two compounds, both isolated from essential oils, was described in 1876 as “Y is already Fig. 2 The spectra from Fig. 1 in greater detail distinguished physically from X by the fact that it penetrates much less into cork, and does not show the property of gnashing when rubbing on a glass bottleneck, like X and other thin-liquid essential oils” [ 3 ]. Despite the difficulty in determining the structure, the empirical formula was determined correctly as C10H14O. Today, structural analysis is facilitated greatly by use of spectroscopic methods. First, we look at the 1H-NMR spectrum of the substance, measured in dimethyl-d6 sulfoxide. In Figs. 1 and 2, the 1H-NMR spectra of both optical forms are shown in an unusual mirrored manner, in which the spectrum of the (+)-compound is face up and the spectrum of the (−)-compound is face down. The similarity of both spectra is shown in detail in Fig. 2. Only a small incongruity is observed here, caused by an enantiomeric impurity of the (−)-compound. In Fig. 3, the infrared spectra of both optical isomers are given in the same manner. No deviation between the two enantiomers is observed here as well, as expected. Note the strong absorption at 1669.4 cm-1. A set of two-dimensional NMR spectra are provided to make possible structural analysis (Fig. 4). In addition to the 1H correlated spectroscopy (COSY) spectrum, the 13C heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) spectra are also shown here. Whereas the COSY experiment generally correlates protons via a geminal spin coupling (2JH–H) or a vicinal spin coupling (3JH–H), in the heteronuclear correlated spectra the direct couplings (1JH–C) and couplings over two (2JH–C–C) or three (3JH–C-C–C) covalent bonds are indicated. All twodimensional spectra were pictured with comparable 1HNMR chemical shift (F2 dimension) scaling in the range from 1 to 7 ppm. For easier interpretation, note the 13C distortionless enhancement by polarization transfer (DEPT) spectrum in F1 projection of the HSQC spectrum. (+) - unknown compound Fig. 4 The two-dimensional 1H correlation spectroscopy spectrum (top), and the 13C heteronuclear single quantum correlation spectrum (middle) and multiple bond correlation spectrum (bottom). The spectra of both isomers were superimposable; therefore, only the spectra of the (+)compound are pictured here Here, all CH2 carbon atoms show negative signals, whereas the quaternary carbon atoms are not detected (as in the HSQC spectrum). The HMBC spectrum was truncated for clarity in the 13C-NMR chemical shift (F1) dimension. Can you identify the two compounds described in this challenge? We invite our readers to participate in the Analytical Challenge by solving the puzzle above. Please send the correct solution to by December 1, 2017. Make sure you enter “Through the looking-glass 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 1. van't Hoff JH . Sur les formules de structure dans l'espace . Arch Neerl Sci Exactes Nat . 1874 ; 9 : 445 - 54 . 2. Le Bel J-A. Sur les relations qui existent entre les formules atomiques des corps organiques et le pouvoir rotatoire de leurs dissolutions . Bull Soc Chim Paris. 1874 ; 22 : 337 - 47 . 3. Flueckiger FA . xxx. Ber Dtsch Chem Ges . 1876 ; 9 : 468 - 74 .

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Reinhard Meusinger. Through the looking-glass challenge, Analytical and Bioanalytical Chemistry, 2017, 5795-5798, DOI: 10.1007/s00216-017-0539-8